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Density, body condition, and movement of coastal cutthroat trout (Oncorhynchus clarki clarki) in logged and forested headwater streams of southwestern British Columbia De Groot, Jennifer D.

Abstract

Coastal cutthroat trout (O. clarki clarki) rear in small, headwater streams that may be particularly susceptible to impacts from land-use activities such as logging. Headwater populations of coastal cutthroat trout were trapped from 1997 to 2002 to investigate: (i) the effects of second-growth logging on trout densities and summer body condition over summer and winter months; (ii) the influence of physical habitat characteristics on summer trout densities; and (iii) the characteristics of winter trout movement within different habitat conditions. Trout mean summer densities in logged streams showed no decline following harvest, nor were there any large changes evident to physical habitat. However, among years, mean summer trout densities were higher in streams with deeper pools. Though summer trout densities remained unchanged in logged streams, trout densities in control streams declined. This suggests that trout may actually have benefited from logging, potentially due to enhanced levels of primary productivity arising from the removal of riparian canopy. Winter trout densities were similar to summer values, indicating that these headwater streams provided suitable conditions for trout populations year-round. Low flows during late summer, which reduce pool depth, may be more limiting to age 1 and older cutthroat trout found in coastal headwater streams than winter high-flow events. Increases in stream temperatures following logging activities were not associated with enhanced summer body conditions of trout, largely because minor temperature effects of logging (average increases of 1-2° C) were offset by a colder-than-average post-logging period. Winter movement (average 2-7 habitat units or 21-40 m) was found to be a common behaviour of coastal cutthroat trout in headwater streams and did not seem to be mediated by dominance hierarchies, influenced by riparian forest condition, or confer a growth advantage/ disadvantage for trout. Winter movement, however, was somewhat limited and may be a reflection of the need for trout to seek deeper pool habitats that provide cover and refuge from high-discharge events. Land-use management practices that protect existing pools and facilitate the processes that create pools may ensure the continued persistence of coastal cutthroat trout populations in headwater streams of southwestern British Columbia.

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