"Schools Only for the Young"
"...It is known that young tunas grow quickly and remain near the surface of the ocean for their first three or four years. The survivors of a single spawning often swim and feed together in schools. Some of these schools may contain several species of young tunas, for the young generally prefer the warmer surface waters while the adults seek the cooler temperatures of the depths. This schooling behavior may protect the young because the constantly shifting and shimmering mass of the school makes it difficult for a predator to single out an individual to attack... 

"...A tuna remains in surface schools for about five years. Then, for reasons scientists have yet to discover, it abandons the schools and becomes a more solitary, deepwater swimmer. Information concerning the swimming depths of the important commercial tunas has primarily come from depth measurements taken during fishing operations and research fishing tests. It has been found that the species composition will vary according to the season and geographic area being fished and the vertical distribution of the tunas. Different tuna species have different tolerances to ocean temperatures and oxygen levels. These tolerances will help stratify the tunas within a given area, not only by species by also by age group within a species..." 

Note that schooling behavior dictates which gear types are appropriate for different species. For example, some pelagic species -- like sardines, mackerel and squid -- swim in very large, distinct, tight formations. The most practical, efficient method for catching those kinds of pelagic fish is to encircle them with purse seine nets. On the other hand, albacore do not swim in tight schools, and encirclement gear is therefore not appropriate for catching them. Because albacore swim relatively far away from one another, fishermen must therefore catch them one-by-one with troll, longline, or pole & line gear.