Exotic brown trout impacts : the case of a “novel” predator

1. Exotic brown trout impacts: the case of a “novel” predator Gary P. Thiede, Utah State University Phaedra Budy, US Geological Survey, Utah Cooperative Fish and Wildlife Research Unit Christy Meredith, Utah State University, now with US Forest Service W. Carl Saunders, Utah State University

2. Identifying extent and degree of effects of invasive species is critical • Understanding factors that determine success • Ability to effectively prioritize control efforts Often, effects remain poorly understood and overlooked when invader has “value”

3. What makes a successful invasive species ? • Depends on invader traits • Depends on characteristics of environment • Broad feeding niche • + Choose high-energy prey • + Establish high densities • ____________________ • “ SUCCESS ”

4. Brown trout • Wide array of life-history expressions and plasticity • Occupy wide niche for a trout • Optimal temps 12 – 18 oC; however, max thermal tolerance near 30 oC • Perform greatest in prey-rich streams, but perform well in low productivity streams • And... extremely high propagule pressure (introduction effort)

6. Objective Determine the predatory impact of this exotic predator on the native fish community in the Logan River, Utah

7. Study area Stronghold for Bonneville cutthroat trout Cold, snowy winters Hot, dry summers Spring-snowmelt floods (15.7 cms) Base flow @ approx 2.8 cms Average summer stream temperatures range from 9.2 - 12.1 oC Summer diel fluctuations are as high as 8.8 oC Long-term monitoring since 2001

8. Area of Detail Salt Lake City UTAH N 0 2 4 6 8 1 0 Km 2000 m 1350 m

9. Methods Field experiments: McHugh, Saunders, et al. Laboratory experiments: Saunders, Burbank Field evaluations:Wood, Meredith, Saunders, Gordon, Hafen, et al. Long-term monitoring: Budy, et al. Diet analyses: de la Hoz Franco, McHugh, VanDyke, Wood, Meredith, Gordon, et al.

10. Methods Field experiments: McHugh, Saunders, et al. Laboratory experiments: Saunders, Burbank Field evaluations: Wood, Meredith, Saunders, Gordon, Hafen, et al. Long-term monitoring Diet analyses: compilation of findings from 2001 - present

11. Methods Long-term monitoring Collect fish during annual e-fishing surveys Diet analyses Sample fish at all elevations: take guts, gastric lavage In laboratory: identify, weigh, and measure prey Using bioenergetics model, estimate predatory impact (how many prey fish can brown trout consume)

12. Results Very high densities (some of the highest in the universe) High growth rates High condition High survival

13. Results mice snakes hot dogs

14. Gape limit Results Capable of eating large prey fish Eat prey fish smaller than gape limit

15. No sculpin present Consume sculpin where present Cannibalism Consume cutthroat

16. Temperature Growth Bioenergetics modeling Diet Output = biomass of prey consumed

17. Results Bioenergetics modeling of consumption: An average age-2 brown trout (18 – 26 cm; 7 – 10 inches) consumes 13 sculpin a year Expanded by abundance, that is up to 2500 sculpin in a single stream reach

18. Results Distinct negative relationship between cutthroat and brown density Not so clear with sculpin and whitefish density(between dams)

19. Considerations Sculpin eat small brown trout and likely eat brown trout eggs Are these naturalized, non-native brown trout fulfilling the niche of extirpated fluvial cutthroat trout ?

20. Comparison: diets of stream-dwelling brown trout in native vs exotic locations

21. Comparison: maximum size of brown trout in native vs exotic locations

22. Summary One of the world’s most successful and “deleterious” invaders, brown trout are superior competitors are highly aggressive are cannibalistic prey on native fish consume thousands of sculpin are restricted to lower elevations ... but for how long ? are not as benign as previously thought

23. Very preliminary findings After three years of removal work “between the dams”, where all species co-occur Sculpin abundance continues to fluctuate widely (likely due to sculpin behavior, removal techniques) Stocked rainbow trout numbers have increased Whitefish abundance has increased ( first sign of whitefish recruitment in over 10 years ? ) Cutthroat trout abundance has increased

24. Management Anglers love them Science shows they impact native fishes Managers must balance the trade off

25. Acknowledgements • Utah DWR • Sport Fish Restoration • USGS Utah Coop Unit • S.J. Quinney Foundation • NSF Advance Program • Ecology Center @ USU • ICRRR @ USU • Fish Ecology Lab @ USU • Pete McHugh • Jeremiah Wood • E. de la Hoz Franco • Erin Van Dyke • Wes Gordon • Konrad Hafen • Nora Burbank • Dan Weber

26. AFS Policy Statement –#15 Introduction of Aquatic Species: ...Documentation of predation by introduced species on native species serves as the most definitive example of impacts on communities...