Bear Predation on Pacific Salmon Facilitates Colonization of Carcasses by Fly Maggots

Abstract

The carcasses of Pacific salmon can be an important food resource for aquatic and terrestrial organisms, depending on where the carcasses are located. We hypothesized that the availability of sockeye salmon (Oncorhynchus nerka) carcasses to ovipositing flies (Family: Calliphoridae) would be facilitated by brown bears (Ursus arctos) moving salmon from the water to terrestrial environments. We further hypothesized that the proportion of carcasses in terrestrial environments (hence accessible to flies) would vary among streams as a function of stream size. In three small southwestern Alaska streams during 2 y, flies oviposited on about half of the carcasses in terrestrial habitats (44% of those on gravel, 54% in grass and 57% in the forest). Only 28% of the partially submerged carcasses and 0.5% of the fully submerged carcasses were colonized. The proportion of carcasses deposited outside the stream was strongly and positively influenced by the level of bear predation and negatively affected by stream complexity (in deeper more complex streams more carcasses remained in the water). Based on data from the past 12 y on sockeye salmon abundance and predation by bears (averaging about 40% of the salmon), we estimated that on average 204, 551 and 839 carcasses were colonized per km annually in the three streams (12 to 24% of the total number of carcasses). In these sites, where salmon are abundant and highly available to bears, a large fraction of the tissue is not consumed and so is available to flies. Fly maggots, numbering up to 50,000 per carcass, can consume virtually an entire salmon within 5 d. Thus bears as consumers and transporters of carcasses facilitate the scavenging of carcasses by maggots, increasing the transfer marine-derived nutrients from aquatic to riparian habitats.