After nearly eight years, a project started in 2008 by BLM-Utah, Utah-DWR and Trout Unlimited to continues to successfully increase native fish populations in northern Utah. Image Credits: Wikimedia Commons.
The result was therefore not simply a case of fish numbers rising after a stocking exercise. After the brown trout were eradicated, the researchers introduced relatively small numbers of cutthroat trout eggs and young fish. They introduced a small number of eggs and young fish of cutthroat trout into the ecosystem.
The long-running experiment took place in Right Hand Fork, a tributary where non-native brown trout had become established alongside the native Bonneville cutthroat trout, according to the US Geological Survey (USGS) . Researchers had monitored the fish community for a decade before beginning the removal programme. The four-year effort included two years of mechanical removal followed by two years of chemical treatment. Within six years of the eradication, the native population had recovered to close to the stream’s estimated carrying capacity. After the removal of brown trout from the ecosystem, however, the scientists did not leave the recovery process to chance. In this way, the experiment evaluated the influence of both factors – elimination of the competitor and the introduction of a new source population for the native fish. Results came fast. Scientists calculated the growth rate and the carrying capacity of the population based on monitoring results during several years. After six years of eradication and seeding of brown trout population reached the carrying capacity of the stream. At least four age classes existed, including adult fish suitable for anglers. A field experiment published in Transactions of the American Fisheries Society examined brown trout’s effects on native cutthroat trout at both individual and population levels, providing evidence that interactions between the two species can influence the native fish population.
The recovery was also consistent with earlier research showing that brown trout can affect the performance of native Bonneville cutthroat trout. The restoration removed a competing non-native population and then allowed the native fish to establish themselves in habitat that had previously been occupied by brown trout.
Because the model showed that there was a 95% chance that the cutthroat trout population would have attained at least 90% of its carrying capacity after 10 years of brown trout eradication, this is. The modelling of the cutthroat population by the researchers gave an optimistic long-term outlook. This does not mean all the streams will show a similar trend. The study location had healthy habitat, and the researchers stated that removal of invader species could be influenced by the size and status of the ecosystem, among other factors. However, they also noted that such small tributaries as the Right Hand Fork were constrained by size. Any larger benefits might result from their ability to serve as source populations that enable native trout to thrive in the downstream reaches of a river system. The research provided an informative illustration of how the native fish might fare in conditions when they have the opportunity for restoration. Four years of brown trout elimination and six years of monitoring resulted in the population of Bonneville cutthroat trout being on par with the maximum capacity of the stream to support them. The restoration did not take several decades but was fast.