Most of fishes are grown in a mesh cage submerged in the open water of oceans or bays. Fish farming, a relatively inexpensive way to provide cheap protein to a growing world population, now supplies, by some estimates, 30 percent of the fish consumed by humans. But growing fish in the pens can have some nasty side effects, especially when the pens are set near sensitive coastal environments. All those fish penned up together consume massive amounts of commercial feed, some of which drifts off uneaten in the currents.
The waste of the crowded fish created another unpleasant waste stream and it can carry disease, causing damage directly. Or the phosphate and nitrates in the mix may feed an algae bloom that sucks the oxygen from the water, leaving it uninhabitable, a phenomenon long associated with fertilizer runoff.
Jeffrey Koseff, a professor of civil and environmental engineering and co-director of Stanford’s Woods Institute for the Environment, observed that the effluent from pens would be benignly diluted by the sea if the pens were kept a reasonable distance from shore. He told that the fine details of modeling the flow of dissolved fish poop from a submerged cage are not as simple as they may seem. The design of the cage itself can affect the outcome.
It is explained that the fish farmer would prefer that currents flush out his pens frequently, but as those currents take out the garbage they might unfortunately deliver it to a mangrove ecosystem or a public beach. On the other hand, insufficient flow through the pen can create a “dead zone” on the ocean floor as the fecal matter and uneaten food pile up beneath the fish.
