One fish, two fish, three fish, more: novel resighting method produces precise and cost-effective estimates of abundance.

Abstract

Estimates of abundance are essential to manage and conserve marine species. Numerous methods are available to determine population size, but the suitability of methods for schooling fishes and the associated precision can vary depending on the species and system. Here, we developed and compared three mark-recapture/resight methods to assess the most robust method to estimate the abundance of silver trevally (Pseudocaranx georgianus). While the recapture rate was similar across the methods, the swim pass method (resighting) recorded the largest number of individuals (mean ± standard error 211 ± 14.9) and had the lowest coefficient of variation (CV; 4.5%-12%) compared to 360-video (resighting, 45 ± 2.1 individuals surveyed, 14.8%-22.2% CV) and large-scale capture methods (recapture, 30 ± 3.8 individuals surveyed, 17.3%-26.5% CV). The inclusion of individual identification into the abundance estimator models for large-scale capture did not change the abundance estimates and showed similar resolution between the models (CV 18.2%-26.7%). We showed that the swim pass method is logistically easy to implement and generates precise estimates of silver trevally abundance. This new method provides a low-cost, time-efficient resighting method that can be adapted to suit similar aggregating pelagic species interacting with wildlife tourism operations, enabling researchers to rapidly estimate the abundance of species that have been previously difficult to count.