Research Stories: A mission to tag giant mantas in northern Peru

Authors Kerstin Forsberg and Samantha Andrzejaczek recall their team’s efforts working with NGOs and local government, scientists and community to produce their latest research on the behaviour and conservation of manta rays in northern Peru.

It was a sunny day in May 2018, and we were out in the waters of Tumbes, northern Peru, searching for endangered giant oceanic manta rays (Mobula birostris). Our team included Planeta Océano staff and volunteers, Stanford University researchers, invited local government officers and scientists, and local fishermen.

Our mission: equip mantas with satellite tags to better understand their behaviour and help guide their conservation.

The tags would help us to understand where manta rays were going, as well as record the depth and temperature of their movements. Understanding these are critical considering the importance of Peru’s manta population. Although legally protected in Peru, very little is known about this species, and the likelihood of mantas incidentally interacting with artisanal fisheries in this low-income tropical region of Peru is high.

The fishing village of Zorritos in the Tumbes region (northern Peru), where our team set out each day © Juan Zacarías

The plan to unite Stanford’s scientific expertise with Planeta Océano’s (PO) community-based manta ray efforts had arisen in a conversation two years ago, between Rolex Laureates Professor Barbara Block and Kerstin Forsberg at the 2016 Rolex Awards ceremony. Our joint team was now five days in the water searching for mantas aboard local fishing vessels. Yet despite our best efforts and the large size of these animals (getting up to seven meters in diameter!), we were not finding any individuals.

We would set out every day before 6 am and navigate over 15 nautical miles off the coast. Together with local fishermen’s expert eyes, we would patrol the area hoping to spot mantas at the surface. It was our final day on the water, we had our return flight booked for that evening, and we were heading back to land as we finished our last patrol without any sightings.

Then suddenly, one of the fishermen shouted… ‘Manta!’

Expedition crew members, on-board a local fisherman’s boat, observe a manta ray swimming at the surface of the water © Juan Zacarías

Robbie Schallert, in charge of tag deployment, swiftly got in the water. Swimming against time as the wind started picking up and the flight departure time edged closer, Robbie free-dived, accurately positioned himself and tagged the manta. Incredible excitement erupted on board as we witnessed a manta being tagged for the first time in Peru. Enthusiasm was shared between the scientists, young volunteers and local fishermen, some of which had even harvested mantas in the past, prior to being involved in PO’s conservation efforts.

We would later run a public contest to name this manta, afterwards called ‘Mylo’, short for its Family Myliobatidae. Mylo (MR1 in our paper) was tracked for 3 months migrating back and forth between Tumbes and the coast of Ecuador.

Co-authors Kerstin Forsberg, Robert Schallert and Natalie Arnoldi exchange ideas during the expedition © Juan Zacarías

Tempted to re-book flights to continue tagging, we finally opted to organise a second expedition two months later.

This second time we encountered more mantas, but tagging wasn’t any easier. Our boat would stay at a distance and Robbie would swim to tag the manta. Yet this time the mantas, reported to swim at speeds of up to 20 km/hour and constantly diving up and down to the seabed over 40 meters below, were skittish and would quickly dive away once approached.  We had to repeat this process numerous times over the week to tag an additional four mantas.

A giant manta ray interacts with a freediver in northern Peru © Joost van Uffelen

The tags ultimately provided information on the diving behaviours carried out by the manta rays, as well as transboundary and long-distance movements towards continental Ecuador and the Galapagos Islands. Although two mantas generally remained around the coast, one moved a long way offshore (>1000 km!), being the longest migration recorded by one of these individuals in the region to date.

When on the coast, we found that mantas displayed patterns of reverse diel vertical migration, meaning their movement patterns changed on a daily cycle. During the day, the mantas mostly swam in warm surface waters. At night, however, they began continuously diving up and down between the surface and depths of about 40 meters. Why were they doing this? We aren’t too sure, but guess it’s a strategy to dive down to feed on zooplanktonic prey that can only be found in deeper waters.

However, given the drop in temperature with movement into deeper depths, we think they need to come back up to the surface to rewarm at the surface in between dives, similar to ducking into a freezer to grab some food and having to come out and warm up afterwards.  When offshore, movements were deeper, with one manta diving down to a maximum depth of 648 meters!

Understanding the diving patterns carried out by manta rays is critical to support their conservation © Joost van Uffelen

The exciting information provided by this study published in Ecological Solutions and Evidence aims to inform management and conservation strategies for the region. Although oceanic manta rays are protected in both Peru and nearby Ecuador, they remain threatened by a number of human activities. The high use of surface waters recorded by our study increases the risk of interaction with vessels moving through the area, as well as the risk of being caught in fishing nets.

Such interactions threaten not only the mantas, but also the local fishermen. Mantas can be accidentally caught in nets and, given their large size, are difficult to disentangle, causing socio-economic impacts to small-scale fishermen through gear damage and loss of fishing gear, with a loss of up to USD 1300 per interaction according to some fishermen.

Even more, our results highlight the need to further strengthen collaborative management and conservation strategies between both countries to effectively protect the population, as well protect mantas offshore as they move through Areas Beyond National Jurisdiction.

We aim to continue our work in northern Peru to continue understanding how and why oceanic mantas use this area on an annual basis, as well as raising awareness for the importance of these animals in our oceans. Together with PO’s Marine Educator’s Network of Northern Peru, an educational module based on this research is also being implemented in schools, where students can simulate being researchers by analysing data, and learning about manta ray conservation. Hopefully, some of them will be inspired to become marine scientists and conservationists in the future!

By teaching children about manta ray tagging at a local school in Zorritos, we promote their interest in science and conservation © Juan Zacarías

Read the full article: “Reverse diel vertical movements of oceanic manta rays off the northern coast of Peru and implications for conservation” in Issue 2:1 of Ecological Solutions and Evidence.

Some more photos of Manta rays:

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