Evaluation of scuba assessment methods for shallow-water, nearshore black rockfish (Sebastes melanops)

Management of black rockfish, Sebastes melanops, in Southcentral Alaska currently relies on historical fishery indicators, resulting in uncertainty on the population status. Because of the lack of fishery-independent data, an index to monitor population trends is needed. This study evaluated the utility of using strip transect sampling techniques to estimate black rockfish density within specific study areas. Four scuba transect surveys were conducted during the three-year study. Inter-annual density index estimates were highly variable, with coefficients of variation ranging from 22 to 33%. In addition, between-ob server estimates differed by 60%. Responsive movement of black rockfish to the presence of divers was detected during the first survey, prompting the development of a separate experiment designed to further assess responsive movement. During the experiment, black rockfish counts increased significantly with the time divers spent at stationary locations, while there was not a significant between-ob server effect. Thus, strip transect sampling for black rockfish will likely yield estimates that are highly variable, not repeatable, and biased high. As an alternative to strip transect sampling, timed counts are recommended for indexing black rockfish abundance because of the less subjective counting method.