Every summer in waters off the town of Laguna in
Brazil’s southeast corner, fishers wade out into estuary canals to cast their
nets in hopes of catching migratory mullet fish. The water is murky, and the
fish are difficult to spot. However, the fishers have help from an unexpected
quarter: bottlenose dolphins that push the prey toward the nets.
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The two species of predators have coordinated their fishing
for generations.
“The experience of fishing with dolphins is unique,” said
Wilson F. Dos Santos, a Laguna fisherman who has been casting nets alongside
the dolphins for 50 years. He learned the practice when he was 15, taking his
father coffee and food as he fished and watching the aquatic partners at work.
He added that working with the dolphins “helps with our family income”, because
human families eat what they catch.
In research published earlier this year in the Proceedings
of the National Academy of Science, a team of Brazilian scientists reported
that dolphins might benefit from the cooperation as much as their terrestrial
mammal counterparts did. Those that fish with humans seem to live longer than
other dolphins in the area do.
“Human-wildlife cooperation in general is a rare phenomenon
at a global scale,” said Mauricio Cantor, a biologist at Oregon State
University and an author of the paper. “Usually humans gain the benefit, and
nature pays the cost. But this interaction has been happening for over 150
years.”
“The dolphins know what they are doing,” Daura-Jorge said. “They’re taking advantage of the fishers’ actions to actively forage.”
Humans have worked with other species to find food for
millenniums, including honeyguides in southeast Africa and Indigenous American
accounts of hunting alongside wolves. And net-casting fishers working with
dolphins is not unique to Brazil — the practice has also been observed in
Mauritania, Myanmar, and India, but the bottlenoses of Laguna are the most
famous. The local population of about five dozen cooperative dolphins has been
systematically monitored since 2007, said Fábio G. Daura-Jorge, a biologist at
the Federal University of Santa Catarina in Brazil and an author of the paper.
In 2017, the team began surveilling both the fish and dolphins with GPS,
drones, and sonar.
Dolphin signalsThe team found that the dolphins gave a cue — usually a
sudden, deep dive — to signal that they had driven prey within reach of fishers’
nets. Eighty-six percent of the successful catches during the study period came
from fishers reading dolphins’ behaviors, Cantor said. Careful observation and
timing were key: Those who cast nets too late or missed dolphin cues were less
likely to catch anything.
The dolphins also time their foraging carefully. The team of
researchers used hydrophones to measure the animals’ echolocation clicks, the
rate of which increased as the nets hit the water. When a fisher made a
successful cast, the dolphins homed in on disoriented mullet or plucked a few
fish from inside the net. When fishers mistimed their cast or failed to respond
to the dolphins’ cues, the dolphins did not strike.
“The dolphins know what they are doing,” Daura-Jorge said.
“They’re taking advantage of the fishers’ actions to actively forage.”
The human participants proved to be keen observers, too. The
fishers shared a wealth of experience with researchers about how the dolphins
and fish behaved, and they knew how to recognize the dolphins that made good
fishing partners. The fishers identified their partners’ clicking and told
researchers that they felt the buzz of the echolocation in their legs when the dolphins
“cry out”.
SymbiosisThe strategy has clear benefits for the dolphins, Cantor
said: Those that hunted with humans were 13 percent more likely to survive to
adulthood. Cooperative dolphins tended to linger near the fishing grounds that
they shared with humans, while other dolphins that traveled more widely
throughout the area’s water were three times more likely to end up tangled in
illegal fish nets.
While this partnership helps both sides, the practice has
been in decline over the past decade, Cantor said. Commercial operations
overfish mullet stocks across southern Brazil. As fish numbers drop, individual
dolphins and fishers hunt together less.
“Safeguarding the cultural tradition of fishers and dolphins is critical to keep them cooperating and also important to conserve the population of dolphins.”
Because the whole system depends on both sides having a
careful understanding of each other’s cues, Daura-Jorge said, it can easily
break down. That could make Laguna’s bottlenose dolphins appear to be more
competitors than partners, placing them under further threat.
“Safeguarding the cultural tradition of fishers and dolphins
is critical to keep them cooperating and also important to conserve the
population of dolphins,” he said.
Regulation of industrial mullet fishing and a crackdown on
illegal fisheries could help to ensure that there are enough fish to go around,
Cantor said. Area participants including Dos Santos are also committed to the
practice, which is a point of local pride and draws in tourists.
“It’s an interaction that goes beyond only the material
benefits,” Cantor said. “Trying to preserve cultural diversity is an indirect
way of preserving biological diversity, too.”
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