Piscicola salmositica Meyer 1946 is widely distributed throughout the Pacific coast of North America from central British Columbia to northern California. Habitual ectoparasites of teleost fishes, these piscicolid leeches are ordinarily encountered in association with spawning adult salmon (genus Oncorhynchus) and other Salmonidae; the common name "salmonid leech" is therefore applied. Piscicola salmositica, in common with its teleost hosts, occurs in a characteristic lotic environment — streams with moderate to swift currents, generally low temperatures, high dissolved oxygen content, and gravelled beds.An annual cycle of abundance is evident. Salmonid leeches appear in early fall attached to their hosts as they ascend into hatchery and natural spawning areas. From late September to February, during a period of feeding, growth, and reproduction, the leech populations attain a great abundance. By late winter, subsequent to the death of their hosts, the leeches have become virtually nonexistent, apparently as a result of high mortality among the mature individuals, some downstream displacement, and limited prédation by sculpins (Cottus sp.). Only the small, immature leeches are believed to survive the summer and return attached to spawning salmon the following fall.During the period of abundance in Soos Creek, a western Washington study stream, the lengths of feeding leeches increased at a rate of 2–3 mm per week. Weight, in relation to length, was dependent upon the amount of salmon blood ingested.Reproduction of salmonid leeches in Soos Creek was correlated with low water temperatures. Breeding began in temperatures of about 12 C in October and continued during December and January even in temperatures dropping to 5 C. The leeches attained sexual maturity at a length of about 15 mm; mature individuals then copulated and fastened cocoons to the lower surfaces of stones in the streambed. Deposited cocoons were dark brown and averaged 1.4 by 1.2 mm. In the laboratory, the average rate of reproduction ranged from 1.6 to 2.5 cocoons per leech per day, but the largest leeches (23–30 mm) produced the greatest number of cocoons. One young leech developed in each cocoon, emerging at a length of about 5.3 mm. Complete embryonic development and hatching occurred in about 7 days.
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