Specific growth rate of channel catfish and goldfish decreased with increasing salinity
in the range from 0x to 9x (Altinok and Grizzle, 2001), which may be related to the
corresponding increase in urea-N excretion observed for these species in the present study.
The effects of salinity on growth rates of these species, especially channel catfish, were not
explained by alterations in basal metabolic rate (Altinok and Grizzle, 2003). A
proportional increase in urea-N excretion compared to ammonia-N could decrease specific
growth rate because excretion of ammonia is energetically less expensive than converting
ammonia to urea (Mommsen and Walsh, 1991; Korsgaard et al., 1995). A greater energy
expenditure on excretion could influence energy balance and therefore growth efficiency.