Swimming performance and morphology of juvenile sockeye salmon, Oncorhynchus nerka: comparison of inlet and outlet fry populations

We raised two populations of sockeye salmon fry from fertilized eggs in the laboratory and tested the hypothesis that outlet fry populations, fish which must migrate upstream to reach rearing lakes after yolk-sac absorption, have better swimming ability and morphological characteristics conducive to enhanced swimming performance than inlet fry populations, fish which migrate downstream to rearing lakes. Despite being of identical age, fry from the outlet population were larger (approx. 6.7% longer, ~5 mm on average) and more laterally compressed than inlet fry at the time of our initial experiments. Using an open-top box flume, we found that the burst-swimming performance (in cm s⁻¹) of the outlet population was 31% better. We found no differences between populations in prolonged-swimming performance. We were unable to find any direct relationships between measures of swimming performance and size or shape variables, suggesting that the larger, more robust morphology of outlet fry was not responsible for the superior burst ability. Recent biochemical studies indicate outlet fry may be metabolically better provisioned for burst swimming than inlet fry. It is possible that the morphological differences between the populations of fry reflect adaptations needed by adults during their migration and spawning.