THE EFFECT OF WEED-CONTROL PRACTICES ON MACROINVERTEBRATE COMMUNITIESIN IRISH CANALS

Macroinvertebrates in aquatic habitats form an integral part of the di et of many freshwater fish. It is therefore important to understand th e effects that weed control practices have on this community in canal fishery watercourses. The principal forms of weed control operated in the Grand and Royal Canals include mechanical cutting, using a variety of boat-mounted and land-based apparatus, and chemical treatment usin g dichlobenil. The community composition and relative abundance of mac roinvertebrates in control, mechanically cut and dichlobenil treated c anal sites was recorded on three to five occasions between 1993 and 19 94. The results indicated that Asellus aquaticus was the dominant orga nism at all canal locations. The land-based Mowing Bucket effected the greatest reduction in macroinvertebrate numbers in the immediate afte rmath of the cut. This reflects the capacity of the machine to cut veg etation to canal bed level, thereby removing any substrate for colonis ation. At all eight sites examined, macroinvertebrate numbers increase d relatively rapidly following treatment and no adverse effect on depe ndent fish life resulted. The Office of Public Works policy of removin g obstructive vegetation from a central navigation channel, while pres erving weeded marginal fringes, minimises che impact of weed control o perations on the macroinvertebrate fauna.