TIDAL EFFECTS ON HABITAT SELECTION AND AGGREGATION BY JUVENILE POLLOCK POLLACHIUS-VIRENS IN THE ROCKY INTERTIDAL ZONE

Fucoid macroalgae in the rocky intertidal zone are a potentially impor tant foraging and refuging habitat for juvenile fishes. A dominant fea ture of this habitat is that its availability changes with the tides. Vegetated habitat availability changes in many other systems as well, yet little is known about the effects of these changes on animal distr ibutions. We addressed this problem by studying young-of-the-year poll ock Pollachius virens using visual transect surveys in the rocky inter tidal zone. We examined tidal effects on pollock distribution, their d epth and habitat preferences and the relationship between habitat use and patterns of aggregation. Nearly all the pollock population in our study area moved into the intertidal zone and alternated between aggre gation in the open and dispersal in the algae. On rising tides, polloc k moved from the subtidal zone to the open intertidal zone in large sc hools then dispersed among available depths and throughout algal habit ats in small schools or as solitary fish. When in algae, pollock prefe rred the dense algal habitat over the sparse algal habitat. On falling tides, pollock schooled in the open habitat in downshore intertidal a nd subtidal zones. The main seasonal difference, from early to late su mmer, was an overall distributional shift towards the downshore open h abitat and the subtidal zone and a preference for greater depths in th e intertidal zone. These results support the hypothesis that pollock w ere using both refuging and schooling antipredator tactics during inte rtidal zone migrations. We suggest that rocky shores are important nur series for juvenile pollock.