Initial Changes in Use of Space and Food by Experimentally Segregated Populations of Dolly Varden (Salvelinus malma) and Cutthroat Trout (Salmo clarki)

Abstract

Dolly Varden (Salvelinus malma) (N = 881) and cutthroat trout (Salmo clarki) (N = 1571) were captured from a native lacustrine sympatric stock and transferred to two nearby lakes, formerly fishless, to establish allopatric populations with genetic backgrounds similar to the donor stock. Nearshore (5- and 10-m depth contours) vertical distributions of segregated Dolly Varden and cutthroat trout, between spring and autumn of the first 18 mo after transfer, did not differ significantly from those in their sympatric lake. Dolly Varden were captured mainly near the bottom while cutthroat trout were more evenly distributed throughout the water column. Nevertheless, initially the allopatric Dolly Varden rapidly became highly planktivorous, virtually eliminating midwater populations of Chaoborus trivittatus and C. americanus (third and fourth instar larvae) within a year. They later ate mainly benthic organisms as in the sympatric lake. Although allopatric cutthroat trout had a similar effect on Chaoborus larvae in their lake, they also ate more large benthic prey than did those in sympatry with Dolly Varden in the donor lake. These results suggest that while there may be genetic control restricting broad vertical use of space by formerly sympatric Dolly Varden shortly after being placed in allopatry, such control is flexible enough that a highly benthofagous stock in sympatry quickly becomes planktivorous in the absence of cutthroat trout.