Investigating zooplankton populations in the North Sea several times, a spring-peak density of herbivore copepod populations was observed in May/June, followed by a decline in June/July. This was for instance observed in 1973 in the Southern Bight (when a peak density of fish larvae coincided with that decline). The decline can be caused by lack of food (phytoplankton) and/or by heavy predation. The goal of this doctoral research was to assess the role of predation in the, in June/July expected, decline of herbivore copepod populations. We investigated the following possible predators: fish larvae, Hydromedusae, ctenophores and scyphomedusae. From June 14 to July 5 we sampled weekly in an coastal area and in a more offshore region along the Dutch coast. We determined the density and the length composition of the predator-species. Unfortunately we missed the decline of the herbivore copepod populations (in connection with the fixed sail-scheme). The development of the zooplankton may be retarded by the low (water) temperatures in the spring of 1977. This research cannot give an answer to the earlier stated problem. What we were able to do was to investigate if we could find a relationship between the amount of presence and production respectively mortality of the several predator-species on the one side and the copepods on the other hand. We found that the fish larvae can develop when the copepod densities are low. In that case they will presumably eat other animals. However when the densities of the copepods are high, as observed in the coastal area, the relative developmental rates are higher for both the hydromedusa Phialidium hemisphaericum and the ctenophore Beroë gracilis and the fish larvae. Compared with preceding research the development of the fish larvae was rather moderate (the density was as much as hundred times lower than in 1973 during the same period in the same area). A possible reason for that is the competition for food by for instance Phialidium hemisphaericum, that abounded in 1977. Another reason can be that Phialidium hemisphaericum as the Scyphomedusae Aurelia aurita, Cyanea lamarckii and Chrysaora hyoscella also can eat fish larvae, so that they may have caused by predation the low densities of the fish larvae. |
