Sensitivities of mosquitofish and black mudfish to a piscicide: could rotenone be used to control mosquitofish in New Zealand wetlands?

The comparative sensitivities of mosquitofish (Gambusia affinis) and black mudfish (Neochanna diversus) to the piscicide rotenone were investigated to determine whether it is feasible to use rotenone to control mosquitofish i n wetlands inhabited by mudfish. Mosquitofish were exposed to four nominal rotenone concentrations (56, 79, 112, 158 mu g.L-1) and the time taken for 50% (ET50) and 90% (ET90) of individuals to gasp at the water surface was r ecorded. Fish were removed from the rotenone at this stage and placed in ro tenone-free water to recover. Mudfish were exposed to a single nominal rote none concentration (100 mu g.L-1), and removed when hanging at the surface and gulping air. The response time of mosquitofish decreased with concentra tion. Mean ET50 values were 158 and 26 min for 56 and 158 mu g.L-1 rotenone , respectively. Mean ET90 values were 246 and 36 min, respectively. Mudfish were approximately twice as sensitive as mosquitofish to 100 mu g.L-1 rote none. All mudfish recovered following exposure to rotenone. The management implications of the findings are discussed. We suggest that if rotenone wer e to be used to control mosquitofish in wetlands, it could be applied in su mmer to remnant standing water as repeated doses of 100-200 mu g.L-1.