Pacifastacus fortis

This species is endangered and known from a limited range (<13 sq. km) that remains as wide as it was historically but is now reduced to scattered, disjunct occurrences with declining numbers of individuals and declining habitat quality mostly in headwaters where it was formerly continuous throughout but is now fragmented by dams. An inference of population decline has been made which determined this to be at least 50% in the last 10 years, and is continuing to decline. It is threatened by habitat modification and introduction of non-native species of fish and crayfish. The construction of the Pit I hydroelectric project also poses a threat, therefore further research into the ecology, distribution and abundance of this species is required.

Known only from tributaries (Fall River and Hat Creek subdrainages and a spring tributary of the Pit River at Pit Power House III) of the Pit River in Shasta County, in northeastern California; apparently with a relict distribution of a once much more continuous one (Bouchard, 1977; Eng and Daniels, 1982; Daniels, 1980; Light et al., 1995; USFWS, 1998; Rogers, 2005).

By 1990, it was restricted to seven isolated localities, mostly in the headwaters of spring-fed tributaries of the Pit River. Relict distribution exist today but no evidence the species had a wider range during historic times (Eng and Daniels, 1982). Historically, distribution was more or less continuous throughout the Fall River, Hat Creek, and the segment of the Pit River that joins these drainages (Light et al., 1995). It was described in 1898 from the Fall River at Fall River Mills and Hat Creek at Cassel with subsequent collections from the Fall River system in 1934, 1964, 1973-1974; with collections in 1975 from all three river systems (headwaters of the Fall River, Sucker Springs Creek on the Pit River, Crystal Lake on Hat Creek) (Bouchard, 1977). In 1978-1980 it was found in numerous locations in the Fall River system including the type locality as well as Sucker Springs Creek and the Pit River and in Crystal, Baum, and Rising River lakes on the Hat Creek system. Resurveys in 1985-1986 found no major changes in distribution but noted a declining population in Crystal Lake occurring with a large population of introduced Pacifastacus leniusculus in only five years plus new observations in Eastman Lake at the Lava Creek overflow, the Fall River at the mouth of Spring Creek, and Tritton Reservoir. Most recently in 1990-1991, Light et al. (1995) found it in a total of 14 sites (Crystal, Baum, and Rising River lakes in the Hat Creek subdrainages; Fall River, Big Lake, Spring Squaw, and Lava Creeks, and Crystal and Rainbow Springs in the Fall River subdrainage; and in Sucker Springs, a tributary of Pit River between the two subdrainages) comprising seven noncontiguous subpopulations separated by dams, gradient barriers and many stream km; mostly in the same areas as historically but now isolated in patches rather than distributed continuously; however the marginal locations (Fall River, Pit River mainstem near sucker Springs, and Baum Lake) have not yielded specimens since 1980 but these represented washdowns or strays from nearby populations anyway.

Currently threatened by habitat fragmentation (there are now many small, genetically isolated populations remaining from a formerly continuous distribution), and introduced populations of aggressive Orconectes virilis and particularly Pacifastacus leniusculus leniusculus and small and large-mouthed bass in the area seriously threatened the continued existence of this species (Daniels, 1980; Light et al., 1995). This species is endangered by habitat loss from water diversions, predation, and competition with the exotic signal crayfish (Pacifastacus leniusculus) and other species. Two entire populations have been extirpated since 1978 and although range extent remains what it was historically, occupied area is significantly reduced as the species is reduced to isolated patches mostly in headwaters where it was once distributed continuously (USFWS, 1998; Light et al., 1995). Preliminary data show invasion of P. leniusculus reduce densities of P. fortis following invasion in streams, reduce densities of snail (Juga sp., Fluminicola sp.) prey as well as other benthic invertebrates (chironomids, water boatman, zooplankton), P. leniusculus become more aggressive towards P. fortis as density of P. leniusculus increases (this is even more pronounced for the highly aggressive Procambarus clarkii), and at high densities, P. leniusculus individuals dispersed more than at lower density (Kats et al., 2006).

Historically this species was threatened by water diversion and impoundments from four hydroelectric power facilities (which fragmented populations and resulted in secondary effects such as increased siltation and water temperature and decreased dissolved oxygen plus reduced flow), introduction of non-native competitive crayfish species such as Orconectes virilis and Pacifastacus leniusculus leniusculus as fishing bait, associated introduction of crayfish pathogens (limited but present), non-native game fish introductions that prey on crayfish, alteration of habitat for trout spawning (including fish pathogen control, stream channelization, impoundment, substrate modification), crayfish fishing (historical only- banned in 1981), siltation and loss of larval substrate (through land reclamation activities, channelization, dredging, logging, forest fires, culverts and bridges, agriculture, grazing, and introduced muskrat activity). (USFWS, 1998).