Vertical distributions, diurnal migrations, and sampling problems of hyperiid amphipods in the North Pacific central gyre

Abstract

Over 14,000 hyperiid amphipods were identified to species from replicated, stratified opening-closing net tows (n = 79) in the North Pacific central gyre. Most hyperiid individuals live in the upper 100 m; details of their resource partitioning are not yet known, but differences in either diurnal migratory behavior or preferred depths could help species avoid competition. Each of the two most abundant species accounts for 25% of the hyperiids captured. One species appears to show normal (up at night), the other reverse migration on a fine vertical scale, but statistical treatments of the data only marginally support these conclusions.

Details of vertical distributions are shown for the 10 most abundant species (accounting for 80% of the individuals captured). Variability among replicates is large, making it difficult to draw statistically defensible conclusions about vertical distributions. At the 5% significance level only two species show diurnal migration; at the 10% level, only four do so. These difficulties occur even with well replicated sampling in an extremely stable environment. The difficulties are discussed and applied to the question of niche dimensionality among hyperiid species.