The decline in walleye pollock (Gadus chalcogrammus) stocks in Korean waters is a major concern for fishery conservation and management. However, the causes and mechanisms of this collapse remain unclear. This study investigated the complex dynamics influencing the abundance of walleye pollocks in the East Sea of Korea over several decades, by analyzing data from long-term changes in biological factors including composition of length and sex, catch, and oceanographic condition. Prior to the mid-1980s, the catch ratio of juveniles was higher than that of adults, with a higher proportion of females in both juvenile and adult catches compared to males. Especially, high fishing pressure on female individuals can be an important factor contributing to declining reproduction. Consequently, after the mid-1980s, there was a sudden decline in juvenile pollock catches. In the late 1980s, there was a rapid increase in sea surface temperature (SST) in the spawning grounds, resulting in a decrease in both the duration of suitable temperature for spawning and the regional proportion for suitable spawning conditions. Consequently, the decline in pollock stocks after the late 1980s due to overfishing of pollock in the mid-1980s was further exacerbated by the effects of SST warming after the late 1980s. These findings highlight the impact of overfishing and environmental factors on pollock stocks and indicate the need for appropriate fishery management practices to ensure the sustainable use of fishery resources.

The feeding characteristics of the chub mackerel, Scomber japonicus from Jeju Island waters and the Yellow Sea were compared by analyzing the stomach contents. Large and small purse seines were used to collect 1437 individuals from Jeju Island waters and 1066 individuals from the Yellow Sea. The fork length of S. japonicus ranged from 21.0 to 43.6 cm for individuals from Jeju Island waters and 19.6 to 41.5 cm for those from the Yellow Sea. Based on the index of relative importance and variation in stomach contents according to fish size, S. japonicus was identified as a piscivore and a spatiotemporally opportunistic feeder. The proportion of fish in the diet increased as the body sizes of S. japonicus from both Jeju Island waters and the Yellow Sea increased, whereas the proportion of copepods, amphipods, and euphausiids gradually decreased. Differences in stomach content compositions were observed due to interactions between size and location. Thus, the feeding characteristics of S. japonicus are affected by the abundance and composition of prey in its habitat.

This study aimed to evaluate the mesh selectivity of gillnets used in olive flounder (Paralichthys olivaceus) fishing by examining the distribution of total length (TL) across various mesh sizes. A marine fishing experiment was conducted at Gajin Harbor in Goseong County, Gangwon Province, employing experimental gear with four different mesh sizes: 90, 105, 135, and 150 mm. Mesh selectivity analysis involved a comparative assessment of selectivity curve methods through normal, log-normal, and bi-normal functions. Among these, the bi-normal function demonstrated the best fit for estimating the mesh selectivity curve within the SELECT model. The critical finding was the net size at which 50% of the minimum landing size (35 cm in TL) of olive flounder was selected, ranging from 108.7 to 121.1 mm. According to the bi-normal function curve, this size was estimated as 114.0 mm. Our findings indicate that using a mesh size larger than the theoretically estimated size in mesh selectivity analysis can further decrease the catch of immature individuals. These findings provide essential data for devising strategies to efficiently utilize and manage olive flounder resources.