Buoyancy regulation by hatchery and wild coho salmon during the transition from freshwater to marine environments

One aspect of diadromy that has received little attention is buoyancy regulation in fish moving between freshwater and marine environments. Because of density differences between the two water types, fish must alter their whole-fish density (WFD) or they will become positively (float) or negatively (sink) buoyant as they change environments. This idea was first suggested over 80 year ago but has been largely overlooked by the scientific community. To explore how fish regulate buoyancy during this important transition, I measured WFD and lipid levels and estimated swim bladder volumes (SBVs) of juvenile coho salmon Oncorhynchus kisutch collected from freshwater and marine environments. These fish exhibited increased WFD with increasingly dense environments, suggesting active buoyancy regulation. Most of the WFD increase was attributable to decreases in SBV, although hatchery coho salmon also exhibited decreased lipid levels with increasing WFD. Hatchery coho salmon had significantly higher lipid levels than wild coho salmon in both freshwater and marine environments. These high lipid levels may impede the ability of hatchery fish to regulate buoyancy and may increase their vulnerability to surface predators. Furthermore, lipid levels that vary with both environmental water density and fish origin clearly complicate the interpretation of this variable during the important transition from freshwater to the ocean.