Effects of Impoundment Upon Trout (Salmo trutta) in the Basin of Cow Green Reservoir

Abstract

(1) The biology, numbers, growth and production of indigenous brown trout, Salmo trutta L., in Cow Green Reservoir, upper Teesdale, are described for a period of c. 10 years following impoundment in 1970. Comparisons are made with pre-impoundment results. (2) Reservoir trout spawned in autumn in afferent streams up to 7 km from the reservoir. Most returned after spawning. (3) Trout entered the reservoir from afferent streams at c. 2 years of age compared with 3 years for entry to the River Tees before impoundment. Trout growth before impoundment is described by a Walford Plot with l/o = 39 0 cm and gradient (k) = 0 86. After impoundment l/o was c. 40 cm, but the value of k (0 53-0 68) was lower than before impoundment (implying higher growth rate). (4) On average, males matured earlier than females, but an appreciable proportion of both sexes were mature in age-group III. These fish formed 17-50% of spawners in most years. The female: male sex ratio within the reservoir was 1 67: 1. Individual fecundity increased in the initial years of impoundment, but by 1980 it had returned to the preimpoundment level. The average size of female spawners increased after impoundment and this resulted in an increase in the mean number of eggs female-1, in the mean weight egg-, and in the population fecundity. (5) The total mortality rate (Z) of trout aged III and older varied from 0-67 to 1 93 year-1 (mean 1 19+0-16) during the years 1972-80. The instantaneous rate of natural mortality (M) was 0 70+0 37 year-'. (6) Estimates of the littoral population of trout e 15 cm varied from 1199 in 1972 to 3375 in 1975, with a mean value of 2010 from 1972 to 1980, minimum estimates of trout numbers < 15 cm in July ranged from 136 in 1972 to 1509 in 1974 (mean 905 between 1972 and 1980). The littoral population probably formed c. 22% of the total population. (7) Trout production in the reservoir was c. 1 0 g m-2 year-', which is similar to the value in the River Tees before impoundment.