8086

• UCREFRP Catalog Number: 8086
• Author: Valdez, R. A. and B. R. Cowdell.
• Title: Lake Powell Inflow Study. Annual Summary Report - 1993.
• USFW Year: 1994.
• USFW - Doc Type: 384-1,
• Copyright Material: NO

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3.2.2 SeinintJ

Small mesh seines were used in backwaters, shorelines and return channels to capture age-O and juvenile fish.
The area seined was measured and the catch per effort (CPE) was computed as number of fish per 100 square
meters of area sampled (number fish/100m2). Water depth, temperature and substrate type were recorded for
each seine haul.

The pooled fish/area method was used to calculate catch rates for seining. This method is simply the total
number of a fish divided by the sum of area seined. The advantage of this method is that it considers all area
sampled and it is the same method used in previous investigations. The disadvantage is that it does not allow
for a determination of statistical variability.

The geometric mean of the sample CPE's was used to compare densities of age-O squawtish. This method is
being used by the Service's Database Manager on ISMP data (Personal communication, C. McAda, February
24, 1992). It involves a log transformation of CPE's, primarily because catch data for Colorado squawtish in
backwaters are highly variable and not normally distributed.

The geometric mean calculation method was as follows: First, a CPE for age-O squawtish was computed for
each seine haul (age-0/loom2). Then a '1' was added to the CPE of each seine haul to account for zero
catches. This adjusted CPE was transformed to a natural logarithm. The transformed values were averaged,
by reach and year, and the antilog of the average minus '1' yielded the geometric mean. Standard deviation
was computed from log-transformed values, and the antilog taken to provide bounds around the geometric
mean. The computation is expressed as:

GMa>E = antilog [(l/n) ~ ln (CPE + 1)] - 1

where:

GMcPE =
CPE =
n =

geometric mean catch per effort
catch per effort of age-O squawtish (age-0/loom2)
total number of seine hauls (with and without target fish)

The advantage to this approach is that it reduces variation by making the variance independent of the mean.
However, log transformations tend to weight higher CPE's less and geometric mean CPE's do not necessarily
reflect actual fish densities.

3.2.3 Nettini!

Gill and trammel nets were used to capture predatory fish for stomach analysis. Gill nets were 100 ft long,
6 ft deep with a mesh size of I-in. Trammel nets were 75 ft long, 6 ft deep with an outer panel of 12-inch
mesh and an inner panel of I-in. Nets were constructed of double knotted nylon multifilament line. Nets
were fished along eddy lines and in return channels, and checked at 2 hour interval to minimi7.e stress and
reduce mortality of entangled fish.

3.2.4 Trappinl!

Clear plastic light traps of the quatrefoil design were used in the Lake Powell inflow to capture larval fish.
Larval light traps were first used in the Lake Powell inflow during a separate NPS sampling trip, conducted
June 20 -22, 1993. Light traps were incorporated into this study during post-runoff sampling and will be used
on both sampling trips in 1994.

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