Potential for top-down control on tropical tunas based on size structure of predator-prey interactions

The ecological consequences of widespread fisheries-induced reductions of large pelagic predators are not fully understood. Tropical tunas are considered a main component of apex predator guilds that include sharks and billfishes, and thus may seem unsusceptible to secondary effects of fishing top predators. However, intra-guild predation can occur because of size-structured interactions. We compiled existing data of apex predator diets to evaluate whether skipjack tuna Katsuwonus pelamis and yellowfin tuna Thunnus albacares might be vulnerable to top-down control by large pelagic predators in the eastern tropical Pacific Ocean. We identified potentially important predators of tunas by the frequency, quantity, and size and age of tunas in their diets and considered the degree that predated tunas could have potentially contributed to the reproductive output of the population. Our results indicate that the proportion of predator diets consisting of skipjack and yellowfin tunas was high for sharks and billfishes. These predators also consumed a wide size range of tunas, including subadults of the size capable of making a notable contribution to the reproductive output of tuna populations. Our study suggests that in the eastern tropical Pacific Ocean tropical tunas act as mesopredators more so than apex predators. Sharks and billfishes have the potential to play an important role in regulating these tuna populations. This study sets the stage for future efforts that could ascertain whether diminished levels of large predators have enhanced the production of tuna stocks and whether the trophic interactions of skipjack and yellowfin tunas should be explicitly accounted for when their population dynamics are assessed.