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During three consequtive years, 1975–1977, Individually tagged Baltic salmon Salmo salar smolts of sexually immature male and female fish (n = 35027, mean size: 15.2 cm) and precocious males (n = 6518, mean size: 14.2 cm) were released into Umeälven (Ume river), northern Sweden. Rate of survival (% captured adults) based on 3714 recoveries was significantly higher (p < 0.01) for smolts from immature fish (10.2%) than those from smolts of early maturing males, i.e. precocious males (2.2%). corresponding to an average yield of 474 and 85 kg per KHX) smolts released, respectively. Gain in survival was on average 2.5% and 1.4% per cm increase in smolt size for immature smolts and smolts from precocious males, respectively. The poor survival among smolts of precocious males is suggested to he related to an interaction between sexual maturation and smolting linked to incompletely resorbed gonads leading to a non migratory behaviour. These non migratory males are then suggested to suffer heavily by predation in the river. The two smolt categories had a similar growth pattern in sea. Smolts from precocious males did not mature early in sea indicating no relation to grisling, i.e. sexually maturing fish returning after first winter in sea. Adult weight of fish returning the fourth summer after release was related to smolt size (P < 0.05). Our Response Surface Model (RSA) predicted that large smolts (19.0 cm) had a higher specific growth rate over their life‐span compared to small smolts (<15.0 cm), 0.86% d−1 and 0.46% d−1, respectively. Large smolts (19.0 cm) attained a size of 3.0 kg during their second winter in sea about six months earlier than small smolts (13.0 cm). The paper discusses alternative release strategies that can be employed if the ultimate goal of salmon stocking is maximizing yield.

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1992. Circannual cycle of seawater c3daptability in Oncorhynchus nerka: genetic differences between syrnpatric sockeye salmon and kokanee. Can. J. Fish. Aquat. Sci. 49: 99-109.Anadromous sockeye salmon and nonanadromous kokanee (Oncorhynchus nerka) spawn syrnpatrically in locc3-tions across British Columbia, yet remain genetically distinct. To investigate the possibility that these differences are maintained by selection against hybrids of the two forms owing to differences in seawater adaptability, we raised pure and reciprocal crosses of Shuswap River sockeye and kokanee in controlled hatchery conditions and tested their seawater adaptabilities nine times over 2 yr. Sockeye, kokanee, and reciprocal hybrids all showed a seasonal cycle in their seawater adaptability, which was lowest in midwinter and highest in the spring. However, clear differences were evident among cross-types in the spring s f their second year. Sockeye were the first to show increased seawater adaptabilities, hybrid groups next, and kokanee last. The observed differences could not be attributed to tank effects, or to differences in sine, sex, or state s f maturity. Le saurnon nerka anadrome et sa forme non anadrome, le kokani (Oncorhynchus nerka), fraient de facon sympatrique en plusieerrs lieux de la Colombie-Britannique, mais demeurent genktiquernent distincts. Afin d'etudier la possibiliak que ces differences soient maintenues par une selection s'exer~ant 21 ['encontre des hybrides des deux formes, pour des raisons d'adaptation a I'eau de mer, nsus avons eleve en pisciculture et dans des conditions contr8lees des produits de croisements purs et r6ciproques de saumons nerkas et kokanis de la rivi6re Shuswap et vkrifik leurs capacitks $'adaptation A l'eau de mer a neuf reprises au cours d'uns periode de 2 aws. Les saumons nerkas et kskanis de meme que leurs hybrides rkciproques ont tous prksente u n cycle saisonnier d'adaptation a I'eau de mer qgli etait le moins kleve au milieu de I'hiver et le plus eleve au printernps. Des diffkrences nettes etaient cependant evidentes entre les poissons de types de croisement differents au cours de la deuxieme annee.Les nerkas ont 6t4 les premiers a presenter une capacitk accrue d'adaptation a I'eau de mer, venaient ensuite les hybrides et, en dernier, les kokanis. Les kcarts observes ne pouvaient @tre attribues 2 des effets lies aux cuves csu a des differences de taille, de sexe ou de stade de maturite.

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