Development of aquaculture for any species requires absolute control of reproductive function and the production of large numbers of fertilized eggs of good quality for larval rearing. Reproduction of almost all fish species in aquaculture exhibit some form of dysfunction, ranging from (a) lack or incomplete gametogenesis (vitellogenesis and spermatogenesis), (b) failure of oocyte maturation and ovulation at the end of vitellogenesis, (c) reduced sperm volume and/or quality and (d) failure of spawning after ovulation and spermiation.
In Mediterranean aquaculture, the major species of interest include the gilthead seabream (Sparus aurata), European seabass (Dicentrarchus labrax), meagre (Argyrosomus regius), sharp-snout seabream (Diplodus puntazzo), Senegalese sole (Solea senegalensis), Japanese red seabream (Pagrus major), recently renamed as “short-finned porgy” for the European market, and recently greater amberjack (Seriola dumerili). All species have asynchronous development of the ovary and spawn multiple times during their annual reproductive season, and produce pelagic eggs (~1mm in diameter). Of these species, only gilthead seabream -and to a lesser extend short-finned porgy- reproduce consistently without any biological or production issues and are considered fully domesticated.
Hormonal therapies based on GnRHa injections or implants have been tested in all Mediterranean aquaculture species, including some past and present candidates such as the shi drum (Umbrina cirrosa), dusky grouper (Epinephelus marginatus), wreck fish (Polyprion americanus) and Atlantic bluefin tuna (Thunnus thynnus). These treatments induce the pituitary release and further synthesis of the gonadotropin hormone responsible for maturation (Luteinizing hormone, LH). Released in the circulation, LH reaches the gonads and stimulates steroidogenic changes in the synthesis of sex steroid hormones, which are the final effectors of oocyte maturation, ovulation and spermiation. If environmental conditions are appropriate (e.g. tank size and depth, water temperature and salinity, other factors that may not be known yet, etc.), then spontaneous spawning takes place, otherwise in vitro fertilization is necessary (e.g. in wreckfish and Senegalese sole).
A major issue in the reproduction of fish in aquaculture is the limited parental contribution of males during a particular day’s spawning. This results in only a limited number of families being produced and a gradual reduction in the genetic variability of the stock of each hatchery, as not all of the males contribute equally to the produced offspring. The small number of families produced is very important and is limiting the implementation of breeding selection programmes based on mass spawning protocols, and communal rearing of the eggs and larvae. The use of GnRHa-based hormonal therapies may be used to increase parentage contribution of a given stock by (a) synchronizing the maturation, ovulation and spawning of a larger number of females at a given time and (b) increasing the number of males that contribute to the fertilization of larger numbers of eggs.
Future research will enable control of reproduction without the use of hormones, to allow production of organic fingerlings for grow out.