Population dynamics and life history characteristics of Rio Grande cutthroat trout in the upper Rio Costilla watershed, New Mexico
Principle Investigators
Dr. Zachary Klein, New Mexico State University
Dr. Carter Kruse, Turner Institute of Ecoagriculture
Graduate Student
Maret Smith-Miller, MS candidate
Rationale.
Rio Grande cutthroat trout currently occupy about 12% of their historical distribution. To restore Rio Grande cutthroat trout, natural resource management agencies have instituted various conservation efforts, including habitat restoration, species reintroductions, and removal of competing nonnative fish. These conservation efforts have been successful as evidenced by a 6% increase in the number of populations. However, conservation questions remain, including the value of high mountain lakes in conservation planning. Rio Grande cutthroat trout have been stocked into previously fishless lentic systems throughout their distribution. For instance, there are currently 16 lakes in the upper Rio Costilla watershed at Vermejo Park Ranch that now support or could potentially be stocked with Rio Grande cutthroat trout. These systems can serve as important refugia populations. Additionally, many of the lakes provide important fisheries that help to increase anglers’ appreciation for the species and support for conservation efforts. Unfortunately, Rio Grande cutthroat trout populations in lakes have received relatively little attention. Baseline population demographics and dynamics data for lentic populations are largely lacking. These data are necessary for understanding how lentic populations function and for guiding management actions. Furthermore, population-level data can help to identify potential threats (e.g., unsuitable habitat, overexploitation) to the persistence of these critically important populations.
Outcomes.
In 2022 and 2023, 13 high-mountain lakes on Vermejo Park Ranch were sampled from May to June with gill nets. Relationships between vital rates and habitat features were evaluated using nonmetric multidimensional scaling.Eight of 13 high-mountain lakes supported RGCT. Although no statistically significant relationships were identified between vital rates and habitat features, general patterns were observed. Rio Grande Cutthroat Trout mean length-at-age-3 in each system varied from 187 to 240 mm. Mean length-at-age-3 was largest in high-mountain lakes with relatively high morphoedaphic index values (> 22). Mean annual survival rates varied between 0.28 (SE = 0.08) and 0.53 (0.05) for age-3 and older RGCT. Annual survival was higher and less variable in warm (minimum annual temperature > 1.00°C at 75% depth) and deep (> 1.35 m) high-mountain lakes that occurred at low elevations (< 3,100 m). Mean annual recruitment variability index (RVI)values varied between -0.09 and 0.81 and were positively associated with lakes that had high-quality inlets.Population demographics and dynamics data were collected from a subset of six lakes and used to construct astochastic, age-structured, Leslie-matrix model that evaluated the response of a hypotheticalRGCT population to stocking regimes. Thirty-one scenarios were modeled with varying stocking frequencies (1-, 2-, 3-, 5-, and 10-year intervals), age compositions (age-0 and adult RGCT), and densities. In the absence of management actions, RGCT are expected to be quasi-extirpated (≤ 5 mature females) within 25 years. However, stocking 10 adults at 2-year intervals resulted in a 97% reduction in the probability of quasi-extirpation. Additionally, elasticity analysis indicated that survival of age-0 through age-2 RGCT had the greatest influence on population growth of any of the estimated parameters.Collectively, these results suggest that high-mountain lakes in the upper Rio Costilla watershed can support RGCT populations. Lake elevation, water temperature, water depth, inlet quality, total dissolved solids, and morphoedaphic index appear to be important to the growth, survival, and recruitment of RGCT. Stocking 10 adult fish every two years is an effective approach for conserving RGCT in high-mountain lakes. These results provide baseline data on RGCT and may help inform translocations and potential restoration that will likely benefit the species. However, they also highlight the fact that information on RGCT in high-mountain lakes is lacking, and additional research is likely needed to fully understand the value of high-mountain lakes as a conservation tool for the species.
Smith-Miller, M.H. 2024. Population dynamics, habitat use, and life-history characteristics of Rio Grande cutthroat trout in the upper Rio Costilla watershed, New Mexico. MS Thesis, New Mexico State University, Las Cruces, New Mexico.
