Spawn specifics: The reproductive potential of common Mississippi fish

It’s the start of spawning season, the time of year when a pulse of new fish are added to the population. 

But how many are added? 

That depends on many things, but it’s a lot. Here are the results of some simple calculations with available data — sometimes robust, sometimes scant — on fish fecundity, age at maturity, reproductive life span, and growth rate.

A little biology

Fish achieve sexual maturity and spawn when they reach a certain size. Commonly, the age of sexual maturity is reported, but age has little to do with it. Rather, sexual maturity occurs when fish reach a certain size. This is biologically important, because it takes a lot of energy to develop eggs and sperm, and to complete the spawn, energy that is not available in the body of a small fish.

Fecundity is the number of eggs produced by a mature female fish for a single spawning cycle. Fish grow throughout their life, and larger fish produce more eggs. To accommodate this, biologists measure relative fecundity: the number of eggs per pound of female.

All fish have a life span. Although only occasionally measured, biologists and hatchery workers commonly report that older fish are less reliable spawners; the fecundity may decline, and the eggs and sperm may not be as viable.

In a lifetime

In the early days of fisheries management, minimum-length limits were established based on size at maturity. The reasoning was to forego harvest until the fish had a chance to spawn. This makes sense in a fishery in which most of the population is harvested annually. This may occur in some commercial and subsistence fisheries, but it is rarely the case in recreational fisheries.

A more meaningful estimation of a population’s spawning potential is to estimate the reproductive contribution of a fish throughout its reproductive life. Fishery managers use a similar approach — the spawning potential ratio — to establish harvest guidelines to prevent recruitment overfishing.

Sport vs. forage

Estimates of the reproductive contribution for common sportfish and a keystone forage fish — gizzard shad — are summarized in the table. The calculations are based on the assumption that the average of the limited number of relative fecundity estimates are reasonably accurate for fish in Mississippi waters, and that all eggs are spawned. 

The relative fecundity estimates are multiplied by fish weights during each year of approximately the first half of their reproductive life span. Summing the calculated numbers of eggs produced provides an estimate of lifetime reproductive contribution.

Clearly, an individual fish can produce a lot of progeny during its active, reproductive years. Remember, these numbers are estimates of potential progeny. Fecundity is measured by counting mature eggs in the ovary. I doubt that all of the eggs are spawned during each annual cycle. Furthermore, not all eggs are fertilized, develop and hatch. Therefore, the estimates of total reproductive output may not accurately predict the number of offspring. On the other hand, I have used low estimates for reproductive life span, partially offsetting high estimates of the possible number of progeny produced.

What should be obvious is that if good habitat is available for spawning, Mississippi’s native sportfishes can produce more than enough young to sustain their populations.