Never thought about it so I did the standard Web Research. Found this:
http://au.answers.yahoo.com/question...9224842AAbFB1h
Freshwater fish secrete a very dilute urine while most saltwater fish secrete a very concentrated urine (which is why pollution is always higher in marine aquaria with a similar fish volume.
The exceptions to this are Sharks, Rays, and Skates: Instead of passing urea (which is mostly composed of organic salts) out of their bodies, it is put into their blood stream, effectively raising the concentration to that of sea water! Even with this method, they must still eliminate excess sodium This is performed by a special gland known as the rectal gland.