Laboratory investigations on the effect of prey size and concentration on the feeding behaviour of Sardina pilchardus

Laboratory experiments were conducted to study the effects of different prey types and concentrations on the feeding behaviour of the Iberian sardine Sardina pilchardus. Known concentrations of different prey types (both single prey type and a mixture of prey types) were provided to a shoal of sardines acclimated to laboratory conditions and their feeding behaviour was observed. Data on feeding mode choice, feeding selectivity and filtration efficiency were collected, and clearance rates for different prey types and sizes were estimated. Sardines use 2 feeding modes and switch between them depending on prey size. Filter-feeding was used to capture prey <= 724 mu m and particulate feeding to capture prey >= 780 mu m; therefore the feeding mode switch occurs within these limits. Sardines are able to feed on nanoplankton and can retain prey items as small as 4 mu m, and filtration efficiency increases from 20% at this prey size to close to maximum for prey > 200 mu m. Sardines show selective feeding, preferentially ingesting fish eggs compared to other prey types (even larger fish larvae) when fed cultured, mixed prey assemblages and selecting copepods and decapods over other zooplankton prey when fed wild-collected, mixed prey assemblages. Clearance rates were generally low compared to other clupeids, arising from the smaller mouth gape and lower swimming speed of this species. Results obtained from this study suggest that filter-feeding is the dominant feeding mode of S. pilchardus and that it is able to efficiently utilize microplankton prey, and corroborate previous dietary studies indicating that small zooplankton and chain-forming diatoms dominated stomach contents.