AVAILABILITY OF MICROHABITATS AND THEIR USE BY BROWN TROUT (SALMO TRUTTA) AND GRAYLING (THYMALLUS THYMALLUS) IN THE RIVER VOJMÅN, SWEDEN

Abstract

Microhabitat preferences of brown trout and grayling were studied in the upper portion of the River Vojmån, northern Sweden in 1990–1993. Microhabitat preference was quantified for open water fish during the day and at dusk and for fish residing under stones during the day. In total, measurements were made for 665 trout and 230 grayling, ranging in size from 2.5 to 50 cm total length. Relationships were found between most microhabitat variables and fish length. Consequently, fish were divided into three size groups for each species. For diurnally active open water fish, small brown trout and grayling preferred slower moving water with finer substrates than large brown trout and grayling. In addition, small trout preferred shallower water than large trout, whereas grayling showed no size segregation with respect to depth. Interspecific microhabitat segregation for diurnally active open water fish was pronounced, with distance to instream cover, distance to shore and depth the major segregating factors. Brown trout that occurred under stones were generally ≤12 cm in length and preferred faster mean currents than similar sized trout in the open water. Trout had stronger preferences for bedrock and slightly faster mean current velocities at dusk than during the day. Grayling did not occur under stones and relatively few individuals were observed at dusk. These results indicate that microhabitat preferences vary with body size, species and time of day. Consequently, when developing water regulation strategies it is important to maintain a broad diversity of microhabitats at different flows, at least until we know more about the mechanisms responsible for this variation in microhabitat preference.