Stocks and fishery of cod (<i>Gadus macrocephalus, Gadidae</i>) in the northwestern Bering Sea in 1965–2022

Abstract

For pacific cod in the northwestern Bering Sea, biomass of the summer-fall feeding aggregations in the specified area of their highest density (polygon) is considered as a stock unit. The fish in these aggregations belong to the populations spawn in both local and adjacent areas. The stock values, biological parameters, and annual catches are determined for the period from 1965 to 2022 on the materials collected in research surveys and commercial expeditions, as well as on the data of fishery statistics and cited data. The average length of cod caught by different commercial fishing gears is determined as 42.01 cm for Danish seine, 53.01 cm for bottom trawls, 58.15 cm for midwater trawls, 61.94 cm for longlines. The midwater and bottom trawls used in research surveys are equipped with the rigging for better account of juvenile fish, therefore the average length of cod in their catches is 46.42 and 36.52 cm, respectively. The cod aged from 3 to 12 years are presented in the longline catches, with the modal group of 5 years old, but the age range is wider for bottom trawls, from yearlings to 12–13 years old fish, with a modal group varying from 2 to 5 years old in different years. The parameters of von Bertalanffy equation of linear growth for pacific cod are determined as: L∞ = 111.993 cm, K = 0.159, and t0 = –0.024 and for the equation of weight growth rate: W∞ = 14.006 kg, K = 0.245, and t0 = 1.221. The age of mass maturation is 4.4 years. The instantaneous coefficient of natural mortality, calculated by Tyurin, is age-dependent, with the minimum for 5 years old fish (0.345 year–1), 1.245 year–1 for yearlings, 1.231 year–1 for 11 years old fish, and 1.860 year–1 for 12 years old fish. History of the cod fishery in the northwestern Bering Sea is traced since its start in 1965. The cod fishery was realized with various types of fishing vessels (from small seiners to supertrawlers) and fishing gears, but the bulk of annual catch was landed with longlines (65.9 % in the 2010–2022, whereas only 21.4 % was landed with bottom trawls and 12.7 % with Danish seine). Overfishing happened in 1971, when the annual catch increased to 91,600 t on the background of poor replenishment, after that the catch decreased to 760–1420 t in 1976–1979. Then the stock had recovered and the catch increased from 3890 t in 1980 to 58,480 t in 1986. Next decline to 13,810 t in 2009 was followed by a new growth to the record value of 106.2 . 103 t in 2020, but recently the annual catch decreased again (61.5 . 103 t in 2022). This dynamics corresponded to the stock fluctuations: according to the trawl surveys data, the spawning stock within the polygon ranged between 66.5–282.6 . 103 tons in the 1965–1974, decreased to 24.5 . 103 t in 1977 because of overfishing, gradually recovered to 108.1 . 103 t in 1980, 173.0 . 103 t in 1982, and 561.6 . 103 t in 2000, then declined again to 151.4 . 103 t in 2001 and grew in the last two decades to the maximum values of 1,297.9 . 103 t in 2018. Recently a rapid decline of the stock is observed to 475.0 . 103 t in 2022. The changes of the assessed spawning stock generally precede for two years the CPUE changes due to relatively younger age composition in the catches of research bottom trawl used for the stock calculation, in comparison with the age composition in commercial catches. The stock dynamics within the polygon in the northwestern Bering Sea is driven by variations of the cod feeding migration to this area that correlates with the extension of the coldwater pool at the sea bottom around St. Lawrence Island. From the other hand, the migration depends on the cod abundance on the spawning grounds that could be negatively impacted by the bottom trawl fishery in the spawning season. Since 2002, the fishing pressure was moderate, and fishing mortality did not exceed the fishing exploitation target. However, overfishing is possible in the next 3–4 years, if the fishing mortality goes beyond the target benchmark.