Choosing the optimal method of stock assessment for Saccharina japonica in the northwestern Tatar Strait

Abstract

Experiments on comparing the methods for assessment of Saccharina japonica commercial stock were conducted in 2021. In total, 11 sites in the northwestern Tatar Strait southward from the Sovetskaya Gavan Bay, along the 44 km stretch of coastline between Cape Krasny Partizan (48о58′ N 140о23′ E) and Cape Korovin (48о37′ N 140о11′ E), were surveyed aboard RV Ubezhdenny and small boats. This is a traditional area of Saccharina japonica commercial harvesting. Four methods were compared: 1) visual assessment of projective cover from the sea surface by two observers independently; 2) assessment of projective cover by two observers independently on the images obtained from a remotely operated underwater vehicle (ROV); 3) estimation of projective cover by a diver; and 4) assessment on the data of quantitative diving samples. The former three methods are comparable and differ by 1.2–1.7 times. The estimates made by the latter method of diving sampling exceed them by 2.4–4.1 times because of artifacts specific to this method. To eliminate inaccuracies, diving samples should be collected under control from the sea surface by ROV. Evaluation of the projective cover by diver is inadvisable. Visual assessments of the projective cover from the sea surface are simple and cheap but inconvenient for documentation. The stock assessment with micro-ROV is recognized as the optimal method provided an adequate accounting of the kelp abundance and complete documenting of the survey materials.