Cautious considerations for using multiple covariate distance sampling and seafloor terrain for improved estimates of rockfish density

Distance sampling is one of the most widely used methods for adjusting fish counts for detectability, which allows the estimation of absolute density. Further, abundance can be estimated spatially beyond the transect width with density surface models. The present study used a density surface modelling approach with survey data from line transects and terrain metrics from multibeam acoustic bathymetry data, integrated with a Geographic Information System, to estimate the population density of Pacific Ocean perch Sebastes alutus (POP) within 6 sampling areas in the Gulf of Alaska (GOA) at 2 sampling scales: local and landscape. For adult POP, the final density model included depth as a significant predictor for density estimation at both the local and landscape scale. Additional factors included sponge coverage (local scale), aspect eastness and seafloor slope (landscape scale). Predicted densities of adult POP are highest on low (<5 degrees) eastern-facing slopes with high sponge coverage (>75%), at depths of 100 to 200 m. By using habitat-based density models with data collected from line transect distance sampling and multibeam acoustic seafloor mapping surveys, we provide an alternative method for estimating rockfish abundance in untrawlable areas, which may help improve rockfish stock assessments and essential fish habitat descriptions and maps.