Saturday, 9 November 2019

Landing Obligation and the law of unintended consequences

For years, fishermen at sea have been talking about the consequences for sea birds from the very first days of the Fish Fight campaign to introduce a total ban on discards and the possible consequences of depriving seagoing bird populations of a food source. Since January of this year a total ban on discarding quota'd fish has been in place. The industry has come under some criticism for appearing not to conform with the legislation.

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

The paper has modelled such consequences, an extract is given below - the full paper is available here:


Every 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.

Seabirds are the best‐studied consumers of fisheries waste. At least 52% of species feed on discards to some degree, and fisheries waste is the dominant dietary item in some populations. Discarding has shaped many aspects of seabird ecology, including their movements demography , and community structure. However, there is still relatively little known about the number of seabirds that discards can support, how this has changed over time and what the likely consequences of changes in discard availability may be for seabird communities. A fuller understanding of the wider ecosystem‐level impact of fisheries requires that we address this knowledge gap.

The North Sea is one of the world's largest discard‐producing regions and also supports an internationally important seabird assemblage . This makes it an excellent system in which to estimate the number of scavengers that could be supported by discards. During peak production in the late 1980s and early 1990s, North Sea fisheries generated 500,000–900,000 tonnes of discards . These were estimated to support ~6 million seabirds in 1990 by Garthe et al in one of the only studies to attempt such a calculation. However, the model used to arrive at that figure did not account for the precision of its input variables; had that uncertainty been propagated, the final estimate could have been very imprecise, with 95% confidence intervals of 2.2 and 16.3 million birds. At the time, the largest source of variation in the model came from very poor estimates of discard production. More reliable discard data for the region have recently become available which show a steep decline in the overall quantity of fish discharged over the period 1978–2011. Moreover, catches have also shifted away from fish that are easy for seabirds to swallow (~80% roundfish) to less easily ingested fish (>50% flatfish), and the North Sea seabird community has also changed since the 1990s. A robust estimate—acknowledging the inevitable uncertainties and accounting for long‐term changes in discarding practices—of the size and composition of the discard scavenging seabird community in the North Sea is therefore overdue.

Here, 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 seabirds 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). We implement our model in a Bayesian framework, so that we can carry over input parameter uncertainty, and present our estimates split by species and by breeding and non‐breeding seasons. We also consider our work in the context of global fisheries so that a broader understanding of the size of scavenging communities might be achieved. Finally, we discuss the ecological implications of our results in light of potential changes in practise and policy around discarding—the least studied component of fisheries impacts.


3.1 North Sea discard production

We estimate that North Sea mixed demersal fisheries generated 509,840 (284,619–788,105) tonnes of discards in 1990 and this declined by 48% to 267,549 (138,627–436,251) tonnes in 2010. Roundfish, which are particularly important food for seabirds because they can be easily swallowed, declined by 52.9% (from 120,768 to 56,819 tonnes). When taking account of the energetic content of different discard types, this represented 1,884 (1,471–2,348) and 1,014 (767–1,305) billion kJ of biomass discarded from North Sea mixed demersal fisheries in 1990 and 2010, respectively.

3.2 Number of seabirds supported by North Sea discards

After accounting for assimilation efficiency and consumption rates in 1990, 720 (499–984) billion kJ were available to support an estimated 1.24 (0.68–2.65) million birds during breeding and 4.55 (2.65–7.82) million during the non‐breeding season (Figure 2A). In 2010, 385 (257–548) billion kJ of discards could be used, potentially supporting 656,255 (373,084–1,132,250) birds during breeding and 2.89 (1.62–4.96) million individuals during the non‐breeding season (Figure 2B). Combining the seasonal posteriors gave a total of 5.66 (3.33–9.74) million seabirds potentially supported in 1990 versus 3.45 (1.98–5.78) million individuals—or 39% fewer—able to consume fishery discards in the North Sea in 2010 (Figure 3A).

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. These findings are important in the context of global discard declines and bans, including the EU's Landing Obligation, as this gradual reduction may lessen any deleterious impacts of an abrupt drop in discards. Nevertheless, discards still have the potential to support large numbers of scavenging seabirds in the North Sea and the general lack of empirical data on the impacts of fishery discards at an ecosystem level makes it difficult to predict the real ecological consequences of the landing obligation. Further work is needed to monitor the response of seabird scavengers to changing fishery practices where discard bans are implemented and to quantify their numbers globally, particularly if we are serious about adopting an ecosystem‐level approach to fisheries management. Data on global discard rates are available (Zeller et al., 2018), but information on seabird community composition, their diet and how this varies over time are also required in order to construct robust bioenergetics models.

RBS was supported by the Leiden Conservation Foundation, Bristol Zoological Society, Zoological Society of San Diego and the Pew Fellows Program in Marine Conservation at The Pew Charitable Trusts. The views expressed are those of the authors and do not necessarily reflect the views of The Pew Charitable Trusts. We thank Robin M. Cook for posterior probabilities for the landings and discards from the mixed demersal fisheries in the North Sea. The authors have no conflicts of interest to report.