한국 남동해역 멸치(Engraulis japonicus) 미성어 및 성어 자원평가

Abstract

Pacific anchovies (Engraulis japonicus) are small pelagic fish at the intermediate trophic level in the marine ecosystem and a commercially important target species in south Korea. A total fishery yield of the anchovy stock in 2020 was 216,748 MT, which was the largest yield over fishery yields of the other coastal species. About 67 % of the fishery yield of the anchovy stock was caught in the southeastern (SE) coastal waters of Korea. Anchovies are often sold as dried anchovies. Dried anchovies are classified, according to total length size: Se-myeol (minimum size anchovy; ≤ 1.5 cm); Ja-myeol (tiny anchovy; 1.6 – 3.0 cm); So-myeol (small anchovy; 3.1 – 4.5 cm); Joong-myeol (medium anchovy; 4.6 – 7.6 cm); Dae-myeol (large anchovy; ≥ 7.7 cm). Data about yields caught in the SE waters and length compositions in those yields by the anchovy drag net fishery were available from 2009 – 2018 (data source: Statistics Korea and Korea Maritime Institute). I made semiannual catch per unit effort (CPUE) data, using values of catch and efforts uploaded on the National Institute of Fisheries Science website (https://www.nifs.go.kr/), where semiannual yields (catch part) from the drag net fishery and the number of vessels (effort part) for the fishery from the corresponding time were available. The number of fishing vessels was used as the fishing effort data because the other information such as fishing hours was not available. I assumed that Se-myeol, Ja-myeol, So-myeol and Joong-myeol were reproductively immature, while assuming that Dae-myeol was mature. I applied a state-space surplus production model for the stock assessment in the SE coastal waters, treating timely biomass as random effects. A new feature of my model lies in predicting each biomass of immature and mature fish by time as well as estimating the other parameters. I used R package, TMB for implementing the state-space model. The maximum sustainable yield was estimated as about 255,898 MT. The carrying capacity was estimated as about 765,189 MT. Predicted biomass of immature fish was larger than that of mature fish. I also performed model validation by simulation. The result of model validation was that bias of all parameters ranged from -23.5 to 42.8 %. Additionally, I applied a different assumption regarding immature and mature fish: immature fish (Se-myeol, Ja-myeol and So-myeol); mature fish (Joong-myeol and Dae-myeol). Under this new assumption, predicted biomass of immature fish was smaller than that of mature fish. Therefore, Joong-myeol is at a key stage, which determines immature vs. mature population sizes. If maturation information such as gonad weights were available, this confusion would be clarified. Results in my study should not be used for actual management because the CPUE data do not represent a relative population size. I would like the reader to pay attention to my methods per se, especially the new feature, which is different from the traditional surplus production model.