Canned with bluefin tuna

A tireless traveler, bluefin tuna cover thousands of kilometers in the course of their lives. What does it do during its peregrinations? To lift the veil on the intimacy of this large fish and better understand its population dynamics, marine biologists and microelectronics and microelectronics researchers.

It can grow to over 3 meters and weigh over 600 kilos, "the size of a cow". It lives for almost 40 years and travels up to 200 kilometers in a day. Bluefin tuna: a migratory fish whose name has long been associated with overfishing, but which today is doing well. "Atlantic bluefin tuna was overfished from the mid-1990s onwards, but since 2012 the stock has been doing better," explains Tristan Rouyer. Overexploited, yet still little-known. "There's still a lot we don't know about tuna population dynamics," explains the researcher from the Marine Biodiversity, Exploitation and Conservation Laboratory (Marbec*). The reason? The difficulty of tracking such an imposing and mobile animal.

"To study wild animals such as bluefin tuna in the wild, we use electronic tagging," explains Vincent Kerzérho. Problem: the systems used to track fish are very expensive. " Between 3,000 and 4,000 euros per animal", explains the researcher from the Montpellier Laboratory of Computer Science, Robotics and Microelectronics (Lirmm**). Quite a budget for a single-use device. "And quite an obstacle when you consider that to study a population on a large scale you need to collect a lot of data." To overcome these difficulties and improve our knowledge of these big fish, researchers from the two laboratories decided to work together.

Fishing for experts

A collaboration that began in 2015 with the Popstar and FishNship projects. "We set out to find experts in microelectronics to develop new solutions," recalls Tristan Rouyer. With hisIfremer colleague Sylvain Bonhommeau, the biologist went fishing for collaborators and hooked up with Serge Bernard and Vincent Kerzérho's team at LIRMM. "The contact went very well, both scientifically and on a human level.

Their mission? To develop less expensive, more complete marking devices. " Commercially available devices only incorporate 3 sensors to measure pressure, temperature and light in order to determine the fish's position," explains Vincent Kerzérho. While these measurements enable us to track the tuna's movements, they provide no information on the animal's physiology. "We know where it is, but we don't know what it's doing there. Is it hunting? Breeding? Is it building up reserves or consuming them? This is vital information for understanding the life cycle of tuna and their migratory dynamics," explains Tristan Rouyer (see box).

Fish scale

The micro-electronics specialists have therefore concocted a new sensor that not only geolocates the fish, but also provides information on its fat content. " We use bio-impedance, a technique developed in the medical field, the same one used for scales that give fat levels. The sensors will also have to withstand the hostile environment of salt water, with its pressure at depths of up to 1,000 meters, where tuna frequently make incursions. And they will also have to meet the challenge of autonomy. "Tuna spend a lot of time at depth, which makes continuous data transmission impossible. So we designed a system that unhooks itself after 6 months to 1 year. It then returns to the surface, where it finally transmits the information collected," explains Vincent Kerzérho.

Timing

The technical challenge has been met, with a micro-electronic device measuring just a few square millimeters, connected to electrodes that have to be implanted in the fish's flesh. This is where another challenge begins... Because as Tristan Rouyer points out, "tagging a 250-kilo tuna is complicated". And with good reason: you have to get this enormous fish out of the water. "We did this with the help of the SATHOAN fishermen from Sète. Tuna fishing is carried out using a seine net, a huge net with which boats encircle schools of fish. Researchers take advantage of the opportunity to capture individuals in a carefully timed operation. A line fitted with a hook is cast into the seine, the tuna that bites it is hoisted aboard in a stretcher and then intubated on the boat, where it spends less than two minutes, "the time it takes to deploy the equipment". The animal is then released and continues on its way, with the researchers in its wake.

Transatlantique

Since 2018, scientists have tagged 8 tuna, gathering valuable data in the process. " One of the tuna tagged in the Mediterranean went as far south as Iceland, then on to Canada, before crossing the Atlantic again and returning to the Mediterranean," explains Tristan Rouyer. A year-long journey that provides more precise information on the migratory routes taken by these great travelers.

And this is just the beginning: the researchers would like to tag more tuna to gather even more data. " This information is invaluable, because a better understanding of the migratory dynamics of these populations enables us to better understand the use of their habitat, particularly in relation to environmental conditions", explains Tristan Rouyer, who also stresses the importance of this knowledge in anticipating possible changes linked, for example, to global warming. "The more we know about the species, the better we can preserve it.

Highly migratory

A long-distance traveler, bluefin tuna have long kept the map of their routes secret. Their migrations meet two essential needs: to feed and to reproduce. "Bluefin tuna reproduce in the warm waters of the Mediterranean. Outside this breeding zone, they scour the colder waters of theAtlantic for food, even going as far as the northern areas near the coasts of Norway and Canada, where they hunt herring and mackerel," explains Tristan Rouyer. It can then gain 30% to 40% of its mass, which it then loses during the breeding season.

*Marbec (UM - CNRS - IRD - Ifremer)
**Lirmm (UM - CNRS)