Title Studies on the Stock Enhancement Technology of the Black Rockfish Sebastes schlegeli
Authers Masahiro NAKAGAWA
Keywords Sebastes schlegeli, stock enhancement, broodstock management, juvenile production, stocking effectiveness
Citation Bull. Fish. Res. Agen. No.25, 223-287, 2008
Many species in Sebastes genus are important as a commercial and recreational fisheries resource. Juvenile of six rockfish species (Sebastes schlegeli, S. inermis, S. vulpes, S. pachycephalus, S. oblongus and S. thompsoni)are currently produced for stocking in Japan. Growth of black rockfish, Sebastes schlegeli, is more rapid than that of other rockfish species; also, their range of movement is limited. For these reasons, black rockfish is considered to be a promising target fish species for stock enhancement. Stock enhancement is a management tool that might augment depleted wild populations of marine living resources by supplementing or restocking the stocks. This study aimed to develop systematic technology for the stock enhancement of this species which includes broodstock management, juvenile production, nursery growout, marking method and release strategy.
 Effects of rearing temperature on growth and maturation of black rockfish were examined in captivity. Rockfish were reared at three different temperature conditions, natural temperature (control group), natural temperature + 12℃in winter-spring (12℃group)and natural temperature + 15℃in winter-spring (15℃group), during the period from 7 months after birth(MAB)in December 1997 to 48 MAB in May 2001(4 years old). The lowest natural water temperature ranged from 4 to 6℃in February. Rockfish reared at higher temperature in winter-spring showed significantly larger total length (TL)as 15℃group > 12℃group > control group up to 15 MAB. However, significant difference in TL between treatments was not found after 16 MAB. This was caused by higher growth rate of control group > 12℃group > 15℃group during the following warm period of 13 (July 1998) to 18 MAB (December 1998). This phenomenon is thought to be compensatory growth of fish whose growth was limited due to low temperature in winter-spring season. Female fish grew faster at higher temperature treatment up to 2.5 years old, but there was no difference in growth rate between treatments for male. A slightly higher maturation rate was found at higher temperature treatment in male and no temperature effect on maturation in female fish. Four-year-old female at natural temperature and 12℃treatments bore healthy larvae. However, those at 15℃treatment had abnormal parturition. Within the present experimental conditions, the artificial increase of winter-spring temperature from natural one to 12℃or 15℃is concluded to be of no use in order to enhance growth and maturation of black rockfish broodstock.
 The gonadal development in rearing black rockfish were monitored for one full year. Female fish began vitelogenesis from November and completed it in March. Gestation occurred from April, and parturition occurred in June. The reproductive cycle of female fish was divided into the following five periods. Vitellogenesis onset period: November-December, Vitellogenic period: January-March, Gestation period: April-May, Parturition period: June, Resting period: June-October.
 Male fish began spermatogenesis from June, and matured in November and December. The reproductive cycle of male fish was divided into the following five periods. Spermatogonial proliferation period: March-May, Early spermatogenic period: June-July, Late spermatogenic period: August-October, Functional maturation period: November-December, Recovery period: January-February.
 Five hatchery-reared female black rockfish(7 years-old)were separated from male fish and reared in an experimental tank before the mating season in order to prevent copulation. Egg diameter was observed weekly using a cannulation method. Egg diameter increased from the start of the measurements and then decreased 2-3 weeks after ovulation. Subsequently, all individuals released unfertilized eggs. The prevention of copulation was thought to be a cause of abnormal spawning of eggs during the birth season.
 Developmental stages of intra-ovarian embryos in individual female black rockfish reared in an indoor tank were examined using a cannulation method on almost the same day (April 24 or 25)for three successive years. The estimated delivery occurred near the predicted date regardless of the year. This indicates that black rockfish have individually specific annual cycle of fertilization and embryonic development. Understanding individual cycle of parturition is useful for efficient broodstock management and juvenile production in aquaculture.
 Because size difference in black rockfish larvae induces increased cannibalism, preparation of a sufficient number of larvae with the same or similar daily age is necessary for efficient aquaculture. We developed a method to concentrate the parturition period of female broodstock by examining the developmental stage of embryos using cannulation and selecting females, which are predicted to have similar parturition dates. We set up two groups examined over a 3-year period: one for selected broodstock by observation of embryos and the other as unselected control groups. The parturition period of selected group was significantly shorter than that of the control group and greater numbers of larvae with the same parturition date were obtained in the selected group over the 3 years of experiments. The differences between the predicted dates and the actual dates of parturition were 1-3 days for the day parturition began and 0-2 days for the day parturition ceased. These results indicate that selecting broodstock female on the basis of the inspection of the embryonic developmental stage using cannulation will enable us to obtain sufficient number of newly released larvae for systematic and efficient aquaculture.
 Effects of feeding schedule without rotifer on growth and survival of black rockfish larvae were examined in captivity. Newly born larvae of black rockfish were reared under two feeding schedule, rotifer-Artemia (control group)and Artemia group from onset of feeding to 0 day after birth. There were no differences in feeding rate, growth rate, and survival rate between treatments. Feeding schedule rotifer without providing was proved to be feasible in rearing black rockfish juveniles in a hatchery.
 Results of mass production of the black rockfish conducted from 1982 to 2004 were reviewed. Growth, development and mortality rate of black rockfish larvae and juvenile were examined. High mortality rates were observed from 3-7 days after birth (DAB), 18-25 DAB and 35-40 DAB. During the period from 3 to 7 DAB larvae died due to starvation caused by deformity in mouth and vertebrae. Larvae from 18 to 25 DAB were thought to die of gasbalance disorder. Mortality of larvae from 35 to 40 DAB occurred because of cannibalism. The highest mortality of larvae was found from 3 to 7 DAB indicating that prevention of deformity is essential to develop an efficient aquaculture system of juvenile fish.
 The effects of rearing density and feeding frequency to hatchery costs were studied in order to reduce costs during the intermediate culture of black rockfish in which fish are reared from 40 to 100 mm in TL. Juveniles were reared at 200, 400, and 800 individuals per kL, and fed formula feed once or twice per day. The experiment lasted for 42 days and no significant differences were observed in survival rates(99.9-100 %)and daily growth rates(3.3-3.5 %). Feed efficiencies, recorded as food intake per individual, in the onceper- day feeding groups were significantly higher than those in the twice-per-day feeding groups. Based on these results, once-per-day feeding and rearing under the highest density (800 ind./kL)were determined to be the most effective methods for economic rearing.
 Juvenile black rockfish were marked by fin removal method in four size classes(40, 60, 80, 100 mmTL). The survival rate and duration of the mark were investigated over two years. The survival rates 2 years after marking was 95 % for 40 mm and 80 mm fish and 97 % for 60 and 100 mm fish. The mark retention rates 2 years after marking was 97 % for 40 mm fish and 100 % for 60, 80 and 100 mm fish. There was no difference in survival rate, mark retention rate and growth rate between treatments.
 Effective conditions for alizarin complexone(ALC)staining of otoliths were examined in black rockfish to establish the optimum ALC marking methods. Juvenile black rockfish (45 mmTL)were immersed in three immersion concentrations(15, 30 and 50 mg/L)of ALC for three treatment durations (6, 12, 24h). The survival rate, ALC mark visibility and duration of the mark were investigated over two years. The survival rates 10 days after ALC immersion for 15 mg/Ltreatment were 99.9 % in two water tanks, showing significantly higher values than the 52.4 % and 81.7 % for 30 mg/L, and 13.7 % and 3.9 % for 50 mg/L. In addition, the 15 mg/L treatment showed the highest value in visibility of ALC marks on otoliths. The ALC marks were visible for over two years without polishing the otolith. Costs of ALC marking per fish under the conditions of 15, 30 and 50 mg/L were 1.50 yen, 4.47 yen and 57.0 yen, respectively. Among the concentrations used in this experiment, 15 mg/L was concluded to be the optimum concentration of ALC for otolith marking of black rockfish.
 This study evaluated the stocking effectiveness of black rockfish released in Yamada Bay, off the northeastern Pacific coast of Japan, using three indices estimated from survey data collected on all business days at the Yamada Fish Market over a period of 11 years. Eight annual cohorts of hatchery-raised, marked juveniles were released into the bay from 1989-1991 and 1993-1997. During the market census, all landed black rockfish were investigated for TL and presence or absence of marks. The census indicated that 51,512 fish out of 447,400 hatchery-released juveniles were landed commercially and recorded at the market. Of the returned fish, 1-year-old fish accounted for 61.2 %, 2-year-old fish for 30.3 %, and ≧3 year-old fish for 8.6 %. Return rates of each year class ranged from 9.3 % to 15.4 % showing stable and rather high values. However, the economic return rate(landed sales values/hatchery costs)ranged between 0.68 and 1.25; 5 of the 8 year classes showed rates of less than 1, where hatchery costs exceeded their associated benefit. To make stocking of black rockfish feasible, raising the market price of this species through fishery management, decreasing hatchery production costs, and developing effective release strategies such as optimum release size are needed.
 Effects of size at release on return rate and economic return rate at fish market of black rockfish were examined. Black rockfish were released at four different sizes at release into Miyako Bay: 100 mmTL(control groups), 80 mmTL group, 60 mmTL group, 50 mmTL group, from 1999 to 2001, respectively.
 Return rate of control groups ranged between 13.8 % and 22.9 % (average 19.0 %)showing stable and rather high values. Of the experimental groups, the return rate was 9.5 % for 8 cmTL group, 9.6 % for 60 mmTL group and 3.4 % for 50 mmTL group. A positive correlation (r2 = 0.76)was observed between size at release and return rate. Economic return rate of control groups ranged between 0.93 and 1.18 (average 1.03)showing stable and rather high values. Of the experimental groups, it was 0.78 for 80 mmTL group, 0.58 for 60 mmTL group and 0.29 for 50 mmTL group. A positive correlation(r2 = 0.93)was observed between size at release and economic return rate. Return rate and economic return rate increased with size at release. Released(range 70-202 mmTL, N = 211)and wild juveniles (range 47-198 mmTL, N = 522)were collected using cage setnet in the most inner area of Miyako Bay. This indicates that juveniles used shallow vegetated habitats with seaweed and eelgrass in the most inner area of the bay as nursery grounds.
 Increase of catch of black rockfish was recognized by release of hatchery-raised juveniles into Yamada Bay. However, compared with red sea bream and Japanese flounder, economic effectiveness is not sufficiently high enough to support the program financially. One important reason for the low economic effectiveness is low commercial value of black rockfish, because small 1-year-old fish accounted for the majority of landed fish. Although market landings of black rockfish increased due to stocking of hatchery fish, it quickly decreased after stop of stocking.
 This suggests that stocking did not contribute to construct spawning stock population on Yamada Bay because of over fishing of young fish.  We can conclude that appropriate management of recruited population and conservation of nursery grounds are crucial for successful stock enhancement program.
URI http://www.fra.affrc.go.jp/bulletin/bull/bull25/nakagawa.pdf