There have been workshops, articles and publications on the subject of how to obtain the perfect egg. Yet, identifiers of the perfect egg elude us. While this debate continues, the aquaculture industry seeks to perfect a supply of high-quality seed to keep it going.

To define egg quality is to swing blindfolded at an egg-sized piñata suspended in a halibut incubation silo with a chopstick. However, there are a few tries at it: developmental and biochemical.

In most species of fish, milestones of development have been taken as points of determining egg quality. In this measure, fertility is measured as cell divisions after 12 or 24 hours for salmonids. Alternatively, the occurrence of the primitive streak after seven days is a primary indicator of fertility.

These indicators of fertility and development imply a direct relationship with egg quality. There is good evidence to suggest that good fertilization rates are a reflection of good egg quality and this high performance carries on through to hatching. While it stands to reason that a good egg will have a good fertilization rate, additional factors may contribute: great eggs and bad milt do not an eyed egg make. And then there’s the environment, including culture technique (or lack thereof).

Biochemical measures are terrific for giving detailed analysis of what was out of whack. However, if the eggs were rotten, no analysis will detail when or how the rotting occurred. Despite this, there is some valuable information that can be garnered from biochemical analysis. For example, in the 1980’s, Chinook eggs had poor fertility compared to their wild counterparts. After determination of the fatty acid profiles and mineral content of the eggs, it was shown that the eggs were full of butter from beef tallow and low in the fish oils. As well, there were low levels of the element selenium, which has a proven role in fertility. This was the genesis of broodstock diets.

While biochemical analysis gives accurate profiles of stuff in the eggs, it takes a little time. Until a litmus test of egg chemistry is available, chemical analysis will remain something that tracks changes or is in response to problems of egg development.

In marine eggs, the battle of egg quality determinants is even less clear. This war wages over the ‘goodness’ of eggs based on whether they float or sink. This can change with composition: the eggs from one female can change over a spawning season. As well, some warm water species have semi-floaters that are every bit as viable as the top-floaters. In halibut circles the formation of the blastodisk, is a key milestone event. In sablefish, Anoplopoma fimbria, abnormal embryos seem to be related to egg quality, but it is really unclear whether or not adult nutrition or holding conditions contribute to the phenomenon. As with salmonid eggs, all this assessment is after the fact, there is no predictive indicator of egg quality prior to adding milt.

Genetics can play a part in egg quality. Some of the fastest growing and lowest-grilsing stocks have the poorest fertility rates. Nutrition plays a big part in egg quality, especially during the viltellogenic, or egg-growing stages of maturation. If that’s not bad enough, culture conditions, transport and handling can also affect the hatchery bonus. But, every broodstock manager’s nightmare is overripening.

Certainly, a key feature of overripened eggs is poor fertility. This occurs sometime after ovulation where constituent articles of the egg are no longer able to perform or contribute to the development of an embryo. Overripe eggs may look fine, but they do nothing but cause trouble.

In salmon, maturation of the egg occurs after ovulation. This period can depend on many factors, but in general, fertility increases for about four days post ovulation and declines after about seven (see Bromage et al., 1994) after which the eggs are deemed overripe. If fertility can be seen as a measure of egg quality, then letting a fish sit for a few days after she has ovulated is a good thing.

For the first while after killing the hen, where the egg is kept is not important so long as the eggs are in ovarian fluid. For the most part, this means that if eggs are kept in low temps and in ovarian fluid (away from light and mechanical stress), there is a period of grace to transport or delay fertilizing eggs before post-spawn overripening occurs. This varies among salmonids, but within 24 hours, high fertility rates are usually sustained.

When fertility rates are high, eyed rates are high and survival is high. All that is needed now is some quick, simple and cheap method to determine which eggs are good and which are not. Better yet, identify the gene for egg quality and select for it.

Literature Cited

Bromage, N. et al., 1994. Egg quality determinants in finfish: the role of overripening with special reference to the timing of stripping in Atlantic halibut Hippoglossus hippoglossus. J. World Aquacult. Soc. 25: 13-21.