Effects of invasive piscivorous Brown Trout (Salmo trutta) on native fish populations of the Jemez River, New Mexico
Invasive predators can harm native fish populations in both marine and freshwater systems. Brown trout (Salmo trutta) are among the most harmful invasive species and have been introduced in lakes and rivers throughout the world. Brown trout often feed on or compete with native fishes for food or space. The predatory effects of invasive Brown trout can also be altered by the presence of non-native prey since such prey can possibly alleviate or intensify predation on native species. Invasive Brown trout and Northern crayfish (Faxonius virilis) have established stable populations in many locations in streams of northern New Mexico (USA). The interactions of trout and crayfish may affect native fish species such as Rio Grande chub (Gila pandora) and Rio Grande sucker (Catostomus plebeius); both are species of conservation concern across their native range. The effects of invasive predators and prey on Rio Grande chub and sucker have not been studied. Therefore the goal of my thesis was to study the predator-prey interactions of trout, crayfish, and native fishes. I tested the general hypothesis that the presence of invasive Northern crayfish affects predation on native fish populations through a shared predator. Specifically, I addressed three main questions. First, were Brown trout eating native fishes and non-native crayfish? Second, did the presence of crayfish alleviate or intensify predation pressure on native species? Finally, what were the patterns of ontogeny in Brown trout? To test my hypothesis, I sampled fish populations and collected 2,499 Brown trout diets in streams with and without crayfish during 2021. I evaluated Brown trout diet composition, predation rates, the influence of prey density on predation rates, and the sequence of predator-prey interactions (which prey species is consumed first, native or invasive?). Brown trout were documented eating Rio Grande chub and Rio Grande sucker as well as Northern crayfish. Overall piscivory (% of diets) varied from 0-10.6% across sampling locations. I found that Brown trout consumed crayfish before consuming fish. Individual Brown trout were more likely to have crayfish in their diets than prey fish. However, diet compositions revealed the consumption of Northern crayfish did not exclude fish from being consumed suggesting the alleviation of predation on native fishes is not occurring. Additionally, individual Brown trout in streams containing crayfish were more likely to consume prey fish suggesting interactions between Brown trout and Northern crayfish are negatively affecting native fish species. Collectively my findings provide support for my hypothesis and indicate that Brown trout and Northern crayfish are negatively affecting native fish populations through apparent competition. I recommend the removal of Brown trout and Northern crayfish to reduce predation and protect threatened populations of Rio Grande chub and Rio Grande sucker.