Abstract
Commercial capture fisheries produce huge quantities of offal, as well as undersized and unwanted catch in the form of discards. Declines in global catches and legislation to ban discarding will significantly reduce discards, but this subsidy supports a large scavenger community. Understanding the potential impact of declining discards for scavengers should feature in an eco-system based approach to fisheries management, but requires greater knowledge of scavenger/fishery interactions. Here we use bird-borne cameras, in tandem with GPS loggers, to provide a unique view of seabird/fishery interactions. 20,643 digital images (one min−1) from ten bird-borne cameras deployed on central place northern gannets Morus bassanus revealed that all birds photographed fishing vessels. These were large (>15 m) boats, with no small-scale vessels. Virtually all vessels were trawlers, and gannets were almost always accompanied by other scavenging birds. All individuals exhibited an Area-Restricted Search (ARS) during foraging, but only 42% of ARS were associated with fishing vessels, indicating much ‘natural’ foraging. The proportion of ARS behaviours associated with fishing boats were higher for males (81%) than females (30%), although the reasons for this are currently unclear. Our study illustrates that fisheries form a very important component of the prey-landscape for foraging gannets and that a discard ban, such as that proposed under reforms of the EU Common Fisheries Policy, may have a significant impact on gannet behaviour, particularly males. However, a continued reliance on ‘natural’ foraging suggests the ability to switch away from scavenging, but only if there is sufficient food to meet their needs in the absence of a discard subsidy.
Highlights
Commercial capture fisheries generate huge quantities of discards in the form of offal, unwanted or over-quota catch – during 1992–2001 an average of 7.3 million tonnes of fish were discarded each year [1]
Device failure arose because of electronic faults, water ingress or the lens becoming obscured by gannet plumage
Based on these findings we might predict that gannets would be severely impacted as discards decline, but analysis of foraging behaviour reveal that most individuals showed a mixture of scavenging and ‘natural’ foraging
Summary
Commercial capture fisheries generate huge quantities of discards in the form of offal, unwanted or over-quota catch – during 1992–2001 an average of 7.3 million tonnes of fish were discarded each year [1]. This level of discarding is not sustainable and has been shown to negatively impact ecosystem functioning and biodiversity [2,3]. Understanding the impact of fisheries reforms is important under an ecosystem approach to fisheries management but is hampered by our poor understanding of scavenging ecology
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