Native trout species such as bull trout, westslope and Yellowstone cutthroat trout may exhibit three distinct life history patterns that can encompass sizeable geographic areas. Resident fish reside entirely in natal streams, while fluvial and adfluvial fishes out-migrate from natal streams as juveniles to larger rivers (fluvial), or lake environments (adfluvial) growing to maturity before returning to spawn.
Fish can reach sexual maturity faster in lakes than rivers and streams due to increased productivity. The spatial distribution and diverse habitat needs of native fluvial and adfluvial fishes emphasize the importance of sustaining watershed health and habitat connectivity. It also highlights the vulnerabilities of migratory fishes to anthropogenic influences throughout their habitat range. Influences affecting habitat, access to habitat, water quality and species composition (competition, hybridization or predation with non-native species) throughout a watershed ultimately affect the health and viability of native fish populations in streams, rivers and lakes.
Native and Non-Native Species
Many non-native fish and aquatic species have been legally introduced by agencies into our lakes and rivers to enhance angling opportunities. In fact, early fisheries management in the United States centered on the introduction of non-native game fish. Some planned introductions have benefited angling opportunities in certain waters, while others have had unexpected and damaging results. Introduced non-native species can cause problems because they are often more aggressive and more adaptable to ecosystem changes. Often, they reproduce more rapidly than native species giving them a competitive edge.
The introduction of Mysis shrimp into the Flathead Watershed is one example of an unexpected and drastic food web change caused by an introduction. This introduction was intended to provide a better food source for kokanee salmon—a popular fish amongst anglers. Instead of boosting kokanee salmon, Mysis shrimp are the primary cause for the collapse of that salmon population in Flathead Lake and gave rise to a fish community dominated by non-native lake trout and non-native whitefish.
Our understanding of the ecological impacts of introduced species has continuously advanced. Legislation, like the Endangered Species Act, now places a high value on native species and mandates their protection. Today, fisheries managers carefully weigh the costs and benefits associated with new species introduction or the continued stocking of non-native fishes to our waters.
In Montana, fisheries managers emphasize preserving and enhancing wild fish populations. They also give special consideration to populations of native fish. Montana is home to many valuable native species, including white sturgeon, mountain whitefish, grayling, bull trout and Montana’s State Fish—the black spotted (westlsope) cutthroat trout.
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