Adaptive cluster sampling of Gulf of Alaska rockfish

Abstract

National Marine Fisheries Service trawl surveys result in more variable biomass estimates for long-lived Gulf of Alaska rockfish than researchers expect. Adaptive cluster sampling (ACS) was investigated to improve these surveys. In August 1998 east of Kodiak, AK, a sampling cruise tested ACS for Pacific ocean perch (POP), and shortraker and rougheye rockfish (SR/RE). In each of six strata, simple random sampling was conducted, then ACS was performed on top stations. Stopping rules prevented sampling from continuing indefinitely. Results did not resolve whether ACS alone was better than simple random sampling. ACS, combined with stratification, increased precision of POP estimates by 30% over random sampling, suggesting that the spatial distribution has both fine-scale and habitat-scale patterns. Variograms indicated that the expected aggregation was not encountered for POP, but that POP are more aggregated than SR/RE. Some diel movement of POP was evident. Both species were concentrated at specific depths.