In crowded net pens they can become epidemics.

In response to concerns, Mr. Entis has said the company would only sell fish that have been rendered sterile for use in net pens.

He said 50,000 to 100,000 fluke would initially be in net pens.

They take care of ponds and floating net pens.

Natural currents bring fresh, oxygenated water to the net pens and dissolve soluble wastes.

It is not uncommon for farmed salmon to escape from the net pens they are contained in while living in open waters.

Aquaculture that uses naturally occurring salmon, mostly Atlantic salmon, cultivates the fish in net pens.

Sea cages, also called sea pens or net pens, are usually made of mesh framed with steel or plastic.

Increasing quantities of high-grade tuna are reared in net pens and fed bait fish.

He said the farms can be managed without widespread outbreaks of disease, and noted that in Norway the net pens increased their production 50 percent last year.