Production of a food commodity is based on the supply of seedstock. Terrestrial livestock production has well-established mechanisms for insuring that the supply of stock is perpetual; the growing cycle is continuous.

The basis is a breeding and broodstock programme ensuring the continual supply of quality seed.

In comparison, aquaculture breeding programmes are mainly in their infancy, and those for salmon largely based on swine and poultry programmes.

As these programmes are comparatively new, great gains are realised in short times owing to the limited selective pressure on various traits. Often, these genetic gains are 10-15% per breeding generation.

But where terrestrial systems are perpetual, salmon seed production is largely cyclical. 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 despite significant advances in salmon production and seed supply.

One of the most commonly used mechanisms to increase seed supply is photoperiod and temperature manipulation. In some stocks, the breeding season is extended, or put out of season by controlled photoperiod regimes.

Temperature control serves to preserve egg quality while compressing and advancing spawning times. However, few facilities can manipulate fish in such a way. Most 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 – maturing fish spawning at different times.

This can mean delays further down the production. Even so, males are notorious for being early, late or spent when females are ready.

One solution was developed in the 1930’s. Researchers took pituitaries from ripe fish and fed, implanted or injected them into later-spawning fish (hypophysation).

The idea was to take the residual and indigenous gonadotropins in the pituitary of one fish and augment those of the soon to be spawner.

More recently, dried carp pituitaries or their extract (CPE) were administered to other species of fish to induce spawning. This works well for carp, but may require larger doses for other species.

With CPE, the dose is variable depending upon the maturity of the donor and cross-species effectiveness can further vary the results. Yet to many, CPE remains the standard.

Fish reproductive systems evaded scientific investigation for some time. The identification of the brain (hypothalamus)-pituitary-gonadal axis (HPG 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, GnRH was not identified until 1984.

Once the controlling mechanism for fish was equated with mammals, research into induced maturation developed.

The first wide-scale use of brain peptide analogues (GnRHa) was directed at carp. It was soon obvious 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 around the world with great success for over 15 years.

Further studies 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 in order to coordinate and synchronize spawning times in large groups of salmon. At first 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 salmon GnRHa was approved 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.

Synchronized spawning among selected pairs for breeding purposes accelerated the gains in breeding programmes. The issue of milt supply for breeding programmes and production was negated 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.

While salmon producers have not secured a continuous production of seed on a permanent scale, there have been great gains in technological development.

Induced maturation with GnRHa is the newest in the arsenal of management tools which when combined with photoperiod, temperature manipulation and breeding programmes, are helping producers make progress towards the goal of year-round egg supply.

While the idea of a breeder facility for salmon may seem far-fetched; it is clearly possible and will be a reality in the near future.