The Enigmatic Spawning Strategy of Bluefin Tuna

Bluefin tuna (BFT) is an icon of the oceans, with a special place in the minds of people, also up along the Norwegian coast. Armed with a mesothermal strategy of warm blood and a cold heart, it is a powerful predator on mackerel and herring in the Northern seas. Still, their eggs and larvae need warm water to hatch and grow and therefore the adults migrate south to mate and spawn. However, while the eggs hatching success and larval growth rates peak above 25 degrees C, BFT spawn surprisingly early in the Mediterranean Sea, just as temperatures pass the minimum hatching temperature of 20 degrees C. What is it that forces BFT to place their newborns into these apparently sub-optimal conditions? Here, we present three hypotheses to explain this strategy: 1) the need to match the occurrence of larval zooplankton prey; 2) an evolutionary cannibalistic arms race where earlier spawning batches can surf-ride on the size spectrum of younger siblings; and 3) an internal life-history mismatch where constraints on the time-budget of adults are more important than egg fitness. These hypotheses are not mutually exclusive, and we evaluate their validity with a combination of data and theoretical models.