Abstract
Juveniles of different salmonid species often co-exist along environmental gradients, making them a useful model for identifying dominant trade-off axes and their stability within a biological hierarchy (e.g., from individuals to populations to species). In this perspective, we use multivariate trade-offs among juvenile coho salmon (Oncorhynchus kisutch) and rainbow trout (Oncorhynchus mykiss) as a case study to explore broader-scale patterns of trait association. Multivariate ordination identified a dominant trade-off axis between high growth, consumption, and growth efficiency versus high aerobic scope and active metabolism among individual juvenile rainbow trout and three ecologically divergent rainbow trout populations. This pattern suggests a dominant trade-off between growth and active metabolism among individuals and populations, facilitated by simple developmental controls on increased energy intake (larger digestive tract, potentially more risky foraging behavior). In contrast, the adaptive trade-off differentiating species appears to have shifted to maximizing growth and consumption (rainbow trout) versus maximizing growth efficiency (coho), based on evolved differences in foraging strategy and digestive physiology. The generality of these patterns remains uncertain, but trade-offs related to growth efficiency may be an underappreciated dimension of adaptive differentiation in fishes, and their relationship to digestive strategy and active metabolism warrants further investigation.