Evaluating the Piscicide Rotenone As an Option for Eradication of Invasive Mozambique Tilapia in a Hawaiian Brackish-Water Wetland Complex

Mozambique Tilapia Oreochromis mossambicus were recently discovered in ‘Aimakapâ Fishpond, a 12-hectare brackish-water wetland complex in Kaloko-Honokôhau National Historical Park, Hawai’i. As a possible eradication method, we conducted laboratory tests to assess rotenone’s efficacy in killing tilapia and effects on non-target species. Tests involved exposing organisms to various concentrations of CFT Legumine (5% rotenone) in static trials of 48-h to 72-h duration. Test organisms included: Mozambique Tilapia, non-native Guppy Poecilia reticulata, the non-native odonate Rambur’s forktail Ischnura ramburii, native feeble shrimp Palaemon debilis, and native ‘ôpae’ula shrimp Halocaridina rubra. All organisms and water used in tests were obtained from ‘Aimakapâ (12.6–12.7 ppt salinity), or, for H. rubra, an anchialine pool (15.0–15.2 ppt salinity). Survival analyses indicated CFT Legumine concentrations >3 ppm (>0.15 mg/L rotenone) achieved 100% mortality of tilapia and 93% of guppies within 24 h. No mortality was observed for ‘ôpae’ula shrimp and little mortality among other invertebrates. Results demonstrate rotenone’s potential for non-native fish eradication in brackish-water habitats, with benefit of low mortality to certain macro-invertebrates. Although rotenone’s effects on freshwater organisms have been well studied, our research represents one of only a few controlled laboratory experiments demonstrating rotenone tolerance of brackish or marine fauna.