New Zealand Journal of Marine and Freshwater Research abstracts
Assessing the accuracy of drift-dive estimates of brown trout (Salmo
trutta) abundance in two New Zealand rivers:
a mark-resighting study
ROGER G. YOUNG
JOHN W. HAYES
Cawthron Institute
Private Bag 2
Nelson, New Zealand
email: roger@cawthron.org.nz
Abstract We compared raw drift-dive counts of adult brown
trout (
Salmo trutta Linnaeus 1758) with population estimates calculated
from mark-resight techniques on two occasions in two back-country rivers near
Nelson, New Zealand. Trout were caught by angling and marked with colour-coded
dart tags between 1 and 7 days before drift dives were carried out. Drift-dive
counts in the Owen and Ugly Rivers were 57-66% and 21-43%, respectively, of the
population estimates calculated using the mark-resight techniques. Although
underwater census is the cheapest and easiest way to census adult trout
populations in clear-water rivers, a large proportion of the trout population
may not be seen in rivers with abundant in-stream cover. Raw drift-dive counts
provide an index of relative abundance that is useful for monitoring changes in
trout populations of specific river reaches over time. However, for comparisons
between rivers more accurate population estimates are required. We recommend a
combined approach of raw drift-dive counts and the use of mark-resight
techniques to provide the best estimates of trout numbers. As many trout as
possible should be tagged to improve the accuracy of mark-resight studies.
Keywords drift diving; underwater census; brown trout;
Salmo trutta; population estimate; sampling efficiency; mark-recapture;
river
M00028
Received 15 June 2000; accepted 29 January 2001
New Zealand Journal of Marine and Freshwater Research, 2001, Vol. 35:
269-275
0028-8330/01/3502-0269 $7.00 (c) The Royal Society of New Zealand
2001
PDF file of entire paper: medium quality (583K); (scanned from paper original: notes about this process)
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