Several nests have been found in the wild, all constructed from shredded bark. There are no breeding data for the Pacific Water Shrew in British Columbia. In other parts of its range, the breeding season extends from late January to late August, with most young born in March. The litter size is three or four. The number of litters produced by females has not been documented. Males do not breed in their first summer.
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This shrew's diet is rather specialized, consisting of aquatic insects (water beetles and the nymphal stages of stoneflies, mayflies and alderflies), craneflies, ground beetles, spiders, harvestmen, centipedes, earthworms, slugs and small terrestrial snails. Aquatic insects may account for 25 per cent of the prey types eaten. Much of what we know about the feeding behaviour of this species comes from Donald Pattie's observations of several captive individuals. Pattie's captives fed readily on earthworms and termites, attacking earthworms ferociously with a series of swift bites along their bodies, and tearing apart pieces of rotten wood with their teeth, then pushing their long snouts into the termite tunnels. When hunting under water, they seized an animal in their jaws and carried it to land before eating. The Pacific Water Shrew probably relies on touch to detect prey under water, using the sensitive vibrissae on its long snout. Evidently this shrew will cache excess food. Pattie's captives deposited excess earthworms in a corner of the cage to be consumed later.
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Throughout its range, the Pacific Water Shrew appears to be rare. There are no actual estimates of populations, but in most small mammal studies, it represented less than one per cent of the small mammals captured. In 1992, Carlos Galindo-Leal and Gustavo Zuleta conducted an intensive survey of this species in southwestern British Columbia using more than 19,000 trap nights in 55 locations. They captured nearly 1,000 small mammals, but only 3 were Pacific Water Shrews, taken at 3 widely separated locations.
The only shrew with similar aquatic habits is the Water Shrew. Competition between the two is probably rare in British Columbia. Although they co-occur at a few locations (e.g., Haney and Cultus Lake) their distributions in southwestern British Columbia are usually separated by elevation, with the Water Shrew inhabiting areas above 1,000 metres.
A semi-aquatic species, the Pacific Water Shrew can swim for several minutes underwater or on the surface, propelling itself with alternating movements of the hind legs. Waterproof pelage keeps it warm in water, and air trapped in the fur provides buoyancy. Evidently this species can run for several seconds across the surface of water. Several specimens captured during recent environmental surveys near Sumas Mountain were taken inadvertently in submerged minnow traps set in small creeks.
No studies have been done on predation, but owls, the Pacific Giant Salamander and fish are the most likely predators of this shrew.
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