Vertical distributions and susceptibilities to vertebrate predation of the marine copepods Metridia lucens and Calanus pacificus

Abstract

We present results of a 2‐yr field study of the vertical distributions and diel vertical migrations (DVM) of Metridia lucens Boeck in Dabob Bay, Washington, and compare them to previous findings for co‐occurring Calanus pacificus Brodsky. M. lucens was found deeper than C. paciftcus , both day and night, and always exhibited DVM behavior, whereas C. pacificus exhibited more variable migration behavior, occasionally occurring in the surface layer in the daytime. These results led us to test the hypothesis that M. lucens exhibits greater avoidance of the well‐lit surface layer than C. pacificus because the former is more susceptible to predation by planktivorous fish. A 2‐yr field study of the diet and ambient prey fields of planktivorous fish in Dabob Bay indicated that 31 of 39 species/size classes of fish exhibited greater electivity for C. pacificus than M. lucens. A series of laboratory experiments corroborated these findings by showing juvenile Pacific herring to exhibit a highly statistically significant preference for C. pacificus over M. lucens. On the basis of both studies we reject the hypothesis that M. lucens is more susceptible to visual predation than C. pacificus and we conclude that the deeper distribution and greater avoidance of surface waters of M. lucens must be due to some other factor.