Alteration of buoyancy by some Central American stream fishes, and a comparison with North American species

Of 23 species (nine families) of Central American fishes examined, nine (eight physostomes, one physoclist) did not reduce buoyancy by decreasing swim-bladder volume when current was encountered. The remainder (six physostomes, eight physoclists) decreased buoyancy, usually within 24–48 h, but three of these species (one physostome) were unable to maintain a reduced buoyancy for as long as 72 h once the minimum was achieved. All species have an excess internal pressure of swim-bladder gas. All of the North American species that have been studied do show a buoyancy alteration in response to changes in water current and adjust to a greater extent, although rates of adjustment appear similar to those of Central American fishes. Differences in flow regime between North and Central American streams are described and may partially account for these differences in reaction to water current.