How Fish Stocking Works

Let's say that we're considering whether to stock a local river with­ trout. We want our trout to survive and thrive, so first we should consider what type of environment we're tossing them into. Could these waters support trout? Is there enough food and plant cover? And if so, why aren't there trout swimming about right now? It's important to consider whether there's some reason that specific populations can't survive in certain bodies of water.

One of the main factors related to the new species' success is the fish that already call those waters home. To figure out what's down below, local fishermen's catches should be monitored; this information will provide a rough idea of how many fish live in the area, what kinds of fish there are and if they're in good health. After all, you don't want to stock new fish only to have them catch a disease from the existing fish (the fish that are to be stocked should also receive a health check). Taking into account the local competition also helps determine whether the new fish will be able to have adequate access to food.

Access to food brings us to our next issue: overstocking. If the local river is already home to many fish, then it may not be able to support much more. Each body of water has a carrying capacity, which is related to how much life that area can sustain. Stocking the water above and beyond the given capacity does damage to all the fish as they compete for food, plant cover and other resources.

If the water and all of its inhabitants check out, then it's time to plan a fish stocking. You can't just go throwing fish into the water whenever you darn well please. The best time for fish stocking is usually during seasons in which the water temperatures are low and there's a higher quantity of oxygen in the water [source: Environment Agency]. Fish need less oxygen at that time, and they also won't have to fend off quite as many parasites and disease-causing pathogens during those cooler temperatures. These conditions create the most conducive environment for the success of the new fish. Fish stocking may occur throughout that time period, because studies indicate that releasing small numbers of fish at regular intervals may portend greater success for the new species than one large dump of fish [source: Cowx].

A carefully evaluated fish stocking can reduce the risks associated with fish stocking. On the next page, we'll dive into these risks and examine why some people would prefer not to give these hatchery fish new homes.

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It's important to consider caref­ully whether fish stocking is necessary and plan out all the steps of the implementation, because once the fish are stocked, it's nearly impossible to reverse the process. To further compound the difficulty of the planning process, however, you have to consider that success of a fish stocking will mean different things to different people. To a recreational fisherman, having a record-breaking fish at the end of the line is a victory, while an environmentalist may bemoan the impact that record-breaking fish had on the existing populations. And sometimes everything dies no matter what you do.

One impact to the existing populations relates to genetics. Say we added one species of trout to waters that already had another kind of trout. If the two different types of trout were to mate, the genetic purity of each line would be lost, and there would be less genetic diversity in the water. In 1959, stocking was banned at Yellowstone National Park for this reason; the cross-breeding was erasing the uniqueness of existing fish populations [source: Wuerthner].

But beyond a seeming inability to fend off the amorous advances of the stocked fish, the native fish face other struggles as well. Stocked fish consume resources that the native fish need as well. But to add insult to injury, stocked fish may often end up dying, so not only do they eat and run, the native fish are left with nothing and eventually die as well.

The native fish aren't the only ones dying -- fish stocking can have an impact on the greater ecosystem as well. For example, when gulls in the Great Lakes area were studied after a fish stocking, they were found to have consumed more terrestrial food items, including garbage. This is likely because the stocked fish preyed on the native fish that otherwise would have been the birds' meals [source: Ecological Society of America]. One study indicates that stocked fish in the Pacific Northwest spread a disease that resulted in a 15 percent increase in amphibian embryo death [source: Meadows].

For these reasons, not everyone is on board with fish stocking. While fishermen enjoy an easy hunt, some say that there are plenty of fish available naturally for an enterprising fisherman without the government and fisheries getting involved [source: Ingold]. Indeed, if stocking fish ends up killing off the native populations, then the stocking ultimately may ruin the practice altogether. Fishing restrictions are sometimes considered as an alternative to fish stocking.

But is there an alternative to fish stocking for a guy who loves his rod and reel? After banning fish stocking, Yellowstone has had great success with a catch-and-release method. Park officials have found that native fish are caught and released approximately 10 times per season; the stocked fish were only caught once and were generally smaller compared to the natives [source: Spooner]. Some other alternatives to stocked fish rely on the principle, "If you build it, they will come." These methods involve making waters more naturally attractive to fish by promoting aquatic plant growth (thus providing more food) and creating shallow, shaded areas for spawning (thus providing a love nest for the fish).

For more on fishery management, see the links on the next page.

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