Vertical distribution of zooplankton > 39 μm in relation to the physical environment off the west coast of South Island, New Zealand

Abstract

Abstract Data sets of pump‐sampled zooplankton > 39 μm were obtained for early August 1987, late July 1988, and late June 1990, along with environmental data in 1987 and 1990, off the west coast of South Island, New Zealand. The environmental circumstances favourable to zooplankton species known to be food for hoki larvae (Macruronus novaezelandiae) were investigated. We found that a major proportion of the vertical distribution of the epipelagic zooplankton off Westland is influenced by passive interaction with physical processes against a background of the distribution of autotrophic particles. Multivariate analysis classified stations into continental slope (Group I), outer‐mid shelf (Group II), and neritic (Group III) stations/depths. Contrasting with other stations in Group I (the habitat of hoki larvae), the physical conditions of one station differed in that winter mixing had hardly begun (indicated by relatively low nutrients), concentrations of 20–200 μrn autotrophic particles were low, as were concentrations of copepod nauplii, and Calocalanus spp. Deep mixing in early winter, in subtropical water along the outer shelf and slope of the west coast of South Island, may be necessary to promote the growth of zooplankton species important in the diet of hoki. This promotion of copepod growth may have been mediated through the growth of autotrophic particles > 20 um in a higher nutrient environment and/or the changing light environment which could have differentially favoured the growth of various phytoplankton size fractions, and therefore their predators. The 1990 year class of hoki did not make a strong contribution to the fishery for adults. In 1990 the onset of winter mixing (from model results) occurred 2 weeks later than in 1987 and 1988, years when hoki year classes contributed strongly to the fishery.