The effects of a land drainage scheme on the salmonid populations of the River Camowen, Co. Tyrone

Annual electrofishing surveys were carried out over an 11-year period at 5 sites on an upland river which was subject to dredging operations in connection with a land drainage scheme. Four of the sites were dredged 3 years after the start of the survey and one control site remained undredged throughout. The results indicated that dredging operations initially reduced salmonid (Salmo salar , S. trutta ) densities and that there was subsequently a progressive downstream recovery, with fry densities taking up to 6 years to improve at the most downstream site. Yearling and older fish recovered to pre-drainage scheme levels more rapidly than fry. Increased fish growth rates were also observed at all sites over the experimental period, and this may have been correlated with enrichment of the river system by leaching from increased fertilizer usage within the catchment.