Selection of an Optimum Method for the Assessment of the Saccharina japonica Stock in the Northwestern Tatar Strait

An experimental study aimed at comparing different methods for assessing the Saccharina japonica commercial stock, a target commercial kelp species in the Russian Far East, was carried out in 2021. Samples were collected from 11 sites from aboard the R/V Ubezhdenny and small-displacement vessels. Study sites were located in the northwestern Tatar Strait within an area stretching for 44 km south of Sovetskaya Gavan Bay, between Cape Krasny Partizan and Cape Korovin (from 48°58′ N, 140°23′ E to 48°37′ N, 140°11′ E), in areas of traditional commercial harvesting of S. japonica . Four assessment methods were compared: (1) based on independent visual estimates of plant cover (PC) by two observers from the surface; (2) based on independent estimates of PC by two observers using a series of images obtained from a small remotely operated underwater vehicle (ROV), or drone; (3) based on PC estimates made by a diver; and (4) based on quantitative samples made by a diver. The stock assessment values based on the first three methods are comparable and differ by 1.2–1.7 times. The stock values based on diver-collected samples are 2.4–4.1 times higher than the first three due to artifacts inherent in the diving method. To eliminate inaccuracies, samples should be collected by divers under the control from the surface using an underwater drone. PC estimates by divers are impractical. Visual PC estimates from the surface are simple and inexpensive, but do not allow documentation of survey results. To assess commercial kelp resources, the use of an underwater drone, which provides adequate abundance assessment and complete documentation of materials obtained, is the optimal approach.