The Role of Whirling Disease in Structuring the Greenback Cutthroat Trout Population in Zimmerman Lake
Introduction: Factors influencing the structure and dynamics of native greenback cutthroat trout populations in high elevation habitats are poorly understood. Information is needed for managers to make sound decisions regarding reintroduction and recovery efforts of this threatened species. Whirling disease has become established throughout Colorado in many trout habitats and has severely impacted the size structure of trout populations. Water temperature has been documented to limit the perseverance of greenback cutthroat trout in high elevation habitats. However, the effects of whirling disease on the dynamics of greenback cutthroat trout populations and the mechanisms of water temperature on recruitment of trout have not been studied in detail. The purpose of this study is to provide insight into the effects of whirling disease and water temperature on the reproductive success, recruitment, and production of greenback cutthroat trout in Zimmerman Lake and Zimmerman Creek. The study will help managers evaluate the appropriateness of high elevation habitats as potential reintroduction and broodstock sites for greenback cutthroat trout.
Objectives: To (1) determine if whirling disease has become established in the lake and creek; (2) estimate the extent of infection; (3) evaluate the impacts of whirling disease on survival; and (4) evaluate direct and indirect effects of water temperature on distribution and recruitment of greenback cutthroat trout in Zimmerman Lake and Creek.
Progress: Approximately 450 greenback cutthroat trout live in Zimmerman Lake. All fish were > 175 mm in total length, spawn in the inlet creek, and appear to be remnants from stockings in 1996, 1998, and 2000. Growth was initially rapid after stocking, but has declined over time and is dependent on the year of stocking and strain of fish stocked. Cold water temperatures caused fish in the lake to spawn late in the spring, which led to small fry emerging late in the fall. Approximately 6811 fry migrated from the inlet creek into the lake in the fall of 2002, but only 72 fry migrated in 2003 and 16 in 2004. Shorter growing seasons and cold-water temperatures lead to substantial reductions in the numbers of emerging fry. After the spawning period, adult trout were distributed over 97% of the lake area having depths > 6 m. However, fish became concentrated near the inlet creek in 2002 when fry dispersed from the spawning stream. We did not observe adult fish concentrating near the inlet in 2003 or 2004. Fish densities in the lake were generally low compared to other high elevation lakes and body condition was generally better than average for greenback cutthroat trout, despite feeding primarily on small zooplankton during the summer. No trout in the lake or inlet creek have been positive for whirling disease, but twelve trout in the outlet creek downstream of the lake were heavily infected. TAMs have not been observed in our filtrations and oligochaete densities are low. Most oligochaetes were found within 20 m of the inlet creek. Our results suggest that whirling disease is not affecting recruitment of greenback cutthroat trout in Zimmerman Lake. Production of greenback cutthroat trout is potentially limited by interaction between the length of growing season and fry predation by adults.
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