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Fishes and Stream Habitat in Tributaries of the Klamath River in Crater Lake National Park, With Reference to the Sun Creek Bull Trout.


Materials and Methods


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Fish Population Estimates

Each stream survey began at the park boundary and progressed upstream into the tributaries until fish were judged to be in low abundance or absent (Fig. 2). Estimates of fish abundance were made by direct observation by a single snorkel diver in 10 percent (every tenth unit of a habitat type) of all habitat types, except in Sun Creek where 20 percent of the pools (every fifth pool) were surveyed. These counts were extrapolated to estimate the total number of fish in each stream (Hankin & Reeves 1988). A flashlight was used to illuminate shaded areas beneath undercut banks and woody debris.

Age classes of fish based on measured body length were: < 60 mm, age 0; 60 - 100 mm, age 1; > 100 mm, age 2 and older. Brook and bull trout hybrids were identified by spots on dorsal fins and often by small vermiculation patterns on dorsal flanks and weak tri-coloration on pectoral and pelvic fins (fins with slight or no orange cast and a black stripe next to a white leading edge). Bull trout had solid-colored fins and lacked body vermiculations. Brook trout as small as 50 mm were distinguished from bull trout by dorsal fin vermiculations.

In Sun Creek, the position of each fish in the channel was assigned to one of three locations: (1) mid-channel without cover; (2) mid-channel in lee of cover; (3) and channel edge. Cover was considered to be any obstruction to stream flow, e.g., woody debris and larger than average substrate.

Fish collected using electroshocking were compared with the number of fish counted during snorkel counts in Sun Creek. Ideally, ten or more comparisons were needed to calculate reliable correction factors for abundance estimates based on snorkel counts (Hankin & Reeves 1988). Only a single 70 m section of stream channel was successfully electroshocked in a rigorous manner. Other electroshocking efforts met with failure due in part to logistical problems and equipment failure. In the 70 m section, three brook trout and eight bull trout were estimated from electroshocking, whereas two brook trout and nine bull trout were estimated from diving. Other less-precise electroshocking comparisons were in general agreement with the dive counts, although in some instances, abundance of age-1 brook trout were underestimated. Because there were no reliable estimates of dive count errors, visual estimates of fish numbers in all streams were not adjusted and should only be considered as relative estimates of population size.

Electroshocking was used to sample selected sites to verify presence or absence of fish and to give a general impression of fish abundance in reaches not surveyed by snorkel diving. Three categories were used to describe fish abundance from snorkel diving and electroshocking surveys: high abundance - > 5 fish per habitat unit; moderate abundance - 2-5 fish per habitat unit; and low abundance - <2 fish per habitat unit.

Ivlev's electivity index (Ivlev 1961) was used to describe habitat utilization by the fishes. The index in this application was defined as:


where E was the value of electivity, r was the proportion of fish in a habitat type i and p was the proportion of the area in habitat type i The index had a possible range of -1 to + 1 and was asymptotic towards its extremes. Negative values indicated avoidance, positive values indicated preference, and values near zero indicated no selection.





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