As soon as intensive aquaculture platforms started to emerge in the early 1970s, it became clear that the fact that many commercially important fish fail to reproduce in captivity was a critical bottleneck in the industry. In order to enable efficient and cost effective aquaculture, the full life cycle of the fish of interest must be closed in confinement so eggs and juveniles (“seeds”) can be available to the growers all year-round. Intensive research in fish reproductive biology ensued, whi ch led to the discovery that captive fish fail to spawn because of a dysfunctional brain hormone , gonadotropin-releasing hormone (GnRH) , which is the key regulator of reproduction. This understanding drove the development of a technology to induce spawning in farmed fish, consisting of the exogenous, sustained-release administration of supe r-potent GnRH analogs (GnRHa ). The GnRHa were selected based on their resistance to enzymatic degradation in the fish tissues, increased bin ding to the pituitary GnRH receptors, and enhanced in vitro and in vivo bioactivity in triggering the pituitary-gonadal axis leading to ovulation and spawning. This technology has since been optimized for dozens of farmed fish species and used to induce/synchronize final oocyte maturation, ovulation, spermiation and spawning in commercial hatcheries around the world.
In an effort to further u nderstand the fish GnRH system in the context of fish reproduction and aquaculture, many discoveries were made that establish ed fish as a vertebrate reproductive model . Fish have been shown to possess 2-3 forms of GnRH , novel fish GnRHs were discovered, and the functional significance of GnRH multiplicity was studied. New functions of GnRH isoforms were described , including coordinating reproduction with feeding, which may have great relevance to aquaculture. Basic studies also led to the discovery of additional players in the control of reproduction that may ultimately lead to better control of the reproductive cycle in aquaculture, from sex differentiation to puberty and gonadal maturation. The advancement of novel platforms of molecular biology, genomics and biotechnology enables the application of recombinant and gene delivery technologies to the control of reproductio n. Germ cell transplantation methods provide innovative strategies for gender control or spawning of fish that are difficult to culture in captivity until maturation.
As much as fertil e broodstock are essential for successful seed production, reproductively sterile fi sh are desirable for optimal grow-out and for genetic containment. The emergence of gene silencing and genome editing platforms paves the way for highly efficient approaches for generating reproductively sterile fish for culture and harvest, while maintaining a fully fertile parent stock for efficient spawning .