Assessing the effects of anticoagulant rodenticides on non-target fish species
Authors:   , Riegerix Rachelle Catharine-Rose 
Published: April 2021

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Invasive rat species have negatively affected island ecosystems contributing to the decline and extinction of many endemic species. The eradication of rodents on islands can leads to restoration of native ecosystems. Anticoagulant rodenticides (ARs) have been an effective tool for rat eradication on many oceanic island systems (Wilmer et al 2007). The Hawaiian Islands, USA have used first generation AR, diphacinone (DPN), for pest and rodent control. Rodent populations, found on the Hawaiian Islands, have not only caused the extinction small shorebird and plant populations but have also been in direct competition for resources. ARs, in the form of a bait pellet containing 0.005% DPN, have been aerially and hand broadcast across the islands. Aerial broadcasting via helicopter has allowed for remote islands with uneven and dangerous terrain. Successful Hawaiian rodent eradications have occurred on Kure Atoll, Mokoli'i Island, and Mokapu Island. Lehua Island, a remote and uninhabited island near Ni'ihau, successfully eradicated rabbits in 2006 but was unable to eradicate the rodent population via aerial broadcasting of DPN in 2009. Following this attempt in 2009, a fish mortality event was reported on Ni'ihau and a dead whale calf also was found around the same time. Analysis of the affected fish were done and found no rodenticide in tissues. Despite these results, concerns of AR exposure to non-target species such as fish were voiced by the public. To determine AR effects on coral reef fish found in Hawaii, U.S. Geological Survey was asked to address their sensitivity as a prerequisite of another DPN aerial broadcast over Lehua Island. In December 2016, we presented our preliminary findings, which were used in the planning processes of an aerial broadcast which was conducted in August 2017. My dissertation research used acute toxicity testing, tissue analysis, and pharmacokinetics to determine the relative sensitivity of fish species to ARs. The primary objectives for my dissertation were (1) determine the median lethal dose (LD50) of three common ARs in four fish species. (2) Assess AR concentrations in fish tissues over time, effects on clotting, and AR half-lives in fish tissue. and (3) Evaluate diphacinone concentrations in coral reef fish species muscle and liver pre and post-aerial broadcasting. Direct toxicity to ARs does not appear to be a primary concern as not only did fish in this study refuse to eat rodenticide dosed bait pellets in the lab aquaria but in comparison to target and non-target species, they appear to be less sensitive. Exposure to ARs delays clotting in fish species. There is a lag-time of effects to clotting time (up to 24 hours) but clotting times return to baseline or control values between 5-7 days post dosing. DPN and BROD half-lives in fish tissues, plasma, muscle, and liver were fairly rapid. DPN and BROD half-lives in plasma were longer than half-lives in muscle and the target organ, liver. I found no detectable or quantifiable amount of DPN in the sixteen species of coral reef fish liver and muscles samples examined from Lehua Island, Hawaii. Overall, these studies suggest that the use of rodenticides near waterways with a possibility of accidental exposure to fish species, should have little to no effect on fish communities.