The effects on the size selectivity for Muraenesox cinereus caught by coastal longline fishery were investigated in the southern coast of Korea from June 2 to 17, 2019. Four sizes of hooks (sizes 15, 17, 18 and 19) and two sizes of bait (sizes 9.9 g and 18.3 g) were tested in seven and three fishing trials, respectively. Such results revealed that smaller hook and bait size improved capture efficiency. And our results demonstrate that there was no significant size selectivity effect for hook size (ANOVA, p>0.05), but small bait improved on catching smaller fish (ANOVA, p<0.05).

The purpose of this study is to identify economic situation on scale of tuna distant longline fishery by analyzing its economies of scale using the cost function. To analyze its economics of scale, the deep-sea fishing statistics were used from 2012 to 2016. In detail, the number of panels for estimating the cost function was 68 tuna distant longline vessels from 2012 to 2016, and the total number of observations over the five years were 340. As a final model, the two-way fixed effect model based on the translog cost function was adopted through the F test, the Breusch-Pagan test and the Hausman test. As a result of the analysis, it was found that tuna distant longline fishery between 2012 and 2014 was diseconomies of scale, the fishery between 2015 and 2016 was economies of scale. However, the economic indicators of the scale from 2012 to 2016 were almost close to zero, indicating that the constant returns to scale, the optimal scale, were reached. Therefore, in the situation where the amount of fishery resources in the world continues to decrease, it is necessary to prepare a method to obtain economic benefits through scale maintenance and reduction rather than indiscriminate scale expansion.

The species composition and abundance variations of fishery resources in the adjacent marine ranching area, Tongyeong, Korea, were investigated by shrimp trawl, gill net, and longline during the period of July, September, and October in 2016. During the study period, the total catch were 8,522.9 kg with 34 species from the shrimp trawl, 32 species from the gill net, and nine species from the longline. The dominant species were different by gear, which were Hypodytes rubripinnis and Parapercis sexfasciata in the shrimp trawl, Platycephalus indicus and Raja kenojei in the gill net, and Conger myriaster and Scomber Japonicus in the longline. In terms of spatial distribution, Yongchodo showed the highest total catch in number as well as of in weight while Jukdo showed the lowest total catch in number and Bijindo showed the lowest total catch in weight. The amount of demersal fish resources in the survey area estimated as 301 ha, was 99,396 individuals which was converted to be 8,552.9 kg. The amount of demersal fish resources by gear were of trawling area, which area is 127 ha, were 76,251 and 3,489.5 kg, 74 ha in the gill net and longline survey area was 16,213 and 3,457.3 kg, and the other 100 ha area was 6,932 and 1,606.1 kg. In this study, the minimum resources for demersal fish is 61,687 and 4,265.2 kg, and the maximum is 149,439 and 14,197.9 kg.

The species composition and variation in abundance of fishery resources near Oenarodo, Go-heung Peninsula, Korea, were investigated by gill net, trap net, and longline in May, July, and October 2015 and 2016. During the study period, the total catch included 14 species in the gill net, 11 species in the trap net, and 4 species in the longline. The dominant species were Portunus trituberculatus and Raja pulchrain the gill net, Charybdis japonicaand and Octopus vulgarisin in the trap net, and Muraenesox cinereusin in the longline. The Catch Per Unit Effort (CPUE) per individual and per weight in the gill net were similar in May and July of 2015 and 2016. In October 2015, the CPUE per individual was 2.1 ind./h and the CPUE per weight was 505 g/h higher than the results in 2016, but there was no significant difference in the total CPUE between 2015 and 2016. In the trap net, the CPUE per weight was similar in both 2015 and 2016, but the CPUE per individual was 2.7 ind./h higher in October 2015 than in October 2016 and the total CPUE was not significantly different from 2015 to 2016. The CPUE per individual and weight in the longline were significantly higher in July and October 2015 than in the same months of 2016, but the total CPUE in 2015 and 2016 did not show a significant difference.

Korean distant water tuna longline fishery commenced in 1966 in the Atlantic Ocean. Since then, it has become one of the most important fisheries of Korea. By early of 1970s, total amount of tuna caught in the Atlantic Ocean was the highest among other Korean distant water tuna fisheries, but has become minor since 1990s. The annual catch of tuna and tuna-like species by Korean tuna longline fishery in the Atlantic Ocean was about 1,900 mt in 2013. Bigeye tuna was the predominant species in species composition followed by yellowfin tuna, Albacore tuna, Blue shark and Swordfish. Korean distant water tuna longline fishery have mainly operated in the tropical area of the Atlantic Ocean (20°N~20°S, 20°E~60°W), fishing ground was almost similar as in the previous years. The length frequency of major species (Bigeye tuna, Yellowfin tuna, Albacore tuna, Blue shark and Swordfish) were estimated. As the result of length (size) frequency data on main species caught by lonline fishery in the Atlantic Ocean, main length intervals of bigeye tuna caught in 2011, 2012 and 2013 were 120~125 cm, 160~165 cm and 130~135 cm, respectively. For yellowfin tuna, those were 125~130 cm, 150~155 cm, 145~150 cm and for albacore, 109 cm, 102 cm, 109 cm and 106 cm respectively. For swordfish caught in 2011, 2012 and 2013, main length intervals were 130~135 cm, 125~135 cm and 125~130 cm, respectively, and for blue shark, 195~200 cm in 2011 and 185~190 cm in 2012, 2013.

Catches, CPUE, fishing season, disc width composition and fishing ground were studied to rebuild skate ray, Raja pulchra resource by the offshore longline fishery in Heuksan-do, Korea. Annual catches and CPUE of skate ray were increased from 2003 to 2010 (CPUE: from 2007 to 2010) continuously. The main fishing season of the skate ray was from December to next May. Disc width at 50% maturity for Raja pulchra by sex were estimated to be 65.83cm, 57.01cm by female and male respectively. The range of disc width was 22.3-112.0cm and the average disc width was 66.9cm, 60.0cm by female and male of Raja pulchra respectively during 2007-2010. Annual average of disc width of female skate ray was decreased from 2007 to 2010. The fishing grounds were formed from the coastal water off Hong-do to the coastal water off Gyeogyeolbiyeol-do, Korea. The center of fishing ground in October was formed in the northwestern coastal water off Gyeogyeolbiyeol-do and separated from that of the other months.

We conducted experiments to compare the catch rate of bigeye tuna and yellowfin tuna between circle hooks and straight shank hook in the Korean tuna longline fishery at the eastern and central Pacific Ocean from 2005 to 2007. We analyzed difference of fork length, survival and hooking location between a circle hook and a straight shank hook for both tunas, respectively. There was no difference in the mean fork length size of yellowfin tuna caught on the two type of hook but bigeye tuna was significant. In case of survival, there was no difference between two hook type, but the difference of hooking location was significant for both species. We also analyzed to find determinants of both tunas catch rate using generalized linear models (GLMs) which were used latitude, longitude, year, month, depth, hook type, bait type and so on as independent variables. Spatial factors, latitude and longitude, and temporal factors, year and month, affected catch rate of bigeye tuna and yellowfin tuna. And also, depth such as a marine environment factor was influenced on catch rate.

Ecological risk assessment(ERA), developed in Australia, can be used to estimate the risk of target, bycatch and protected species from the effects of fishing using limited data for stock assessment. In this study, we employed the ERA approach to estimate risks to tunas, billfishes, sharks, sea turtles and other species by the Korean tuna longline fishery in the Western and Central Pacific Ocean using productivity and susceptibility analyses of the ERA based on low(<1.30), medium(1.30-1.84) and high risk(>1.84). Albacore, bigeye, yellowfin, skipjack and bluefin tunas were generally evaluated in the medium risk. The susceptibility of tuna species, however, had higher risks than the productivity. Billfishes were also at medium risk, while sharks were at high risk by the tuna longline fishery. The risk of productivity was generally high, because most sharks caught by the tuna longline fishery have high longevities, i.e., over 10 years, including ovoviviparous species. Susceptibility, which is related with the selection of fishing gear, was also high, because the longline fishery has no gear modifications to prevent bycatch of protected species. Not only target tuna species were influenced by the tuna longline fishery in the Western and Central Pacific Ocean, but also nontarget species, such as pomfret, mackerels rays, sea turtle were done. Ecosystem-based fishery assessment tools, such as productivity and susceptibility analysis(PSA), have the ability to provide broad scientific advice to the policy makers and stakeholders.

This study standardized catch per unit effort(CPUE) of the Korean longline fishery, which has been used to assess the status of stock as an index of abundance, for bigeye and yellowfin tunas in the Indian Ocean. The Generalized Linear Model(GLM) was used to analyze the fishery data, which were catch in number and effort data collected each month from 1971 to 2007 by 5 × 5 degree of latitude and longitude. Explanatory variables for the GLM analysis were year, month, fishing area, number of hooks between floats(HBF), and environment factors. The HBF was divided into three classes while the area was divided into eight subareas. Although sea surface temperature(SST) and southern oscillation index(SOI) were considered as environmental factors, only SST was used to build a model based on statistical significance. Standardized CPUE for yellowfin tuna showed a declining trend, while nominal CPUE for the species showed an increasing trend.

Data collected by on-board observers and from experimental surveys during 2004-2006 were analyzed to figure out the status of shark bycatch in Korean tuna longline fishery. Results obtained from 10 surveys indicated that 14, 13 and 1 species of shark were incidentally caught in Korean tuna longline fishery operated in the Pacific, Indian and Atlantic (Mediterranean) Ocean, respectively, and that shark bycatch accounted for about 29-31% of the total catch. Sharks brought aboard were processed in 3 ways; out of 1,127 sharks observed, 575 sharks (51.0%) were discarded after finning, 299 sharks (26.5%) were stored frozen after finning for future use and 253 sharks (22.4%) were released into the sea immediately after caught. The fin to body weight ratio of sharks was estimated to be about 4.7% which is similar to the guideline of 5% established by the international fisheries organizations. The underestimate of shark bycatch in Korean tuna longline fishery was significant because it was general practice that fishermen on-board did not count the discarded shark as a catch.

The circle hook experiments were conducted to compare the catch rates of target and bycatch species between J hook and circle hooks in the tuna longline fishery of the eastern Pacific Ocean between 1˚48'S-7˚00'S and 142˚00'-149˚13'W from July 15 to August 12, 2005. In the target species group no significant differences among 3 types hook, between size 4.0 traditional tuna hooks(J-4) and size 15 circle hooks(C15), and between C15 and size 18 circle hooks(C18) were revealed, but significant differences were found between J-4 and C18. In the bycatch species group significant differences were found among 3 types hook, between J 4 and C15, and between J-4 and C18, but no significant differences were revealed between C15 and C18. Large circle hook(C18) had the lowest catch rate for tunas and for other fishes, and the small circle hook(C15) had lowest rate for billfishes and sharks. The length distributions for bigeye tuna are very similar for the 3 hook types. There were very slight differences in length size between hook types in the bycatch species.