Discard-ban policies can help improve our understanding of the ecological role of food availability to seabirds

Abstract

Discards from fisheries are the most important predictable anthropogenic food subsidies (PAFS) that are being incorporated into marine ecosystems. Changes on their availability and predictability can help us to understand the role that food availability (i.e. an important indicator of the carrying capacity) plays at different ecological levels, from individual fitness to community dynamic and ecosystem functioning. For several reasons, seabirds are an excellent model for evaluating the ecological effects arising from a lack of discards: 1) they are one of the most important discard scavengers, 2) they are easy to monitor and 3) they are apical predators are globally distributed, which makes them suitable health indicators of ecosystems. Here we review the existing information on seabird-discard interactions to identify the main knowledge gaps and propose new challenges for improving our understanding of the general role of food availability. We conclude that the new policies on the ban of fishery discards that are being progressively implemented in the European Union, Norway, Chile and New Zealand offer a suitable experimental scenario for improving our understanding of how a large decrease in the carrying capacity may alter demographic parameters such as survival, dispersal and reproduction, the resilience of populations against perturbations and the role of individual specialization in the foraging process.

Conservation status and overview of threats to seabirds

“…Commercial fisheries provide enormous quantities (>10 million tonnes) of supplementary food for scavenging seabirds in the form of discards (unwanted catch, spent bait, and offal), including from large, demersal species that would otherwise be unavailable (McInnes et al 2017;Real et al 2018;Tasker et al 2000; Chapter 3). Discarding can be highly beneficial, depending on the balance between seabird bycatch that may result from increased vessel interactions, and higher breeding success associated with diets that include a high proportion of discards (McInnes et al 2017;Oro et al 1995;Phillips et al 1999).…”

“…Discarding can be highly beneficial, depending on the balance between seabird bycatch that may result from increased vessel interactions, and higher breeding success associated with diets that include a high proportion of discards (McInnes et al 2017;Oro et al 1995;Phillips et al 1999). As such, there are concerns that reductions in discarding associated with better fisheries management could have negative repercussions for some seabird populations (Bicknell et al 2013;Real et al 2018). However, availability of discards affects food web and community structure by increasing dietary overlap (Bugoni et al 2010;Jiménez et al 2017), and potentially influences distributions by concentrating seabirds in productive areas favoured by fisheries, exacerbating bycatch risk (Ryan and Moloney 1988).…”

Interspecific synchrony on breeding performance and the role of anthropogenic food subsidies

“…Such policies include the Landfill Waste Council Directive (LWCD, [ 23 ])—aiming for the termination of open landfilling—and the Landing Obligation Directive (LOD, [ 24 ])—enacting a ban of fisheries discards. The implementation of these policies is a fantastic opportunity to test our hypothesis since they ultimately aim to modify the availability of anthropogenic food subsidies for wildlife [ 25 , 26 ]. Here, we focus on the effect of the transformation of Mallorca’s single open-landfill (Son Reus) into an incinerator following LWCD on the changes in the egg volume of two sympatric avian top predators.…”

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

“…2013). However, we are unable to understand fully the impact of changing discards because of knowledge gaps about the number/ biomass of scavengers supported, how this has changed over time, or might vary in the future (Heath et al, 2014;Real et al, 2018).…”