Passive and active behavioural contributions to patchiness and spatial pattern during the early life history of marine fishes

Multiple ichthyoplankton surveys of Placentia Bay, Newfoundland during the spring and summers of 1997 and 1998 indicated that both passive and active processes contribute to spatial pattern in pelagic eggs and larval fish. Distribution patterns of pelagic eggs and early larvae in coastal Newfoundland waters were consistent with passive drift during development, but older larvae were associated with areas of high production. Recently hatched larvae from pelagic spawners decreased in size throughout the spawning season, reflecting seasonal decreases in egg diameter. Larvae from demersal eggs increased in size over the same time period, possibly reflecting retention and growth. Lloyd's index of patchiness calculated for pelagic eggs generally increased during development, suggesting passive retention. Patchiness decreased after hatch, then increased at approximately 10 mm. We hypothesize that the observed spatial patterns in older larvae, seasonal size increases in larvae from demersal eggs, and ontogenetic changes in patchiness reflect active processes. Predator patchiness, ontogenetic changes in spatial patterns, and published larval consumption rates support the hypothesis that mortality did not generate this pattern. Furthermore, mean crowding of capelin larvae by potential prey increased with development. Increases in patchiness during late larval development coincided with the size at which swimming ability was predicted to exceed ambient flow. This pattern suggests that larger larvae may actively contribute to their spatial distribution. We conclude that swimming ability and behaviour become increasingly important in determining spatial distribution patterns as pelagic larvae develop.