One of the most frequently asked questions in salmon broodstock seminars is "How do I know when my fish are ready?"

There are two types of ‘ready’. One is ready to manipulate spawning date and the second is ready to spawn. In both cases, there is a need to determine where the fish are in terms of their maturation process. The real question is "How do I determine the maturative state of my fish?"

One measure of how fish are progressing is gonado-somatic index or GSI, where the weight of the gonad is expressed at a percentage of the total body weight. As the gonads increase in size during the summer, the GSI increases. Once the gonads are nearing the end of being built, the GSI levels off even though the fish is not fully mature; the gonads have to go through final maturation before they are ripe.

Generally, GSI is used to determine early spawning segments of a population or to separate maturing from silver fish. Those fish that are ‘late bloomers’ are usually culled or harvested. This practice generally makes for a shorter and earlier spawning season, but does not tell you when the fish are ripe.

Egg size is a similar indicator of vitellogenesis winding down. The end of summer and about one month prior to ovulation there is no further increase in egg size. At this point, the egg must mature, not increase in size.

The fishes’ hormones that control egg size and maturation are GTH I & II respectively. The measurement of these two hormones is not ‘farm ready’; an ELISA or radio-immunoassay is necessary for measurement. Even if kits were farm ready, it may not help. Spawning is an individual thing that has many variables and a kit would be able to tell you how mature that particular fish was, but it wouldn’t be an accurate prediction of the population. GTH determination would also mean sampling blood in all the fish. So for now, GTH kits are used mainly in the labs.

Because steroids ultimately control final maturation of the egg, this would seem a more accurate method of determining ‘ready’. However, sample size and ‘farm ready’ assays get in the way. As with the GTH assays, steroid assays are a thing of the lab.

Part of the maturation process in salmon is tolerance to FW. It would stand to reason that this would be a good non-invasive indicator of maturation. Unfortunately, the window for FW entry is large for salmon but, it does indicate that the fish are going to spawn.

Physical signs such as colouration and the formation of a kype (hooked jaw) are likewise indicators of spawning, but not readiness. Although physical characteristics are a valuable tool for sorting spawners from silvers, they are not a reliable indicator of ripeness.

An accurate method for determining ‘ready’ is historical records. Historical data is valuable for predicting the start and duration of spawning, but some stocks will have a variation in spawning time that centers on crosses and year classes. As well, some stocks have an earlier spawning date than others and some year classes within a stock have different spawning dates. This is a ruse done by fish to confuse the issue, but accurate records limit the guesswork.

Of importance are the historical average date of first spawning and the duration of the spawning season. These two factors are key indicators of when to manipulate spawning date and when to be ready to take eggs.

Generally speaking, out-crossed stocks have a wider spawning window and a start date that is median to the two crosses. Predictable dates come from pure strains that usually have a smaller spawning window. As another generality, 3SW fish will spawn after 2SW fish.

Probably the best method for determining ripeness and timing of treatments can not be bought or sold. There is no replacement for experience. Some folks have the ‘touch’, others don’t. And touch is what it’s about.

Ovulation and maturation are not the same thing. The egg can mature after days the ovary releases it. It takes a talented touch to determine when all the eggs have been released from the ovary and for how long. One good indicator is the hyperal bones in the tail (Fig. 1). These are the bones that hold the fin rays of the tail. When a fish is ready, the hyperal muscles become very soft and the bones become ‘crushable’ or squishy.

In Figure 2, the female spawner is displaying classic signs of being ready. The eggs are resting in the forward end of the body cavity and move freely when palpated. (It is prudent to note that at the time the photo was taken the fish was euthanized using high levels of anesthetic). The lines of skin form folds that are oblique away from the ovipositor that is large, pink and pronounced. The ribs of the fish are visible indicating thinness to the belly walls. Fat belly walls, a heritable trait, confuse the issue when feeling for free eggs.

There is a difference between FW and SW spawned fish. Fish in SW tend not to undergo the loss to body integrity that hits FW spawned fish. That is, SW fish have firmer flesh that makes it harder to judge if the eggs are completely ovulated. SW spawned fish also tend to be a bit livelier, at least at first sort.