• Korean Journal of Fisheries and Aquatic Sciences
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• v.23 no.6
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• pp.443-450
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• 1990

The virtual population analysis technique was applied to the silver croaker, Otolithes argenteus, stock based on length frequency composition, age-length key and nominal catches taken by the trawl fleet and fixed-stake nets in the Kuwait's waters during $1981\~1988$. One-year-old fish was dominant, whereas three-year-old and older fish were at a very low level. Fishing mortality was much higher in the age groups of 2 and 3 than in the others. A strong year-class occurred in 1980 and 1981. Population size of fishable stock markedly decreased from 1982 to 1985 and remained at a low level during $1986\~1987$. The effects of changes in fishing effort showed that an increase in fishing effort would not lead to benefits in yield and would comprise more young and fewer old fish, whereas a reduction in fishing effort to a certain level (by $20\~40\%$ ) from the fishing effort level on the average during $1981\~1988$ could bring some small advantage to the fishery consisting of fewer young and older fish.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.48 no.4
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• pp.445-451
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• 2012

We developed an age-based spawner-recruit model incorporating environmental variables to forecast stock biomass and recruits of pelagic fish in this study. We applied the model to the Tsushima stock of jack mackerel, which is shared by Korea and Japan. The stock biomass of jack mackerel (Trachurus japonicus) around Korean waters ranged from 141 thousand metric tons (mt) and 728 thousand mt and recruits ranged from 27 thousand mt to 283 thousand mt. We hind-casted the stock biomass to evaluate the model performance and robustness for the period of 1987~2009. It was found that the model has been useful to forecast stock biomass and recruits for the period of the lifespan of fish species. The model is also capable of forecasting the long-term period, assuming a certain climatic regime.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.50 no.4
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• pp.467-475
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• 2014

This study was performed to estimate optimum fishing mortality (F) and the age at first capture ($t_c$) for small yellow croaker in Korean waters. We first estimated optimum F and $t_c$ using traditional yield-per-recruit (YPR) analysis, and the results were 0.8/year and 2.5 years old, respectively. However, the individual fish price per unit weight of small yellow croaker in Korea increases dramatically by size. Thus, we developed an alternative method, which is called as production value-per-recruit (PPR) analysis. We developed two types of the PPR analysis, that is, the discrete function and the continuous function method. We estimated optimum F and $t_c$ using the two types of the PPR analysis and compared the results. The optimum F and $t_c$ from the discrete function method, were 0.3/year and 5.0 years old, respectively, while those from the continuous function method were 0.5/year and 3.5 years old, respectively. These PPR estimates were much more conservative for the stock management than the traditional YPR analysis, which can prevent the fish stock from the economic overfishing. As a result, the PPR analysis could be more proper approach for stock assessment in the case that the individual fish price per unit weight increases dramatically by size like small yellow croaker in Korea.

• Proceedings of the Korean Society of Fisheries Technology Conference
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• 2000.05a
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• pp.327-327
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• 2000

In the 1990's several economically important marine fisheries collapsed or showed signs of extreme hardship owing to overcapitalization and excessive fishing pressure on the resources. Public concern was raised and demands voiced for more prudent fishery management practices. The United Nations responded and developed international guidelines, such as the FAO Code of Conduct for Responsible Fishing and the UN Straddling Fish Socks and Highly Migratory Fish Stocks Agreement These guidelines task fishery managers to take prudent steps to ensure the long-term viability of fisheries and fishery resources. Included in the guidelines are two particular demands that will challenge stock assessment scientists in the new millennium. They are application of the precautionary approach and the inclusion of ecological considerations in assessments and management advice. High-lighted in the presentation are examples from the central-western Pacifi skipjack tuna fishery and the eastern Pacific thresher shark fishery where insufficien information is affecting stock assessments. The shortcomings are further linked t the new challenges of applying the precautionary approach, such as reference points, and ecological considerations, such as predator-prey and oceanographic-regime shift.(omitted)

• Korean Journal of Fisheries and Aquatic Sciences
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• v.52 no.6
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• pp.719-724
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• 2019

This study is an extension of our previous model for a size-based fish stock assessment. In the previous model, we applied an allometric length-weight relationship (W=α·L^β) to convert lengths of fish to weights, and estimated those parameters α and β, using data about lengths and weights aggregated over years. In this study, we focused on whether consideration of temporal (e.g., year-to-year) variability in those estimates (i.e., ${\hat{\alpha}}$ and ${\hat{\beta}}$) would contributive. After calculating year-specific estimates (i.e., year-specific pairs of ${\hat{\alpha}}$ and ${\hat{\beta}}$) by applying data about lengths and weights separated by year, we evaluated the contribution of those year-specific pairs of ${\hat{\alpha}}$ and ${\hat{\beta}}$ to the performance of the size-based stock assessment model. The model with such year-to-year variability being considered (lower AIC) outperformed that with the variability being ignored (higher AIC). We illustrated this study using data on Korean chub mackerel Scomber japonicus from 2005-2017.

• Journal of Fisheries and Marine Sciences Education
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• v.27 no.2
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• pp.363-370
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• 2015

Olive flounder, Paralichthys olivaceus is one of the most target fish for aquaculture and national stock enhancement program in Korea. In order to find more information about hatchery fish stocking for stock enhancement we investigated the sex ratio of wild-caught fish populations which were artificially released into wild seas. The recapture rate was 9.0% to 94.5% in studied areas. The proportion of sex ratio ranged from 7.8% to 64.6% in female and from 34.4% to 92.1% in male respectively. This result suggests that the sex ratio of wild populations might be effected by the released fish. So we need more careful stocking program of the fish to preserve the balance of the sex ratio in the wild population.

• Korean Journal of Fisheries and Aquatic Sciences
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• v.51 no.1
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• pp.79-90
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• 2018

Fish ages are critical information in fish stock assessments because they are required for age-structure models such as virtual population analysis and stochastic catch-at-age models, whose outputs include recruitment strengths, a spawning stock size (abundance or biomass), and the projection of a fish population size in future. However, most countries other than the developed countries have not identified ages of fish caught by fisheries or surveys in a consistent manner for a long time (e.g.,>20 years). Instead, data about fish body sizes (e.g., lengths) have been well available because of ease of measurement. To infer age compositions of fish in a target group using fish length data, we intended to improve the length frequency analysis (LFA), which Schnute and Fournier had introduced in 1980. Our study was different in two ways from the Schnute and Fournier's method. First we calculated not only point estimates of age compositions but also the uncertainty in those estimates. Second, we modified LFA based on the von Bertalanffy growth model (vB-based model) to allow both individual-to-individual and cohort-to-cohort variability in estimates of parameters in the vB-based model. For illustration, we used data about lengths of Korean mackerel Scomber japonicas caught by purse-seine fisheries from 2000-2016.

• Journal of Fisheries and Marine Sciences Education
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• v.15 no.1
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• pp.81-100
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• 2003

It is undeniable that IUU Fishing are threatening so many legal fishermen' economic livelihood, negatively impact on conservation and protection of the fishery stock and ecosystem itself. Especially, negative impact of IUU Fishing resulted from the increasing fishery activities on the high seas. The Korea case of Coastal and Off-shore Fisheries, difficulties in conserving and controlling the fishery stock was brought about. Simultaneously, it is the fact that there are so many damage such as the reduction of fish Stock management program's effect, dissatisfaction of legal fishermen, over-exploiting of fish stock. Related with this kind of problem, FAO had adopted "International Plan of Action to Prevent, Deter and Eliminate Illegal, Unreported and Unregulated Fishing(2001)". From this reason, Korea also needs to make actual efforts to prevent IUU Fishing. i.e. each nation should develop Korea action plan by Feb. 2004 and impliment it, report on implementation toward FAO. This Paper will review the definition of the IUU stipulated by "International Plan of Action on Illegal, Unreported and Unregulated Fishing" and study Korea cases of the IUU fishing. Finally, the analysis of Korea's implementation will be done, centering around the contents stated on the International Action Plan. The significance of this paper is to grope the political countermeasures against international movement of the IUU fishing prevention.

• Korean Journal of Fisheries and Aquatic Sciences
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• v.56 no.1
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• pp.1-6
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• 2023

Since 2019, Korea Ministry of Oceans and Fisheries has set the annual total allowable catch (TAC) by fish species, and has allocated the annual TAC into each fishery vessel. Also the Korea government plans to adopt the individual transferrable quota system in time. The TAC allocation is similar to the individual fishermen/fishing quota, which the US National Oceanic and Atmospheric Administration has implemented. However, the TAC based management faced fishermen's complaints and a debate is still underway about how to allocate TAC. Because the ideas of the Korea policy are from those in the foreign countries, I intended to provide the Korea fisheries community with an overview about how the TAC has been developed, what problem it caused, and foreign examples of how to allocate it. Furthermore, I pointed out a substantial room for improvement in their current practice of stock assessments, because, otherwise, their current methods for estimation of TAC by species cannot be trusted. Finally I made specific suggestions about what they need to do to reform their current stock assessments.

• Journal of Environmental Science International
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• v.13 no.5
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• pp.449-467
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• 2004

Interannual and decadal scale changes in body size of Pacific saury, catch and catch per unit effort were examined to investigate the environmental effects on the stock structure and abundance in the Tsushima Warm Current region. Interannual changes in thermal conditions are responsible for the different occurrence (catch) rates of sized group of the fish. Changes in body size due to environmental variables lead the stock to be homogeneous during the period of high abundance, while one of the reminder cohorts supports the stock during the period of low level of abundance. Migration circuits of two cohorts of saury stock are hypothesized on the basis of short life span and spatio-temporal changes of the stock structure in normal environmental conditions. Changes in upper ocean structure and production cycles by the decadal scale climate changes lead changes in stock structure and recruitment, resulting in the fluctuation of saury abundance. Hypothesized mechanism of the effects of climate changes on stock structure and abundance is illustrated on the basis of changes in thermal regime and production cycle.