Effects of Impoundment Upon Fish Populations in Afferent Streams at Cow Green Reservoir

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Abstract
(1) The numbers, growth, production and movement of brown trout (Salmo trutta L.) and bullhead (Cottus gobio L.) are described for the streams flowing into Cow Green reservoir after its impoundment, and comparisons are made with pre-impoundment results. (2) There was no change in the mean length for age of trout, but there was a small, statistically significant, increase in that of bullheads. (3) The population density of trout fry increased in most streams by 300-1100%, probably because of an increase in egg production through the immigration of large reservoir trout to spawn. There was a statistically significant (P < 0.05) increase in the instantaneous rate of loss of fish in most streams. (4) In five afferent streams, trout production, from August of the first year of life onwards, increased from 0.73-3.7 g m-2 year-1 to 0.91-8.2 g m-2 year-1 following impoundment, and there was an increase in the contribution of the O group. Bullhead production in Red Sike and Near Hole Sike was estimated at 1.08 and 1.86 g m-2 year-1 respectively but there are no pre-impoundment results for comparison. (5) Some reservoir trout moved into the afferent streams during the spawning run of mature fish. Recruitment to the upper reaches of most streams was largely through the upstream movement of trout from the nursery areas in the middle and lower zones. (6) The numbers of bullheads in the lower reaches of most streams were erratic, but most adults overwintered and spawned in the reservoir, and some moved into the streams during summer. Exceptions are Red Sike and Near Hole Sike in which resident spawning populations became established. (7) A preponderance of males was found amongst the sexually mature trout resident in the afferent streams, but within the reservoir there was a preponderance of female trout.