The populations of fish in the River Tees system on the Moor House National Nature Reserve, Westmorland

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Abstract
The age, growth, population densities and annual production of an isolated population of brown trout (Salmo trutta L.) and bullhead (Coitus gobio L.) were investigated over a 5 year period in Trout Beck in the northern Pennines. Additional data were obtained from two other sites; Great Dodgen Pot Sike and the River Tees.Trout growth rates were low, particularly in Dodgen Pot Sike, but ages up to VIII were recorded. Bullhead growth rates were similar to those in the Tees at Cow Green (Crisp et al., 1974), though the Moor House bullheads had a higher survival rate. Population densities of trout ranged from 0.10–0.22/m2, with a fry contribution of 0.80%. The bullhead population in Trout Beck was sparse, generally 0.1/m2, though an 0 group value of 0.46/ma was estimated in 1967. At Tees Bridge values ranged from 2.5–7/8/m2. Most trout matured by age III, though in Great Dodgen Pot Sike only half of the females were mature at age IV.Annual production was estimated from Allen graphs. Trout production ranged from 1.02–3.50 g/m2/year, the fry contributing up to 51.5% of the total. A value of 0.48 g/m2/year was obtained for bullheads in the Trout Beck system in a year of good recruitment, whilst bullhead production at Tees Bridge was 7.43 g/m2/year, with fry and I group contributing 48.7 and 37.5% respectively. The high survival rate, irregular recruitment, and poor growth of these high altitude populations is discussed. Growth and reproductive peculiarities of the Great Dodgen Pot Sike trout are also discussed.