The Influence of Life History Trade-Offs and the Size of Incubation Gravels on Egg Size Variation in Sockeye Salmon (Oncorhynchus nerka)

Abstract

Egg size is a critical life history trait, reflecting female investment and affecting offspring fitness. We investigated several factors which may influence variation in egg weight for sockeye salmon (Oncorhynchus nerka). Comparisons were based on collections from 18 Alaskan populations, among which adult migration distance and juvenile rearing habitat were similar but the size composition of incubation gravels was different. Among populations, most of the variation in egg weight could be explained by a positive correlation with different measures of the size composition of incubation gravels (Pearson's r = 0.45-0.91). In contrast, egg. weight was poorly correlated with female body length and with female snout length, a morphological feature used during intra-sexual competition. Within each of the Alaskan populations, however, egg weight and snout length were positively correlated with female body length and hence with each other. A positive association between snout length and egg weight was still evident even after the effects of covariance with body size were removed using residuals analysis : for all of the fish pooled and within 6 of the 16 populations. A significant relationship was not detected in the other populations but the trend was nevertheless positive in 8 of the other 10. Examination of reproductive traits (gonad weight, egg weight, egg number, snout length and hump size) within another population identified a trade-off between egg weight and egg number for females of a given body length. In contrast, positive correlations between reproductive traits were more common, suggesting that energy-rich individuals produce large eggs and large secondary sexual characteristics rather than sacrificing one for the other.