Excretion of ammonia and urea by phylogenetically diverse fish species in low salinities

Two species of freshwater stenohaline fish (channel catfish, Ictalurus punctatus; and goldfish, Carassius auratus) and four species of euryhaline fish (rainbow trout, Oncorhynchus mykiss; brown trout, Salmo trutta; striped bass, Morone saxatilis; and Gulf sturgeon, Acipenser oxyrinchus desotoi) were raised in salinities from 0‰ to 9‰, and then their resting ammonia and urea excretion were compared. The ammonia excretion of channel catfish, goldfish, rainbow trout, and brown trout was higher in 9.0‰ salinity than in the other tested salinities, but ammonia excretion by Gulf sturgeon and striped bass was lowest in 9.0‰ salinity. The urea excretion of goldfish and channel catfish increased with increasing salinity, while that of Gulf sturgeon did not change and that of all other species decreased. In the higher salinities, excretion of urea-N as a percentage of the total nitrogen excretion (ammonia-N plus urea-N) increased for channel catfish, goldfish, and Gulf sturgeon but decreased for the other species. Overall, the effects of salinity were similar for two stenohaline species, while the euryhaline species were more variable.