Factor Analyses of a Hyperiid Amphipod Assemblage from the North Pacific Central Gyre

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Abstract
Abundances of the 20 most common species of hyperiid amphipods in the North Pacific central gyre (28.degree. N, 155.degree. W) were studied with factor analysis. Samples (79) were collected with opening/closing nets during 10 days each net tow was about 1000 m long. Depth intervals were 0-25, 25-50, 50-75, 75-100, 100-350 and 350-600 m. R-mode analyses grouped species with similar patterns of numerical variation across samples. Q-mode analysis grouped samples with similar patterns of variation in species abundances. Orthogonal and oblique factor analyses delineated identical groups of species. Abundances of species within a group vary in parallel-ways across samples: abundances within a group are believed correlated with a single underlying factor. Among the 20 spp. there were about 5 major patterns of variation, each presumably corresponding to an important environmental factor or process. Nineteen species each had a high correlation with 1 of these factors; the remaining species was consistently highly correlated with more than 1 factor. The factors are hypothetical constructs derived from the data base, but have yet to be identified with real-world variables. The groups of species were not dominated by patterns of species vertical distributions, diurnal vertical migrations or relative abundances. The groups of samples were dominated by time of day and depth. The results of factor analyses were compared to an earlier recurrent group analysis of the same data: recurrent group analysis uses only presence/absence. The comparison showed that abundance and presence/absence of species are separable, but not independent, aspects of hyperiid distributional pattern.