Dames & Moore, 1999 - USDA Forest Service

were not developed to ascertain the presence of specific age classes to judge presencelabsence of
recruitment, but were developed to compare with the size distribution from previous Railroad Creek
investigations.
Population estimates were calculated using the Seber LeCren two-pass removal method (Everhart et al..
1975). This method assumes constant sampling effort for both passes, the population sampled is closed
(i.e., no fish enter or exit the sample location during sampling), and that the chance of capture is equal for
all individual fish and remains constant from sample to sample. The Seber ~ e~ren two pass removal
method equation incorporates the number of fish collected during each pass as follows:
Where:
N
- population size
c I
- .
the number of fish captured in the first pass
c2 = the number of fish captured in the second pass
Certain situations arose requiring more that two passes to provide a more accurate estimate of the trout
population. Estimating populations with data from multiple (three or more) passes requires application of
linear regression analyses. When multiple passes are required, the Leslie Method (Everhart, 1975) was
used to estimate trout populations. This method is based on the understanding that the population at the
start of any pass (excluding the first pass) is equal to some original population less what has been caught
up to the beginning of that pass. This linear relationship implies that if catch-per-unit effort is plotted
against cumulative catch, up to that pass, the result will be a straight line with slope equal to catchability
and intercept equal to the original population times catchability (Everhart et al., 1975).
A third pass was conducted at 4 of the 8 locations electrofished. Only one location lent itself to the Leslie
method. The relationship between catch per unit effort and cumulative catch at this location was not
realistic (i.e., the slope of the regression w& 0.0002), therefore, the total catch (of the 3 passes) was
considered a reasonable estimate of the population. At two of the four locations, the third pass produced
no fish and, as such, the total catch (of the first two passes) was considered a reasonable estimate of the
population. At the remaining location, a third pass was conducted only because the first pass produced
fewer fish than the second pass. The third pass at this location also produced no fish and, as such, the
total catch (of the first two passes) was considered a reasonable estimate of the population.
In addition to calculating trout population numbers, several other parameters were quantified during this
investigation. These parameters include, as appropriate: .
Total number of species present
Total numbers of individuals in sample
Number of non-trout species present
Proportion of individuals as omnivores
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17693-005-019Uuly 19.1999:2:07 PM;DRAFT FINAL RI REPORT