Trout population response to habitat enhancement in six northern Colorado streams

Abstract

We examined the effects of log drop structures on trout populations in six small, remote Rocky Mountain streams. Angling pressure was low on all streams, and most anglers killed no fish. Log drop structures were installed in 250-m treatment sections in summer 1988, and results were compared with adjacent 250-m control sections during 1987–1990. The structures caused marked changes in habitat, including greatly increased pool volume, decreased current velocity, and increased depth and cover. After the structures were installed, abundance and biomass of age-2 and older trout (and often age-1 trout) increased in all six streams, but there was no evidence that trout were in better condition or grew to larger sizes in most streams. Recaptures of tagged trout in two streams showed that the logs did not result in increased growth or survival of resident trout, although recaptures of fin-clipped trout in other streams suggested that apparent survival may have increased temporarily in treatment sections. Low recapture rates of marked trout and high percentages of unmarked adults indicated that immigration rates were high, suggesting that movement is an important mechanism to account for the increase in trout populations after habitat enhancement in these streams.