MacKenzie, Graham. 2001. Trophic relations between coho salmon carcasses, Oomycetes and select caddis fly lavae. M.S.
Anadromous salmonids are important vectors for the transfer of marine nutrients into freshwater and terrestrial ecosystems. The saprophytic oomycetes (water molds) may be important intermediates in the transfer of these nutrients from salmon carcasses to caddisfly larvae. In the Pacific Northwest, the decay of salmon carcasses coincides with the later stages of development of several taxa of caddisfly larvae, which consume these oomycetes. This research not only demonstrates the importance of salmon carcasses as a supplemental food resource for caddisfly larvae but it also documents a mechanism of nutrient transfer via the oomycetes and adds understanding to the feeding ecology of caddisfly larvae.
A variety of methods were used to investigate the trophic relations between coho salmon (Oncorhynchus kisutch) carcasses, oomycetes and select caddisfly larvae. A growth experiment showed no significant difference in growth rates of Onocosmoecus (fam. Limnephilidae) caddisfly larvae fed a variety of diets: conditioned red alder (Alnus rubra) leaves, conditioned alder leaves with oomycetes and conditioned alder leaves with salmon meat. Stable isotope studies showed that oomycetes obtain most of their nutrients from salmon carcasses and caddisfly larvae in salmon bearing streams also are enriched with these marine derived nutrients. Onocosmoecus larvae had higher levels of marine carbon and nitrogen compared with Lenarchus (fam. Limnephilidae) larvae. 13C analysis of lipids extracted from Onocosmoecus larvae showed no significant difference between larvae taken from salmon bearing and non-salmon bearing streams. Evidence of consumption of oomycetes by late instar Onocosmoecus and Lenarchus caddisfly larvae is provided through gut content analysis and photo-microscopy techniques. Oomycetes from the family Leptomitacea (order Leptomitales) were identified as being the primary mold associated with salmon carcasses in this study. Time-lapse videotapes of food preference tests revealed that Lenarchus and Onocosmoecus larvae preferred oomycetes initially to conditioned red alder leaves and salmon flesh.