1. Sandeel Fisheries Compensation

1.1. Executive Summary

  1. Poor fisheries management can have important negative effects on fish stocks that many fish-eating seabirds depend upon. There are numerous examples of the benefits of improvements to fisheries management to the demography of breeding seabirds.
  2. In the North Sea the sandeel (Ammodytes spp.) is a very important forage fish and a key species for the whole ecosystem. Current fisheries management of the sandeel stocks in the North Sea has depleted these stocks below the level necessary for healthy fish-eating seabird populations.
  3. This report demonstrates that fisheries can deplete fish stocks and that this depletion has negative effects on seabird demography. Further, it shows that improvements to fisheries management can increase stocks and improve the demography of seabirds that depend upon those stocks.
  4. A review of published material has provided multiple examples of the application of Marine Protected Areas (MPAs) or No-Take Zones (NTZ’s) that have increased fish stocks and seabird populations have benefited from these increases.
  5. Based on the information in this review, further analyses of the potential effects of improved fisheries management of sandeel stocks in the North Sea Sandeel Area 4 (SA4) as a suitable compensation measure for predicted impacts from the Proposed Development was warranted.
  6. Sandeels in SA4 have been exploited as a commercial fishery for many decades. Predicted sandeel Total Stock Biomass (TSB) in SA4 declined through 1990’s and early 2000’s. A no take “box” was added to SA4 in 1999 as a tool to manage the sandeel stock for fisheries exploitation. The stock in SA4 showed signs of recovery from 2006 to 2018, but recent increases in fishing outside the box has resulted in poor sandeel stock recovery in recent years.
  7. Multiple studies have demonstrated the importance of sandeel stocks for breeding seabirds in the North Sea. Both productivity and adult survival have been shown to be negatively affected by low sandeel availability for the species being assessed here. There was also strong evidence that the foraging range for breeding seabirds is affected by prey availability, including sandeels in the North Sea. Seabirds in SA4 are likely to rely on sandeel abundance across a large part of the area, not only the area inside the box closed to fishing.
  8. Evidence of strong relationships between sandeel TSB in SA4 and population size, adult survival and productivity was shown in this study from seabird data collected by UKCEH on the Isle of May for kittiwake, guillemot and puffin. For razorbill strong relationships were shown for population size and adult survival, though not for productivity. This was thought to be due to the difficulties of studying this species, which nests, partly, in cracks in cliffs, under rocks, etc. making data gathering challenging.
  9. It was shown that there was a strong relationship between fishing effort and sandeel spawning stock biomass in SA4, suggesting that fishing effort negatively effects the sandeel stocks in SA4. In addition, previous modelling work has shown that removal or reduction in fishing pressure on sandeels in the North Sea would result in increases in sandeel populations. This is a pattern typically seen in other fisheries on other fish species around the world.
  10. There is evidence that the sandeel box only provides limited benefits to seabirds and that the appropriate spatial scale of management of the fishery to benefit seabirds is at the whole stock level (i.e., SA4). Therefore, the best available evidence shows that improvements to the management of the SA4 stock could be used as a compensation measure for predicted impacts from the proposed development.
  11. The relationships between sandeel TSB and seabird demography from the Isle of May were used to predict the likely gains to seabirds from increasing the TSB in SA4. The additional number of birds per annum predicted from these relationships was compared with the predicted impacts on the relevant SPA populations from the proposed development alone. Five compensation scenarios were used to show the range of potentially realistic changes as a result of the proposed compensation measures. The scenario that produced the smallest benefit was the change in sandeel TSB from 300,000 to 400,000 tonnes. The largest predicted change to SPA population was from the change in sandeel TSB from 100,000 to 200,000 tonnes. This was due to the shape of the relationships between species survival and productivity on the Isle of May and sandeel TSB in SA4.
  12. Across the range of likely changes in sandeel TSB in SA4, it was found that reducing or removing fishing pressure would have positive effects on adult survival and productivity. The positive effects of these predicted changes in demographic parameters were compared with the negative effects of the three predicted impact scenarios from the Proposed Development alone. Three different approaches to were made:
  • Predicted increase in number of adult birds in each SPA population and in the SPA network based on increased adult survival;
  • Predicted change in populations growth rate and size due to the effects of compensation and impacts combined using PVA; and
  • Predicted relationship between CRG and impact level compared with three impact prediction scenarios, using PVA.
  1. For all three approaches, for all species and all SPAs, it was clear that the predicted minimum benefit from reducing or removing fishing pressure in SA4 was sufficient to compensate for all predicted impact scenarios.


1.2. Introduction

  1. There are numerous examples from around the world of the negative effects of poor fisheries management on fish stocks. In many cases these fish stocks are important for the breeding success and/or survival of seabird populations that forage on these stocks. There have been numerous examples of the benefits of improved fisheries management on fish stocks and consequent benefits to seabird populations.
  2. In the North Sea a key species for the whole ecosystem is sandeels. This lipid rich fish occurs in suitable sandy habitat across the North Sea. Sandeels feed on plankton and are predated by seabirds, marine mammals and other fish. Their life cycle is well known, and populations tend to be driven by occasional especially good years. Sandeel populations can be limited by availability of plankton to feed on, but also by predation from a variety of species.
  3. There has been a commercial fishery for sandeels in the North Sea since the 1960s. At present the fishery in the North Sea is almost entirely from Danish fishing vessels, though there are some Norwegian vessels which fish in the Norwegian sector of the North Sea. Fishing mortality on sandeel populations in the North Sea has been high, with peaks of one million tonnes per annum being landed in the 1990s. The stocks are now under management, but this is based on a “surplus production” model, which aims to remove the maximum sustainable yield. This management approach maintains stocks well below the level needed for seabirds to maintain high breeding success. The stocks are recognised as seven discrete populations, with the population off the east coast of Scotland (Sandeel Area 4 – SA4) including a closed “box”. This box was introduced as a management measure in response to declining total stock biomass (TSB). While stocks have begun to recover from lows in the early 2000s, fishing activity has increased in recent years. The stock is still managed as a single entity, with no adjustment for the presence of the box.
  4. Many seabirds in the North Sea are known to be dependent on sandeels, particularly in the breeding season. Evidence from the east coast of Scotland and in Shetland has shown that breeding success of many species can be affected by low sandeel abundance. There is also evidence from kittiwakes that adult survival may be reduced if sandeel stocks are sufficiently depleted.
  5. This report demonstrates that fisheries can deplete fish stocks and that this can harm seabird populations. It will also be shown that improved management of fisheries can increase fish stocks and improve the health of breeding seabird populations dependent on those fish stocks. This report will show that current management of sandeel stocks in the North Sea, and in SA4 specifically, has reduced sandeel stocks and that this has had a negative effect on seabird populations. However, recovering sandeel stocks will be shown to have a positive effect on both adult survival and productivity of four key seabird species by comparing the TSB of sandeels in SA4 with demographic parameters from the long-term study on the Isle of May by the UK Centre for Ecology and Hydrology (UKCEH). This study will show that the current management of the SA4 fishery using the sandeel box is not at a large enough spatial scale to provide a sufficiently reliable stock for seabirds on the east coast of Scotland. Berwick |Bank Windfarm Limited (the Applicant) therefore proposes that further removal/reduction of fishing pressure on the sandeel population in SA4 is a suitable, and sufficient, compensation measure for the predicted impact from the Berwick Bank wind farm (hereafter the Proposed Development) on Special Protected Areas (SPAs) protecting:
  • Black-legged kittiwake (hereafter “kittiwake”) Rissa tridactyla;
  • Common guillemot (hereafter “guillemot”) Uria aalge;
  • Razorbill Alca torda; and
  • Atlantic puffin (hereafter “puffin”) Fratercula arctica.
  1. The report assesses the levels of uncertainty in the assessment, including the uncertainty in reaching robust conclusions from correlative data. This uncertainty affects the precaution taken in assessing the efficacy of the proposed compensation measures. Through the approach of ameliorating the effects of uncertainty by applying adequate precaution, the assessment will show that there is sufficient compensation available through the proposed reduction or removal of fishing pressure on sandeels in SA4 to a high level of confidence in the conclusions reached.

1.3. The effectiveness of Marine protected areas

  1. Compensation measures may be needed for SPA seabird populations due to predicted impacts from the Proposed Development. One of the potentially beneficial measures would be to improve the demographic parameters of seabird populations predicted to be impacted by the Proposed Development through the closure or management of fisheries of seabird prey species. As such, the influence of prey fish abundance on seabird demographics was reviewed. This review is summarised here, and the complete review is available in Annex A - Review of Marine Protected Areas.
  2. There are numerous reviews of the evidence that protected areas benefit the conservation of species, communities, and ecosystem services. In particular, fishery closures strongly promote the recovery of fish stock biomass following heavy exploitation (MacNeil et al. 2015, Cabral 2020). Fishery closures can take the form of technical measures (specified constraints on gear use within a fishery; McClanahan et al. 2014, Campbell et al. 2018), periodic or seasonal closures (Cohen and Alexander 2013), or rights-based controls on access into the fishery. Such constraints on fishing may be the most effective measures to achieve conservation objectives of marine protected areas (MPAs) (Campbell et al. 2018, Cabral et al. 2020, Vilas et al. 2020).
  3. From analysis of 87 MPAs worldwide, Edgar et al. (2014) defined five key factors that determine the effectiveness of an MPA; the extent to which fishing is limited, the level of enforcement of fisheries constraints, MPA age, MPA size, and presence of continuous habitat allowing spill over of fish or shellfish from the MPA into surrounding waters. Similarly, Zupan et al. (2018) found that the designation of MPAs alone may not result in the lessening of some human threats, which is highly dependent on management goals and the related specific regulations that are adopted.
  4. Baskett and Barnett (2015) concluded in relation to fishery no-take protected areas “Responses at each level depend on the tendency of fisheries to target larger body sizes and the tendency for greater reserve protection with less movement within and across populations. The primary population response to reserves is survival to greater ages and sizes plus increases in the population size for harvested species, with greater response to reserves that are large relative to species' movement rates. The primary community response to reserves is an increase in total biomass and diversity, with the potential for trophic cascades and altered spatial patterning of metacommunities. The primary evolutionary response to reserves is increased genetic diversity, with the theoretical potential for protection against fisheries-induced evolution and selection for reduced movement.” The potential for the combined outcome of these responses to buffer marine populations and communities against temporal environmental heterogeneity has preliminary theoretical and empirical support. However, while the benefits from many MPAs have been widely recognised, not all MPAs have successful outcomes. Giakoumi et al. (2018) reviewed 27 detailed case studies from around the world and concluded that the most important factor determining the success or failure of a MPA was the level of stakeholder engagement. This conclusion was also reached in a comparison between two MPAs for coral reef fish in the Philippines, one successful and one unsuccessful, because constraints on fishing failed at one site due to a lack of community support (Russ and Alcala 1999).
  5. Many examples of the effects of fisheries on fish stocks and the effects of MPAs on those stocks were found (e.g. Fernandez-Chacon et al. 2020, Jaco and Steele 2019, Ballantyne 2014). These are described in detail in Annex A - Review of Marine Protected Areas.
  6. One of the key benefits of MPAs is to create “spillover”. Many studies present evidence that spillover occurs from MPAs and so supports fisheries in the region (e.g. McClanahan and Mangi 2000, Gell and Roberts 2003, Abesamis and Russ 2005, Goni et al. 2008, Harmelin-Vivien et al. 2008, Stobart et al. 2009, Goni et al. 2010, Vandeperre et al. 2011, Florin et al. 2013, Huserbråten et al. 2013, Kerwath et al. 2013, Rossiter and Levine 2014, Alos et al. 2015, Di Lorenzo et al. 2016, Sackett et al. 2017, Kleiven et al. 2019, Kough et al. 2019, Marshall et al. 2019, Cabral et al. 2020, Di Lorenzo et al. 2020, Vilas et al. 2020, Sala-Coromina et al. 2021Further studies are reviewed in Annex A - Review of Marine Protected Areas.
  7. While there is much empirical evidence of increases in sizes and numbers of animals within MPAs compared with control areas outside the MPA, another approach to assessing the benefits of MPAs is to use scenario modelling. Several examples of scenario modelling are described in Annex A - Review of Marine Protected Areas.
  8. The success of very many MPAs and No Take Zones (NTZs) around the world has led to a more strategic approach to marine conservation designations in some countries. Examples are provided in Annex A from the State of Victoria, Australia, (, the northern Channel Islands, California. and the Great Barrier Reef, Australia (Sobel and Dahlgren 2004).
  9. On the high seas, 286,200 km2 of the North-East Atlantic was designated as six MPAs in international waters under the Convention for the Protection of the Marine Environment of the North-East Atlantic (the OSPAR Convention) in 2010, which is considered to be the start of a process of developing an ecologically coherent and representative MPA network in that ocean (O’Leary et al. 2012).
  10. In England, in addition to existing and new SPAs and SACs, 91 Marine Conservation Zones (MCZs) have been designated between November 2013 and May 2019 as an ecologically coherent network in terms of representation of species and habitats. In Scotland, a combination of marine extensions to SPAs originally designated for breeding seabirds, designation of marine areas as SPAs for nonbreeding seabirds, designation of SACs for marine mammals, MPAs for marine mammals, fish and marine invertebrates, comprise 225 sites providing protection over more than 37% of Scotland’s marine waters. Many of these sites have been designated within the last few years, so too recently for any assessment of changes that may follow as a consequence of management. Not all of these MPAs involve establishment of fisheries restrictions, depending on the objectives for individual sites. In addition to SPAs, SACs and MPAs, five other area-based measures include a temporary no-take zone for sandeel fishing off east Scotland, which has remained in force without any suggestion that this will be revoked.

Case studies of NTZs that influence seabird demography

  1. The review found multiple case studies of MPA’s or NTZ’s that had an influence on seabird demography. These are detailed in Annex A. Very few MPAs/NTZs have been designated with the objective to enhance conservation of seabird populations (Ronconi et al. 2012, Hentati-Sundberg et al. 2020). However, that outcome could arise if MPA/NTZ designation resulted in a reduction of seabird bycatch in fisheries, or if the MPA/NTZ resulted in a bottom-up increase in energy flow through the food web up to seabirds (i.e. increased the abundance or quality of their preferred foods; Hentati-Sundberg et al. 2020), or if MPA/NTZ designation improved the quality of breeding habitat for seabirds (for example by reducing human disturbance, removing threats from alien invasive mammal predators, or improving nest site quality).
  2. Several studies have focused on the potential of designating or managing marine protected areas for seabird conservation (Lascelles et al. 2012, Ronconi et al. 2012, Sherley et al. 2017). Studwell et al. (2021) presented a habitat prioritization approach for identifying critical areas for wildlife conservation action, including seabirds. Silva et al. (2020) investigated spatial overlap between a key forage fish species (sandeel) and humpback whale and great shearwater in the Gulf of Maine, USA. Both the cetacean and the seabird showed very strong and consistent match in spatial distribution with that of sandeel. They proposed managing protected areas for these top predators on the basis of the key role of sandeel habitat in determining predator distributions in that system.
  3. In a review of the pressures and threats to global populations of penguins, Boersma et al. (2020) identified marine spatial planning as the highest ranked conservation need to conserve endangered penguin populations, for which they particularly emphasize the need for MPAs to manage fisheries to ensure that adequate prey resources for penguins remain in areas critical to their breeding success (i.e. close to colonies) and survival (i.e. over larger spatial scales when penguins are dispersed from the colony sites).
  4. Requena et al. (2020) used tracking data from nine seabird species and one marine mammal to identify marine hotspots around Tristan da Cunha, South Atlantic Ocean. They concluded that tracking data provide reliable information that could be used to define MPAs for these top predator populations. Analyses of seabird tracking data in UK waters was considered to provide effective identification of seabird hotspots that could be designated as MPAs (Cleasby et al. 2020). Critchley et al. (2019) used seabird tracking data to test whether simple foraging radius models from colonies provide a cost-effective alternative to large-scale surveys or tracking studies. They showed that foraging radius distribution broadly matched foraging areas identified from tracking breeding adults from colonies or from aerial surveys..
  5. Perrow et al. (2015) also used a combination of tracking of breeding adults, a boat-based survey, and a foraging radius approach to define the at-sea MPA (in this case a SPA marine extension) for breeding little terns. Similarly, tracking data from marine mammals have been used to justify decisions on boundaries of MPAs, in some cases providing retrospective justification (e.g. Kirkman et al. 2016). Arias-Del-Razo et al. (2019) showed that MPAs with large populations of marine mammals still provided large gains in fish biomass (which increased with the age of the MPA), despite the presence of marine mammals that could be a major predator on those fish. However, Kelaher et al. (2015) concluded that reef fish increased less in MPAs with large seal populations than in MPAs without large numbers of seals and suggested that if the aim is to recover reef fish populations, designating MPA sites away from seal colonies may be preferable. An implication of this, of course, is that if the aim is to improve conditions for top predators, then marine habitat management that enhances populations of fish on which the predators can feed will be an effective conservation measure.
  6. Bertrand et al. (2012) showed that the foraging efficiency of breeding seabirds in Peru may be significantly affected by not only the global quantity, but also the temporal and spatial patterns of fishery removals of forage fish (in this case, anchoveta). They concluded that, together with an ecosystem-based definition of the fishery quota, an ecosystem approach to fisheries management should limit the risk of local depletion around breeding colonies using, for instance, adaptive marine protected areas around colonies of forage-fish dependent seabirds.
  7. Hentati-Sundberg et al. (2020) developed a bioenergetics model linking top predators (such as seabirds) breeding biology and foraging ecology with forage fish ecology and fisheries management. They applied their framework to the case study example of common guillemots and razorbills at a Baltic Sea colony where they depend on sprat and juvenile herring as key prey species. They showed that a fishery management target of ‘one-third-for-the-birds’ (Cury et al. 2011; see section 1.6 for further details on the “Cury threshold”) is sufficient to sustain successful breeding by the seabirds. However, the results also highlight the importance of maintaining sufficient prey densities in the vicinity of the colony, suggesting that fine-scale spatial fisheries management is necessary to maintain high seabird breeding success, and therefore indicating the value of a MPA that limits forage-fish fishery harvests in areas close to the seabird colony.
  8. The following example case studies are reviewed in detail in Annex A - Review of Marine Protected Areas:
  • British Indian Ocean Territory (BIOT) MPA;
  • South Georgia and South Sandwich Islands (SGSSI) MPA and seabirds;
  • Mediterranean MPAs and foraging Yelkouan shearwaters (Puffinus yelkouan);
  • The Namibian Islands’ Marine Protected Area, designated specifically for its seabirds; and
  • Habitat management plans to conserve African penguins in South Africa

Discussion and Conclusions

  1. This review found numerous examples that changes to fisheries management improved prey species populations resulting in positive demographic change for seabird population that foraged on those prey. This was across a wide variety of seabird taxa, fish taxa, fisheries type and locations around the world. These results suggest that changes to the fisheries of key prey fish populations has a good potential provide suitable compensation measures for impacts predicted to affect protected seabird populations.
  2. This review therefore establishes a good baseline to explore the potential benefits to sandeel stocks from changes to fisheries management in the North Sea and whether this could result in positive outcomes for seabirds, as carried out in the remainder of this report.

1.4. Sandeels in the North Sea

  1. There are several species of sandeel, but the abundant species in UK waters is the lesser sandeel Ammodytes marinus. This is a lipid-rich short-lived shoaling fish, widely distributed in UK waters where suitable sandy seabed habitat occurs (Heessen et al. 2015). The biology of sandeel is well understood. It is a key component of North Sea marine ecosystems. It feeds on small planktonic animals and itself is eaten by seabirds, marine mammals, and many large predatory fish (ICES 2017, Wright et al. 2018, Furness & Tasker 2000, Dunn 2021). Adult sandeels (mostly two years old or older) spawn in late winter, the eggs drift with local currents, and the larvae that hatch then feed on small plankton, grow rapidly and store lipids. In July-August the larvae metamorphose into adult fish and bury themselves in sandy areas of seabed to overwinter, living off their stored lipid reserves. The timing of this varies considerably, presumably depending on food availability and therefore how quickly the larvae grow and how much lipid they are able to store. These juvenile sandeels tend to remain in the water column much longer than the older fish do, so that seabirds mostly switch to feeding on these juveniles later in the breeding season. They may continue to feed on juveniles post-breeding, but there is less evidence on diet after the breeding season. Adult sandeels emerge to spawn in winter so may be available to seabirds then, but some diving species can dig sandeels out of the seabed so can potentially continue to feed on this food supply throughout the winter. In spring they start feeding on the seasonal zooplankton bloom, moving between their sandy sediment and the plankton layer near the sea surface where they feed. Once they have accumulated enough lipid, they bury themselves in the sand and live off their stored lipid; usually this is in June-July, before the young of the year have completed their development. In some years survival of sandeel larvae is low, and relatively few join the adult population. In some years high survival leads to high recruitment. The biomass of the sandeel stock therefore tends to be driven by occasional especially good years (ICES 2017). In sandeel stocks with low fishing mortality, there is evidence of density-dependence; years with high stock biomass tend to show low recruitment, whereas high recruitment is more likely when adult stock biomass is lower (ICES 2017, Lindegren 2018). That implies “bottom-up” control, with sandeel abundance constrained by plankton production (which is in turn influenced by climate change) (Lindegren et al. 2018, Frederiksen et al. 2007). In 1950s-80s stocks of large predatory fish in the North Sea (cod, haddock, whiting) were depleted by high fishing effort. Those predatory fish fed heavily on sandeels and their depletion will have reduced natural mortality of sandeels (Sherman et al. 1981). There is evidence that sandeel abundance in the North Sea increased in the 1970s-80s, particularly after stocks of herring and mackerel had been reduced drastically by overfishing (Sherman et al. 1981). Herring and mackerel compete with sandeels for zooplankton food, but adult herring and mackerel also feed on sandeel larvae. That implies “top-down” control, with sandeel abundance constrained by impacts of predation (Frederiksen et al. 2007, Sherman et al. 1981). The evidence suggests that strength of “bottom-up” and “top-down” control of sandeel abundance may vary in different years and different regions, but each can be important (Frederiksen et al. 2007).

The North Sea sandeel fishery

  1. The North Sea sandeel fishery began in the 1960s, to provide raw material for production of fish meal and fish oils after the North Sea herring and mackerel stocks had been overfished and depleted, so could no longer provide that resource. The fishery is almost entirely fished by Danish boats, with a small catch taken by Norwegian boats. At its peak in the 1990s this largely unregulated fishery was removing a sandeel biomass of one million tonnes per year. In the early 2000s the stock biomass declined rapidly, and so did landings and profitability of the fishery (Engelhard et al. 2014, Lindegren et al. 2018, Hill et al. 2020). Stock biomass has remained low and not returned to the productive levels of the 1980s despite the fishery now being under quota regulations (ICES 2017, Lindegren et al. 2018). However, despite regulation, the imposed fishing mortality on sandeel stocks has often been high (ICES 2017). This is because management of the fishery is based on a “surplus production” model. That model aims to remove as many sandeels as possible, consistent with leaving enough adult fish to produce future recruitment (ICES 2017). The threshold quantity thought to be necessary to achieve future recruitment (known as Blim) is a spawning stock biomass that is far below the abundance needed to allow kittiwakes to achieve high breeding success (ICES 2017, Dunn 2021, Cury et al. 2011, Hill et al. 2020). Current management of the sandeel stock permits a reduction of sandeel biomass well below levels needed by dependent seabirds, but not below the much lower threshold of Blim. However, because there is considerable uncertainty in the modelling of short-lived fish such as sandeel, in practice the stocks can even be reduced below Blim for some years. This problem was identified and highlighted in the ICES Benchmark Working Group on sandeels (ICES 2017) but the problem persists (Dunn 2021, ICES 2022).
  2. The North Sea sandeel is now managed as seven distinct stocks (ICES 2017; Figure 1.1   Open ▸ ). ICES sandeel area 1r (SA1r) is relevant to seabirds in the southern North Sea and much of the fishery is on that stock, especially around Dogger Bank. ICES SA7r is around Shetland. ICES SA4 is off east Scotland and north-east England and is the area of relevance to the Proposed Development.
  3. The stock in ICES SA1r was subject to high fishing mortality levels (F=0.8 to 1.2) during 1999-2009 and declined considerably in abundance. In recent years the spawning stock biomass (SSB) in this area has been less than 10% of its highest historical level and slightly below the limiting spawning stock biomass at which ICES should recommend closure of the fishery (Blim of 110,000 tonnes SSB) because there is an increased risk of recruitment failure in this stock (ICES 2021).  The SSB was below Blim in 2019 and 2020 and only marginally above that limit in 2021. In 2021, ICES advised that the total allowable catch (TAC) should be less than 5,464 tonnes and the TAC agreed was 5,351 tonnes (ICES 2022). However, the catch taken was 16,944 tonnes (ICES 2022), nearly three times the size of the set TAC. A hindcast analysis of the ICES SA1r sandeel stock to assess the consequence of the high fishing mortality estimated that sandeel spawning stock biomass would have been about twice as large now as it is, if the fishery had maintained fishing mortality (F) at F=0.4 rather than at the levels of F=0.8 to 1.2 as seen during 1999-2009 (Lindegren et al. 2018). By implication it would have been even larger if there had been no fishing on sandeels.
  4. The Shetland stock was the first to collapse, in the late 1980s (Furness & Tasker 2000). At the time, the collapse was attributed as possibly due to climate change and changes in ocean currents bringing sandeel larvae to Shetland from Orkney, as it was thought that fishing pressures would not affect short-lived fish with recruitment driven by environmental factors. The fishery was abandoned due to low sandeel biomass and the stock is no longer monitored, but seabird breeding success has been very slow to recover at Shetland (JNCC 2021 and annual Shetland Bird Reports and Fair Isle Bird Observatory reports). It is possible that recovery of this stock has been inhibited by increased abundances of adult herring and adult mackerel in the northern North Sea and increases in stocks of large predatory fish, as well as by high natural mortality imposed on the depleted stock by marine mammals and seabirds (Saraux et al. 2020). Lack of any assessment of sandeel stock at Shetland (because it is no longer of commercial importance) makes further interpretation speculative.

Figure 1.1:
Sandeel in the North Sea: Stock areas for the seven sandeel stocks. The border of the Norwegian Exclusive Economic Zone (EEZ) is also shown. The closed part of Sandeel Area 4 is shown with hatched markings.

Figure 1.1 Sandeel in the North Sea: Stock areas for the seven sandeel stocks. The border of the Norwegian Exclusive Economic Zone (EEZ) is also shown. The closed part of Sandeel Area 4 is shown with hatched markings.

 

  1. The stock in ICES SA4 declined between 1992 and 1999 when subject to high fishing mortality ( Figure 1.2   Open ▸ ). An area off east Scotland was closed to sandeel fishing as a result of evidence from breeding failures of kittiwakes at colonies in the region (especially the Isle of May where this was studied in detail) indicating that the sandeel stock had been severely depleted. The closure was intended to protect the depleted sandeel stock to allow recovery, not specifically to protect kittiwakes and other seabirds. However, fishing was allowed within ICES SA4 outside the closed box, so that fishing mortality on the stock was not necessarily reduced by the box but was concentrated on an area of the same stock that was more distant from seabird colonies. The stock in ICES SA4 recovered from a very low level in 2000-05, with a progressive increase in abundance up to 2018 ( Figure 1.3   Open ▸ ).

Figure 1.2:
Abundance (TSB in tonnes) of sandeels in ICES SA4 (which includes the no-take zone off east Scotland that was established in 2000) in the period 1993 to 2001. Data from ICES 2022.

Figure 1.2 Abundance (TSB in tonnes) of sandeels in ICES SA4 (which includes the no-take zone off east Scotland that was established in 2000) in the period 1993 to 2001. Data from ICES 2022.

Figure 1.3:
Abundance (TSB in tonnes) of sandeels in ICES SA4 (which includes the no-take zone off east Scotland that was established in 2000) in the period 2007 to 2018. Data from ICES 2022.

Figure 1.3 Abundance (TSB in tonnes) of sandeels in ICES SA4 (which includes the no-take zone off east Scotland that was established in 2000) in the period 2007 to 2018. Data from ICES 2022.

 

  1. The sandeel box has somewhat limited fishing impact on the SA4 stock since 2000 and the TSB in SA4 has increased from a low of 33,000 tonnes in 2008 to 481,000 tonnes in 2020 (ICES 2022). While it seems unlikely that TSB in SA4 would be allowed to drop as low as the levels in 2008, there is no mechanism in place to prevent the fishery from returning to TSB levels this low. Indeed, in 2021 the fishery extracted 51,882 tonnes of sandeels from the open part of SA4 from an estimated TSB of 288,685 tonnes in the whole of SA4 (including the closed box) before this catch was taken. Despite ICES setting a cap on fishing mortality for this stock for the 2021 season, fishing mortality increased to a level far above that cap and was therefore at a level considered by ICES to be unsustainable, even if fishing effort had been distributed across the whole of ICES SA4 ( Figure 1.4   Open ▸ ). The red line in Figure 1.4   Open ▸ shows Fcap, the limit on fishing mortality that, according to ICES, should not be exceeded because the escapement strategy [by which sandeel stocks are managed] is not sustainable for short-lived species unless the strategy is combined with a ceiling (Fcap) on fishing mortality. Fcap for ICES SA4 is set at 0.15. The cap on fishing mortality should be incorporated into stock management. However, this is impossible as the value of fishing mortality is computed from the stock assessment after the fishing has occurred. Thus, a cap on fishing mortality cannot be enforced under this management regime. Since a large part of ICES SA4 is closed to sandeel fishing, the fact that fishing mortality greatly exceeded the cap set for the whole area, while limited to only the open part of the area, was recognised by ICES as unsatisfactory but was still not taken into account within the existing management and assessment process (ICES 2022). Following the high fishing mortality in 2021, sandeel TSB fell dramatically, and spawning stock biomass was assessed to be so low at the end of 2021 that the TAC advised for 2022 was set at zero in January 2022 (ICES 2022).

Figure 1.4:
ICES estimates (ICES 2022 Table 9.5.10) of imposed fishing mortality (F1-2) on the ICES SA4 sandeel stock (including the part of the stock that is in the ‘closed’ box).

Figure 1.4 ICES estimates (ICES 2022 Table 9.5.10) of imposed fishing mortality (F1-2) on the ICES SA4 sandeel stock (including the part of the stock that is in the ‘closed’ box).

 

  1. RSPB reviewed the latest evidence regarding impacts of sandeel fishing on seabirds in the North Sea (Dunn 2021) and stated, “we identify three serious flaws in the current management of the fishery:
  • The sandeel fishery is permitted to operate within the foraging range of red-listed species like kittiwake and puffin breeding at internationally important and legally protected seabird colonies on the UK coast.
  • The current approach to setting maximum annual catch levels aims to protect the sandeel stock itself, but not the wildlife that depends on it. Even fishing in accordance with the scientific advice can lead to depletion of sandeel stocks to levels likely have a negative impact on top predators like seabirds.
  • Scientific advice on catch levels within sandeel management areas takes no account of zones that are closed by law to sandeel fishing, thus fishing effort is concentrated into a smaller area, potentially leading to overfishing and localised depletion of sandeel.”
  1. In a recent prepublication study for Defra (unpublished), Natural England have shown that full closure of the North Sea sandeel fishery would increase sandeel biomass by 40%. This was predicted to result in a 42% increase in seabird populations in the North Sea, and a 20% increase in predatory fish that feed on sandeels.

Discussion and Conclusions

  1. There is strong evidence that the sandeel population in the North Sea, including in SA4, is negatively affected by high levels of fishing mortality. Hindcast modelling by ICES (Lindegren et al. 2018) showed that the sandeel population in SA1r would be larger if fishing mortality had been lower. The Ecopath with Ecosim model of the North Sea sandeel population (unpublished Natural England report to Defra) shows that the sandeel population would be 40% larger if the fishery were closed. In SA4 the sandeel TSB declined prior to the introduction of the sandeel box. The introduction of the box showed that the stock in SA4 would recover if the fishery were closed, but subsequent fishing management illustrates that the population remains low due to the current management approach and is even below the precautionary minimum TSB of 100,000 tonnes.

1.5. Seabirds dependent on sandeels

  1. Many of the seabird species that breed in the UK feed mainly on sandeels during the seabird breeding season (Furness & Tasker 2000). In the nonbreeding period (August-March), sandeels are mostly buried in the sand so are unavailable to surface-feeding seabirds (terns, skuas, gulls). While these species may continue to feed on juveniles post-breeding, there is less evidence on diet after the breeding season, as birds cannot be trapped or observed at the nest site. However, some seabirds can dive to the seabed throughout much of the North Sea and will dig sandeels out of the sand. That includes shags and guillemots. Those species have been found to have sandeels in their stomachs even in mid-winter (e.g. from autopsies of oiled seabirds). Seabirds that cannot access sandeels when they go deep tend to migrate away from the UK to overwinter where other foods are available, e.g. kittiwakes migrate to Canada, puffins to the mid-Atlantic, terns and skuas to West Africa. Some seabirds can switch from a diet of sandeels to a diet of sprats or juvenile herring, but sprats and juvenile herring tend to be distributed in different parts of the North Sea from sandeels (Heessen et al. 2015) and relatively few seabird species and colonies in the North Sea can manage well in the absence of abundant sandeels (Furness & Tasker 2000).
  2. There is strong evidence that kittiwake breeding success at North Sea colonies is greatly influenced by sandeel abundance within that part of the North Sea (Furness & Tasker 2000, Carroll et al. 2017, Cury et al. 2011, Frederiksen et al. 2004, Frederiksen et al. 2005, Furness 2007, Olin et al. 2020, Saraux et al. 2020, Sydeman et al. 2017). Indeed, spatial synchrony in breeding success of kittiwakes at different colonies led to the realisation that there are several separate stocks of sandeels in different areas of the North Sea with different dynamics. There is also evidence that kittiwake adult survival is influenced by sandeel abundance; that has been shown at Shetland (Oro et al. 2002) and (for a different sandeel stock) at the Isle of May (Frederiksen et al. 2004). There is evidence indicating that breeding success of terns (especially Arctic terns), skuas, shags, fulmars and puffins is influenced by sandeel abundance (Furness & Tasker 2000, Cury et al. 2011, Furness 2007). In contrast, some seabirds can switch to other prey and their breeding success is not influenced by changes in sandeel abundance. That applies to gannet in particular. Some seabirds appear intermediate in response, with reduced breeding success when sandeel abundance is extremely low, but little effect on their breeding success where sandeel abundance declines slightly. Broadly, the relationship between breeding success and sandeel abundance tends to be non-linear ( Figure 1.5   Open ▸ ), with a collapse in seabird breeding success once sandeel abundance falls to a low level, but there can be a wide range of sandeel abundances where the amount of food is more than enough to sustain high breeding success of dependent seabirds, and seabird breeding success can vary for many other reasons, making these relationships statistically noisy. But the evidence that many UK seabirds fare better when sandeel abundance is good is robust and extensive.

Figure 1.5:
Breeding success of kittiwakes at Foula, Shetland, in relation to the Shetland sandeel (SA7) total stock biomass for the years 1976 to 2004.

Figure 1.5 Breeding success of kittiwakes at Foula, Shetland, in relation to the Shetland sandeel (SA7) total stock biomass for the years 1976 to 2004.

  1. There is evidence that many seabird populations in the UK increased considerably in numbers during the 1970s-80s in response to increased abundance of sandeels in the North Sea (Mitchell et al. 2004). More recently, many populations of sandeel-dependent seabirds have declined as sandeel abundances have fallen in response to both bottom-up and top-down influences and fishing pressures (JNCC 2021).
  2. Tracking studies show that breeding seabirds can travel large distances from their nest sites to search for food (Woodward et al. 2019). Foraging ranges differ among species according to their ecology. Breeding kittiwake maximum foraging range has been reported to be 770 km (Woodward et al. 2019) although most forage within 100 km of their nest site and often much closer. Foraging ranges tend to increase when sandeel abundance is reduced and tend to be larger around larger colonies.

Seabird foraging range during the breeding season

  1. Seabird foraging during the breeding season, while they are central point foragers, is likely to be influenced by a number of factors. The seasonal and spatial availability of prey will depend on the behaviour and annual cycle of the prey, the effects of oceanographic conditions, and fisheries. Adult birds need to forage to maintain themselves through the breeding season, with the added need to provision chicks after they hatch. The balance of these needs will change through the breeding cycle as the need for chicks to be provisioned and protected from predators or weather conditions changes. During incubation there is less need for return visits to the nest and so adult birds should be able to forage further from the nest. In addition, adult birds can feed on larger prey items than chicks can, so fewer, longer, foraging trips may be possible. During the chick phase the requirements for both prey size and frequency of foraging trips will change, with more frequent trips for smaller prey when chicks are young, but potentially less frequent trips as chicks age, but when larger prey may be preferred. Additionally, the need to protect chicks from predators, and adverse weather conditions, will also vary as chicks age. Smaller chicks will require adults to spend more time at the nest site than when they have larger chicks, constraining foraging range during the early chick-rearing period. Furthermore, several studies have demonstrated that density-dependent prey depletion occurs close to large seabird colonies within a breeding season (known as the ‘Ashmole’s halo’ effect; Weber et al. 2021) such that birds are likely to need to search for food further away from the colony late in the breeding season (i.e. when chicks are close to fledging). 
  2. A clear example of these effects on seabird foraging was shown by Jouventin et al. (1994) who satellite-tracked king penguins (Aptenodytes patagonicus) at different stages of breeding. Birds with large chicks had the longest foraging range. Birds with small chicks had the shortest foraging range. During incubation, foraging range was intermediate between the extremes found with small and large chicks.
  3. Bass Rock gannets travelled about 50% further from the colony during foraging trips in the incubation period than during chick-rearing, males a mean maximum of 293 km in incubation, 205 km in chick-rearing; females a mean maximum of 308 km in incubation, 224 km in chick-rearing, with these differences being statistically significant (Lane et al. 2020).
  4. Robertson et al. (2014) showed that kittiwakes breeding on Coquet Island in north-east England foraged much further from the colony during the incubation phase than during the chick rearing phase. Their results showed the mean of the maximum foraging range during the incubation phase could be more than 5 times the distance during chick-rearing (9.03 ± 1.17 km during incubation, 50.95 ± 12.99 km during chick rearing). This reduction in foraging range during the early chick rearing phases was due to the requirement to feed chicks regularly and the shift from adults feeding themselves larger (1+ year group) sandeels during incubation to feeding chicks smaller (0 year group) sandeels. As the breeding season progresses, adult sandeels begin to move into deeper water (further from the colony) and settle into the substrate, while juvenile sandeels remain in the water column (Rindorf et al. 2000).
  5. In another study on the same species, kittiwakes in a good year with high breeding success showed no difference in foraging range during chick-rearing and during incubation; both were short. However, in a poor year with low breeding success foraging ranges were longer but were very much longer during incubation and only slightly longer during chick-rearing (Osborne et al. 2020). The authors concluded that partners of incubating kittiwakes could make very long foraging trips during the incubation period if food was scarce, but that parents needed to return regularly to provision chicks so were less able to extend foraging range during chick-rearing even though food supply was poor. Food abundance may therefore influence the magnitude of differences in foraging range between incubation and chick-rearing.
  6. Razorbills breeding on Skomer, South Wales, were shown to have different foraging ranges between incubation and chick rearing (Shoji et al. 2016). This study found that, incubating birds had longer foraging trip durations than chick rearing birds (23.7 ± 12.9 hours and 14.6 ± 13.4 hours respectively) and foraged further from the colony (34.3 ± 9.6 km and 25.49 ± 7.3 km respectively).
  7. Cairns et al. (1987) reported that common guillemots in their study colony in Newfoundland had a foraging range during incubation (median 38 km) that was more than seven times greater than during chick-rearing (median 5 km). 
  8. Oppel et al. (2018) reported shorter foraging ranges of guillemots and kittiwakes during chick rearing at the colony than during incubation. However, they reported little difference in foraging range of razorbill between breeding stages. Thaxter et al. (2010) noted that guillemot foraging range during the chick rearing phase at the colony was shorter than razorbill, which was likely due to the combination of higher wing loading in guillemot and their provisioning of single prey items to chicks. Razorbills forage further than guillemots but have a lower wing loading and capture multiple prey for chicks on each foraging trip. Thus guillemots, at this narrow part of their annual cycle, will most likely be foraging closer to the colony than other species. However, chick rearing phase of guillemots (and razorbills) annual cycle does not only occur on the breeding colony with fledging occurring relatively early in the breeding season while chicks are still flightless and nutritionally dependent on their male parent. These birds disperse away from the breeding colony very quickly to areas further offshore than those used for foraging during the phase of chicks being fed at the nest (Camphuysen 2002). This behaviour avoids predators, such as large gulls, that occur in greater densities closer to the coast, and allows adults to take chicks to where food is concentrated, but results in offshore waters being a critical part of the breeding season. Since male guillemots moult and become flightless while feeding their dependent chicks at sea, their energetic needs will be relatively high during this part of their annual cycle and being flightless it is critically important that they are in a location with reliable and high food availability. As such, the presence of suitable prey availability in waters further offshore may be more important to the productivity and survival of guillemots and at present the likelihood of these waters being fished is higher, and the fishing mortality on sandeels in those areas is higher, due to the current fisheries management practice in SA4.
  9. Fayet et al. (2021) tracked puffins from four colonies in a single year with contrasting population demography. They found that puffins foraging from declining populations with low productivity in Norway foraged further from the breeding colony, during the early chick phase, than puffins breeding at stable colonies in Iceland and Wales. Harris et al. (2012) showed that chick rearing puffins undertook two types of foraging trips from the Isle of May. Shorter trips were made during the day to areas near the colony (9 – 17 km), while longer overnight trips were made to areas 38 – 66 km from the colony.
  10. Among other, related, seabird species similar patterns can be seen. Little auks (Alle alle) have been shown to undertake a dual foraging strategy, with shorter foraging trips for provisions chicks and longer foraging trips for adult self-maintenance (Wojczulanis-Jakubas et al. 2018). Brunnich’s guillemots (Uria lomvia) were shown to forage in deep, offshore, waters further from the colony during incubation, while foraging in shallower, inshore, waters closer to the colony during chick provisioning (Ito et al. 2010).

Discussion and Conclusions

  1. The available evidence indicates that for seabirds, habitat management zones would be intended to enhance food supply, and the mobility of seabirds means that to be effective any such zones would need to be large. Outside the breeding season, seabirds tend either to migrate substantial distances to overwinter in areas with good food supplies and benign conditions, or in those species that do not migrate, the birds disperse from the breeding area.
  2. In the breeding season, seabirds become central-place foragers, commuting from their nest site to foraging habitat. While this constrains their spatial distribution, the maximum foraging ranges of many breeding seabirds are large.
  3. There is strong evidence that sandeel stocks are important for several species of seabirds foraging in the North Sea during the breeding season, including kittiwake and puffin. There is strong evidence that kittiwake breeding success and survival are influenced strongly by sandeel abundance. There is also good evidence that sandeel abundance has an influence on the breeding success of other seabirds. There is strong evidence that the foraging range of seabirds is strongly affected by the need to gather appropriate prey and that this varies during the breeding season. Most tracking studies used to inform the connectivity between foraging areas and breeding colonies has been based on the early chick rearing phase of the season, and that this is likely to be at a time when the foraging range is most constrained. Thus, seabirds in SA4 are likely to rely on sandeel abundance across a large part of the area, not only the area inside the box closed to fishing.
  4. These conclusions suggest that evidence of the effects of sandeel abundance on the demography of SPA qualifying features predicted to be impacted by the Proposed Development should be analysed. The following section considers the evidence that increasing sandeel abundance results in increases in seabird demography and sandeel abundance can be increased by reducing or removing fishing pressure.

1.6. Seabird compensation through changes in sandeel fisheries

  1. In order to determine whether changes to management of sandeel fisheries in the North Sea can be used as compensation for impacts from the Proposed Development on breeding seabird SPAs it is important to understand the evidence that sandeel abundance can influence seabird populations, that reducing or removing fishing pressure will have an effect on the abundance of sandeels and the appropriate spatial scale for the compensation measures.

Evidence that higher sandeel abundance results in higher seabird breeding success, survival rate or population size

  1. There is a strong relationship between kittiwake breeding success and the abundance of sandeels in the area (Furness & Tasker 2000). This was first shown for Shetland (ICES SA7r), with similar results for common guillemot, fulmar, Arctic tern, shag, great skua and Arctic skua, but also applies for kittiwakes in ICES SA4 and ICES SA1r. The relationship is non-linear for all species except Arctic tern. Considering a range of seabirds and different prey fish (but including data from Shetland) models indicate a tendency for seabird breeding success to be reduced once forage fish abundance falls below one-third of the long-term maximum abundance of the prey stock. This has been proposed as the “Cury threshold” to maintain forage fish stocks above levels required by dependent predators. Some seabirds do not show such a relationship; those species are ones that can readily switch to alternative prey types (species such as gannets). For some species (although not kittiwake, guillemot, razorbill or puffin which are the subject of this analysis), data are too limited to be certain if there is a relationship. The closure of sandeel fishing within the box in SA4 provided experimental evidence for the mitigation of fishery impact by closing the fishery. There was an increase in kittiwake breeding success at colonies within the closed area compared to those outside (Daunt et al. 2008, Frederiksen & Wanless. 2006, Frederiksen et al. 2008). Data also indicate that Sandwich tern breeding success also benefitted from the closure.
  2. Demonstrating a relationship between sandeel total stock biomass and adult survival of seabirds is difficult, in part because few studies have long-term data on adult survival rates of seabirds, but also because survival is likely to be buffered compared to breeding success (long-lived birds protect their survival by abandoning breeding effort when times are bad). However, kittiwake adult survival at Shetland was strongly affected by sandeel stock biomass (Oro & Furness 2002). Survival of Arctic skuas at Shetland was increased by supplementary feeding, implying that low sandeel abundance was responsible for low survival in that species (Davis et al. 2005).
  3. For each of the focal species in this study the available evidence on changes in sandeel TSB in SA4 was compared with evidence of changes in the demography of the populations breeding on the Isle of May. Available seabird demographic evidence, adult return rate (as a proxy for survival) and productivity) was collated from publicly available data collected by UKCEH and NatureScot.

Kittiwake

  1. Studies at the Isle of May show a strong relationship between sandeel abundance in SA4 and kittiwake return rates, which are a simple proxy for adult survival ( Figure 1.6   Open ▸ ). Resighting rates of marked birds from the Isle of May studies are sufficiently high from year to year that the return rate will have a very high degree of correlation with apparent survival, and therefore provide a reasonable proxy for survival. In a recent study (Daunt et al. 2020) the return rate (or resighting probability) on the Isle of May was 92% for kittiwake, 99% for guillemot and 97% for razorbill. Plotted residuals for each plot are shown in ANNEX B.

Figure 1.6:
Annual return rates of adult kittiwakes at the Isle of May (UKCEH data from online annual reports) in relation to ICES estimates of annual sandeel total stock biomass (tonnes) in SA4 between 2004 and 2019 (ICES 2022). Data for 2020-2021 are not yet published.

Figure 1.6 Annual return rates of adult kittiwakes at the Isle of May (UKCEH data from online annual reports) in relation to ICES estimates of annual sandeel total stock biomass (tonnes) in SA4 between 2004 and 2019 (ICES 2022). Data for 2020-2021 are not yet published.

 

  1. There was also a clear relationship between the productivity of kittiwakes on the Isle of May and the sandeel stock in SA4 across the same time period ( Figure 1.7   Open ▸ ). This relationship was less clear than that for survival. This was likely due to the greater influence of weather and predation events strongly affecting the survival of eggs or chick, compared with the effects of these factors on adult survival. Indeed, the data shown in Figure 1.7   Open ▸ has a single value removed, as predation on chicks on the Isle of May in 2010 reduced productivity to a very low level (0.29 fledglings per nest)[1].

Figure 1.7:
Annual productivity rates of adult kittiwakes at the Isle of May (UKCEH data from online annual reports) in relation to ICES estimates of annual sandeel total stock biomass (tonnes) in SA4 between 2004 and 2019 (except 2010) (ICES 2022). Data for 2019-2021 are not yet published.

Figure 1.7 Annual productivity rates of adult kittiwakes at the Isle of May (UKCEH data from online annual reports) in relation to ICES estimates of annual sandeel total stock biomass (tonnes) in SA4 between 2004 and 2019 (except 2010) (ICES 2022). Data for 2019-2021 are not yet published.

 

  1. Sandeel stock biomass in SA4 impacts on demographic parameters appear to have had an effect on the abundance of kittiwakes nesting on the Isle of May, as there is a significant positive correlation between breeding numbers and sandeel total stock biomass ( Figure 1.8   Open ▸ ). It is likely that sandeel stock also had an influence on other demographic rates, particularly immature survival and age at first breeding. However, there were no data available to make those comparisons.

Figure 1.8:
Population size (Apparently Occupied Nests – (AON)) of adult kittiwakes at the Isle of May (UKCEH data from online annual reports) in relation to ICES estimates of annual sandeel total stock biomass (tonnes) in SA4 between 2004 and 2021 (ICES 2022).

Figure 1.8 Population size (Apparently Occupied Nests – (AON)) of adult kittiwakes at the Isle of May (UKCEH data from online annual reports) in relation to ICES estimates of annual sandeel total stock biomass (tonnes) in SA4 between 2004 and 2021 (ICES 2022).

Guillemot

  1. Return rates of common guillemot also showed a relationship with sandeel abundance ( Figure 1.9   Open ▸ ). It is important to note that, as with breeding success, these relationships are non-linear. There can be large differences in sandeel abundance between moderate and high abundance over which range adult survival remains high; survival falls much faster when sandeel abundance falls below a critical threshold level. This means that gains depend on the starting point on the graph. A 10% increase in sandeel abundance may greatly improve breeding success and survival of dependent seabirds if it occurs at low sandeel abundance but will have much less effect if it occurs at high sandeel abundance. Residuals for each plot are in ANNEX B.

Figure 1.9:
Annual return rates of adult guillemots at the Isle of May (UKCEH data from online annual reports) in relation to ICES estimates of annual sandeel total stock biomass (tonnes) in SA4 between 2004 and 2019 (ICES 2022). Data for 2020-2021 are not yet published.

Figure 1.9 Annual return rates of adult guillemots at the Isle of May (UKCEH data from online annual reports) in relation to ICES estimates of annual sandeel total stock biomass (tonnes) in SA4 between 2004 and 2019 (ICES 2022). Data for 2020-2021 are not yet published.

 

  1. There was also a relationship between the productivity of guillemots on the Isle of May and the sandeel stock in SA4 across the same time period ( Figure 1.10   Open ▸ ). This relationship was less clear than that for survival. This was likely due to the greater influence of weather and predation events strongly affecting the survival of eggs or chick, compared with the effects of these factors on adult survival. It may also be influenced by the fact that guillemot chicks fledge when only partly grown and continue their growth and development at sea; in this species productivity only measures success to shortly before chicks leave the nest sites.

Figure 1.10:
Annual productivity rates of adult guillemots at the Isle of May (UKCEH data from online annual reports) in relation to ICES estimates of annual sandeel total stock biomass (tonnes) in SA4 between 2004 and 2019 (ICES 2022). Data for 2020-2021 are not yet published.

Figure 1.10 Annual productivity rates of adult guillemots at the Isle of May (UKCEH data from online annual reports) in relation to ICES estimates of annual sandeel total stock biomass (tonnes) in SA4 between 2004 and 2019 (ICES 2022). Data for 2020-2021 are not yet published.

 

  1. It appears that the effects of sandeel stock biomass in SA4 on demographic parameters also influence the abundance of guillemots nesting on the Isle of May ( Figure 1.11   Open ▸ ). It is likely that sandeel stock also had an influence on other demographic rates, particularly immature survival and age at first breeding. However, there were no data available to make those comparisons.

Figure 1.11:
Population size (individuals on land) of adult guillemots at the Isle of May (UKCEH data from online annual reports) in relation to ICES estimates of annual sandeel total stock biomass (tonnes) in SA4 between 2004 and 2021 (ICES 2022).

Figure 1.11 Population size (individuals on land) of adult guillemots at the Isle of May (UKCEH data from online annual reports) in relation to ICES estimates of annual sandeel total stock biomass (tonnes) in SA4 between 2004 and 2021 (ICES 2022).