한국 연근해 고등어 (Scomber japonicus)의 생식 특성 및 난과 자어의 분포와 수송

Abstract

We investigated the maturity and spawning dynamics of chub mackerel S. japonicus in Korean waters using samples collected by a large purse seine fishery from January 2013 to December 2017. We analyzed sex ratio, monthly maturity stage, gonadosomatic index (GSI), Fulton's condition factor (K), and fork length at 50% group maturity. We detected a significant difference in sex ratio (χ2 test, P<0.05). Spawning occurred from March to June, with a peak in May. The relationship between Total length(TL) and body weight(BW) was expressed as TL=1.0827FL-0.3393(r2=0.9886). The relationship between fork length (FL) and body weight (BW) was expressed as BW=0.0025FL3.4693 (r2=0.9314). At 50% group maturity, average fork length was estimated to be 29.3 cm, while it was 31.1 cm at 75% maturity and 35.5 cm at 97.5% maturity.

Reproductive characteristics (spawning fraction, batch fecundity) of S. japonicus (462 mature females) were investigated based on samples collected by large purse seine in Korean waters from May to June 2017(45days). Spawning fraction and batch fecundity were determined by histological methods. The average daily spawning fraction was 14.4% during this sampling period. We estimated that S. japonicus spawned every 6.9 days (6.5 times) during these 45 days. Also, 72 of 462 females spawned almost every 1.7days. Results of daily spawning fraction in maturity S. japonicus females showed that smaller females have a lower spawning fraction than larger females; accordingly, larger females spawned more frequently than smaller ones. Proportion of females who have finished spawning were tended to decrease as the fork length (FL) and Fulton's condition factor (K) increased. In other words, as the FL and K increase, the spawning continues for a long period of time. Nineteen females which had migratory nucleus stage oocytes in their ovaries were used for estimating batch fecundity. The average batch fecundity was 96,100 oocytes per female, and the relative batch fecundity was 158 eggs per gram female wet weight. thus, we estimated the potential annual fecundity range were 624,433±83,255 (mean±SE) (batch fecundity×spawning frequency). The batch fecundity was significantly correlated with ovary weight and GSI, whereas there were no significant differences between batch fecundity and FL, BW (without ovary), K. We propose that it is necessary to conduct long-term reproductive characteristics monitoring for better understand the mechanisms of reproduction and recruitment of this species because reproductive characteristics are dependent on female size and age.

The horizontal distributions of eggs and larvae of chub mackerel S. japonicus were extensively surveyed in the vicinity of Korean waters between 31°75'N and 36°50'N during May and June in 2016 and 2017 (total four surveys). We used a coupled bio-physical model (DisMELS) that combines an individual-based model (IBM) incorporating vertical migration of larvae and temperature-dependent survival to understand transport processes of the early life stage of S. japonicus. Using the distributions of eggs and larvae from surveys, the potential spawning grounds were estimated at the northwest and southeast of Jeju Island and the central East China Sea in May, and at the southwestern East Sea and southern Yellow Sea in June by running the model backward in time. In forward experiments within 30 days from the backward results, most larvae were transported to both the Korean and Japanese sides of the East Sea through the Korea Strait. However, the larvae released in the central East China Sea were transported to the Japanese side only, while those released in the southern West Sea were retained within that region. The survival rates at 30 days after release based on the simulation incorporating temperature-dependent survival throughout May and June were 29.7% in 2016 and 28.8% in 2017.