Tahoe Research Group : Annual Report

Impact of Lake Trout Predation on Prey Populationsin Lake Tahoe:

A Bioenergetics Assessment

G.P. Thiede¹, D.A. Beauchamp¹ M.W. Kershner¹, B.C. Allen², and C.K. Gemperle^1
1

Utah Cooperative Fish and Wildlife Research Unit, Department of Fisheries and Wildlife, Utah State University, Logan, UT 84322-5255

²Tahoe Research Group, Department of Environmental Science & Policy, University of California, Davis, CA 95616

Introduction

Since their introduction over 100 years ago, lake trout have become established as the top predator and primary sport fish in Lake Tahoe. The lake has since undergone major changes in trophic structure with introductions of kokanee (Oncorhynchus nerka) in the 1940s and mysids (Mysis relicta) in the 1960s (Richards et al. 1975; Goldman et al. 1979). While these introductions have altered lake trout feeding ecology (Morgan et al. 1978; Richards et al. 1991), quantitative assessment of their influence on lake trout behavior, and consequently, the Lake Tahoe food web, has not been conducted. Similarly, these food web changes may affect the native fish assemblage and kokanee populations, which may have important ecological implications for the Lake Tahoe basin.

Changes in Lake Tahoe’s food web also have major implications for resource managers seeking to balance human development in the basin with aesthetics, water quality, and the sport fishery. The last study to evaluate the status of the sport fishery in Lake Tahoe was initiated in 1960 (e.g. Cordone and Frantz 1966, Cordone et al. 1971). Thus, there is need for current knowledge on the population dynamics of lake trout and their trophic interactions with the associated food web to better understand and manage the production potential of this fishery.

In this study, we quantified seasonal predation rates by lake trout on individual prey types to determine if predation influenced prey population dynamics in Lake Tahoe. Specifically, we investigated how prey availability affects growth, diet, and distribution of lake trout, and conversely, how lake trout predation affects prey population dynamics.

A bioenergetics model was used to assess the effects of predation by lake trout (Salvelinus namaycush) on prey populations in Lake Tahoe, California-Nevada. This model was combined with seasonal diet, growth, and thermal experience of lake trout during 1992-1995 to estimate seasonal consumption by the lake trout population. These results were compared to abundance and biomass of prey populations.

Results

From spring 1992 through winter 1995, mean total annual consumption amounted to 460 metric tons (tonnes) of mysid (Mysis relicta), 140 tonnes of native fishes (suckers, minnows, and sculpin), 6.7 tonnes (0.13 kg/ha) of kokanee (Oncorhynchus nerka), 22 tonnes of lake trout, and 100 tonnes of other prey, mostly signal crayfish (Pacifasticus leniusculus). Consumption of kokanee varied both seasonally and annually. Predation removed an estimated 22,900 adult kokanee, or 58% of potential kokanee spawners in autumn 1992 and 33,700 kokanee or 21% of the potential spawners in autumn 1994. Consumption estimates of mysids remained similar in all three years despite considerable seasonal variability in mysid abundances. Predation removed an average of 15% (range: 7-32%) of the mysid standing stock each season. Each season, predation removed 20-48% of native adult fish biomass. Consumption rates on native fishes and signal crayfish tracked changes in the availability of native prey fish and crayfish. In general, predatory demand by lake trout represented significant, but likely sustainable, fractions of the prey population biomass in Lake Tahoe.

Conclusion

In oligotrophic systems, predation can play a significant role in determining food web structure, particularly through its influence on available forage; however, this is not always the case. Our study indicates that lake trout can have substantive effects on significant components (but not all; i.e., mysids) of the forage base (e.g., kokanee, native prey fish, signal crayfish) in Lake Tahoe, despite its status as a highly oligotrophic system. In contrast, predation is often the principal cause of mortality for cyprinids and other forage fish. This appears to be the situation in Lake Tahoe, where consumption by lake trout accounted for 20-48% of seasonal native fish biomass during the three study periods. Overall, there is no indication that current lake trout populations (>250 mm) can exert sufficient predation pressure to control mysid population size in Lake Tahoe.

Overall, this study demonstrated that lake trout predation was differentially important for populations of individual prey types. Lake trout appear to be responsible for a large proportion of native prey fish mortality, possibly contributing to substantial declines in native fish abundance during the past 30 years. Similarly, predation by lake trout may have significant effects on kokanee population fluctuations, as well as on their own recruitment (via cannibalism). While lake trout had little apparent impact on mysid biomass, mysids were the most important prey for lake trout and likely contributed to high growth in this lake trout population. Thus, mysids remain the dominant strong interactor in the Lake Tahoe food web, given their well-documented influence in this lake (Richards et al. 1975, 1991; Morgan et al. 1978; Goldman et al. 1979), and the fact that lake trout, the system’s top predator, have little effect on their population dynamics.

References

Cordone, A.J., and T.C. Frantz. 1966. The Lake Tahoe sport fishery. California Fish and Game 52:240-274.

Cordone, A.J., S.J. Nicola, and P. Baker. 1971. The kokanee salmon in Lake Tahoe. California Fish and Game 57:28-43.

Goldman, C.R., M. Morgan, S.T. Threlkeld, and N. Angeli. 1979. A population dynamics analysis of the cladoceran disappearance from Lake Tahoe. Limnol. Oceanogr. 24:289-297.

Morgan, M.D., S.T. Threlkeld, and C.R. Goldman. 1978. Impact of the introduction of kokanee (Oncorhynchus nerka) and opossum shrimp (Mysis relicta) on a subalpine lake. Journal of the Fisheries Research Board of Canada 35:1572-1579.

Richards, R.C., C.R. Goldman, T.C. Frantz, and R. Wickwire. 1975. Where have all the Daphnia gone? The decline of a major cladoceran in Lake Tahoe, California-Nevada. Internationale Vereinigung für theoretische und angewandte Limnologie Verhandlungen 19:835-842.

Richards, R.C., C.R. Goldman, E. Byron, and C. Levitan. 1991. The mysids and lake trout of Lake Tahoe: a 25-year history of changes in the fertility, plankton, and fishery of a subalpine lake. American Fisheries Society Symposium 9:30-38.