Fish larvae: zooplankton relationships in microcosm simulations of earthen nursery ponds. II. Brackish water system.

摘要

A simulation of the effects of predation intensity on zooplankton composition in brackish water nursery ponds was carried out to address the problem that commercial fish nurseries encountered in obtaining enough zooplankton of adequate species composition and size when fish larvae started to feed. The experimental system consisted of 12 containers (130 litres) with treatments of 4 densities (0, 1, 2 or 4 larvae/litre) of common carp (Cyprinus carpio), stocked on the 6th day after filling the containers. Zooplankton-environment relationships were explored using factor analysis. Factor analysis allowed the identification of several groups of zooplanktons that responded in different ways to fish larva predation pressure. The first factor represented a general measurement of rotifer abundance, and the second identified the direct effect of size-selective fish predation. Since no rotifers were present in the filling water, all these species were autochthonous populations that hatched from resting forms in the sediment and reproduced. In the absence of fish predation, this led to a steep rotifer increase. Fish predation started when the rotifer concentration was just starting to increase, and their direct predation reduced and delayed the rotifer abundance peak. This effect increased with the elevation in fish larval density. Estimations of rotifer consumption by fish larvae in this experiment were higher than similar calculations from literature data, which led to the testing of the hypothesis that factors other than direct predation affected the rotifer population dynamics. The mechanisms involved in rotifer population regulation were discussed. It was concluded that in commercial nurseries, increased production of larvae could be achieved by keeping the larval density at an intermediate level and stocking fish to match the increasing phase of the rotifer peak. Under reasonable larval density (up to 2 litres), it seemed that the direct predation effect of fish larvae on rotifer dynamics was minor compared to fish induced self-regulation..