DECLINE OF THE SHASTA CRAYFISH (PACIFASTACUS-FORTIS FAXON) OF NORTHEASTERN CALIFORNIA

Native freshwater faunas in North America are declining, principally d ue to the combined effects of habitat degradation and introduced speci es. Relatively little attention has been directed to the decline of fr eshwater invertebrates, which may be even more threatened than fishes. This paper chronicles recent changes in the distribution and abundanc e of the Shasta crayfish (Pacifastacus fortis Faxon), a state and fede rally endangered species endemic to the midreaches of the Pit River sy stem in northeastern California. We made snorkeling and SCUBA surveys for crayfish in 1990 and 1991 and examined various records for histori c distributions. Shasta crayfish have been extirpated from much of the ir historic range by water impoundment and diversion, and they are fur ther threatened by two introduced crayfish species (Pacifastacus leniu sculus Dana and Orconectes virilis Hagen). By 1990 Shasta crayfish wer e restricted to seven isolated populations, mostly in the headwaters o f spring-fed tributaries to the Pit river. P. leniusculus had become e stablished throughout much of the study area in about 12 years. In one site P. leniusculus probably contributed to the precipitous decline o f Shasta crayfish, from 2000-3000 in 1980 to about 370 (+/- 135) in 19 91. O. virilis, which occurred in only the most disturbed parts of the system, showed little range expansion in 30 years and had been replac ed in a large stretch of the Pit River by P. leniusculus. P. leniuscul us occupied sites with a broad range of habitat variables (temperature , pH, turbidity, substrate size) partly or wholly over lapping measure s of sites with the other two species. The decline of the Shasta crayf ish, like the extinction of its closest relative P. nigrescens in the San Francisco Bay area earlier this century, reflects the decline of i ts habitat and probably pressure from the aggressive exotic P. leniusc ulus.