it's something that roberto mielgo has seen firsthand. he was around when purse 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, w