The production of brown trout, Salmo trutta in tributaries of the Upper Wye, Wales

Abstract

The population density, age structure, biomass, growth and production of brown trout were investigated in four tributaries of the upper River Wye. The populations at each site were largely maintained by immigration from nursery areas. Abundance of separate year classes at sites on the three largest tributaries reached a peak at age 2+. On the smallest stream numbers reached a peak at 1+. Recruitment occurred throughout the year but decreased with age of year classes. Maximum O+ densities ranged from 0.04 to 0.89 m−2, and >0+ densities from 0.13 to 0.59 m−2. Average total biomass in 1975 ranged from 2.6 to 14.2 g m−2. Within the study sites annual trout production in 1975 ranged from 2.9–19.7 g m−2. Production values were dependent on age structure and population mobility at the study sites. In the three largest streams 2+ and 3+ fish contributed 66.3–88.3% of total production whilst 1+ and 2+ fish contributed 74.5–84.5 % of the total in the smallest stream. The mobile (non‐resident) component of the population accounted for up to 60–70% of production at certain times of the year, but over the year (1976) accounted for =30 % of total production. The resident component of the highest annual production value (19.7 g m−2) was estimated to be between 15.0 and 18.2 g m−2.