Sukgeun Jung

Fishing vs. Climate Change: An Example of Filefish (Thamnaconus modestus) in the Northern East China Sea

The main cause of annual fluctuations in catch and species composition of fisheries is usually uncertain, but a prevailing view has been that fishing effects are more critical than environmental variability. Filefish is a good anecdote: many Korean fisheries scientists have attributed the sudden collapse of Korean filefish fisheries in the early 1990s to overfishing, especially by trawl fisheries in the northern East China Sea (NECS). However, interdisciplinary researches have revealed that climate-driven, multi-decadal variability in oceanic conditions impacts both fish and fisheries around the world. To test the two alternative hypotheses (i.e., fishing and climate) as the major cause of the sudden decline of filefish, we compared fisheries data of filefish from the adjacent countries of the NECS and analyzed oceanographic conditions in relation to changes in species composition of fish assemblages in the NECS. Results suggested that the basin-wide, 1989 regime shift in the North Pacific and the subsequent shrinkage of habitat range to the southwest were the major cause of the sudden decline of filefish catch in the NECS. Locally, shifts in water temperature and currents were identified in the NECS for the early 1990s, but further physiology-oriented researches are required to understand the detailed mechanism of climate-change effects on filefish stocks in the marginal seas of East Asia.

Estimating the Abundance and Fishing Mortality of Pacific Cod Gadus macrocephalus during the Spawning Season in Jinhae Bay, Korea, Using a Mark-Recapture Method

We estimated the population size and fishing mortality of Pacific cod Gadus macrocephalus during the spawning season in waters off Woipo, Geoje Island, Korea, using a mark-recapture method. We marked and released 51 cod>50 cm in total length; six were recaptured by local fishermen during the period from December 15 to 31, 2009. The estimated population size was ca. 180,000 and the fishing mortality of the exploitable cod was 26%. Although we could assume a closed population due to the short survey period, we evaluated the uncertainty in the estimates by applying bootstrap resampling because the sample size was small. The estimated 95% confidence interval was 94,000-568,000 for the population size and 8-49% for fishing mortality. Our study demonstrated that the application of mark-recapture methods and bootstrap resampling can be useful in stock assessment for fisheries management in Korea, but requires a larger sample size, spatially extensive coverage, and sophisticated mark-recapture models based on a refined sampling design for reliable stock assessment and biological reference points in sustainable cod management.

Simulation-based Yield-per-recruit Analysis of Pacific cod Gadus macrocephalus in Southeastern Korean Coastal Waters

We derived biological reference points for Pacific cod Gadus macrocephalus in southeastern Korean waters by applying a yield-per-recruit analysis based on a daily simulation that adopted size-dependent fecundity, growth, and natural mortality functions. This showed that the yield per recruit of Pacific cod can be maximized at an instantaneous rate of fishing mortality (F)