A recent video making the rounds on social media shows a large pod of dolphins on the KZN north coast. The appearance of dolphins in the video brought much delight to marine mammal fans along the coast who were happy to see them back after a short hiatus. For many anglers that watched the video, something much more exciting may have been noticed. Namely, the large school of dolphin-sized yellowfin tuna travelling alongside the dolphin pod.
Tuna fishermen (pole and net) have long tracked down dolphin pods, associating them with schools of tuna. This led to a significant amount of dolphin by-catch causing public outcry among cetacean (whales, dolphins, and porpoises) lovers. Then began the "Dolphin-Safe" marketing, which meant that fishers didn't purposely circle a dolphin pod with a purse seine not to intentionally trap them. The purse seine nets cause a multitude of bycatch to vulnerable and endangered species (including sea turtles, sea birds, sharks or anything that happens to be in the area really), but hey, at least the dolphins are safe? So that answers the shore-observer question as to why ski-boats and kayaks are often chasing down a dolphin pod, but I think we are getting a bit sidetracked here. We know that tuna like to travel with dolphins, but why? What causes a fish, which is actually food for some cetaceans, to travel with a pod of "intelligent" oceanic super-predators?
Initially, scientists thought it might have something to do with feeding habits. This is the "feeding hypothesis", which suggests that the two species work together to find food sources. This theory has been put down by many scientists who note that dolphins and tuna tend to feed at different depths, at different times, and often on different prey.
An alternative hypothesis is that tuna school and travel together with similarly sized companions due to the old "safety in numbers" platitude, known as the "predation hypothesis". Travelling in large groups may offer some safety due to the dilution effect (whereby the risk is lessened by spreading it over a larger number of individuals), the confusion effect (whereby predators have increased difficulty in tracking a potential target within a large group of similarly coloured and rapidly moving individuals), the encounter effect (whereby a single predator would be less likely to encounter prey that is concentrated in a few large groups rather than dispersed in many smaller groups), and the vigilance effect (whereby predators can be detected more readily by integrating the senses of a large number of individuals) (Scott et al. 2012).
Large sharks, billfishes and larger toothed whales have been known to predate on both tuna and dolphins and researchers have noted that yellowfin tuna would stop feeding to follow spotted dolphins that were attempting to avoid their research ship, suggesting that ‘fleeing with dolphins would be advantageous to tuna if, as a general tactic, it results in escaping predators most of the time’.
A study by Scott et al. 2012, (click here to read it) that tracked both dolphins and tunas simultaneously found evidence to support the predation hypothesis, noting that both species show increased group sizes during the day, likely for the same reason, as both are potential prey for large sharks and small whales.
Now the only question we are left with is who is following who?