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Life History Strategy

As a primitive fish, sturgeon also retain a life history strategy that differs from the one adopted by most species of modern fish. Compared to modern freshwater fish, sturgeon grow exceptionally large, have very low mortality, live a very long time and produce far fewer offspring. Modern fish grow to maturity quickly and produce massive numbers of offspring. The vast majority of these offspring usually do not survive, however their very existence offers the potential to rapidly increase the size of the population if either the opportunity or the necessity arises. Perhaps the difference can best be described as follows: sturgeon expect to live forever and do not worry much about raising a replacement generation, whereas most fish expect to die almost any minute so they better produce as many offspring as they can. The significance of this difference in approaches is that the sturgeon are going to do well only as long as most of them do live forever. If too many are killed, they do not produce enough young sturgeon to maintain the population.

Where Did All The Manitoba Sturgeon Go?

Only a century ago sturgeon were found in large numbers throughout their range in North America. Although they are not in imminent danger of extinction, their populations are only remnants of what they once were. Where did they all go? The story starts only a hundred years ago when a combination of intensive commercial fishing and alterations to habitat caused by industrial development rapidly depleted their numbers. They have not recovered.

Environmental Factors

Differences between the life history strategy of sturgeon and other species of fish affect their ability to deal with both commercial fishing pressure and environmental change. Sturgeon tend to be more vulnerable to environmental impacts such as dams and pollution than other species.

While dams can significantly affect sturgeon movements and their habitat, it is interesting to note that the major decline in the Nelson River sturgeon population occurred before any dams were constructed.

The North American Commercial Sturgeon Fishery

While sturgeon were considered a valuable food fish by North America's aboriginal people, they were considered a nuisance by commercial fishermen prior to 1860. Sturgeon were so large that they tended to destroy fishermen's nets and there was no demand for their meat. Sturgeon that were caught would normally be used for fertilizer or dried and burned as firewood.

In the mid 1800's the Lake Erie port of Sandusky, Ohio was considered the largest fish market in the world. Although sturgeon eggs were being processed into caviar by 1855, the big change occurred in 1860 when they discovered that sturgeon could be smoked and sold as a substitute for smoked halibut. By 1866, between two and three million pounds of smoked sturgeon were produced in Sandusky. By 1880, sturgeon were a major fishery, and by 1890 populations were already in decline in many areas. As eastern populations declined, sturgeon fisheries moved west as the railways opened up new areas.

This early decline was caused by a commercial fishery that seriously over-fished stocks, an experience that almost all sturgeon stocks in North America shared. Typically a fishery opens with a relatively high initial catch followed by a rapid and permanent decline to very low catches. Throughout the history of the commercial fishery, this has been the case for every major sturgeon fishery. For example, the Lake Erie catch fell from five million lbs. to one million lbs. (80%) in only ten years (1885-95). The Lake of the Woods catch dropped 90% in only seven years.

Sturgeons are unique among commercially fished species in this sudden and permanent decline to very low population levels. Most other species decline somewhat when fished but relatively high catches can be maintained over the long term. Sturgeons have a unique life history that makes them particularly vulnerable to fishing pressure. While sturgeon actually grow fairly quickly, they take far longer to reach commercial sizes and sexual maturity than any other species of North American freshwater fish. While a walleye in northern Manitoba might reach maturity within six or seven years, a female sturgeon will not reach maturity until it is twenty to twenty-five years old. Even after they reach maturity they only spawn once every four to seven years rather than spawning every year like other species.

Adult sturgeon are so large that there are almost no nonhuman predators that can prey on them. Smaller sturgeon are similarly protected from predators by five rows of sharp scutes that run the length of their body. Once they survive the first few years of their life, their mortality rate is extremely low. They can also live to a ripe old age for a fish (over 150 years). With few things that can kill them, including old age, a sturgeon population depends more on the survival of adult fish, than it does on producing young sturgeon.
This is precisely the opposite of the life history strategy that most fish use. They depend on producing large numbers of eggs to ensure that there is always potential for a large number of young fish to grow up and replenish the population. Mortality of eggs, fry and young is high, which is why they must produce large numbers to ensure that enough survive to maturity to maintain the population.

Most species of fish are adapted to withstand high levels of mortality, so they can maintain their population even if they are fished heavily. Since sturgeon would normally have very low levels of mortality they are not adapted to maintain their population if they suffer significant mortality from fishing. This lesson has been taught repeatedly in commercial sturgeon fisheries across North America for the past century. Sturgeon populations can only be fished very cautiously or they will decline. Once a sturgeon population declines, they will take so long to recover that the decline appears permanent in terms of human life spans.

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