The wave-drag hypothesis: an explanation for size-based lateral segregation during the upstream migration of salmonids

During their spawning migration, large salmon, e.g., chinook (Oncorhynchus tshawytscha), swim upstream further from the bank than smaller ones, e.g., sockeye (Oncorhynchus nerka). This pattern is counterintuitive because natural selection should favor behavior that minimizes migration costs, yet by traveling further from the bank, large fish will have to swim against faster currents. Existing theory predicts that they will expend more energy than necessary as a result. One explanation for this apparently paradoxical behavior is that large fish swim further from the bank to avoid wave drag, the resistance associated with the generation of surface waves when swimming close to the surface. Wave drag was incorporated into existing theory, and the resulting model was tested to determine whether it explained size-based lateral segregation of chinook and sockeye salmon in the Nushagak River, Alaska. The wave-drag model accurately predicted the migration corridor for both species. Existing theory worked well for sockeye but not for chinook. The key to these predictions is that wave drag scales according the ratio of maximum body diameter to submergence depth, so bigger fish need to swim deeper to escape its effects.