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A Long‐Term Comparison of Redband Trout Distribution, Density, and Size Structure in Southwestern Idaho
Author(s) -
Zoellick Bruce W.,
Allen Dale B.,
Flatter Brian J.
Publication year - 2005
Publication title -
north american journal of fisheries management
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.587
H-Index - 72
eISSN - 1548-8675
pISSN - 0275-5947
DOI - 10.1577/m04-102.1
Subject(s) - trout , tributary , rainbow trout , fishery , drainage basin , juvenile , biology , environmental science , ecology , zoology , geography , fish <actinopterygii> , cartography
During 1993–2003, we reexamined the density and size structure of populations of Columbia River redband trout Oncorhynchus mykiss gairdneri at 43 sites in sagebrush desert basins of southwestern Idaho. Trout density had originally been sampled at these sites during 1977–1982. Populations were sampled in four major drainages of the Snake River basin: Bruneau River, Jordan Creek, Owyhee River, and Snake River tributaries. Redband trout distribution remained relatively stable between the two time periods, trout being present at 79% of the sites in 1977–1982 (1970s) and 84% of the sites in 1993–2003 (1990s). The mean density of redband trout did not differ between the 1970s (29.9 ± 6.3 trout/100 m 2 (mean ± SE)) and the 1990s (26.4 ± 5.5 trout/100 m 2 ), but the differences in redband trout density between these time periods were correlated with elevation. Densities were stable or declined at elevations less than 1,500 m, and densities generally were stable or increased at elevations of 1,500 m or greater. Densities increased from 19.7 ± 6.3 trout/100 m 2 in the 1970s to 39.3 ± 12.0 trout/100 m 2 ( n = 12) in the 1990s for the Jordan Creek drainage, where most sites were at higher elevations. Mean density declined from 37.5 ± 9.5 to 17.8 ± 6.0 trout/100 m 2 ( n = 16) from the 1970s to 1990s for Snake River tributaries, which were located at low elevations. The proportion of juvenile trout contributing to Snake River tributary populations declined from 48% to 25% from the 1970s to the 1990s. In contrast, the size structure of redband trout populations in the higher‐elevation Jordan Creek drainage was similar between time periods, juvenile trout constituting 44– 47% of the fish in those populations. Static to declining populations at low elevations indicate the importance of retaining or restoring connectivity between redband trout populations in desert basins.