Scavenger communities and fisheries waste: North Sea discards support 3 million seabirds, 2 million fewer than in 1990

Richard B Sherley,Hannah Ladd‐Jones,Stefan Garthe,Olivia Stevenson,Stephen C Votier

doi:10.1111/faf.12422

Richard B Sherley, Hannah Ladd‐Jones + Show 3 more

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https://doi.org/10.1111/faf.12422

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Abstract

AbstractEvery year fisheries discard >10 million tonnes of fish. This provides a bounty for scavengers, yet the ecological impact of discarding is understudied. Seabirds are the best‐studied discard scavengers and fisheries have shaped their movement ecology, demography and community structure. However, we know little about the number of scavenging seabirds that discards support, how this varies over time or might change as stocks and policy change. Here, we use a Bayesian bioenergetics model to estimate the number of scavenging birds potentially supported by discards in the North Sea (one of the highest discard‐producing regions) in 1990, around the peak of production, and again after discard declines in 2010. We estimate that North Sea discards declined by 48% from 509,840 tonnes in 1990 to 267,549 tonnes in 2010. This waste had the potential to support 5.66 (95% credible intervals: 3.33–9.74) million seabirds in the 1990s, declining by 39% to 3.45 (1.98–5.78) million birds by 2010. Our study reveals the potential for fishery discards to support very large scavenging seabird communities but also shows how this has declined over recent decades. Discard bans, like the European Union's Landing Obligation, may reduce inflated scavenger communities, but come against a backdrop of gradual declines potentially buffering deleterious impacts. More work is required to reduce uncertainty and to generate global estimates, but our study highlights the magnitude of scavenger communities potentially supported by discards and thus the importance of understanding the wider ecological consequences of dumping fisheries waste.

Highlights

Understanding fishery impacts is imperative for understanding marine ecology and conservation
While we focus on the North Sea, seabirds are attracted to feed on discards worldwide with associations reported from the south‐ west Atlantic (Granadeiro, Phillips, Brickle, & Catry, 2011), west‐ ern Mediterranean (Oro & Ruiz, 1997), southern South America (Gonzalez‐Zevallos & Yorio, 2006), Baltic Sea (Garthe & Scherp, 2003), northwest Atlantic (Montevecchi, 2002), southeast Atlantic (Crawford, Underhill, Raubenheimer, Dyer, & Martin, 1992), Canary Current (Camphuysen & van der Meer, 2005) and Australian waters (Svane, 2005)
We estimate that North Sea discards can support ~3.45 million seabirds per annum, but this declined by 39% from close to the period of peak discard production in the 1990s, indicating a shift away from a scavenger‐dominated ecosystem

Summary

| INTRODUCTION

Understanding fishery impacts is imperative for understanding marine ecology and conservation. The North Sea is one of the world's largest discard‐producing re‐ gions (Heath & Cook, 2015; Zeller et al, 2018) and supports an internationally important seabird assemblage (Garthe, Camphuysen, & Furness, 1996; Paleczny, Hammill, Karpouzi, & Pauly, 2015) This makes it an excellent system in which to estimate the number of scav‐ engers that could be supported by discards. We combine data on seabird abundance, diet and energetic expenditure, together with fisheries discard rates and fish energy content into bioenergetics models to estimate the number of sea‐ birds that could be supported by discards in the North Sea around the time of peak production in the early 1990s (hereafter the 1990 model) and following discard declines in the late 2000s (hereafter the 2010 model) (Figure 1).

| METHODS

| DISCUSSION

| CONCLUSIONS