Reproductive characteristics of the male herring in the northern Baltic Sea

The gonad weight and gonadosomatic index of the male Baltic herring Clupea harengus membras L., were highest at the beginning of the reproductive season (April/May), the values decreasing towards the end of it (July/August) during 1988–1991. The decline could not be explained by fish size but may have been due to fish condition. A high individual variation was typical for both gonad weights and gonadosomatic indices in fish of the same size and maturity stage. The mean density of sperm cells was significantly higher in June (34·9 × 10⁹ ml^?1) than in July (19·2 × 10⁹ ml^?1, Mann-Whitney U= 17; P<0·05), the variation among the males being high in both groups. Electron microscope analysis showed a severe disruption of the mitochondrial elements in males spawning at 22°C.