Resident trout consumption of salmon carcass and analog added to tributaries of the N. Fork Boise River, Idaho
After completing spawning, Pacific salmon die and their carcasses provide an important marine-derived subsidy for freshwater and terrestrial ecosystems. Their reduction or absence from some ecosystems in the Pacific Northwest has prompted the use of nutrient mitigation techniques as a means of offsetting perceived lost productivity in these systems. We conducted an experiment using salmon carcass (n=3), analog pellets (n=3), and control (n=3) treatments in 500-m reaches of nine tributaries on the North Fork Boise River, Idaho. We hypothesized that salmon carcass and analog material would be directly consumed by resident fishes. Rainbow trout (Oncorhynchus mykiss) and brook trout (Salvelinus fontinalis) were collected by electro-fishing before, and 2, 4, and 6 weeks after treatment additions, and gut contents were obtained via non-lethal gastric-lavage. Diet items were separated into aquatic invertebrate, terrestrial invertebrate, and treatment (i.e., carcass, analog) categories. Both carcass and analog were present in the diets through time and gradually declined. Salmon carcass material comprised 50% of the diets at 2-weeks after treatment and decreased to 10% at 6 weeks. Analog material comprised 30% of the diet at 2 weeks, and decreased to 10% at 6 weeks. Based on this linear decline, we predicted that no treatment would persist in trout diets in carcass streams after 50 days and analog streams after 55 days. Conservative estimates of analog treatment consumption by trout populations ranged from 7 to 18 kg, which was 1.5 to 3.8% total treatment loading. Carcass consumption ranged from 17 to 35 kg, or 1.8 to 2.3% of total loading. Our study shows that salmon carcass and analog treatments provide a food source for resident trout. Further research is being conducted to determine how this alternative diet may affect growth rates and population dynamics.
Idaho Chapter of the American Fisheries Society Annual Meeting
Wipfli, M. S.
Resident trout consumption of salmon carcass and analog added to tributaries of the N. Fork Boise River, Idaho.
Idaho Chapter of the American Fisheries Society Annual Meeting,
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