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Yosemite Nature Notes 46(2) (1977)

The Effect of Introduced Fish on the Amphibian Life in Westfall Meadow

by Douglas Yoon

Many of the higher drainage basins of the Yosemite region were extensively glaciated during the Pleistocene Age, resulting in the elimination of the aquatic vertebrates in these areas.

Steep waterfalls have prevented the natural re-invasion of fish into these drainages, whereas the amphibians, being less restricted to water, have quickly re-invaded these same areas. One such drainage basin is Bridalveil Creek which empties over the south rim of the Yosemite Valley. One particularly interesting spot along this drainage is Westfall Meadow, a large subalpine meadow (about 200 acres at 2,100 m.) through which runs Westfall Creek. This creek is rich in its aquatic life, and for this reason, I have lead nature walks here for the last three summers. One readily finds members of most major phyla of animals (Chordata, Arthropoda, Anellida, Mollusca). The creek meanders through the meadow producing a string of connected ponds which, by late June, become physically isolated as a result of the lowering of the water table. Generally, the larva and adults of the Mountain Yellow-legged Frog (Rana muscosa) and smaller numbers of the larva of the Yosemite Toad (Bufo canorus) and Pacific Treefrog (Hyla regilla) inhabit shallow ponds in the upper (south) end of the meadow. Trout were found in the deeper ponds in the lower (north) end of the meadow.

Over the last four summers the ponds either contained trout and no tadpoles or tadpoles and no trout. Only one pond this year contained tadpoles and trout, and in this case the pond was large (about 12 ft. long - 3.6 m.) and the number of tadpoles very small.

This observed distribution has two possible causes; the first is related to different habitat preferences. Trout prefer deep, well-aerated ponds or streams while tadpoles prefer shallow ponds, rich in algae, which may often become stagnant. The second cause is related to predation. Trout are known to eat almost any small animal life, including tadpoles. In the field, one rarely observes predation of tadpoles by trout, presumably because the supply of tadpoles (or eggs) on an isolated pond is rapidly depleted. In addition, random chance may influence the distribution of tadpole and fish in Westfall Meadow. Large trout have been found in shallow ponds (2.5 ft. deep - .75 m.) and are curiously missing in some of the deeper ponds (4 ft. deep - 1.4 m.). Apparently, the distribution of these large trout depends somewhat upon which ponds they were trapped in when the water table dropped.

As far as is known, Westfall Meadow contained no trout when first discovered and has never been stocked directly. It is reasonable to assume that the trout found in Westfall Creek have migrated one mile (1.6 km.) upstream from Bridalveil Creek which was stocked between 1892 and 1950 with Rainbow, Eastern Brook, Cutthroat, Brown and Golden Trout. In addition, Ostrander Lake, which feeds Bridalveil Creek, was stocked from 1892 to 1967.

By fish-stocking we have created an unnatural situation at Westfall Meadow and as well in many other Sierran subalpine meadows. It is valid to ask what effect the introduced fish have had on the ecology of these meadows.

At Westfall Meadow, the trout probably have eliminated the tadpoles from some ponds in which they might otherwise have been found. However, the effect of fish introduction has not been a simple elimination of the amphibian species, but rather a subtle alteration of their population dynamics. Fortunately, tadpoles continue to survive in stagnant, drying ponds unsuitable for fish.

It is interesting to note that Westfall Meadow has been very dry this year due to light winter snow, and lower ponds have become stagnant killing the trout in them, some up to 10 inches in length. By early summer, many of the upper ponds had dried up completely, and by early August, were devoid of tadpoles and all large fish had died.

This would seem to be a happy ending to the frog story because in the coming spring, frogs may be able to breed unmolested in ponds eliminated of fish. However, small trout still survive in some of the lower ponds and these in time will grow. Additionally there is an influx of trout in Westfall Creek in the spring when the water table is high.

While the rate of influx is unknown, trout can overwinter in some of the small Sierran ponds and large trout have been seen in Westfall Meadow for the last five years. If we fail to see large trout in the meadow next spring, this would strongly imply that the influx of trout into the meadow is low.

Westfall Meadow is an area of prime interest. Here, we have an opportunity to observe at first hand the nature of a species’ response to a new mortality factor and the community’s response to an aggressive introduced species.

Have we, by introducing exotic fish into glaciated stream valleys, merely hastened a process that would eventually have occurred, or have we grossly upset a balance that took thousands of years to establish?

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