Temporal and spatial variation in the diet of a marine top predator—links with commercial fisheries

Abstract

The huge quantities of waste produced by commercial fisheries worldwide attract large numbers of scavengers. Reducing this wasteful practice is desirable, but may have implications for marine ecosystems as scavengers will face a major shortfall in food. Predicting the impact of reduced discarding requires information on the strength of the link between scavengers and fisheries. We analysed sagittal otoliths regurgitated by great skuas Stercorarius skua over 5 yr from 8 different colonies in Shetland, UK, and over 18 yr from 1 of these sites, in relation to spatial and temporal variation in fisheries activity. The proportions of 2 demersal fish not normally available to skuas, haddock Melanogrammus aeglefinus and whiting Merlangius merlangus, are positively correlated with annual variation in discard estimates. The proportion of whiting at 8 colonies is positively correlated with spatial differences in fish biomass estimates, and blue whiting Micromesistus potassou are only eaten in colonies close to the continental shelf edge, where this species is fished. Non-breeding great skuas tend to eat smaller and significantly more variable discarded fish compared with breeders. Our results indicate that great skuas rely heavily on fishery discards and are also sensitive to spatial and temporal changes in fisheries activity. Differences between breeding status indicate that the nonbreeding component of the population may respond differently to changing discard policy. The strength of the link between fisheries and scavengers appear to be species-specific, and testing the strength of these relationships should be an important avenue of future research to interpret the response of scavengers to changes in fisheries management.KEY WORDS: Fisheries · Discards · Seabirds · Diet · Scavenger · Foraging Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 367: [223][224][225][226][227][228][229][230][231][232] 2008 ability has also been shown to directly influence aspects of seabird breeding (Oro et al. 1996), overwinter condition (Hüppop & Wurm 2000) and demography (Oro & Pradel 2000). Global-scale declines in discard availability may leave artificially inflated populations of scavenging seabirds short of food, which has profound implications for aquatic food web dynamics, as well as conservation and ecosystem management (Regehr & Montevecchi 1997, Stenhouse & Montevecchi 1999, Votier et al. 2004a. Thus, it is imperative that we gain a better understanding of the impact of changes in discarding rates for seabird communities.Regardless of the evidence that seabirds scavenge extensively for discards, our grasp of the ways in which they interact with fisheries is still limited. For instance, despite the evidence that fisheries closure or moratoria can have an impact on aspects of seabird ecology (Oro et al. 1996, Hüppop & Wurm 2000, other studies indicate that natural foods may be more important than discards to scavenging species. Although northern gannets Morus bassanus sho...

Research priorities for seabirds: improving conservation and management in the 21st century

“…Although this estimate is lower than in previous decades (Alverson et al 1994), when considered in conjunction with associated overfishing, discards are likely to exert significant impacts on seabirds (Furness 2003). A common and overabundant food source, discards have artificially inflated scavenging seabird populations (Furness et al 1992, Garthe et al 1996, probably due to improved body condition and breeding performance (Bunce et al 2002, Votier et al 2008a). Documented increases in northern fulmars in the North Sea have been ascribed, in part, to food subsidies from fisheries discards (Camphuysen & Garthe 1997, Thompson 2006.…”

“…They mostly breed in colonies and are philopatric to the natal site, although dispersal (both natal and breeding) occurs. For seabirds, estimation of underlying demographic rates is particularly important because declines in breeding populations can be influenced substantially by nonbreeders and immature birds (Grimm et al 2005, Votier et al 2008a. The influence of non-breeders and immature birds is particularly strong when populations are density-dependent and large numbers of sexually mature birds are queuing for recruitment; these birds can rapidly integrate into the breeding population when additive mortality occurs (Tavecchia et al 2007, Votier et al 2008a.…”

“…Ratcliffe et al (2008) demonstrated a promising approach, termed 'virtual seabird' modeling, to estimating population size in a tropical seabird with protracted breeding. The rapid development of data modeling techniques that extend CMR ideas into multi-state models (White et al 2006, Lebreton et al 2009) and spacestate models (Gimenez et al 2008, Patterson et al 2008 opens new avenues for unbiased estimation of demographic variables, including immature survival/ -recruitment (Jenouvrier et al 2008, Votier et al 2008a). Likewise, genetic approaches relevant at the spatiotemporal scale of population demography are now being deployed to estimate immigration/emigration rates (Milot et al 2008, Peery et al 2008, Boessenkool et al 2009).…”

“…For seabirds, estimation of underlying demographic rates is particularly important because declines in breeding populations can be influenced substantially by nonbreeders and immature birds (Grimm et al 2005, Votier et al 2008a. The influence of non-breeders and immature birds is particularly strong when populations are density-dependent and large numbers of sexually mature birds are queuing for recruitment; these birds can rapidly integrate into the breeding population when additive mortality occurs (Tavecchia et al 2007, Votier et al 2008a. The capacity of nonbreeders to mask, mitigate or exacerbate the effect of ecological perturbations (both human and natural) is poorly understood because individuals cannot be easily monitored once outside the breeding site (Oro et al 2006b).…”

Action plan for seabirds in Western-Nordic areas

“…A well-studied example of this concerns great skuas in the Northern Isles of Scotland (home to 60% of the world population (Furness & Ratcliffe 2004)). Great skuas in this region are highly dependent on discards (Votier et al 2008), and in years when the availability of both discards and sandeels (the most important alternative fish prey) is low, they turn their attention to chicks and adults of other seabirds (Votier et al 2004). Predation by great skuas has had negative effects on populations of black-legged kittiwakes (Heubeck et al 1997) and Arctic skuas (Jones et al 2008b).…”