No abstract

An artificial discharge of water (3 .o m 3 /sec), over a 48 h period, from an impoundment into the R . Wye did not substantially affect water temperature or concentrations of dissolved oxygen and suspended solids at a site 16 km below the impoundment . However, the load of suspended material on the second day of the release was about to times greater than the pre-release load . The total number of drifting macroinvertebrates on the first and second days of the release were about 7 and 3 times greater than the number on the day preceding the release . The initial increase in flow at 15 .oo h resulted in an immediate increase in the number of drifting larvae of Rheotanytarsus, a tubicolous chironomid . Subsequently there was an enhanced night-time increase in the total number of drifting invertebrates, particularly the mayfly, Ephemerella ignita (Poda), and this also occurred on the second night of the release . Increases in the number of drifting Rheotanytarsus and Ephemerella, the most abundant invertebrates, resulted in increases in drift density .

The application, rationale and process of habitat evaluation methods are discussed in the context of present day fisheries management. The need to consider habitat features at site and catchment level is stressed.Development of habitat evaluation techniques for assessing brown trout habitat in Welsh streams is reported, and examples of these approaches are given: qualitative (visual assessment), semi-quantitative (a combination of subjective and quantitative measurements) and quantitative (measurements on transect system). Habitat attribute-fish population models were based on functional linear regressions and multiple regression (for the quantitative method). Quantitative models explained up to 80% of the variance in abundance of medium sized (10-20cm) trout in hard water (>25mgI-' CaCO,), but were less effective when data from soft waters were included. Testing of the qualitative and semi-quantitative methods on independent data demonstrated that the latter was more effective in predicting habitat quality.Survey costs varied considerably but, although the more precise methods were more costly per site. they were the most cost-effective because fewer sites were required for a given level of precision.The problems associated with soft-water sites are discussed in terms of factors affecting site carrying capacity; and selection of methods is briefly considered in the light of current management practise.

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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-a, and >O+ densities from 0.13 to 0.59 m+. Average total biomass in 1975 ranged from 2.6 to 14.2 g rn+. 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 743-843 % 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 e 3 0 % of total production. The resident component of the highest annual production value (19.7 g m-a) was estimated to be between 15.0 and 18.2 g m-a.