Fishing mortality versus natural predation on diurnally migrating sandeels Ammodytes marinus

Do large-scale industrial fisheries for sandeels impact 'human consumption' fishes by depriving them of food? In the North Sea, most sandeel fishing takes place on the Dogger Bank dur- ing spring. Here, we studied sandeel-fishery and sandeel-predatory fish interactions in 2 sampling grids, focussing on (1) how localised sandeels are distributed during the day (when feeding pelagi- cally) and at night (when buried in the seabed); (2) how precisely the fishery can target localised sandeel concentrations; and (3) how tightly day/night sandeel distributions are linked to the local abundance of predatory fishes and their sandeel consumption. Sandeel abundance differed widely between both grids. Within each grid, marked day/night differences in distributions indicated exten- sive diurnal migrations, with centres of gravity of day and night distributions up to 15 km apart. Fish- ing effort was tightly matched with sandeel populations during the day and was far higher in the 'high sandeel density' grid. Only at the finest scale examined, effort was concentrated at sandbanks but not significantly matched with local sandeel numbers. This suggested either insufficient know- ledge among fishers or vessel maneuverability issues, possibly relating to high mobility of sandeels. Ten predatory fishes (8 'human consumption' species) preyed on sandeels, most notably whiting and lesser weever. Spatial comparisons showed that the predominant sandeel predators were whiting during the day and lesser weever at night. Tight predator-prey interdependencies were revealed: whiting and weever exhibited 'aggregative responses' to sandeels, and most predators consumed more sandeels in the 'high sandeel density' grid. On the Dogger Bank, sandeel fishing effort is con- centrated when and where sandeels are most available, but also where these are extensively and locally preyed upon by at least 10 predatory fish species. Hence concerns that this industrial fishery may indirectly impact predatory fishes by depriving them of food might have some basis.