It was suggested that in lahontan cutthroat trout, the urea production takes place mainly via the typical teleost pathway of uricolysis, due to the presence of significant level of uricolytic enzyme activity, but not via the urea cycle, since the activity of certain key enzymes in the urea cycle, such as carbamyl phosphate synthetase III (CPS III), are reported to be very low (Wilkie et al., 1993; Wilkie and Wood, 1994). Tarek (C. tarichi) excretes approximately 37% of its nitrogenous wastes as urea-N, but it too has no functional urea cycle (Danulat and Kempe, 1992).