Can fishing actually change a population’s genes
Catching and harvesting fish is a selective form of mortality — it does not operate uniformly or randomly across the population. That fishing may be changing populations has long been known in commercial fisheries, but recent research suggests that recreational fishing can also change populations.
These changes to populations are called ‘fisheries induced evolution” — FIE in shorthand. FIE is not a reduction in the numbers of fish or population size that occurs with overharvest. FIE is a change in characteristics of the population such as growth rate, a physiological parameter, or maybe a behavior such as movement or catchability.
FIE: new term for old concept
In commercial fisheries, in which most fish are caught in nets, capture is size selective. Fish large enough to be retained by the mesh of the net are captured and removed from the population. Smaller fish escape harvest.
If a fishing net removes fish above a certain size, it also selects for faster-growing fish. For example, a 1 ½-inch, square-mesh gill net may retain blue catfish larger than 15 inches. Let’s say, on average, blue catfish in this fishery grow to 15 inches in 4 years. The gill-net fishery will harvest fish 4 years old and older.
But let’s look a little deeper. Not all fish in a year-class grow at the same rate; growth usually looks like a bell-shaped curve, with most fish growing near the average rate, but some growing faster, some slower. The 1 ½-inch mesh gill net harvests by size not age. By capturing fish 15 inches and larger, that 1 ½-inch net, on any given day, also captures some of the fastest-growing 3-year-old cats but does not catch the slower growing 4-year-olds.
Over time, this gill-net fishery will continually select for and harvest the faster-growing fish and leave the slower growers to reproduce. Growth is affected by many factors, but it has a genetic component and, therefore, is an inheritable trait. Over time, harvest can create a slower-growing blue catfish population.
I have used growth as a rather clear-cut example of how harvest can affect a population’s gene pool and affect change. Fishery scientists are learning that other traits — behaviors such as catchability and movement, size at sexual maturity, and physiology — have a genetic component and can be affected by fishing.
Recreation fishing results
Ten years ago, Illinois Natural History Survey fisheries scientists published the results of a study that spanned 18 years. Fish from a heavily fished population of largemouth bass were stocked into a pond and fished intensively. At the end of the year, fish were collected, and the caught fish — dubbed high vulnerability (HV) fish — were stocked into a pond for spawning, and uncaught fish — dubbed low vulnerability (LV) fish — were stocked into another pond for spawning. After growing to catchable sizes, the progeny of the HV and LV fish were marked and stocked into a common pond for angling. This intentional selection process was repeated for three bass generations.
After three generations of selection, the catch rate of the HV fish changed little, but the catch rate of the LV fish decreased significantly. And the differences in catch rate between HV and LV populations increased with each generation of selection. Conclusion: vulnerability to capture is a genetic trait.
These results have obvious implications for bass anglers, but changes in vulnerability are only one measure of the changes that occurred in the population. Other studies have suggested that catch-prone largemouth bass have higher metabolic rates, are bolder and may be less-protective nest-guarding parents. In other words, fishing can affect the population in many ways, some of which may affect the population’s future fitness.
A Connecticut study compared metabolic rates of largemouth bass from two lakes, 437 and 489 acres, that had been open to fishing for 70-plus years to those from nearby similarly aged lakes, 114 and 529 acres, that were closed to fishing. The resting metabolic rates of the unexploited bass populations were 6-percent higher than the exploited populations. The study indicated FIE can occur in more typical fisheries in larger lakes.
In a second study, Connecticut researchers moved bass from the unexploited and exploited populations into a 25-acre lake that was intensively fished. After the bass spawned, genetic analysis revealed the translocated bass bred with the resident bass. The conclusion: bass from an unexploited, HV population will breed with exploited, LV bass.
As catchability is heritable, the hybrids of the HV and LV bass should be more catchable. Intensive monitoring of the fishery will be required to verify this. In the meantime, Kansas is using this science — the broodstock for all bass produced in their hatchery are from unexploited populations.
Do the results of research on largemouth bass apply to other species? Probably yes, and the effects may be greater for a high-harvest fish with shorter generation times like crappie.