Fishermen Bite Off More Than They Can Chew — With A Little Help From Their Dolphin Friends

Credit: Mauricio Cantor, Universidade Federal de Santa Catarina

Let’s say you live in front of your favorite restaurants. How often would you be chasing around an ordinary food truck meal?

Theory predicts that animals—humans included—search for food in ways that maximize net energy intake[1]. Upon finding a food source, the higher its quality and energy content the less likely us animals would wander around for something else to eat. Conversely, the wider one roams looking to grab a bite, the more abundant the food patch should be to compensate the effort. These predictions generally hold true, from nomadic hunter-gathers living in the great wilderness to industrial fishing vessels venturing further offshore after the next profitable fish stock. We found the same principles among dolphins.

In studying how individual dolphins find food, we found they basically use two distinct tactics in Laguna, southern Brazil[2].Some individuals of this small population play the classic dolphin: they form brief groups to prey on passing fish schools. Given the unpredictability of this food source in time and space, these dolphins typically roam over a wide area. Meanwhile, other dolphins remain at specific shallow spots, where a line of artisanal, net-casting fishermen await patiently—they wait for the right moment.

The water is murky. Local fishermen have long figured that the dolphins, with their sonar-like echolocation apparatus, are the only ones that can track fish schools in such an opaque environment. So, fishermen learned to read the dolphins’ behavior. They wait for the dolphins to ‘work the fish’, that is, compact and herd the school towards fishermen. Then there’s the ‘signal’, the apex of their interaction. Experienced fishers know the specific, stereotyped dolphin moves—a sudden dive, a tail or head slap—that give away the right moment to cast the net[3]. Dinner is served.

And a dolphin’s dinner is not a dog’s dinner.

Different from other places where dolphins depredate human fisheries, in Laguna the fishery benefits from dolphins. The human-dolphin fishing happens all year but it’s between April and June, when mullet fish migrate northwards to reproduce, that several enthusiastic and professional fishermen line up in the shallow waters of the Laguna canal, packed in like sardines, to try their luck. On the terrestrial side, the interaction is clearly advantageous: fishermen catch more fish when dolphins are present than when casting their nets at will alone.The lineup is far from a fine kettle of fish; given these benefits, fishermen have their own set of rules that self-organize who gets a chance to fish with the dolphins[4].They are very fond for the dolphins that fill up their nets and help them bring home the bacon. The benefits for dolphins aren’t this clear.

But why would dolphins cue fishermen in?

Why self-serving individuals risk themselves and expend energy to benefit others has been an apparent paradox since Darwin’s seminal work on natural selection. Any cooperative behavior is vulnerable to free-riders; a ‘cheater’ that benefit but do not pay a cost should do better than others, and the two-way cooperation quickly becomes doomed to failure. Theoretically, cooperative behavior should persist when individual benefits overweigh its costs[5]. However, quantifying such trade-offs in the wild remains challenging. Some of these dolphins have practiced this cooperative fishing for decades and more experience than fishermen has had hot dinners. But what is in there for them?

As always, there is no such thing as a free lunch. Dolphins benefit from this fishing business too. But exactly how so is still unclear. The same murky waters that prevents fishermen to track fish make it very challenging for researchers to quantify the dolphins’ catch underwater. The hypothesis is that dolphins should benefit from going after the disoriented fish that break free from the school when the nets are casted. Now, not having to chasing around the dinner is another, yet indirect benefit. The larger the foraging area, the higher the energy expenditure. Thus, if interacting with fishermenis indeed advantageous then cooperative dolphinsshould concentrate their foraging activities around the sites where the interaction occurs. This is what we recently found out.

We spent many hours following dolphins around with our small boat, caring for how they forage and where they spend their time. Piles of GPS and behavioral data later, we sketched their home ranges and favorite feeding spots. We found that the dolphins that interact with fishermen range mostly around the fishing sites and so benefit from not traveling as widely to find food, while dolphins that do not interact mustforage over larger areas[6].

Fishing with humans looks advantageous—so why not all dolphins partake in this tactic?

For dolphins, interacting with fishermen is not an either-feast-or-famine kind of choice. Those who spend most of their time working at the fishermen line—the “good dolphins”, fishermen say—eventually forage by themselves too. Similarly, the dolphins that spend most of their time foraging by themselves, sometimes are seen at the interaction sites. “Bad dolphins” they are called. At first, it looks like “bad dolphins” have a bigger fish to fry, as they cruise along the interactive fishing sites with apparent indifference to the keen fishermen. However, they may not get many opportunities to give this tactic a try.

It’s been long hypothesized that access to know-how is what makes a dolphin become cooperative or non-cooperative—the “good and evil” dichotomy for the fishermen[7]. Dolphins, like fishermen, are notable learners too. They innovate through trial and error; they observe and copy the behavior of their mothers and peers. Decades of research in the wild and in captivity attest that dolphins can learn skills individually and socially. Recently, we found that the structure of these dolphins’ society is very much shaped by the foraging tactic they use: the ones that interact with fishermen have more and stronger social relationships among themselves, just like the dolphins that forage independently form their own clique. If dolphins get the hang of interacting with fishermen from their peers (from noticing fishermen, herding the fish towards them, and targeting loose fish after the net strikes), then it makes sense that a dolphin first needs to become a member of the knowledgeable clique, then practice to become a proficient cooperator. We find a similar dynamic among their Australian cousins. In Shark Bay, bottlenose dolphins that ferret fish from the sea bottom with their beaks protected by soft marine sponges hang out more among themselves than with the “non-sponger” dolphins[8]. The Aussie dolphins also must learn, then practice, to master the tool-using technique. Who one interacts with help defines the foraging traditions they learn—like some of us using chopsticks over forks, and some dolphins using sponges or fishermen.

But learning processes alone don’t tell the full story. Competition for access to the interaction site is also key.Competition is omnipresent in nature and it wouldn’t be different on both sides of the human-dolphin cooperation. As fishermen compete for a spot in line, competition can be fierce among dolphins too. Some are better foragers at the high-quality interaction sites than others, and may competitively exclude the so-called “bad”, non-cooperative dolphins.

Our results combined suggest that the two alternative foraging tactics emerge from the individual trade-offs involving food access, foraging area, learning, and competition. Dolphins may choose between foraging independently over larger areas on unpredictable, passing fish schools; or competing for the opportunity to learn how to fish more efficiently with fishermen at the more predictable interaction sites. From a theoretical viewpoint, human-dolphin interaction highlights how individual behavioral variation can scale up and structure a population, both socially and spatially. After all, those who behave in certain ways spend more time together at certain places.

Lessons from a unique system

This studies also offer us two broader insights. First, in nature no single behavioral strategy is the best. The best strategy may arise from a combination of fate, opportunity, skill, and its efficiency may depend on how many others are using the same strategy. After all, staying in front of your favorite restaurant will only do good if you can find (and afford to buy) a place to live there, otherwise, exploring other foodie corners of the city may turn out an equally good strategy.

Second, it reminds us of the beauty and fragility of cooperating with others. For one, seeing fishermen and dolphins working together is a nice example that even when each party is working for themselves they can mutually benefit. On the other hand, a cooperative interaction will only persist if the number of players is stable, and if the fine balance between costs and benefits is positive for them. The Laguna dolphin population has undergone a recent high mortality, especially due to pollution and entanglement in nets of other fisheries. Likewise, experienced net-casting fishermen have been seeking out more profitable activities, and the lineup has been replaced by amateurs. Our next quest is quantifying the cost-benefit balance for dolphins so to understand the mechanisms that control this unique cooperation, and ultimately assess how fragile it is to these perturbations. Theseare the key steps to conserve an unusual interaction that is ecologically important for dolphins, and culturally and economically important for local fishermen and guarantee that all—humans and dolphins, cooperative or not—have a lot on their plate.

[1]Pyke, G. H. (1984). Optimal foraging theory: a critical review. Annual review of ecology and systematics, 15(1), 523-575.

[2]Cantor, M., Simões-Lopes, P. C., & Daura-Jorge, F. G. (2018). Spatial consequences for dolphins specialized in foraging with fishermen. Animal Behaviour, 139, 19-27.

[3]Simões-Lopes, P. C., Fabián, M. E., & Menegheti, J. O. (1998). Dolphin interactions with the mullet artisanal fishing on southern Brazil: a qualitative and quantitative approach. Revista Brasileira de Zoologia, 15(3), 709-726.

[4]Peterson, D., Hanazaki, N., & Simoes-Lopes, P. C. (2008). Natural resource appropriation in cooperative artisanal fishing between fishermen and dolphins (Tursiops truncatus) in Laguna, Brazil. Ocean & Coastal Management, 51(6), 469-475.

[5]Nowak, M. A. (2006). Evolutionary dynamics. Harvard University Press.

[6]Cantor, M., Simões-Lopes, P. C., & Daura-Jorge, F. G. (2018). Spatial consequences for dolphins specialized in foraging with fishermen. Animal Behaviour, 139, 19-27.

[7]Simões-Lopes, P. C., Daura-Jorge, F. G., & Cantor, M. (2016). Clues of cultural transmission in cooperative foraging between artisanal fishermen and bottlenose dolphins, Tursiops truncatus(Cetacea: Delphinidae). Zoologia, 33(6).

[8]Mann, J., Stanton, M. A., Patterson, E. M., Bienenstock, E. J., & Singh, L. O. (2012). Social networks reveal cultural behaviour in tool-using dolphins. Nature Communications, 3, 980.

These findings are described in the article entitled Spatial consequences for dolphins specialized in foraging with fishermen, recently published in the journal Animal Behaviour. This work was carried out by Mauricio Cantor, Paulo C. Simões-Lopes and Fábio G. Daura-Jorge from the Universidade Federal de Santa Catarina, Brazil.

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