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

The ability to make accurate estimates of fish relative abundance is the ba sis of both ecological and environmental effects studies, and flawed sampli ng methods may give misleading results even in otherwise well-designed surv eys. This paper compares surveys of snapper Pagrus auratus (Sparidae) and b lue cod Parapercis colias (Pinguipedidae) conducted using 3 methods (underw ater Visual census, experimental angling, and baited underwater video) insi de and outside the Cape Rodney-Okakari Point marine reserve in northeastern New Zealand. Angling and baited video consistently detected adult P. aurat us at protected and fished sites, providing estimates of 36.7 and 39.2 time s 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 habi tuated to divers by hand-feeding. Measures of the size structure of P. aura tus were consistent between angling and video, but mean size was significan tly smaller using visual census methods. Relative density of P. colias was similar for all 3 methods, but angling estimated larger mean size, probably due to hook selectivity against smaller fish. The study indicates that met hodological standardisation across all species is not always appropriate fo r environmental effects studies, and that different survey methods should b e considered according to the biology and behaviour of the species of inter est.