Brown trout, Salmo trutta L., production in an Irish coastal stream

Abstract. Brown trout. Salmo trutta L., estimates were obtained for the Owendoher, a tributary of the River Dodder. Data were collected at live stations on 21 occasions from May 1981 to May 1983. Total population densities ranged from 0·07 to 1·29 fish/m² Mean annual densities varied from 0·20 to 0·77 fish/m². Trout in this stream are typically slow-growing fish reaching a maximum length of 7·68, 13·05 and 17·54cm at the end of their first, second and third years respectively. Growth improved in a downstream direction with the highest rates recorded in stretches overlying limestone. There was some evidence of density-dependent growth in the upper reaches Daily instantaneous growth rates were highest for all age groups during the months May to August but became depressed during the winter. There was considerable variation in the total monthly biomass figures with values ranging from 1·32 to 33·46 g/m². Mean annual values ranged from 3·96 to 13·48 g/m². Annual production (somatic) varied from 3·77 to 14·38 g/m². in 1981–1982 and from 6·12 to 12·70 g/m². in 1982–1983. The mean annual production figure for this stream was 11·24 g/m². Production was contributed mainly by 1+ and 2+ trout at all stations. Egg production. accounting for 3–4% of the total estimate, was contributed mainly by resident 1+ and 2+ fish. It ranged from 0·33 to 0·45 g/m² in 1981–1982 and from 0·14 to 0·53 g/m² in the following year. Ratios of annual production to mean biomass (P/B?) were highest for 0+ trout hut decreased with increasing age. The mean P/B? ratios for all age classes varied from 0·95 to 1·17. The mean value for the Owendoher stream was 1·1. The effects on production of changes in age structure, population density and habitat alterations are discussed.