Detection of spatial variability in relative density of fishes:comparison of visual census, angling, and baited underwater video

Abstract

The abiiity to make accurate estimates of fish relative abundance is the basis of both ecological and environrnental effects studies, and flawed sampiing methods may give misleadmg results even in otherwise weil-designed surveys.This paper compares surveys of snapper Pagrus auratus (Sparidae) and blue cod Parapercis colias (Pinguipedidae) conducted using 3 methods (underwater visual census, experimental angiing, and baited underwater video) inside and outside the Cape Rodney-Okakari Point marine reserve in northeastern New Zealand.A n g h g and baited video consistently detected adult P. auratus at protected and fished sites, providing estimates of 36.7 and 39.2 times greater density of fishable P. auratus within the reserve, respectively.Visual surveys provided the least reliable measure of density of P. auratus, with adults only detected at the reserve centre where fish have been habituated to divers by hand-feeding.Measures of the size structure of P. auratus were consistent between angling and video, but mean size was sigruiicantly smailer using visual census methods.Relative density of P. colias was similar for ali 3 methods, but a n g h g estimated larger mean size, probably due to hook selectivity against smalier fish.The study indicates that methodological standardisation across ali species is not always appropriate for environrnental effects studies, and that different survey methods should be considered according to the biology and behaviour of the species of interest.