Numerous predator species were found in the Southern Bight of the North Sea during the period June 18-July 26 (1979). Four subareas distinguished differed in predator density, assumingly due to the abundance of the Scyphomedusa Cyanea lamarckii in both coastal areas. It is not clear how the large jellyfish met their nutritional needs. During the last week they disappeared almost entirely. In the Southern Coastal subarea the density of fish larvae, increased with the decline of Cyanea, but this might be caused by immigration from the south. In both offshore subareas, the predators fed probably mainly on animals other than copepods. In these subareas, Pleurobrachia pileus was one of the predominant species, but decreased significantly during the period of observation. Along the shore the copepods were much more numerous than offshore. A low development rate was distinct in all subareas, possibily due to low food levels. A summer decline was very distinct in the Northern Coastal subarea, starting about July 10. Apparently, the summer decline in the Southern Coastal subarea started one week later. In both offshore subareas, a summer decline of copepods was not observed. The summer decline of copepod populations occurring near the coast was preceded by a shortness of food. The actual decline was probably caused by fish larvae predation and perhaps also by Beroë and Cyanea. |
