Tidal currents, sampling effort and baited remote underwater video (BRUV) surveys: Are we drawing the right conclusions?

Abstract

Estuaries are hydrographically dynamic environments, and such variability can affect the distribution and abundance of estuarine fish. Baited remote underwater video (BRUV) can be used to quantify estuarine species, but BRUV-derived data may be confounded by variable bait plume area and the associated effect on relative sampling effort. This study investigated the potential effects of current velocity on estuarine fish abundance data, and whether associated changes in bait plume size are important for benthic BRUV surveys in estuaries. BRUV sampling was conducted across two zones in two adjacent estuaries, and current velocity measured with a drogue during each BRUV deployment. Current velocity ranged from 0.02 to 0.65ms−1, resulting in potential bait plume areas that varied by orders of magnitude. The maximum number of each species (MaxN) was processed to produce a standardised (by bait plume area) and unstandardised multivariate species data set. The two data sets, whilst developed from identical video footage, yielded contrasting results. Unstandardised data was more variable, but produced a stronger correlation between abiotic variables and community structure. In addition, repeated sampling at some sites revealed significant temporal variance in community structure when data was standardised by bait plume area. Variability in sampling effort resulting from variable current velocity and associated bait plume area may confound interpretation of BRUV data.