On the dynamics of exploited populations ofTisbe holothuriae (Copepoda, Harpacticoidae)

Abstract

A total of 90, weekly exploited populations of the harpacticoid copepodTisbe holothuriae Humes were exposed to 148, 222, 333, 500, 750 or 1125µg Cd⁺⁺ l⁻¹, combined with exploitation rates of 10, 30, 50, 70 or 90% under conditions of surplus food supply at 22° C and 30 ‰ S. All populations exposed to concentrations down to 500µg Cd⁺⁺ l⁻¹ and 3 populations (out of 15) exposed to 333µg l⁻¹ became extinct within the experimental period of 30 weeks. Survival time significantly depended on concentration. A recovery phase from an initially high mortality preceded eventual population extinction after adding 500µg Cd⁺⁺ l⁻¹. In the initial phase, higher nauplii mortality prevailed. During these experiments on acute intoxication, no relationship could be established between survival and exploitation rate. However, experiments on the effects of stepwise increases in Cd concentration (results not yet published) produced evidence of such a relationship. In spite of increased mortality, no significant numbers of dead copepods were detected in weekly samples because of their rapid decomposition and cannibalism, which depends on the amount of food available. Sampling regimes of 5 times per week yielded significant numbers of dead individuals.