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

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