Influence of environmental conditions on the distribution of Pterygosquilla armata capensis (Crustacea: Stomatopoda) off Namibia

Abstract

The distribution patterns of the benthic stomatopod crustacean Pterygosquilla armata capensis were studied off southern Namibia between latitudes 23 and 30°S and between about 50 and 500 m depth. Whereas south of 28°S and off the South African coast this species is mainly a coastal animal distributed on soft substrata at depths shallower than 150–200 m, the maximum densities in Namibia occur in much deeper waters (200–350 m). In winter, maximum relative densities of the species are found shallower (< 200 m depth) than in summer (200–300 m). This density shift is thought to be related to the fact that, in summer, the main upwelling period has passed and inshore oxygen levels on the bottom decrease to highly hypoxic values. The species avoids areas shallower than 200 m north of 27°S, the area where oxygen depletion is strongest during all seasons. Oxygen levels appear to be critical to the inshore distribution of the species, whereas temperatures of < 10–11°C seem to limit its distribution towards deeper waters. Highest abundances occur on sediments with 10–40 per cent silt-clay content. Population size structure did not show any apparent peaks which could be related to age cohorts. Males attained larger size than females. © 1990 Taylor and Francis Group, LLC.