Abstract.-From 1989-91 brown trout Salmo trutta of different sizes (15-41 cm) were caged in the Manistee River and exposed to predation attacks by chestnut lampreys Ichthyomyzon castaneus during spring and summer test periods. Fishery survey and angler catch data was used to estimate percentages of free-living trout attacked by lamprey. Larger trout were more likely to be preyed upon, both in cages, and in the wild. Larger trout in cages, or the wild, were also more frequently observed attacked simultaneously by more than one lamprey. From 0-100% of caged trout were attacked depending upon the test period. Attack rates on caged trout were generally highest during July. Over 76% of the lamprey marks on caged trout during 1990-91 were on the prey's dorsal region. Lamprey marks or attached lamprey were rarely observed on free-living trout in sections of the Manistee River upstream from the County Road 612 bridge in Crawford County. However, from 0-90% of trout observed in river sections between the M-72 bridge in Crawford County, and the Sharon Bridge in Kalkaska County, had been attacked by lamprey.
Mortality of caged trout due solely to lamprey attack could not be reliably estimated due to the confounding effects of such stress factors as confinement, intermittent high water temperatures, and disease. Depending on the year, test site, and test period, from 0 to 100 percent of trout attacked in various test groups died. However, from 4 to 80 percent of brown trout not attacked, also died during the same test periods.