Adjustment of Buoyancy by Longnose Dace (Rhinichthys cataractae) in Relation to Velocity of Water

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Abstract
The buoyancy of longnose dace (Rhinichthys cataractae) is correlated with the velocity of water in which they are living. Swimbladders from fish collected in rapids of streams and rivers are smaller in volume than in those fish collected from still water. Laboratory experiments on yearling and older fish show that individuals alter their buoyancy by changing the volume of their swimbladder. Buoyancy depends on velocity of water and size of fish. Longnose dace adjust their buoyancy rapidly in response to changes in velocity of water. They can decrease their buoyancy to adapt to an increase in velocity of water in 3 hr. A complete adjustment requires 4 days. Fish with access to the surface can increase their buoyancy in response to reduced velocities by completely inflating their swimbladders in less than 24 hr. Fish without access to the surface require 48 hr to complete the same adjustment. Thus, longnose dace have a functional gas gland even though they are physostomes. The gas gland does not function during the early stages of life after hatching. Alevins come to the surface to fill initially their swimbladders. The ability to adjust buoyancy rapidly is an adaption to life in rivers and streams by fishes that live mostly in swift-flowing waters but are sometimes forced to live temporarily in reduced current when the volume of water flowing is diminished.

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