Age, Growth and Diet of the brown trout Salmo Trutta L. in the Roundwood Reservoir System

Abstract

From April to October 1983 monthly samples, totalling 343 trout, were taken from the two reservoirs at Roundwood, Co. Wicklow, using a range of gill nets. In 1984 an extensive electro-fishing programme was carried out in the Vartry River and three other feeder streams, in which 605 trout were obtained. The age data, determined from the scales, showed that there were six year classes in the South Lake and five in the North Lake. The fish from the river and feeder streams were less than 4 years old, 90% belonging to 0 + and 1 + age groups. Trout from the South Lake showed faster growth, attaining 28.7 cm at year 5, compared to 23.2 cm in the North Lake. Only six trout were over over five years old. Sexual maturity was reached during the third year. The dominant food organisms were trichopteran larvae and pupae (Limnephilus vittatus) molluscs (Potamopyrgus jenkinsi and Sphaerium corneum) and chironomid larvae (Endochironomus sp. and Tanytarsus sp.). The species composition of the fauna of both lakes was almost identical, but biomass was greater in the South Lake where the trout populations appeared to be numerically smaller. The high female to male ratios in the lakes, varying between 1.37 and 1.67 to 1 did not appear to be age related. In the Vartry River and other feeder streams the fish were young, immature and predominantly male. These fish contained many more ephemeropteran nymphs than the lake trout. The oligotrophic state of the lakes, combined with the fluctuations in water level make it unlikely that any steps can be taken to improve the sport fishery using the native trout. Development of a put and take fishery is recommended.