Control of reproductive function in captivity is essential for the sustainability
of commercial aquaculture production, and in many fishes it can be achieved by manipulating
photoperiod, water temperature or spawning substrate. The fish reproductive cycle
is separated in the growth (gametogenesis) and maturation phase (oocyte maturation
and spermiation), both controlled by the reproductive hormones of the brain, pituitary
and gonad. Although the growth phase of reproductive development is concluded in captivity
in most fishes-the major exemption being the freshwater eel (Anguilla spp.), oocyte
maturation (OM) and ovulation in females, and spermiation in males may require exogenous
hormonal therapies. In some fishes, these hormonal manipulations are used only as
a management tool to enhance the efficiency of egg production and facilitate hatchery
operations, but in others exogenous hormones are the only way to produce fertilized
eggs reliably. Hormonal manipulations of reproductive function in cultured fishes
have focused on the use of either exogenous luteinizing hormone (LH) preparations
that act directly at the level of the gonad, or synthetic agonists of gonadotropin-releasing
hormone (GnRHa) that act at the level of the pituitary to induce release of the endogenous
LH stores, which, in turn act at the level of the gonad to induce steroidogenesis
and the process of OM and spermiation. After hormonal induction of maturation, broodstock
should spawn spontaneously in their rearing enclosures, however, the natural breeding
behavior followed by spontaneous spawning may be lost in aquaculture conditions. Therefore,
for many species it is also necessary to employ artificial gamete collection and fertilization.
Finally, a common question in regards to hormonal therapies is their effect on gamete
quality, compared to naturally maturing or spawning broodfish. The main factors that
may have significant consequences on gamete quality-mainly on eggs-and should be considered
when choosing a spawning induction procedure include (a) the developmental stage of
the gonads at the time the hormonal therapy is applied, (b) the type of hormonal therapy,
(c) the possible stress induced by the manipulation necessary for the hormone administration
and (d) in the case of artificial insemination, the latency period between hormonal
stimulation and stripping for in vitro fertilization.
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