• The Korean Journal of Malacology
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• v.17 no.2
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• pp.85-94
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• 2001

Spawning, hatching, survival and cannibalism of the octopus, Octopus ocellatus, were investigated at the indoor laboratory. The specimens were collected in the coastal waters of Puan, Korea, from March 2000 to May 2001. Changes in the gonadosomatic index (GSI) during March to May, 2000 were similar to those in 2001. The GSI began to increase in March and reached the maximum in May 2000 and 2001. The mean fecundity of adult individuals in April was larger than those in March and May in 2000 and 2001. In the investigations of survival rates of adults according to the water layers, the mean survival rate in the middle water layer was higher than that in the bottom layer in year 2000 (p = 0.002). The survival rate during 2001 was similar to the result of year 2000 in the water layers (p = 0.018). From the investigations with the purchase dates, the survival (88.8%) in the early spawning period (April 10, 2000) showed larger than those in the mid-spawning (May 11, 2000) and the late spawning (May 23, 2000). For aquaculture, it is suggested that the purchase timing of adults in the early spawning period will have a better result than that of the other spawning periods. In the amount of spawning with water layers, the number of eggs spawned of the adults in the middle layer were larger than those in the bottom layer. In the number of eggs spawned with the purchase dates, the number of eggs spawned in the early spawning period were larger than those in the middle and late spawning periods. The size of eggs spawned were 6.904$\times$2.520 mm. the hatching period required (from spawning to hatched larvae) was 55 days (April 17 to June 7,2000) at water temperatures of 11.2 to 21.1$^{\circ}C$. In case of the hatching rates with water layers, the hatching rate in the middle water layer was larger than that in the bottom one (p = 0.004).

• Korean Journal of Environmental Biology
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• v.38 no.1
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• pp.127-136
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• 2020

This study was conducted to investigate the population characteristics of Nile tilapia (Oreochromis niloticus) in the Daegu Metropolitan City thermal effluent stream (Dalseo Stream) from January to November 2019. The collected fish were identified as 4,247 individuals of 20 species from a total of eight families. The dominant species was O. niloticus with 1,306 individuals and a high relative abundance (30.75%). The water temperature of Dalseo Stream was maintained above 10℃ throughout the year, which means that O. niloticus could inhabit it even in winter. The length-weight analysis showed a regression coefficient b of 3.1496, and a condition factor (k) of 0.0025 with a positive slope. Comparing the water temperature of Dalseo Stream and the total length of O. niloticus per investigation period, the 0-age individuals appeared May 29 when the water temperature was maintained above 22℃. In conclusion, the thermal effluent of Dalseo Stream allowed O. niloticus to survive in winter and maintain stable growth conditions and life cycles. The results of this study will inform ecological information on O. niloticus, which suggests that river management efforts should consider the management of O. niloticus populations for the conservation of fish species diversity.

• Korean Journal of Fisheries and Aquatic Sciences
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• v.20 no.6
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• pp.509-519
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• 1987

Annual reproductive cycle of the Damselfish, Chromis notatus collected monthly at the four coastal areas around Chejudo, Korea are studied on the bases of histological observations of gonadal tissue and various quantitative variables including gonadosomatic index (GSI), fatness, egg diameter composition and the first maturity. The ovary consisted of a pair of saccular structure with many ovarian sacs. Oogonia proliferated along the germinal epithelium of the ovarian sac. Young oocytes with basophilic cytoplasm showed several nucleoli along the nuclear membrane. When the oocytes reached about $450{\mu}m$ in diameter, nucleus migrate toward the animal pole, nuclear membrane disappeared and most of cytoplasm were filled with yolk materials and oil drops. After ovulation, residual follicle and growing oocytes remaining in the ovarian sacs degenerated. But early young oocytes without follicle layer were not degenerated, and growing continuously till the next year. The testis consisted of a pair of lobular structures in the right and left were united in the posterior seminal vesicle. Cortex of testis was composed of many sperm ducts connected with lobuli. GSI began to increase from March, starting season of longer day length and higher water temperature, and reached the maximum value between June and August. It began to decrease from September with the lowest value appearing between October and February without any evident variation. The annual reproductive cycle could be devided into five successive stage : growing(April to Many), mature(May to August), ripe and spent (June to August) and recovery and resting stage(September to March). The spawning peak occurred from June to August. According to the frequency distribution of egg diameter, Chromis notatus was a polycyclic species to spawn twice or more in a spawning season. Fatness, correlated with gonadal phases, was remarkably decreased by spawning. Percentage of the first maturity . in femate and male fish ranging from 7.0 to 7.9 cm were $50\%$ and from 9.0 to 9.9 cm in total length $100\%$.