Is Cannibalism Really an Evolved Survival Strategy of Bluefin Tuna Larvae?
AbstractMarine bony fishes such as bluefin tuna undergo remarkable life-history changes between their brief yet vulnerable time as millimeter-scale larvae to their many years as majestic, meter-scale adults. Throughout ontogeny, specific strategies related to feeding, avoidance of predators, and (for adults) reproduction are highly variable among fish species. A clear link exists between the spawning strategies of adults and the survival strategies of the larvae, as spawning behaviors determine when and where larvae will occur as well as the conditions to which they will be exposed. Adult Atlantic bluefin tuna undertake impressive migrations from their feeding grounds in the North Atlantic to place their vulnerable offspring in one of two locations—the Gulf of Mexico or the Mediterranean Sea—both of which are known to have some of the most unproductive waters in the Atlantic. To explain this paradox, it has been hypothesized that bluefin tuna larvae are cannibalistic, and that larval cannibalism is an evolved strategy that allows for the survival of at least some individuals within the oligotrophic waters in which they are spawned. Yet this hypothesis has never been tested. The goal of this project is to collect the first data on the feeding habits of bluefin tuna larvae in the Gulf of Mexico and to determine whether an ontogenetic diet shift to consuming other fish larvae (and, specifically, other bluefin larvae) occurs during the larval stage. This work will be accomplished using a suite of complementary tools and techniques that include (1) conventional gut inspections, (2) age and growth rate analyses derived from otoliths, (3) DNA barcoding to identify consumed fish larvae, and (4) trophic level determination using bulk and compound-specific stable isotope analyses. Results from these efforts will then be incorporated into a coupled biophysical model to explore what oceanographic features, levels of spawning output, larval mortality rates, and larval behavior would be necessary for cannibalism to be a feasible survival strategy for bluefin tuna larvae. Overall, the project will use novel approaches to fill in some major gaps in our knowledge of the early life history of larval bluefin tuna in the Gulf of Mexico, and it will also address an intriguing theory related to the the spawning and survival strategies of this ecologically and economically important species.