Comparison of fecundity-length-latitude relationships between nonanadromous (kokanee) and anadromous sockeye salmon (Oncorhynchus nerka)

Abstract

This study compared fecundity-length-latitude relationships between 25 kokanee populations (15 natural and 10 introduced) and 48 sockeye salmon populations. Significant differences confirmed the hypothesis that the two Oncorhynchus nerka variants follow different reproductive strategies: (i) fecundity is more highly correlated with length for kokanee than for sockeye salmon; (ii) kokanee have higher fecundity-length regression slopes and lower intercepts than sockeye salmon; (iii) kokanee populations share a common fecundity-length regression slope, but sockeye salmon populations do not; and (iv) average lengths and fecundities of kokanee decrease with increasing latitude, but those of sockeye salmon do not. The first three findings confirm that kokanee maintain a constant egg size while increasing egg number with increasing body size but that sockeye salmon increase both egg number and egg size with increasing body size. Kokanee egg sizes may be less variable than those of sockeye salmon because kokanee have lower and less variable energetic costs of spawning migration and tend to use spawning gravel with smaller and less variable particle sizes. Latitudinal clines in kokanee length and fecundity may reflect latitudinal gradients in temperature and duration of the growing season. Such environmental gradients may explain why kokanee populations are rarely found as far north as Alaska.