New Zealand Journal of Marine and Freshwater Research abstracts
Estimating relative abundance of juvenile brown trout in rivers by underwater
census and electrofishing
JOHN W. HAYES*
NIWA-Marine,
National Institute of Water & Atmospheric
Research Ltd
P.O. Box 8602
Christchurch, New Zealand
DAVID B. BAIRD
Canterbury Agriculture and Science Centre
New Zealand Pastoral Agricultural Research
Institute
P.O. Box 60
Lincoln, New Zealand
*Present address: Cawthron Institute, Private Bag 2, Nelson, New Zealand
Abstract Underwater census and single-pass electrofishing
were compared for estimating relative abundance of juvenile brown trout in the
Kakanui River, South Island, NZ. Mean sampling efficiency was lower, and the
variability of sampling efficiency was much greater, for underwater census (0+
trout: x = 0.38, s = 0.368; 1+ trout: x = 0.62, s = 0.822) than
for single-pass electrofishing (0+ trout: x = 0.61, s = 0.143; 1+ trout:
x = 0.74, s = 0.171). Sampling efficiency of both methods was dependent
on temperature. Electrofishing became less efficient at higher temperatures
whereas underwater census became less efficient at colder temperatures. The
low, and highly variable, sampling efficiency for underwater census of 0+ brown
trout was related to substrate hiding behaviour which is dependent on
temperature. A ratio method for comparing relative abundance estimates is
presented. Minimum significance values for the ratio (R) were derived
for 0+ trout using temperature adjusted sampling efficiencies. To be
statistically significant, relative abundance estimates made by underwater
census had to differ by a factor of 6-7 times, whereas those made by
single-pass electrofishing had to differ only by about 2 times, depending on
the number of fish counted. By confining comparisons of relative abundance
estimates made by underwater census to the summer period, differences of about
3.5-4 times could be detected statistically. It was concluded that single-pass
electrofishing is superior to underwater census for estimating the relative
abundance of juvenile brown trout in shallow (< 1 m) river habitat,
especially when temperature varies widely as with season and time of day.
Keywords brown trout; Salmo trutta; juvenile trout;
electrofishing; underwater census; underwater observation; diving; sampling
efficiency; winter hiding; temperature; rivers
New Zealand Journal of Marine and Freshwater Research, 1994: Vol. 28:
243-253
0028-8330/94/2803-0243 $2.50/0 (c) The Royal Society of New Zealand
1994
PDF file of entire paper: medium quality (852K); (scanned from paper original: notes about this process)
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