Demersal fish communities in the Ross Sea region of Antarctica: comparisons between video and trawl survey methods

Reliable estimates of demersal fish community composition and population densities are essential for understanding the ecological effects of fisheries. Such estimates are conventionally derived from research trawl surveys but video techniques afford an alternative, or complementary, approach. Using data collected during a survey of demersal fish communities in the Ross Sea region of Antarctica, we compared measures of community composition, population density, and biomass derived from three sampling methods; a large demersal fish trawl, a beam trawl, and a towed camera system. Twenty-three sites spanning the continental shelf, northern continental slope, abyssal plain, and two seamounts were sampled using the towed camera and at least one of the trawl types, allowing direct comparisons between sampling methods. Patterns of species turnover between sites were similar across all methods. Estimates of fish population densities from the towed camera and beam trawl data were also comparable but those from the demersal trawl were consistently lower than for the other methods. Macrourus spp. grenadiers were ca. eight times less abundant in the demersal trawl than the video data but more large individuals were sampled by the trawl than the video and biomass estimates were similar. We hypothesise that this pattern could result from size-specific depth preferences in Macrourus spp. and conclude that video and trawl methods provide complementary information and if used together could be effective in routine assessments of demersal fish populations.