Foraging behaviour and prey size spectra of larval herring Clupea harengus

When fish increase in size, the size spectrum of ingested prey increases. This observation has led to the impression that the available food resource increases as fish grow. However, it has been proposed that the size spectrum of prey remains the same on a relative scale and, accordingly, that the available biomass of food as a proportion of the predator biomass will not increase. In order to assess the uniformity of relative prey size spectra of herring larvae and their background in larval foraging behaviour, a set of experimental and field investigations has been carried out. In the experiments, 4 size groups of larval herring Clupea harengus L. were studied when preying on 6 size groups of copepods. Larval swimming and attack behaviour changed with prey size and were related to the ratio between prey length and larval length. The effective search rate showed a maximum when prey length was about 0.027 of larval length. In the field investigations, the relative prey size spectra of 6 size groups of larvae were described. The widths of these spectra were not significantly different and they all attained maxima when the relative length of prey was about 0.028 of larval length. Thus, larval foraging was, to a large extent, determined by the relative size of prey, with significant selection for the preferred sizes. Results imply that only a restricted part of the potential prey (sizes) is of value in larval foraging and consumption and that coincidence between prey size spectra and peaks in the biomass spectra of the environment is important to larval growth and survival.