Anyone who has gone swimming in the ocean knows how the salt can dry you out! Taking a swim in a fresh-water lake or stream, like those sparsely distributed in our Chihuahuan Desert, is gentler to our bodies.
But what about fish? How do their bodies deal with the salt—or lack of it? Strangely enough, sea-dwelling fish drink a lot, while fresh-water fish drink little, but excrete large amounts of urine! The reason for this is what we call "osmosis". The liquid inside and outside a fish "tries" to have an equal concentration of dissolved minerals.
The ocean's salt water has more dissolved minerals than what's
in a fish—so the fluid from the fish seeps out through soft membranes like the
gills. The fish must drink lots of water to make up for this loss. In our local
fresh-water streams and lakes, the liquid inside a fish has more minerals in it than
the water, so fluid enters the fish through those same membranes. These fish could
burst, if they didn't get rid of all that water, and that's why they produce so
much urine.
Listen to the Audio (mp3 format) as recorded by KTEP, Public Radio for the Southwest.
Contributor: Kodi R. Jeffery, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.
A marine fish. Photograph by A.H. Harris.
Page, L.M., and B.M. Burr. 1991. A field guide to freshwater fishes of North America north of Mexico. Houghton Mifflin Company, Boston. 432 pp. (P. 567)