Notes: Nitrogen Excretion in Four Species of Fish from an Alkaline Lake

Abstract

In this study, we examined the ammonia and urea excretion rates in four species of fish native to Pyramid Lake, Nevada (pH 9.4): tui chub Gila bicolor, Tahoe sucker Catostomus tahoensis, Lahontan cutthroat trout Oncorhynchus clarki henshawi, and cui-ui Chasmistes cujus. The activities of liver enzymes associated with the ornithine urea cycle and uricolysis were also measured in all but cui-ui. As in other teleost fish, most of the nitrogen was excreted as ammonia. Ammonia excretion rates ranged from 48 to 342 μmol N·kg−1·h−1. Urea excretion rates varied from 17 to 139 μmol N·kg−1·h−1. The proportion of nitrogenous waste excreted by fish as urea in the alkaline Pyramid Lake water was greater than by fish in neutral water; however, ornithine urea cycle enzyme activities, with the exception of arginase (range, 5.2–41.0 μmol·g−1·min−1), were relatively low. All three of the uricolytic enzymes were present in significant activities. Therefore, it seems that urea is produced in these fish by uricolysis, as it is in other teleost fishes. Although the fish native to Pyramid Lake experience conditions that would inhibit branchial ammonia excretion in other fish, they seem to be able to excrete ammonia and have no unique urea production capabilities.