Assessing the effects of positive buoyancy on rainbow trout (Oncorhynchus mykiss) held in gas supersaturated water

We examined the effects of swim bladder overinflation associated with dissolved gas supersaturation on rainbow trout (Oncorhynchus mykiss). The change in swim bladder volume with increased swim bladder pressure was measured in fish subjected to a decrease in ambient pressure. An expansion of swim bladder volume occurs that is related to the excess swim bladder pressure. The volume change results in a decrease in density and positive buoyancy in the fish. Small fish are adversely affected when exposed to gas supersaturated water because of the high swim bladder pressure required to force gas out the pneumatic duct. Changes in behaviour and depth distribution of fish held in gas supersaturated water were measured in a 2 m deep observation column. A large change in density caused small fish to increase depth and compensate for the swim bladder expansion. Although swim bladder inflation occurs for all sizes of trout held in gas supersaturated water, the impact is greatest for small fish and they must compensate by seeking depth. However, adequate depth to compensate for positive buoyancy may not always exist. In such a case, fish must swim continuously in a head down position to overcome excess buoyancy. The power necessary for a fish to swim with an overinflated swim bladder is greatest for small fish that show the largest change in density.