Fish Stocking and Mountain Yellow-legged Frog Declines in Sierra Nevada High Mountain Lakes

Kathleen R. Matthews US Forest Service Pacific Southwest Research Station, Berkeley, CA

Roland A. Knapp University of California Sierra Nevada Aquatic Research Lab, Mammoth Lakes, CA

All lakes at the upper elevations of the Sierra Nevada were historically fishless, but the majority of large lakes now have one or more species of non-native trout. To improve our understanding of the impacts of non-native trout on native vertebrate and invertebrate species in Sierra Nevada lakes, we compared the aquatic fauna in lakes in the John Muir Wilderness (JMW) to that found in lakes in Kings Canyon National Park (KCNP). These two areas are very similar except with regard to fish stocking. Fish stocking has historically been much more intensive in the JMW than in KCNP and most fish stocking in KCNP was terminated in the late 1970's while this practice continues in the JMW. Therefore, comparisons between lakes from these two areas allowed us to separate the effects of non-native trout from other environmental factors (e.g., acid deposition, climatic conditions, pesticide drift) in altering the distribution of native aquatic species especially amphibians.

Between 1995 and 1997 we surveyed approximately 2200 lakes (1079 lakes in the JMW and 1083 lakes in KCNP. The results of our study indicate that fish distributions have changed dramatically in high elevation lakes throughout the Sierra Nevada. In the areas we surveyed, 316 of 1,079 lakes (29%) in the JMW have introduced trout compared to 207 of 1,083 (19%) in KCNP. The percent of lakes with trout increases markedly when only larger lakes are considered, since small lakes and ponds have typically not been stocked with trout. For lakes larger than 1 ha, 80% contain trout in the JMW but only 40% contain trout in KCNP. We found many more mountain yellow-legged frog populations in KCNP compared to lakes in the JMW. Thirty-five percent of the lakes in KCNP (379 of 1,083) contained mountain yellow-legged frogs compared to only 5% of the lakes in the JMW (56 of 1,079). In addition, the total number of frogs observed in KCNP was much higher: a total of 69,638 adult, subadult, and larval mountain yellow-legged frogs were found in KCNP versus 8,819 adults, subadults, and larvae in the JMW. The mountain yellow-legged frog is particularly sensitive to trout introductions because it is highly aquatic in all life stages, and has an unusual natural history in which tadpoles overwinter 2-3 times before metamorphosing into subadult frogs. This overwintering requirement restricts successful breeding to bodies of water that do not dry up in the summer or freeze solid in the winter. These are the same bodies of water into which trout have most commonly been introduced.

In conclusion, our preliminary results indicate that current levels of fish stocking have had an adverse impact on the mountain yellow-legged frog. Apparently, the lower historical intensity of fish stocking in KCNP and its termination in the late 1970's provided numerous fishless lakes that serve as refuges for mountain yellow-legged frogs. These refuges are nearly absent in the JMW since most of the larger lakes now contain trout. As a result, the mountain yellow-legged frog is now absent from the majority of sites in the JMW where it previously occurred, and if no steps are taken to reverse this decline is likely to be listed under the Endangered Species Act (ESA) in the future. Fish and frog distributions in KCNP could serve as a useful model for future restoration strategies in the JMW. Our study indicates that recovery of the mountain yellow-legged frog in the JMW will require that some currently-stocked lakes be allowed to revert to a fishless condition. If such lakes are selected carefully, the recovery of the mountain yellow-legged frog could be accomplished with only minimal effect on the recreational fishery. Steps taken now to restore the mountain yellow-legged frog to a subset of formerly-occupied habitat could dramatically reduce the likelihood of this species being federally listed in the future. To this end, we recommend that adaptive management projects be implemented in the JMW to evaluate the effect of reducing trout stocking on resident trout populations and native biota.

Back to Program