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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

February 1996

Genes make all the difference

The secrets to successful stocking in natural lakes and streams lie deep within a fish's DNA.

Martin Jennings

Human powers of observation have evolved over time to the point that we now can explore genes, the molecular codes and roadmaps that interpret natural variation. Advances in genetics are helping fish biologists make sound decisions regarding artificial propagation and stocking. As they gather research on the water and experience in the hatchery, biologists are discovering that conserving natural resources means protecting genetic resources, too.

Comfortable hatchery vs. cruel nature

It's a common mistake to assume that the goal of fish genetics is to make a "better" fish. Sport anglers, thrilled by the thought of a bigger, feistier fighter at the end of the line, find the idea of a "superfish" tempting. Fish managers, however, must focus on conserving existing fish populations rather than developing Jaws for lakes. To understand the issues in fish genetics, consider the differences between aquaculture and conservation.

In aquaculture, fish are reared under well-controlled conditions and can be selectively bred for faster growth, conformity, disease resistance, or other attributes, just like livestock. Some put-and-take operations like trout ponds, where fish of catchable size are quickly harvested, use fish that are selectively bred to perform well in artificial hatcheries.

Fish found in lakes and rivers are very different from those raised for aquaculture or a brief existence outside the hatchery. Nature – with its diseases, predators, competitors, unpredictable food supply, and variable physical conditions – doesn't provide the consistent, controlled conditions of a hatchery. No amount of selective breeding can improve the ability of fish to survive all of nature's challenges, which places that nice, plump hatchery fish at a disadvantage in the stream.

Genes – the DNA codes governing physical and behavioral traits – determine a fish's ability to cope with its environment. Wild fish are adapted to the conditions of natural waters; fish produced for aquaculture are adapted to hatchery conditions.

Wild fish adapt through natural selection, meaning some individuals survive and reproduce while others do not. Generally, the best-adapted fish pass on their genes to the next generation. Artificial selection occurs when humans do something to determine which individuals will survive and reproduce – for example, by choosing brood fish with a rapid growth rate.

The traits chosen in selective breeding for aquaculture are not necessarily the same traits favored by nature. In fact, many studies have shown that domesticated (artificially selected) fish perform poorly in natural waters. The concept of producing a "better" fish through artificial selection is valid for an aquaculture pond, but not for lakes and rivers.

It's difficult to produce hatchery fish with genes that will make them behave like wild fish. There's potential for domestication in any hatchery: A worker might inadvertently influence the traits for future populations by choosing to transport a quiet, calm fish for stocking, rather than handle a vigorous fish that is thrashing about. The trout that comes to the hatchery worker's shadow at feeding time is unlikely to flee in response to a blue heron's shadow.

Producing fish that do well in the hatchery is much easier than rearing wild-type fish. But the issue is not how well fish perform in the hatchery or how many fish are stocked – it's how well fish survive after stocking. It's better to produce fewer, smaller wild fish in the hatchery if they will survive better than domesticated strains. Remember: The number of fish stocked is not always a reliable indicator of the number surviving to be caught.

The ins and outs of breeding

Within a population of fish, individuals have different forms of most genes. This genetic variation produces fish that respond differently to the same environmental conditions. A population with lots of genetic variation can cope with change better than a population with little or no genetic variation. For example, if an entire population carries the form of a gene that makes the fish susceptible to a certain disease, an outbreak would cause a catastrophe for fish and anglers alike.

Hatchery fish can quickly lose genetic variation through inbreeding – mating with relatives. If four parents are used to produce 100,000 fish, the offspring will be on average much more similar to each other than if 50 parents were used to produce 100,000 fish. To preserve the long-term ability of the fish to evolve, hatcheries must maintain genetic variation by increasing the number of parent fish. The principle applies whether the parent fish are in the hatchery, or whether eggs and sperm are collected from wild fish.

Genetic variability doesn't only occur within a given population; it also occurs among populations. To recognize differences among populations of a single fish species, fish managers refer to a group of fish with distinctive biological characteristics as a "stock." In a "stock transfer," one group of fish is put into waters containing native fish of the same species, but of a different stock.

Is there any evidence that moving fish stocks around can be harmful? Little work on this topic has been done with warmwater fishes, with the exception of the largemouth bass. Many anglers in north central and southern states have sought to have Florida largemouths stocked in local waters. Compared to largemouth bass typically caught in most northern waters, these fish are huge! But it's important to consider two points: First, is there a sound biological reason to expect the fish to perform well outside their native range, and second, how would a transfer affect the resident bass?

To address these questions, researchers in Illinois studied bass taken from four locations – Florida, Illinois, Wisconsin and Texas. The fish were placed in small lakes with no outlets (so that they could not escape to other waters) located in Florida, Illinois, Minnesota and Texas, and their rates of survival and growth were monitored.

By the criteria of survival and growth, the local fish performed best in each of the four locations. What about reproduction? Again, the local stocks were as good or better than all others in each of the four lakes. Clearly, bass from another region could not improve the local fishery. In each case, nature had already produced the fish best adapted to local conditions.

The next phase of the study addressed the question of how non-native stocks affected the original residents. Although survival and growth of the introduced bass was often poor, the fish that did survive lived to reproduce. (All of the fish in the experiment had distinctive genetic marks that allowed the researchers to tell which stocks produced the offspring that appeared in each lake.) The researchers discovered that, just as with the adults, the offspring of local fish had the edge.

Fish mate randomly, so the native stock in each test lake didn't remain pure for long. Hybrids – the offspring produced from the mating of native and stocked bass – had poor growth and survival rates compared to the pure local stock. Unless all the introduced fish died before reproducing (as they did in Minnesota) the lakes eventually produced nothing but hybrids. The net result diminished the fishing quality of the test lakes by replacing native fish with relatively poorly performing hybrids. Geneticists use the term "outbreeding" to describe what occurs when genetically different populations combine to produce offspring with poor survival rates.

Due to the success of plant and animal hybrids for agriculture, it's easy to assume that hybrids are inherently "superior." This is a common misapplication of genetic theory. Performance is best judged in the context of the animal's environment. When we stock fish in lakes, rivers and streams, the proper time scale for evaluation must extend beyond one generation. A mule may show "hybrid vigor" but doesn't reproduce very well, just as a tiger muskie grows and fights well but fails to reproduce.

Another genetic mechanism was likely at work in the bass study. Over long periods of time, populations evolve groups of genes that work well together. Genes very finely tuned to control the timing of an embryo's development, for example, may not work well in combination with genes from other populations. Breaking up the "gene team" produces an inferior fish, and once that break occurs, it is essentially irreversible – unless you have a few thousand years to let nature sort it out.

In another study, biologists in Illinois, Minnesota and Iowa stocked fry from two walleye populations – one that runs up rivers to spawn and another that spawns in rocky areas of lakes – in a reservoir containing both river and lake habitats. Offspring from the river-spawning fish were found only in the river during the spawning run, while offspring of the lake-spawning fish remained in the lake. The study demonstrated that spawning behavior is inherited, not learned – so stocking fish in a water lacking appropriate habitat is unlikely to produce a self-sustaining population.

Both studies are worth noting for future stocking projects: If the wrong fish are stocked and decline after a year or two, we'll have wasted money that could have been spent on activities proven to help fisheries, such as habitat restoration. And, if even a few introduced fish manage to breed with the existing population, it is possible that the resulting population will be less suited to the lake than the one that was originally there.

Can we conserve genes?

Fish biologists use size, shape, life history characteristics and genetic information to define a fish stock. The genes partially reveal the degree of isolation one stock has from another. If one group contains genetic information different from that found in other groups, the conclusion is that the two groups have not interbred in the recent past. The challenge lies in determining the degree of difference, so that a stock selected for transfer will not put unique genetic resources at risk, and will not be placed in a geographic area to which it is not suited.

Opponents of policies promoting the conservation of genetic resources argue that such policies are simply impractical – so much damage has already been done, they say, it's too late to do anything about it. However, studies of intensively managed fishes such as brook trout in Eastern states demonstrate that stock differences remain despite a history of stock transfers.

Another argument is that hatcheries would be faced with the impossible task of maintaining scores of distinct stocks. Before this issue can be addressed, we need to look at the current stocks of managed species, define the geographic boundaries in which they survive best, and review the genetic impact of past stock transfers. Wisconsin DNR fish biologists are studying the genetics of stocks from Wisconsin, Minnesota, and Illinois to find the best ways to conserve genetic resources.

From the sport angler's point of view, stocking has been a success. Many anglers insist on stocking, even when it is not warranted biologically. Stocking can be a positive, beneficial way to manage fish; however, many lakes produce an excellent fishery without it. If stocking is done without regard for genetics, it can damage the future of fishing.

No single tool, including stocking, is effective or appropriate in every case. Through groups like the Walleye Committee – made up of DNR biologists and representatives of many other organizations with a stake in the future of Wisconsin's fisheries – we hope to develop reasonable guidelines for stocking. The newly renovated Spooner and Woodruff hatcheries will have facilities to better address genetic concerns.

We need to protect the genetic differences occurring within and among fish populations today if we are to have thriving fisheries tomorrow. Once those genetic resources are lost, they are irreplaceable. By maintaining the integrity of distinct fish stocks, the fish that survive will be those best adapted to the waters in which they live. This is the most biologically sound – and cost-effective – approach for long-term fisheries management.

Martin Jennings leads the Northern Lakes Fisheries Research Group for DNR in Spooner, Wis.