Effects of Regulation of the River Tees Upon Fish Populations Below Cow Green Reservoir

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Abstract
The population density, growth, biomass, fecundity and production of bullhead (Cottus gobio L.) and brown trout (Salmo trutta L.) were estimated in the River Tees below the regulating reservoir at Cow Green. Comparisons were made with estimates obtained for the same site before impoundment, and with data from Maize Beck, an unregulated tributary of the Tees below the reservoir. Changes in the growth of trout were negligible, but there was an apparent decrease in the observed lengths of O group bullheads in the regulated Tees. This may reflect a shorter growing season because bullheads spawned later in the Tees after impoundment, possibly in response to the delay in the increase in water temperature during the spring. There were marked annual variations in the numbers of brown trout and bullhead, but population densities of both species increased in the Tees following regulation. The numbers of trout (excluding the O group) increased from 3.5-4.9 100 m-2. For bullheads the largest increases occurred in the I and II age-groups: 9.5-81.5 100 m-2 and 5.2-24.4 100 m-2, respectively. The numbers of both species were significantly higher (P < 0.05) in the Tees than in Maize Beck during both periods. The annual instantaneous mortality rate of bullheads increased from 0.67 .+-. 0.21-1.30 .+-. 0.68 in the Tees following impoundment, corresponding values for Maize Beck were 0.68 .+-. 0.25 and 0.81 .+-. 0.22. The mean fecundity of individual female bullheads of any given size decreased in the Tees, but remained the same in Maize Beck. The increase in population density in the Tees caused a significant increase (P < 0.05) in the number of eggs laid each year by the whole population from 3.70-8.42 eggs m-2. The corresponding values in Maize Beck of 2.04 and 2.28 eggs m-2 were not significantly different (P > 0.05). Only very approximate values were obtained for annual production of trout, Estimated bullhead production increased slightly at Maize Beck and by a factor of three in the Tees, largely as a result of increases in O and I group production. Annual production by older fish changed little. The combination of environmental changes caused by regulation has improved conditions for the downstream fish populations. This was exploited by the bullheads and trout mainly through increased population density and population biomass, rather than through any marked increase in the growth rate of individual fish. This would be expected because growth rate appears to be limited mainly by water temperature.