Abstract
Fisheries compete with seabirds for vanishing marine resources, but also produce fishery waste consumed by seabirds. Marine birds may therefore avoid or seek fishing vessels, and have evolved complex, plastic behavioural responses to vessel presence. Understanding these responses is essential to the conservation of a globally declining seabird community. We studied Cape gannets (Morus capensis), which compete with fisheries for reduced sardine (Sardinops sagax) resources in the Benguela upwelling region off South Africa. Using bird-borne GPS trackers coupled with newly-developed ship-radar detectors we show that foraging gannets seldom attended fishing vessels. Rather, they switched from eating scarce sardines or energetically-poor fishery waste to targeting locally abundant saury (Scomberesox saurus). This pelagic fish is brought into the seascape by warm water influx, and is not commercially exploited by fisheries. Cape gannets thereby show dietary plasticity, allowing them to maintain adult body condition and chick growth rates. This diet switch is a strong indicator that Cape gannets forage in an ecologically perturbed marine environment.
Highlights
Seabirds coexist with fisheries across the world’s oceans, and interact with them in a number of ways [1]
In contradiction with our hypothesis, foraging Cape gannets studied in the Benguela seldom encountered vessels in 2017, and even more rarely attended them for prolonged periods
This surprizing result can be explained through marked dietary plasticity, and has important implications for our understanding of seabird resilience and conservation in perturbed marine environments
Summary
Seabirds coexist with fisheries across the world’s oceans, and interact with them in a number of ways [1]. Seabirds have been guiding fishermen towards profitable harvesting grounds from ancient times. This practice persists to this day, even in industrial fisheries using radar technology to locate seabird aggregations over tuna shoals in tropical waters [2]. Fishing gear may catch or injure seabirds. This contributed to significant seabird population declines, especially in petrels and albatrosses exposed to long-line fisheries [3]. Mitigations measures have been relatively efficient with respect to hook-and-line fisheries [4], yet issues persist with other gear types and because mitigation measures on long-lines are rarely applied in international waters [5]. 10% of fisheries catches are wasted at sea [6], and some of this biomass
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