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.