An ammonia-based tool for chemical control of invasive crayfish

Several species of invasive crayfish have been introduced globally, impacting ecosystem functioning and biodiversity. The opportunistic feeding habits of nonnative crayfish have negatively affected native aquatic species in the southwestern United States, and their ability to travel between isolated aquatic systems make them difficult to control once they become established. Efforts to manually remove crayfish from invaded habitats have met with limited success and few chemical tools are available. With increasing numbers of invasive species in aquatic systems comes a growing need for additional management methods. The use of ammonia has shown promise as a removal tool for introduced fish and has several advantages over other chemical removal methods. An ammonia-based tool could be a costeffective way to eradicate invasive crayfish and support conservation of native aquatic species while utilizing the natural nitrogen cycle to remove the ammonia from the environment and return an ecosystem to baseline conditions. We used laboratory experiments to develop and test lethal concentrations of ammonia for Faxonius virilis (northern crayfish) and Procambarus clarkii (red swamp crayfish). We used a formulation of ammonium sulfate to elevate ammonia concentrations to 50 mg/l, sodium carbonate to increase pH to 9.5, and sodium sulfite to lower dissolved oxygen to 0 mg/l. The formulation achieved 100% mortality of F. virilis in 24 hours in a laboratory setting, but only 90% mortality of P. clarkii. A field trial with P. clarkii achieved 100% mortality of focal crayfish and 99% mortality of additional recaptured crayfish, although 55% were never recovered. These results suggest that an ammonia-based chemical tool could be utilized for invasive crayfish management. We recommend that additional laboratory and field trials focus on further exploring species differences and the effectiveness of ammonia treatments under diverse field conditions.