Some Speculations about the Baltic Herring (Clupea harengus membras) in Connection with the Eutrophication of the Baltic Sea

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Abstract
In this paper the hypothesis is put forward that Baltic herring (Clupea harengus membras) spawning time, spring or autumn, is determined by feeding conditions during the adult phase and thus not genetically fixed. The present "absence" of autumn spawners is thought to be the result of improved feeding conditions during the latest decades as a result of the eutrophication of the Baltic Sea. During two spawning ground studies carried out in 1978 and 1982 unusually high mortality rates were noted for eggs in situ. In 1982, during 4 wk close to peak of spawning, the mortality increased substantially, especially for eggs among filamentous algae. A significant difference was noted between eggs on coarser algae and those among filamentous algae (p < 0.001). During this period the average mortalities were 33 and 75%, respectively. Very low levels of oxygen were measured at night among the filamentous algae. An increase in the amounts of this type of algae as a response to the eutrophication might constitute a new hazard to the reproductive success of the Baltic herring.