Rotenone application and degradation following eradication of invasive roach (Rutilus rutilus) in three Norwegian lakes

Abstract

Roach (Rutilus rutilus) is indigenous to south-eastern Norway and alien to the rest of the country. In Trondheim municipality, in the middle part of Norway, roach was introduced into the Ila watercourse in 1881. Roach has a great potential to alter the ecosystem when introduced to new locations. The potential negative impact on potable water source quality and the prospect of permanently eradicating an alien species resulted in rotenone treatment of six lakes in Trondheim municipality. The rotenone concentration in the lakes was surveyed by water sampling until it could no longer be detected. A lethal concentration of rotenone at all test points was measured in all lakes during the survey period. Fourteen days after treatment, a near homogenous concentration was reached. The concentration reduction was similar in the lakes and relatively quicker during the first weeks after treatment. It was also consistent between depths except for the surface, where the concentration degraded more quickly. Rotenone degradation is a key factor when planning eradication efforts, and reports on this varies considerably between different locations. Despite application of rotenone in different depth strata, it took several days to reach homogenous concentration and several months and a fall turnover for the rotenone to break down and dilute below the detection limit in the lakes described.