Detailed Explanation of Broodstock Induced Spawning Techniques in Turbot Aquaculture

Turbot is found in sea areas from the Mediterranean to the coast of Norway. In Europe, turbot is a valuable cultured fish species. Over the past 10 years, turbot aquaculture has continued to develop. Turbot aquaculture production increased from 150 tonnes in 1985 to 3,700 tonnes in 1998. In the coastal waters of the Atlantic, natural reproduction of turbot occurs in summer; however, by artificially controlling the photoperiod and water temperature, or by controlling only the photoperiod, turbot may spawn year-round.

Turbot spawns in batches, with as many as 10 spawning events during the reproductive period. Under captive conditions, even if turbot ovulates naturally, they do not release eggs and require manual stripping to collect eggs. Moreover, because under captivity the ovulation of females is irregular, the onset of spawning among females is not synchronized, and ovulation cannot be predicted. The variation in spawning timing among females leads to a prolonged reproductive period, which deviates from the requirements of intensive @#@233@#@ production.

Considering the need to ensure an adequate supply of eggs, production enterprises generally maintain large reserves of broodstock. To solve this problem, that is, to shorten the reproductive period and increase the number of ovulating females, French experts as early as 1995 conducted research on implanting sustained-release pellets of gonadotropin-releasing hormone analogue to induce spawning in female turbot and to promote synchronization of spawning in female turbot, and reported this research in 2000 in the recent issue of the journal Aquaculture published in the Netherlands. This article briefly summarizes the main results of that research, for reference by domestic turbot seedling breeding enterprises.

Induced spawning methods

Broodstock rearing: All female fish used in this research came from a commercial fish farm located on the Atlantic coast of France, namely France Turbot Company. The research was divided into 2 experiments. The first experiment was conducted from April to June 1995. To advance reproduction, the female fish used were reared under an artificial photoperiod. There were a total of 24 female fish, 12 in the control group and 12 in the treatment group, with an average body weight of 4.4±0.7 kg. The second experiment was conducted from June to August 1995. The female fish used were reared under a natural photoperiod. There were a total of 51 female fish, 20 in the control group and 31 in the treatment group, with an average body weight of 5.0±0.7 kg. In the 2 experiments, the stocking density of female fish was 8 kg/m~(3). The rearing water temperature for female fish was 14±1°C. At the start of spawning, the water temperature was lowered, and throughout the spawning period, the water temperature was maintained at 12°C to prevent overripening of eggs.

Induced spawning treatment: Each sustained-release pellet used contained 75 micrograms of gonadotropin-releasing hormone analogue (luteinizing hormone-releasing hormone analogue). The implantation site was the dorsal muscle, and the dose used was approximately 25 micrograms per kg of body weight.

Comparison of induced spawning results, female ovulation rate: In the first experiment, no significant difference was seen in the female ovulation rate, that is, the percentage of ovulating females, between the treatment group and the control group; however, in the second experiment, the ovulation rate of females in the treatment group reached 100%, while that in the control group was 47%.

Comparison of spawning synchronization: In the first experiment, in the treatment group, among females in the same group, the delay in the onset of ovulation between the earliest-spawning female and the latest-spawning female was 15 days, while in the control group it was 54 days. In the second experiment, the delay in the treatment group was 28 days, while in the control group it was as long as more than 70 days. In the first experiment, the entire spawning period of the treatment group lasted 4 weeks. In the second experiment, inspection was stopped before the final ovulation was reached; however, the entire spawning period of the treatment group also appeared not to exceed 6 weeks, while the spawning period of the control group was at least 8 to 9 weeks (first experiment) and 13 to 14 weeks (second experiment). In the 2 experiments, the average spawning duration of treated individuals (16 days) was significantly shorter than that of the control group (26 to 35 days). In the 2 experiments, no significant difference was seen in the number of spawning events per female between the treatment group and the control group. However, the spawning rhythm of the treatment group was faster than that of the control group. The average spawning interval of the control group was 2.5 to 2.8 days, while that of the treatment group was 1.6 to 1.8 days.

Comparison of egg collection from females: During the entire spawning period, no significant difference was seen in the total egg collection per female between the treatment group and the control group; however, the average egg collection per spawning in the treatment group (214 mL) was significantly higher than that in the control group (144 mL). During the entire reproductive period, in the first experiment and the second experiment, the total egg collection of the control group was 7 million eggs (12 fish) and 5 million eggs (20 fish), respectively, and the total egg collection of the treatment group was 23 million eggs (12 fish) and 58 million eggs (31 fish), respectively.

Comparison of egg hatching rate: No significant difference was seen in egg size between the treatment group and the control group. In production, the fertilization rate of the treatment group (74.7% to 77.2%, average 76.6%) was significantly lower than that of the control group (88.2% to 92%, average 89.5%); however, there was no difference in hatching rate. Hormone treatment appeared to have some effect on egg quality, at least at the fertilization stage. The hormone dose used in this research can be lowered, presumably without weakening the ovulation-inducing effect, but helping to improve egg quality.

From an economic perspective, the application of gonadotropin-releasing hormone implantation technology can be expected to reduce the number of broodstock reserves by 20 to 30%. In conventional production, during the seedling season, 56% of females ovulate, while the rest either cannot spawn or ovulate too late. Using the hormone pellets described, the percentage of ovulating females may be increased to 90%.