Evaluation of rockfish abundance in untrawlable habitat: combining acoustic and complementary sampling tools

• Main Authors: Jones, Darin T., Wilson, Christopher D., De Robertis, Alex, Rooper, Christopher N., Weber, Thomas C., Butler, John L.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2012
• Subjects: Biology; Ecology; Fisheries; Pacific; Kodiak; Alaska
• Online Access: http://hdl.handle.net/1834/25578

Rockfishes (Sebastes spp.) are an important component ofNorth Pacific marine ecosystems and commercial fisheries. Because the rocky, high-relief substrate that rockfishes often inhabit is inaccessible to standard survey trawls, population abundance assessments for many rockfish species are difficult. As part of a large study to classify substrate and compare complementary sampling tools, we investigated the feasibility of using an acoustic survey in conjunction with a lowered stereo-video camera, a remotely operated vehicle, and a modified bottom trawl to estimate rockfish biomass in untrawlable habitat. The SnakeheadBank south of Kodiak Island, Alaska, was surveyed repeatedly over 4 days and nights. Dusky rockfish (S. variabilis), northern rockfish (S. polyspinis), and harlequin rockfish (S. variegatus) were the most abundantspecies observed on the bank. Backscatter attributed to rockfish were collected primarily near the seafloor at a mean height off the bottom of 1.5 m. Total rockfish backscatter and the height of backscatter off the bottomdid not differ among survey passes or between night and day. Biomass estimates for the 41 square nautical-milearea surveyed on this small, predominantly untrawlable bank were 2350 metric tons (t) of dusky rockfish, 331 t of northern rockfish, and 137 t of harlequin rockfish. These biomass estimates are 5–60 times the densityestimated for these rockfish species by a regularly conducted bottom trawl survey covering the bank and the surrounding shelf. This finding shows that bottom trawl surveys can underestimate the abundance of rockfishesin untrawlable areas and, therefore, may underestimate overall population abundance for these species.