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).
fish abundance were made by direct observation by a single snorkel diver in 10
(every tenth unit of a habitat type) of all habitat types, except in Sun Creek
percent of the pools (every fifth pool) were surveyed. These counts were
estimate the total number of fish in each stream (Hankin & Reeves 1988). A
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
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
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
In Sun Creek, the position of each fish in the channel was assigned to one of
locations: (1) mid-channel without cover; (2) mid-channel in lee of cover; (3)
edge. Cover was considered to be any obstruction to stream flow, e.g., woody
larger than average substrate.
Fish collected using electroshocking were compared with the number of fish
during snorkel counts in Sun Creek. Ideally, ten or more comparisons were needed
calculate reliable correction factors for abundance estimates based on snorkel
(Hankin & Reeves 1988). Only a single 70 m section of stream channel was
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
and eight bull trout were estimated from electroshocking, whereas two brook
trout and nine
bull trout were estimated from diving. Other less-precise electroshocking
in general agreement with the dive counts, although in some instances, abundance
brook trout were underestimated. Because there were no reliable estimates of
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
fish and to give a general impression of fish abundance in reaches not surveyed
diving. Three categories were used to describe fish abundance from snorkel
electroshocking surveys: high abundance - > 5 fish per habitat unit; moderate
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
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,
values indicated preference, and values near zero indicated no selection.