A survey of the macro‐fauna of the River Cynon, a polluted tributary of the River Taff (South Wales)

Abstract

SUMMARY(1) A survey of the macro‐invertebrates and fish in the River Cynon, a trout stream in south‐east Wales receiving industrial and domestic wastes, and nine of its tributaries, was carried out in the summer of 1970. Its principal purpose was to describe the structure of these communities before waste treatment facilities, in course of construction, reduce the degree of pollution.(2) The macro‐invertebrate communities changed dramatically at station C4, there being a very varied fauna upstream and one dominated by chironomids and oligochaetes downstream, principally Nais barbata., Cricotopus bicinctus and Syncricotopus rufiventris. It seems likely that coal particles, influencing the substrate, are largely responsible for this change. Further downstream, and below substantial organic discharges, tubificids and the enchytraeid, Lumbricillus rivalis, became increasingly abundant. The total density of macro‐invertebrates increased from about 2000/m2 in the headwaters to over 20,000/m2 in the lower reaches.(3) The fauna of the clean tributaries, Hir (C16) and Wenallt (C19) was similar to that ofthe unpolluted upper reaches ofthe Cynon (C1‐C3) whereas that of tributaries affected by coal particles was similar to the fauna of the Cynon downstream of C4.(4) Six species of fish were recorded (bullhead, eel, minnow, trout, stickleback and stoneloach) of which tbree (bullhead, eel and trout) were confined to reaches upstream (C8 and above) of industrial eflluents at Abercwmboi. For 0–8 km downstream of these effluents the river was fishless and further downstream, to the confluence with the River Taff, fish density and biomass were reduced.(5) In the upper Cynon and its tributaries the density and biomass of trout were within the range recorded elsewhere in the British Isles. The growth rate of trout in the catchment is low. Diflerences in growth rate and shape of trout in the main river and tributaries suggest that there is little interchange between these areas, except perhaps with very young fish. 0 +* fish were only caught in abundance at two tributary stations and their numbers, even if widely distributed, could not permanently support the current density of older fish throughout the upper catchment.(6) Stone loach and minnow reached a very large size and individuals caught of the latter species approached the maximum recorded length for the British Isles.