it's something that roberto mielgo has seen firsthand.rse seiners first started fishing for tuna in the mediterranean. how many of these vessels are there in the mediterranean right now? >> maybe 39 french, 60 tunisians, i would say 60 croatians, 120 turks, 92 italian. >> you're dealing with an enormous business. >> this is huge business, yes. it's--the stakes are very high. >> mielgo has seen as many as 300 tons of bluefin tuna, worth as much as $2 million, trapped inside one of these nets. divers open a gap and count them as they're transferred into pens the size of a football field. tugboats then slowly drag the pens with the live tuna inside to tuna ranches. tuna ranches? to me, the word "ranching" refers to cattle. >> yes, but you do not breed the bluefin tuna at the ranch. you actually fatten the fish to gain up to 20% weight. >> they feed them sardines and mackerel. they control the color, the flavor. in three to six months, the tuna will be big enough and fat enough to harvest. 90% of them will go to japan, which imports as much t