An Orthokinetic Response to Rates of Salinity Change in Two Estuarine Fishes

The average swimming speed of two estuarine fishes, spot (Leiostomus xanthurus) and croaker (Micropogon undulatus), was determined in the laboratory under a constant and a changing salinity regime. It was found that croaker moved faster under 5 and 10% salinity change hr—1 as compared to the fixed salinity regime. Spot increased its speed of movement only under the 10% change hr—1 condition. These results suggest that (1) those portions of the estuary possessing similar salinity changes may be avoided by spot and croaker and that (2) possibly the rate of change in salinity rather than the actual salt concentration limits the distribution of these species.