Several methods are available to induce maturation in captive fishes, including injection of gonadotropin hormones (GTH), pituitary extracts containing GTH,(') human chorionic GTH, and gonadotropin-releasing hormones (GnRH). All these methods either supply GTH, or GTH-like peptides, or elicit the liberation of native GTHs from the pituitary which then induce maturation.(') Recently, hypothalamic hormones such as native GnRI1 or their analogues have gained favour among commercial fish producers. GnRH peptides are smaller, easier to prepare, and are more effective at inducing maturation than other peptides. Because they are naturally occurring peptides that use the endocrine pathways of the fish, they are more reliable and potent without causing harmful side effects. As well, analogues of both the mammalian and salmon GnRH (sGnRHa) forms are more potent and degrade slower than natural hormones. (3)

Methods for administration of GnRH peptides to fish include topical absorption,(') injection of soluble GnRH in vehicle, intubation,0) or a sustained release preparation .(6) Of these methods, three are commonly used: saline solutions of GnRH (7) injection of commercial preparations that contain a salmon GnRH analogue such as Ovaprim(l) or sustained released pellets (6) (Ovaplant; Syndel Laboratories, Vancouver). Pure synthetic GnR14 analogues have been on the commercial market for over a decade, but require field preparation and refrigeration to prevent degradation. Preparations such as Ovaprim or Ovaplant are preferred because of handling and storage considerations without loss of efficacy.

In a study to demonstrate the efficacy of induced maturation using liquid injectable sGnRHa or implants that contain sGnRHa, coho salmon (Oncorhynchus kisutch) were either injected with Ovaprim, implanted with Ovaplant, or administered both treatments. To determine species differences, rainbow trout (0. mykiss) were treated with Ovaplant to induce spawning. The objectives of this study were to determine whether treatment to induce spawning causes an increase in prespawn mortality, advances spawning date, compresses the spawning season, or has any deleterious effect on the progeny. The study was conducted under controlled conditions and in a commercial production setting.  

The treatments also compressed the spawning season. That is, the duration of the spawning season within the treated groups was less than in the control groups. In the case of trout, spawning was complete by the time the control fish had begun to spawn. This was also the case in coho where the spawning season for treated groups averaged five days compared to 15 days for control fish. A compressed spawning season provides considerable economic advantage as it decreases the amount of time crews must attend adult fish.

Batch spawning of fish also pen-nits batch ponding and first feeding of fry as exemplified by the current study. This results in more uniform growth of fish within a population by decreasing the duration of the first feeding phase. Uniformity in the size of fish is desired because it reduces the need for husbandry practices such as grading and mixing sizes of feeds. It is commonly thought that smolts of a uniform size also perform better in seawater than smolts of varied sizes.

This study has shown that induced maturation and coordinated spawning is a valuable tool for fish culturists. Firstly, the study indicates that greater efficiencies in hatchery operations are possible with regard to operational and capital costs. These savings are realized both immediately with adult fish and later with juvenile production. Secondly, there are implications for fish health as the freshwater residency period of adult fish is reduced; hence the use of therapeutants and the potential for horizontal transmission of disease is reduced. Thirdly, the genetic potential of broodstock can be maximized. By controlling the date of spawning, a greater number of viable gametes can be introduced into the breeding programme. As well, these techniques permit the out crossing of strains and the maximization of single-paired matings, In sum, advanced maturation helps maximize options in broodstock management.