In aquaculture, breeding programmes are, for the most part, in their infancy by comparison to terrestrial production. These salmon breeding programmes are in large measure based upon programmes developed for swine and poultry. As salmon breeding programmes are comparatively new, there are great gains realized in short times owing to the limited selective pressure on various traits. Often, these genetic gains are 10-15% per breeding generation.

Here the similarities between terrestrial and aquatic breeding programmes begin to disappear. Where terrestrial systems are perpetual, salmon seed production is largely cyclical. Indeed, with few exceptions, most salmon spawning still occurs in the autumn. Further, while terrestrial systems have on-demand propagation, salmon reproduction is largely at the mercy of natural cycles. Lest not to seem doom and gloom, there have been significant advances in salmon production and seed supply.

One of the most productive mechanisms to perpetuate seed supply is photoperiod and temperature manipulation. In some stocks, the breeding season is extended, or put out of season by strictly controlled photoperiod regimes. Temperature control serves to preserve egg quality while compressing and advancing spawning times. However, these facilities are very specialized and few in number. The vast majority of salmon ova supply still comes from net cage operations.

While photoperiod and temperature manipulation can delay or advance the spawning season, the problem of asynchronous spawning remains. That is, as one fish matures, it may not be spawning the same time as her cohorts. This can mean delays further down the production line as there may not be enough ova from early or late spawning fish to fill a production lot. As well, males are notorious at being early, late or spent when females are ready.

One solution to answering the asynchrony issue was developed in the 1930’s. Researchers took pituitaries from ripe fish and fed, implanted or injected them into later-spawning fish. This practice was carried over to Indian carp until very recently and is called hypophysation. The idea here is to take the residual and indigenous gonadotropins in the pituitary of one fish and augment those of the soon to be spawner. More recently, carp pituitaries or their extract is administered to other species of fish to induce them to spawn. This can work well for carp, but requires larger doses for other species. Further, there are concerns about immunogenesis or the development of an immune response in repeated use, doses and efficacy in new species. To many, CPE is still the standard. The human equivalent to CPE is human chorionic gonadotropin or HCG. It is available in purified form, but the issues of dose, immunogenesis, and having to increase dose with increase use persist.

In mammals, the endocrine cascade of reproductive hormones was well described in the early part of the twentieth century. Likewise, the tools to induce ovulation in cattle, swine and horses were developed early on. However, fish reproductive systems evaded scientific investigation for some time. In fact, the identification of the brain (hypothalamus)-pituitary-gonadal axis was not recognized and accepted until the late 1980’s or early 1990’s. While the brain peptide that controls reproduction in mammals, LHRH, was identified in 1972; the fish counterpart was not identified until 1984.

Once the controlling mechanism for fish was equated with mammals, research into induced maturation matured. This was supported by crossover work done on LHRH and it’s analogues for use in human pharmaceuticals. The first wide-scale use of brain peptide analogues (GnRHa) was directed at carp because of the history of hypophysation. It was soon elucidated that GnRHa alone was not enough to induce spawning, but that another brain chemical, dopamine, had to be inhibited before spawning could be induced. From this, the product Ovaprim was developed and has been used to great success for over 15 years. Indeed, the induction of maturation in Indian carps by Ovaprim has been accepted as standard procedure and has permitted the expansion of the carp industry in India.

Further studies on the induction of spawning in food fish, primarily salmon showed that sustained-release preparations of GnRHa had superior results over single bolus injections. This work capitalized on the aspirations of researchers to unlock the intricacies of the HPG axis in fish coupled to the need of the salmon aquaculture industry to secure seed stocks. These sustained-release preparations served to continually stimulate the natural endocrine cascade in maturing fish to coordinate and synchronize spawning times in large groups of salmon. At first bulky and awkward to use, more practical and user-friendly forms soon evolved.

By the mid 1990’s, sustained-release preparations of the most potent salmon GnRH analogues were released from testing to the marketplace. By 1998, Ovaplant, a sustained-release implant containing sGnRHa was tested and approved for use in Chile. Ovaplant was soon demonstrated to be a cost effective method of advancing and synchronizing spawning dates in either seawater or freshwater-held broodstock salmon. Further, the issue of milt supply was obviated as milt production was assured and milt volumes increased. This assurance of milt supply has had an added economic benefit to producers in that fewer males are wasted due to non-productivity simply by using the implant.

In photoperiod-controlled spawners, Ovaplant proved effective at synchronizing spawning dates without appreciably sacrificing egg quality. This method of induced maturation has spread from salmon to other species of interest in aquaculture. Halibut producers now routinely use Ovaplant to keep males running through the long spawning cycle. It has also found a niche in preserving the gene pool among threatened or endangered species.

Ovaplant is currently being tested in field trials in Canada and Ireland. These field trials will form part of the requirements for full drug registration in those jurisdictions.

While salmon producers on a permanent scale have not secured a continuous production of seed, there have been great gains in technological development. With the tools of photoperiod and temperature manipulation, breeding programmes and induced maturation, producers are making progress towards the goal of year-round egg supply within companies. While the idea of a breeder facility for salmon may seem far-fetched, it is clearly possible.