EFFECTS OF IMPOUNDMENT UPON TROUT (SALMO-TRUTTA) IN THE BASIN OF COW GREEN RESERVOIR

The biology, numbers, growth and production of indigenous brown trout in Cow Green Reservoir, upper Teesdale, are described for c10 yr following impoundment in 1970. Reservoir trout spawned in autumn in afferent streams up to 7 km from the reservoir. Most return after spawning. Trout entered the reservoir from afferent streams at c2 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∞ =39.0cm and gradient (k) =0.36. After impoundment L∞ was c40 cm, but the value of k (0.53-0.68) was lower than before impoundment (implying higher growth rate). 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 pre-impoundment level. Average size of female spawners increased after impoundment and this resulted in an increase in mean number of eggs female-1, in mean weight egg-1, and in population fecundity. Total mortality rate (Z) of trout aged III and older varied from 0.67-1.93 yr-1 (mean 1.19 ± 0.16) during 1972-80. The instantaneous rate of natural mortality (M) was 0.70 ± 0.37 yr-1. Estimates of the littoral population of trout ≥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 c22% of the total population. Trout production in the reservoir was c1.0 g m-2 yr-1, similar to the value in the River Tees before impoundment. -from Authors