Whale sharks and tunas hunt together

Abstract

The world's largest extant fish, the whale shark (Rhincodon typus), is a docile giant that occurs in tropical and subtropical regions globally. It prefers to feed on plankton, which it filters through its wide mouth. Adult whale sharks have been reported to occur in large numbers during warm summers off the Azores, an isolated archipelago on the mid-North Atlantic ridge (PLoS ONE 2014; doi.org/10.1371/journal.pone.0102060). As this oceanic region is oligotrophic, the whale shark's favorite food is not abundant. Instead, we found that they prey on snipefish (Macroramphosus sp) that have been corralled into “bait balls” at the surface by large schools of bluefin (Thunnus thynnus) and tropical tunas, including bigeye (Thunnus obesus), skipjack (Katsuwonus pelamis), and yellowfin (Thunnus albacares). A feeding frenzy often ensues. With the baitfish corralled, the whale sharks then rely on powerful suction to fill their massive mouths with prey. This shark–tuna feeding association has rarely been observed elsewhere, yet in these islands it is the norm when both whale sharks and tunas are present. We have tagged whale sharks with high-resolution biologgers (with accelerometers, cameras, and gauges to measure location, pressure, and temperature) to help elucidate the ecological importance of this unique associative behavior.