Buoyancy Regulation by Hatchery and Wild Coho Salmon during the Transition from Freshwater to Marine Environments

Abstract

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.

Mechanical challenges to freshwater residency in sharks and rays

“…Teleost fish often compete for the same ecological space with elasmobranchs, but the utility of a gas bladder as a source of upthrust largely negates the buoyancy problem faced by sharks and rays. Juvenile coho salmon (Oncorhynchus kisutch) collected along a salinity gradient display compensation in swim-bladder volume; in marine waters a bladder comprising 5% of whole body volume is adequate to provide near neutral buoyancy and in freshwater this volume only increases to 7% (Weitkamp, 2008). This difference is unlikely to affect parasite drag, because surface area and fineness ratio will remain largely unchanged (Alexander, 1966).…”

“…Because of the minor difference in density of liver tissue (900-1000 kg m −3 ) compared with that of marine waters (∼1027 kg m −3 ), large livers are required to provide necessary force to approach nearneutral buoyancy. Indeed, neutrally buoyant sharks, which are commonly found in the deep sea may have livers that comprise 30% of whole body volume (Corner et al, 1969) compared with only 1-7% swimbladder volume required to provide neutral buoyancy in ray-finned fishes (Alexander, 1966;Davenport, 1999;Weitkamp, 2008).…”

Submerged cage aquaculture of marine fish: A review of the biological challenges and opportunities

“…48 This explains the lack of knowledge surrounding the basic limits of salmon buoyancy to date. Dependent on lipid content, 49 life history stage 50,51 irrespective of the submergence depth tested (Figure 4). 32,34,36 Buoyancy challenges for physostomous fish in submerged culture, however, may not be insurmountable.…”

“…This explains the lack of knowledge surrounding the basic limits of salmon buoyancy to date. Dependent on lipid content, 49 life history stage 50,51 and factors influencing swim bladder volume, buoyancy in fish is dynamic over time. Therefore, understanding how neutral buoyancy limits change across species and life stages is essential to determine the suitability of submerged cages at different stages during production.…”