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Polymorphic population of Oncorhynchus nerka at Babine Lake, B.C. involving anadromous (sockeye) and non-anadromous (kokanee) forms McCart, Peter James


The sockeye and kokanee are, respectively, the anadromous and non-anadromous forms of the Pacific salmon species, Oncorhynchus nerka. Both life history types inhabit Babine Lake, British Columbia, a tributary of the Skeena River system. The purpose of this study was to examine the ecology, morphology and behaviour of sockeye and kokanee in the hope that an understanding of these would provide clues to the genetic relationship of the two forms in Babine Lake. A comparison of the life histories of sockeye and kokanee at Babine Lake revealed a.number of differences. At the time they undertake their seaward migration (usually during the spring of their second summer) sockeye smolts have a mean length greater than that of same-age kokanee. Smolts have an approximately equal sex ratio while, among kokanee, males usually exceed females in abundance. As a result of better growth conditions in the ocean, sockeye, at maturity, are much larger than kokanee. Related to this basic difference in size are differences in fecundity, egg size and testis weight in each of which sockeye exceed kokanee. Laboratory experiments revealed that, regardless of the male parent, progeny of the larger sockeye eggs had an initial size advantage over the progeny of kokanee eggs which they maintained through July of their first year. There was no conclusive evidence of differential mortality to hybrid embryos. There are differences between sockeye and kokanee in two meristic characters: number of lateral line scales and number of vertebrae.In both instances, mean values for sockeye exceed those for kokanee. It is suggested that this difference may not be genetic in origin but rather the result of differences in the amount of yolk incorporated in eggs. The two forms did not differ in gill-raker count. Electrophoretic examination of haemoglobins and muscle myogens revealed no differences between Babine Lake sockeye and kokanee. A detailed examination of the reproductive behaviour of sockeye and kokanee revealed that they spawn sympatrically in a group of streams known as the "early streams." These are small streams which experience considerable fluctuation in water levels and spawning suitability from year to year. Sockeye and kokanee in the early streams overlap almost completely in their spawning season and in their distribution on the spawning grounds. Evidence is presented that hybridization does occur under natural conditions. A study of the homing performance of mature sockeye and kokanee displaced from early streams indicates that they are less likely to home than are sockeye displaced from Pinkut Creek, a large, stable stream in the same area. It is suggested that a reduced homing tendency might be an adaptation to the unstable nature of the early streams. Fish homing to an early stream to which access is blocked, either by low water or by an obstruction, have the alternative of entering other, nearby streams of similar type. The hypothesis which most readily encompasses the available information is that the sockeye and kokanee in the early streams at Babine Lake are part of the same polymorphic population. This polymorphism is presumably maintained by a balance of contending advantages and disadvantages. Kokanee suffer the major disadvantage of smaller size resulting in reduced fecundity, smaller egg size and, probably, reduced spawning success. However, it would appear that they persist in the early streams because they are able to utilize spawning grounds which are unavailable to sockeye under low water conditions. The existence of such a sockeye/kokanee polymorphism and the reduced tendency to home are both thought to be genetically regulated adaptations which enable the early stream populations of O. nerka to maximize their utilization of the available spawning grounds in. the face of extreme fluctuations in the suitability of spawning streams. The hereditary mechanism which would regulate such a sockeye/kokanee polymorphism is not known. Possibly a super-gene is involved. Whatever the mechanism, it would appear that factors other than genotype can influence the tendency to smolt: females are more likely to smolt than males; larger and/or faster growing fish are more likely to smolt than smaller, slower growing fish; immature fish are more likely to smolt than those in which maturation processes have already begun. The applicability of the polymorphism hypothesis to sockeye and kokanee populations in other areas is discussed.

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