Waste from fish farming spreads far

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The salmon you ate last night probably didn’t come from an icy stream in the Pacific Northwest, but from a submerged cage in the ocean where it spent months fattening up on feed before being harvested.

Fish farming, an inexpensive way to supply salmon, cod, trout and other popular fish to consumers, produces an estimated 30% of fish eaten by humans.

But it’s not without harmful side effects. Urine, fecal matter and uneaten feed from fish farms are being carried greater distances than previously thought, according to Stanford researchers.

Uneaten commercial feed gets carried off by currents. And the fish -- often penned up by the thousands -- defecate and urinate, creating waste streams in the ocean.

It had previously been assumed that the waste matter would be diluted by the sea if farming pens were kept far enough from the shore, said Jeffrey Koseff, co-director of Stanford’s Woods Institute for the Environment. But results from new software that simulates fluid dynamics show that waste from fish farming will travel farther and in higher concentrations than scientists thought, Koseff said.

Currents that flush out the pens could ultimately carry the waste to a public beach or mangrove ecosystem. But if not enough currents flow through the pen, fecal matter and uneaten food could pile up beneath the fish and create a ‘dead zone’ on the ocean floor -- an area where there’s not enough oxygen in the water for plants and animals to survive.

-- Catherine Ho