Non-native fish introductions and the decline of the mountain yellow-legged frog from within protected areas

Summary

The mountain yellow-legged frog Rana muscosa has seen huge declines in population size and range despite primarily occuring in within protected consrvation areas. This frog is endemic to the Sierra Nevada (southwest USA)  occuring at elevations of 1,400 to 3,700 m and breeds in historically fishless, deep upland pools and lakes. Once common, it now occurs over less than half of its original range. Extirpation from lakes and ponds is thought to have been a result of widespread introduction of various trout species (and hybrids) to this previously fishless environment.

A large-scale study to analyse the effects of fish introductions on the mountain yellow-legged frog in two protected areas in the Sierra Nevada was undertaken. Differences in fish stocking between these protected areas allowed fish predation to be examined as a possible cause for frog population declines. It was hoped that the findings could then help guide future management policy.

Study site: Two adjacent protected areas in the Sierra Nevada (California), were chosen; the John Muir Wilderness (JMW) and the Kings Canyon National Park (KCNP) encompassing 1,728 pools and lakes. Fish stocking (mostly with salmonids), periodically undertaken since the late 1800s, was terminated in the JMW in 1977, in the KCNP trout introductions are ongoing in 65% of water bodies.

Frog surveys: The presence and abundance of mountain yellow-legged frogs was assessed in all water bodies in the JMW from 23 August-15 September 1995 and 22 July-13 September 1996, and in the KCNP from 29 June-15 September 1997. During the breeding period (July to September) these frogs occupy the shallow waters at the edge of ponds or lakes, therefore, surveys took place along the entire shoreline of all water bodies concentrating on thes shallow marginal areas. Abundance of adults and larvae (tadpoles) were estimated by counting all individuals observed.

Fish surveys: The presence and absence of introduced fish species was recorded by visual searches of the entire shoreline of each water body up to 3 m deep, as well as the first 100 m of each inlet and outlet stream (if present). In water bodies deeper than 3 m, gillnets were laid to determine presence/absence of fish and to establish species composition. Nets were set for 8-12 hours, the reliability of such a procedure was determined by repeated gillnet surveys in a sample of six lakes, and a single gill-netting proved to be ample as it was 100% accurate.

Habitat composition: The following physical characteristics of each water body were recorded: elevation, depth, perimeter, surface area and solar radiation input (the latter measured as exposure to increasing levels of UV radiation was one theory posed for the decline of some upland amphibians). Substrate composition was quantified by estimates of the dominant substrate i.e. silt, sand, cobble, gravel, boulder or bedrock, along 50 transects 3 m in length. Vegetation cover was estimated in the littoral zone, by recording presence/absence along each transect. Stream connectivity and water body isolation were also recorded (as these characteristics would influence potential fish colonisation).

Three species of trout were found: rainbow trout Oncorhynchus mykiss × golden trout O.m.aguabonita hybrids, brown trout Salmo trutta and brook trout Salvelinus fontinalis. The percentage of water bodies inhabited by the three trout species was similar in both protected areas (see Table 1, attached). The percentage of surface area occupied by trout was around 50% in the KCNP (29% of water bodies contained trout) which was nearly half that of the JMW, where around 85% of water body area was occupied by trout (20% of water bodies contained trout).

The presence of trout had a the strong negative effect on mountain yellow-legged frog abundances. Only 4% of all water bodies in the JMW contained adult frogs, which had almost double the amount of trout than water bodies in the KCNP, compared to 31% in the KCNP. There was a similar relationship for frog larvae, with only 3% of water bodies in the JMW and 20% of those in the KCNP containing tadpoles. At the watershed level, the percentage surface area occupied by trout was a good predictor of the water body area containing frogs. As the percentage of water body surface area occupied by fish increased, the percentage area occupied by both adult and larval yellow-legged frogs decreased.

Similarities in the requirements of trout and mountain yellow-legged frogs were evident. Water bodies more than 2 m deep were more likely to contain trout than those less than 2 m in depth. Although mountain yellow-legged frogs breed in shallow waters, they require deep water in order to avoid winter freezing as metamorphosis of larvae can take up to four years. In fishless lakes, numbers of frog larvae increased with water depth, and were more likely to occur in lakes that were deeper than 2 m.

Conclusions: The introduction of salmonid fish into ponds and lakes in the Sierra Nevada has had a significant negative impact on the abundance and distribution of the mountain yellow-legged frog. Introduced salmonids predate heavily on their larvae which has led to the disappearance of mountain yellow-legged frogs from areas were exotic fish have been introduced. Some fish populations in some of the Sierra Nevada lakes can be removed with minimal effort, and this study indicates that the decline of the mountain yellow-legged frog might be relatively easy to reverse. Through intensive gill-netting it would be possible to return some of the lakes containing fish back to something close to their original, fishless state. A preliminary study has confirmed that removal of exotic trout from lakes containing yellow-legged frogs would result in large population increases. Fish stocking in protected areas should be controlled or stopped altogether, in order to safeguard native fauna and flora.


Note: If using or referring to this published study, please read and quote the original paper.