ALLOTUNA

Research Directions:

Reproduction and physiology

Description:

The transition of the bluefin tuna industry from a capture-based industry to a traditional aquaculture industry necessitates the development of (a) broodstock management methods for the production of fertilized eggs and (b) larval rearing techniques for the production of fingerlings for the stocking of grow-out cages. Development of captive broodstocks should be easier to accomplished using juvenile fish collected from the wild. Juveniles will be (a) easier to handle and transfer to rearing facilities, be it sea cages or land-based tanks, (b) more cost effective to maintain since they are much smaller than mature fish captured by the purse seine fishery, and (c) could be easier to adapt to captivity conditions, thus being easier to mature reproductively and perhaps spawn spontaneously in captivity. In order to use juvenile fish for the development of captive brooodstocks, it is important to investigate (a) if juvenile fish do reach reproductive maturity in culture conditions and (b) at which age/size they do so.

In the present proposal, immature fish will be placed in cages and will be reared for 3 consecutive years, under common bluefin tuna rearing conditions. At the time of capture, 12 individuals will be sacrificed in order to obtain blood, which will be used to measure the concentration of important reproductive hormones, such as the steroids 17b-estradiol, Testosterone and 11-keto Testosterone, and 17a-20b-dihydroxyprogesterone. Gonads will also be collected in order to evaluate the gonadosomatic index (gonad weight/body weight) and to determine the sex and reproductive stage of the fish using histological processing. Thereafter and until the conclusion of the experiment, 12 individuals will be randomly sacrificed from the population during the annual reproductive season of bluefin tuna in the Mediterranean. This occurs in June-July. Again, blood and gonads will be collected in order to determine the reproductive status of the fish and determine the age/size of first maturity, as well as the appropriateness of the fish for reproduction in captivity.

In addition, 20 mature bluefin tuna collected during the natural spawning season with a purse seine, will be placed in a cage and reared using standard bluefin tuna rearing conditions. During the reproductive season, and once water temperatures reach appropriate levels (>22 C), fish will be treated with GnRHa implants in order to induce spawning and the production of fertilized eggs