Some aspects of the biology of gudgeon Gobio gobio (L.) in Irish waters

Data on some aspects of the biology of gudgeon were obtained during a programme of research on Irish coarse fishes. Eggs of gudgeon, were collected on 3 occasions—on June 8 and again on June 29, 1965 in a small rivulet entering a hydro-electric reservoir in Co. Cork; and on June 2, 1966 in a millstream on the Grand Canal in Co. Dublin. The eggs were greyish-white in colour, opaque, and 1·375–1·625 mm diam. Newly-hatched larvae from eggs from Co. Cork measured 5·45 mm and were distinguished by the presence of black pigment largely confined to the ventral aspect of the yolk sac and tail. They began to feed after 3 days; in 5–6 days the yolk was fully absorbed, the fish then measuring 6·2 mm. Following the creation of 2 hydro-electric reservoirs on the River Lee in Co. Cork, large populations of gudgeon developed in them for a time. It was found that, in the reservoirs, gudgeon less than 7·5 cm were feeding mainly on Cladocera, especially Chydoridae. The larger reservoir gudgeon and gudgeon in rivers and streams in Co. Cork fed mainly on molluscs, Gammarus, chironomid larvae, caddis larvae and ephemeropteran nymphs. Some gudgeon held a good deal of filamentous algae. Gudgeon were aged by means of scale reading, the results of which were confirmed by length-frequency distribution analysis in the case of the reservoirs. In the Lee system, gudgeon grew relatively rapidly at first, attaining a mean fork-length at 3 years old of 9·5–10·8 cm according to year-class. In the River Allow, a Cork Blackwater tributary, early growth was slower than in the Lee, but the length increments in the third and subsequent seasons were bigger than in the Lee reservoirs. In the Lee, most gudgeon of both sexes spawned for the first time when 2 years old, at a minimum fork-length of 6·9 cm in the case of males and 7·9 cm in the case of females. The trematode parasite Paracoenogonimus ovatus was of frequent occurrence in Irish gudgeon aged 2 years and upwards.