Crater Lake Institute
 

 Home | Site Map | About Us | Donate/Join Us | Contact Us | CLI Store | Press Room

 
 
 You are here: Home > Online Library > Fishes and Stream Habitat > Fish Population Estimates
   

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

 

<< Previous | Table of Contents | Next >>

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.

 

 

 

 

 Site Navigation

  Arts

  Crater Lake News

  Cultural History

  Natural History

  Online Library

     Articles

     Books

        Browse all by Author

        Browse all by Title

        Cultural History

           General

           Historic Structures

           Native American

           Oral Histories

        Natural History

           Flora and Fauna

           General

        Park Management

           General

           Planning

        Research

           Atmosphere

           Fauna

           Fire

           Flora

           General

           Geology

           Limnology

           Visitation

     Nature Notes

     Images

     Maps

  Planning a Visit

  Research