Spawning of herring (Clupea harengus membras L.) in the Archipelago Sea

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Abstract
Spawning of herring ( Clupea harengus membras ) was investigated in the Archipelago Sea, south-west Finland, in 1987–1989 using SCUBA diving, trap-net sampling, and catch statistics. Herring spawned repeatedly on the same beds. Spawning was rather continuous from April to July when the whole spawning area was considered and took place in temperatures between 4 and 18°C. Spawning shoals consisted of fish of different size and age. Egg densities varied between locations, years, and spawnings. Egg size and mortality were dependent on the spawning time. In the early spawnings, eggs were larger than in the late ones. Egg mortality was < 10% in May but increased significantly in June. Temperature, spawning depth, and substratum type partly accounted for the mortality. Perch ( Perca fluviatilis ) was the most important predator of eggs and invertebrate predation was found to be negligible. It is suggested that the annual spawning pattern is variable and dependent on the variation of the start, duration, and intensity of spawning. Elimination of herring offspring at the egg stage may vary considerably each year depending on the time of peak spawning, which probably affects the size of the year-class.

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