'We’re not allowed to fish when the crabs make love’

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Edinburgh Napier research in Brazil is helping to protect a cultural icon, influence policy and save livelihoods. Now a new citizen science app will take this one step beyond.

Ucides coradatus or mangrove crabs are revered in Brazil. Artefacts several thousand years old carry their images while small villages celebrate them through beauty contests where entrants sport dresses made out of hundreds of crab legs. They are also the main source of income for thousands of low-income fisherman who can catch them without the need for expensive boats or equipment. That’s why Edinburgh Napier’s Dr Karen Diele has been working to ensure their sustainability.

“The guys who live from them are very poor, illiterate, marginalised and they can’t even afford fishing boats or nets. To catch these crabs, you don’t need a lot so they are really important for poverty alleviation.

“These crabs live in the mangroves and Brazil has about 5,000km of them. They dig two-metre-deep burrows. I work with fisherman over there who stick their arms into the mud to get them out. They have to protect their arms not because they are scared of the crab but because in the mud there are lots of sharp shells, and they can easily injure themselves - they don’t have any health insurance and can’t afford to lose income. I’ve caught them myself this way.”

Dr Diele from the School of Applied Sciences has been working with the crabs and the fisherman for a number of years to establish the species’ breeding pattern. She has worked out that the crabs always mate at full or new moon, and sometimes both. As a result, capture bans are currently in place at those times.

“That’s policy today. It prohibits catching at full and new moon. At these times, they are not in their burrows and it’s really easy to catch them. Therefore there’s a risk of overfishing. The fisherman are furious because mostly one moon is wrong and they’re not allowed to catch them but they don’t mate and the fishermen lose income. So it’s not a very good solution.”

Dr Diele, her colleagues and partners are now working on a way to improve the placement of these capture bans in order to modify the policy once more. 

“We are pretty sure we can predict it now. We have hides in the mangroves where we’re looking out at night and day. It’s mental work. It’s not magic when it’s happening. Big tides are due when the sun and the moon are in a straight line. There is still another cycle when the moon is closest to the earth. If that happens at new moon then the new moon tides are bigger than the ones at full moon. It’s a complicated cycle but these little things have worked it out.”

Dr Diele already has a network of partners along the Brazilian coast checking that their predictions are correct. Now, they are expanding the surveys to include the fishermen themselves. Built by a student from the School of Computing at Edinburgh Napier, a new app allows fisherman to record where and when they’ve seen the crabs mating. All this data is then recorded on a server built by another School of Computing student.

“It’s real citizen science. The more people that use the app and provide data, the easier it will be to change policy.

“We continue to work with partners and host workshops in Brazil. Now when we ask the fisherman what the most important aspect of crab fishery is, they say: “We're not allowed to fish when the crabs make love.”