In Europe, Oceana is working to reduce the negative effects of bluefin tuna fattening pens.

Sometimes also called “tuna ranching” or “tuna fattening,” this activity involves catching wild tuna and holding them in feeding pens while they are fattened for delivery to different markets, often in Asia.

In the Mediterranean, among other places, juvenile bluefin tuna, a species on the brink of extinction, are captured and held for varying lengths of time in net pens. They are fed frozen prey fish and later sold on the international market for sushi and sashimi, particularly in Japan and Korea.

Bluefin tuna are voracious predators, and require between two and ten percent of their body weight in prey fish every day of the peak summer season. This results in an estimated 225,000 metric tons of prey fish being used in Mediterranean tuna pens alone each year.