ABSTRACT:
We estimated fish production in two tributaries of the Salmon River in Idaho. Fish in Big Springs Creek produced 11.8 g/m2/yr and in the Lemhi River 13.6 g/m2/yr. In Big Springs Creek, juvenile rainbow-steelhead trout comprised most (84-95%) of the biomass present and contributed most of the production (88%). In the Lemhi River, however, a large population of unexploited whitefish made up most (60-80%) of the biomass but they contributed only 52% of the production. The desired yield from Big Springs Creek, an intensively managed stream, was steelhead trout smolts and the weight of the juvenile migrants was 34% of the total production in the stream. In the Lemhi River, Chinook salmon smolts were the primary desired yield and weight of smolts was probably less than 11% of total production. Virtually no yield was obtained from the whitefish population in the Lemhi River.
QUOTES FROM TEXT:
The Idaho Fish and Game Department began reintroduction of steelhead in 1962 with annual releases of fry in Big Springs Creek.
The number of resident trout in Big Springs Creek declined as a result of this steelhead reintroduction (Bjornn 1966).
Big Springs Creek has been managed specifically for production of steelhead trout smolts while the species composition of the Lemhi River was not altered from the natural situation. Whitefish and Chinook salmon have been reduced to abnormally low numbers in Big Springs Creek by a weir which blocked access of these fish to the stream. At the same time, annual releases of up to 330,000 steelhead from complemented rainbow-steelhead recruitment in Big Springs Creek.
A large proportion of the whitefish biomass was in the slow growing, older individuals that did not efficiently produce fish flesh.
Estimation of fish production provides a means of evaluating the efficiency of an aquatic system for producing a yield of desirable species and/or age classes of fish. In Big Springs Creek, the desired yield was steelhead trout smolts or pre-smolts. We assessed production and yield in 1969 when rainbow-steelhead trout comprised the bulk (84-95%) of the biomass present in the stream. The weight of the average number of age 0 and 1 rainbow-steelhead trout which migrated from the stream during the years 1964-1968 amounted to 45% of the production of those age groups in 1969 and 34% of the total production by all species. A relatively high proportion of the total fish production in Big Springs Creek become yield as a result of intensive management.
In the Lemhi River, the primary desired yield was Chinook salmon smolts. If the weight of Chinook smolts which reared in the Lemhi River during 1969 amounted to 50% of the Chinook production, then the yield of Chinook smolts would only amount to 11% of the total production of all species. A yield (harvest and smolts) was obtained from the rainbow-steelhead trout which reared in the Lemhi River, but the whitefish population was virtually unutilized and thus no yield resulted from the 52% of the production they contributed. A relatively small proportion of the fish production in the Lemhi River became yield.
We plan further manipulation of species composition and abundance in these streams to assess the effects of such changes on the yield of salmon and steelhead smolts. In Big Springs Creek we plan to shift from populations of mainly steelhead to a mixture of steelhead and salmon and eventually to mainly salmon. In the Lemhi River we plan to exploit and perhaps eventually remove the whitefish population. Both yield and production should be assessed in these situations to better understand the relationships between species composition, production and yield.