The timing of when this insult or stress occurs in a broodstock’s life is important. From the above, a stress-inducing event will arrest reproduction. There are two major stages of maturation: vitellogenesis and gamete maturation (ovulation). Each stage is governed by a distinct gonadotropin: GtH I for vitellogenesis and GtH II for maturation. As Figure 1 implies, the vitellogenic stage is much longer in duration than the maturation stage. Both stages have critical periods where stress can make a fool of the most astute Broodstock Manager.

Growth is an easy thing to start and stop. If the stress is minor, of small duration or impact, then growth is temporarily halted. Growth is soon back to normal once things improve. Not so with vitellogenesis. This is a big machine that involves several internal organs. Once in motion, there is a momentum that is hard to maintain. So, if a stressor results in stalling or slowing vitellogenesis, egg number is sacrificed. As a rule of thumb, an early stressor, one that occurs before the peak in vitellogenesis, results in a decrease in egg number: fecundity. The energy that would normally go to building eggs from that great broodstock diet now goes towards building a big fish. The result is a beautiful fish, but with fewer eggs than predicted. This throws egg number calculations for a loop because the fish are huge, but bear proportionally fewer number of eggs.

Stressors that occur after the peak in vitellogenesis have a different effect on the fish. Late stress forces the fish to shut down further vitellogenesis resulting in the resorption or abandonment of some of the eggs. This is because there is not enough vitellogenin (yolk) to go around due to an interruption of supply. Again, critical functions are put on hold to ensure survival of the whole organism; egg size, performance and ultimately number are sacrificed to ensure the survival of the fish.

So much for stress during vitellogenesis. Stressors that occur during the short maturation, usually one month prior to spawning of the first fish, cause different responses in maturing fish. These effects range from males that dry up or females that produce early, but poor quality eggs. Males that are thought to be

running, but one week later have gone silver or produce nothing are in this category as are females that give eggs early but have variable quality.

The effect of a late stress can be of three choices for a female fish: 1) the stress is of little effect and things proceed normally, 2) ovulation is advanced and the eggs ‘ditched’, or 3) ovulation is delayed or aborted.

Stress events at spawning are of little consequence to fish or egg performance. Events that occur late in the season or at spawning appear not to effect egg survival. The most important period to influence performance appears to be during the GtH I to GtH II shift when the GtHs change as do the steroids that control vitellogenesis or maturation of the gametes,

Routine husbandry of broodstock is full of stress-inducing events. Early stressful events include sorting and selection, splitting pens, and transporting. Environmental stressors such as temperature changes, water quality issues or holding conditions also can lead to a stress response in the fish. And don’t forget the big-eyed, furry stressors with the high cuddle quotient, seals: otters and sea lions. In all of these events, timing, duration and recovery are essential to limiting impact and outcome.

Husbandry practices that occur late in the maturation process are also important to egg performance. A sliver/brown sort done too early or near the critical GtH I/II changeover period can result in the undesired effects. These include sorting, transporting and routine handling (injections).

Making the best

The doom and gloom scenario of stressors and consequence can be ameliorated. Obviously, some of the biggest stressors can be avoided by site selection. Others can be avoided by performing necessary husbandry operations at the correct times. Generally, for early effects these are when temperatures are lower and before peak vitellogenesis occurs (i.e.: before June). For later stressors, the timing for handling and sorting is as close to spawning as practicality will allow.