Recovery patterns of salmonid populations following a fish kill event on the River Blackwater,Northern Ireland

Abstract A severe fish kill event caused by agricultural pollution impacted 2.4 km of the River Blackwater, Northern Ireland, in August 2007. A post‐kill monitoring programme sampled existing monitoring sites in the fish kill zone as well as upstream and downstream control sites. Post‐kill monitoring in combination with historical background data facilitated an assessment of the extent and severity of the disturbance episode on local salmonid populations. Subsequent recovery patterns indicated that total salmonid abundance returned to control levels within 1 year. Total salmonid biomass recovered within 2 years, whereas brown trout age structure took up to 3 years to return to background levels. Recovery dynamics were dominated by large‐scale directional migrations rather than random local movements. This included an initial autumnal immigration of 0+ age class brown trout, Salmo trutta L., in late 2007 followed by spawning migrations of adult migratory salmonids. The significance of relatively rapid natural recovery patterns is discussed in the context of potential management options such as restocking.