Seabird attraction to fishing vessels is a local process

Seabird aggregation and scavenging around fishing vessels is widely assumed to be a major component of seabird ecology. However, few field data have shown the relative importance of human fishing activities in comparison with the distribution of marine habitats and the availability of natural food sources. Here we perform a spatial analysis of the relative influence of fishing activities, by modelling observed density gradients of working trawlers and attracted seabirds along PCA-derived large scale gradients in hydrographic variables and abundance of small herring Clupea harengus, from shallow estuarine waters to deep oceanic waters across the Baltic Sea-North Sea interface. All hydrographic and biological data. including numbers of attracted seabirds, were collected synoptically from a ship sampling systematically throughout the region. The analysis indicates that a relatively small degree of overlap exists between the spatial distribution of fishing vessels and that of potentially scavenging seabirds. Gradients in the abundance of seabirds attracted to the ship indicate responses to hydrographic features such as upwelling zones and fronts, and gradients in the supply of natural foods such as schools of immature herring, rather than responses to changes in the supply of discards from fishing vessels. Estimates of the scale of attraction of seabirds by the research ship further indicate that attraction in the Baltic Sea-North Sea gradient is a local (< 10 km) process. The small-scale nature of the attraction of seabirds to ships in the region is further tested by displaying observations of trawlers and attracted seabirds obtained from separate cruises in relation to observed salinity and pycnocline characteristics. We suggest that in similar heterogeneous environments at least, attraction of seabirds by trawlers is predominantly a result of local processes. Our results illustrate that the importance of human fisheries in seabird ecology cannot be assessed without detailed knowledge of seabirds' dependence on natural food sources and habitats.