Studies on the Three-Spined Stickleback Gasterosteus aculeatus L. from an Upland Irish Reservoir System

Abstract

Monthly samples were taken using a small-meshed beach seine. A total of 1092 sticklebacks were captured, 725 fish were used to calculate the age distributions and 699 were examined for gut contents. The age data, determined from the otoliths, showed that there were four age classes present. The mean length of first year fish (O-Group fish) was 2.24cm, of second year fish (I-Group) 3.36 and 4.47cm and 5.72cm for II-Group and III-Group fish respectively. The breeding season was June-July. Sex ratios were similar in both lakes and did not differ significantly from 1 : 1. Sexual maturity was reached by all fish above 3cm. The smallest maturing virgin was 2.6cm in length. Egg diameters varied between 1.0 and 1.5mm. The diet was similar in the two lakes studied. In the North Lake Cladocera, chironomid larvae, copepods and molluscs dominated while in the South Lake Cladocera, copepods, chironomid larvae and surface insects were dominant. Sticklebacks were extremely scarce in the Vartry River and feeder streams entering both lakes and would not pose a threat to egg production by trout and minnow in these streams. The influence of sticklebacks is discussed in relation to other fish species.