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Abstract
Tricaine methanesulfonate (MS-222) is one of the most broadly used anesthetic and tranquilizing agents for poikilotherms, and it is currently the only anesthetic approved for use with food fish. To test the preliminary observation in tilapias Oreochromis sp. previously exposed to anesthetizing doses of MS-222 that time to anesthesia was shorter on subsequent exposures, hybrids of white tilapias (Nile tilapia O. niloticus × blue tilapia O. aureus) crossed with Mozambique tilapias O. mossambicus were anesthetized with MS-222. Time to anesthesia was recorded at this initial drug exposure and weekly for 6 weeks after initial exposure. Fish previously exposed to MS-222 did not display a significantly reduced time to anesthesia on the second exposure but did display significant reductions at the third exposure and thereafter. These preliminary results suggest that tilapias do not respond to MS-222 with the typical enzyme induction-mediated tolerance reaction commonly seen with anesthetic chemicals in mammals.