THERMAL TOLERANCE AND VEGETATION PREFERENCE OF ARKANSAS DARTER AND JOHNNY DARTER FROM COLORADO PLAINS STREAMS

The Arkansas darter Etheostoma cragini is declining and now occurs in fragmented populations across its former range in the Arkansas River b asin from Colorado to Arkansas. Regional surveys indicate that the Ark ansas darter primarily inhabits small spring streams with dense aquati c vegetation, suggesting that specific habitat requirements may accoun t for its widespread decline. Thermal tolerance and vegetation prefere nce of the Arkansas darter from the western edge of its range in Color ado were measured in the laboratory and compared with those of the joh nny darter E. nigrum, a more abundant habitat generalist from an adjac ent basin. Critical thermal maxima of Arkansas darters acclimated to 2 0, 25, 27.5, and 30 degrees C were higher than those for johnny darter s (35.0-38.4 degrees C versus 34.0-37.4 degrees C). In a modified uppe r incipient lethal temperature test, Arkansas darters acclimated to 27 .5 degrees C all survived 106 h at 30 degrees C when the temperature w as raised 1.5 degrees C/h, but almost none survived more than 4 h of e xposure to 34 or 36 degrees C. Sixty percent survived 106 h at 32 degr ees C. In contrast, air johnny darters died within 17.5 h when exposed to 32 degrees C and within 4.5 h when exposed to 34 degrees C or high er. In behavioral choice trials in aquaria. Arkansas darters selected dense vegetation over open sand 72% of the time, with higher use durin g day (90%) than night (60%), In contrast, johnny darters showed littl e preference for vegetation (51% overall). Arkansas darters of the siz es we tested (28-55 mm total length) were able to withstand higher wat er temperatures than johnny darters (43-67 mm) and many other fishes o f Great Plains streams, and they have an innate preference for aquatic vegetation.