Causes of spatio‐temporal variation in the patchiness of larval fish distributions: differential mortality or behaviour?

Abstract

ABSTRACTWe studied age‐ and size‐related patterns in the patchiness of distributions of larval capelin (4–21 mm TL) on the Newfoundland Grand Bank. The distribution of yolk‐sac larvae was highly contagious, and was consistent with the distribution of spawning capelin. Lloyd's patchiness index declined fivefold from the yolk‐sac (4–6 mm) to the small post‐yolk‐sac (9–11 mm) stage. The patchiness index then increased steadily and by 2.5‐fold through the medium (14–16 mm) and large (19–21 mm) post‐yolk‐sac stages. This increase in patchiness was accompanied by a steady increase in the strength of the correlation between the abundance of larvae and of the edible zooplankton, and between the centre of mass of the larval and zooplankton vertical distributions through these length classes. In an attempt to distinguish between the alternative hypotheses of differential mortality or behavioural aggregation as the cause of the changes in larval capelin patchiness observed, we compared these changes with changes in the patterns of vertical migration, patchiness, and correlation with food abundances in a co‐occurring, non‐schooling zooplanktivore, Sagitta ekgans. Sagitta vertical migration and correlations between predatodprey abundance were similar to those observed in capelin. However, patchiness indices calculated for Sagitta were similar to the lowest values observed for capelin larvae. We conclude that the initial decline in patchiness observed in small larvae was due primarily to advection and diffusion away from the spawning site. The subsequent increase in larval patchiness, which did not occur in the non‐schooling Sagitta, was attributed to active behaviours of the larvae in response to the vertical and horizontal distributions of their prey. The implications of these findings for the interpretation of larval early life history are discussed.