A practical survey with vertical-longline for characterizing vertical fish species distributions on Hachirigase Sea Hill, Sea of Japan: A case study assuming a complement for echosounder-based fish stock assessment

Abstract

Accurate estimation of fish stocks is crucial for the sustainable use of fishery resources in offshore sea hills. However, bottom trawling is not practical for estimating fish stocks in areas with bumpy or rocky seabed topography. Although quantitative stock surveys using echosounders are effective, the identification of fish species from echograms remains a challenge. This study investigated the efficacy of vertical longlining in identifying fish species distributions near the seabed during an echosounder survey around Hachirigase Sea Hill in the Sea of Japan in June 2006. Seven species were caught at 7 of the 8 stations, resulting in a CPUE（inds./10-minute longlining）of 1.15. Threeline grunts and red lizardfish were the most abundant species, accounting for 43.7% and 37.0% of the CPUE, respectively. Threeline grunts were caught between the seafloor and 9 m above the seafloor, while red lizardfish were caught only in a narrow layer between the seafloor and 3 m above the seafloor. A significant difference in height above the seafloor was revealed between the two species. The study found that vertical longlining could complement echosounder-based surveys and may be an effective approach to determining the abundance of fish on a sea hill where trawl surveys are difficult.