Two ways to advance spawning in broodstock salmon

By Jim Powell, PhD, R.P. Bio

Reprinted Courtesy of Northern Aquaculture, September 1999

There are numerous reasons why salmon producers would like to advance the spawning date of their stocks:

• Earlier eggs mean earlier and bigger smolts,
• Hatchery productivity can be increased if the spawning season is increased,
• Efficiency of the hatchery increases,
• Out-crossing of stocks for hybrids can be achieved, and
• Synchronized spawning saves money and resources.

For salmonids, the key to influencing spawning times is through the fishes’ own natural cycle of maturation. A combination of environmental and physical cues is coordinated in the fishes’ brain, which triggers an endocrine (hormonal) response to start

Male Atlantic salmon receives implant that will advance spawning and improve milt quality.

Salmon take their cues to mature by photoperiod. Photoperiod refers to the amount of daylight relative to darkness in a 24 hour period. After the winter equinox, the daylight hours increase relative to darkness. At the spring equinox, the rate of change from dark to light reaches a maximum. After the summer equinox, the process reverses itself and daylight hours begin to shorten. These rates of change influence maturation in salmonids.

To advance spawning dates, the winter/spring photoperiod must be advanced relative to the natural photoperiod. Supplying artificial light to compress the natural winter and spring photoperiods does this. That is, first the days are artificially lengthened before the real summer equinox and then reduced to a September photoperiod by August. The result is an advancement of spawning date by about one month.

In laboratory situations, trout can be made to spawn six months out of synch by advancing the photoperiod. However, in a cage situation, the winter/spring photoperiod can be increased, but it becomes very difficult to shorten summer days after the summer equinox. Some producers have been successful in advancing Atlantic salmon by a month in land-based systems, but this is not always practical.

Perhaps a more cost-effective method is to induce maturation through direct manipulation of the fishes’ endocrine system. In this manner, the natural cascade of endocrine events is stimulated to begin earlier. To do this, the producer must augment the stimulus that controls maturation. There are a number of ways to achieve this, but perhaps the safest and most effective way is through the use of sustained-released implants that supply minute amounts of peptide (protein) that hurry the maturation process along. These peptides work at the level of the hypothalamus and cue further endocrine events to control gonad maturation.

Implants are injected into the fish during brood sort about 4-6 weeks before the fish normally mature. The delivery site is the dorsal sinus of the fish, the little ridge that runs along the crest of the fishes’ back. This method can only be used to cause final maturation of the gonad, so spawning date can only be moved forward about a month or so.

Besides advancing spawning date, implants also compress the spawning season. In this way, spawning is synchronized among the population and all spawning for the group is over within a shorter time. As the astute will note, this has a downstream effect as eyeing, hatching and first-feeding also occur in a synchronized and ordered fashion, not to mention earlier. Another advantage over photoperiod control is that a small or large number of fish can be treated with implants, not a whole population. As well, photoperiod control moves the spawning period forward; it doesn’t compress the season as implants will.

Both techniques, photoperiod and implants, are used successfully in the industry. They are not mutually exclusive and the two can both be used together. The important thing to remember is that neither is a magic bullet. Of importance is matching expectations with a reasonable cost and effort. We are dealing with captive animals that respond to biological needs. Above else, this must be respected.