Ovarian weight of the Baltic herring (Clupea harengus membras) in relationto spawning time in the Archipelago Sea, northern Baltic

Trends with spawning time in the ovarian weight of the Baltic herring( Clup ea harengus membras) were studied from samples collected over the spawning season in 1988-1991, 1997 and 1998 in the Archipelago Sea, northern Baltic. Usually, ovarian weight was related to the size composition of the spawnin g shoals but there: was also variation unrelated with female length, In the latter cases, ovarian weight first decreased in the beginning of the spawn ing season with decreasing fish length but it then increased again in the m iddle of the season. irrespective of length. In such years. the piecewise r egression analysis suggested a breakpoint for the ovarian weight between th e dates 14 June and 30 June. At the later part of the spawning season, ovar ian weight was associated with temperature conditions and onset of zooplank ton production, which suggests that environmental factors modify the reprod uctive properties of the herring in the Baltic Sea. (C) 2001 International Council for the Exploration of the Sea.