Tuna Belly in Olive Oil. The belly or ventresca of tuna is the juiciest and most tender part, being produced fresh and artisanally during the campaign or "Costera" of the Cantabrian Bonito during the summer months, a true delicacy in canned form. Conservas Costera, a family cannery from Gijón, carries out a production process that preserves its flavor and juiciness for up to 6 years from the preferred consumption date. Gourmet preserves perfect for your table.