Annual Production and Production: Biomass Ratios for Three Species of Stream Trout in Lake Superior Tributaries

—We made estimates of annual production of stream trout in two Lake Superior tributaries in northeastern Minnesota in 1977-1978. The Caribou River (total alkalinity, 34 mg/L as CaCO3) drains pan of the north shore of Lake Superior in an igneous rock geology. The Blackhoof River (total alkalinity, 83 mg/L) drains glacial drift and lake clay at the western end of Lake Superior. Annual production of brook trout Salvelinus fontinalis in the Caribou River was estimated to be 58.0 kg (wet weight) per hectare, reflecting low water fertility. In the Blackhoof River, annual production was 43.5 kg of brown trout Salmo trutta and 95.7 kg of juvenile rainbow trout Oncorhynchus mykiss per hectare. The total salmonid production of 139.2 kg/hectare in the Blackhoof River appeared to reflect an intermediate level of water fertility. Ratios of annual production : mean standing stock (P/B) were calculated to be 2.2 for juvenile rainbow trout, 1.7 for brook trout, and 0.9 for brown trout. These values suggest an inverse relation between P/B and number of age-groups present (three for rainbow trout, four for brook trout, and six for brown trout). Estimates of annual production of stream trout have been roughly associated with water quality indices such as alkalinity, hardness, and conductivity, but mechanisms of cause and effect are not readily discernible. Most studies have been of streams that would be characterized as highly productive and have resulted in annual production estimates in the range of 100-300 kg (wet weight) per hectare (reviews by Mann and Penczak 1986; Waters 1988). Studies of streams of obviously lower productive potential are less common (Cooper and Scherer 1967; Whitworth and Strange 1983). In addition to production rates, of ancillary interest are annual ratios of annual production: mean standing stock (P/B), summarized in reviews by Waters (1977) and Mann and Penczak (1986). Most of the estimates presented in these reviews suggest a fairly consistent level of annual P/B of about 1.0-1.5 for normally growing, stream-resident trout populations, and an inverse relationship between annual P/B and number of age-groups present or maximum age. Our objectives were to record annual production of stream trout populations from an area of 1 Present address: U.S. Army Corps of Engineers, Coeur d'Alene Field Office, 1201 Ironwood Drive, Coeur d'Alene, Idaho 83814, USA. 2 Present address: Department of Natural Resources, Cornell University, Ithaca, New York 14853, USA. presumed low productivity, the northern Lake Superior drainage (Smith and Moyle 1944; Moyle 1956; Krueger and Waters 1983; Waters 1987), and to examine variations in annual P/B ratio among three salmonine species: brook trout Salvelinus fontinalis, brown trout Salmo trutta. and rainbow trout Oncorhynchus mykiss.