Assessing the accuracy of drift-dive estimates of brown trout (Salmo trutta) abundance in two New Zealand rivers: a mark-resighting study

We compared raw drift-dive counts of adult brown trout (Salmo trutta Linnae us 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 an d 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 es timates calculated using the mark-resight techniques. Although underwater c ensus 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 s een in rivers with abundant in-stream cover. Raw drift-dive counts provide an index of relative abundance that is useful for monitoring changes in tro ut populations of specific river reaches over time, However, for comparison s between rivers more accurate population estimates are required. We recomm end a combined approach of raw drift-dive counts and the use of mark-resigh t techniques to provide the best estimates of trout numbers. As many trout as possible should be tagged to improve the accuracy of mark-resight studie s.