Researchers on the high seas
Fishermen have long observed the behaviour of tuna fish, even benefiting from it to make bigger catches. Scientists working on the Fadio project have now drawn inspiration from their methods for other purposes. Sophisticated equipment, tailor-made for their needs, should make it possible to study the mysterious behaviour of these fish, estimate their population and movements, and find out more about their role within the marine ecosystem – and the potential dangers they face.
A thick branch is floating in the middle of the Indian Ocean – it may have been spewed out by a Madagascan river in flood or torn from a tree in cyclone-hit Tanzania. It has been adrift for weeks. Scientists on the Fadio project have just discovered it and are excitedly preparing to launch an armada of instruments and sensors as this apparently insignificant branch has attracted several thousand tuna fish.
|Group of Dolphin Fishes (Coryphaena bippurus) –Indian Ocean © Bertrand Wendling|
Researchers are interested in the behaviour of this emblematic fish, an athletic hunter and tireless migrator, able to swim across entire oceans. The interest is partly due to its economic importance, but above all to its ecological role. At the head of the food chain, this large predator plays a key role in the pelagic ecosystem(1). Given the extent to which it is fished, any indications that can help estimate population size are very valuable.
The behaviour of tuna fish is in itself a fascinating mystery, central to which is the notion of the school. Tuna – and most of their pelagic relatives, and even much smaller species such as anchovies and sardines – regularly come together in schools. Are these concentrations linked to food, reproduction, predatory behaviour or orientation? How long do they last, and how often do they occur? What environmental factors (temperature, available food, potential dangers) play a part in the creation, expansion or dispersal of these schools? Pertinent questions every one, but questions to which we have few answers.
It is not easy trying to monitor the behaviour of the tuna fish which has a long life and is highly mobile, disappearing from view and then resurfacing at the other side of the ocean, where you least expect it. But one aspect of its behaviour does help the researcher: its tendency to collect around floating objects. This can be a piece of wood, rope, a tyre, or even a simple coconut. Whatever the particular nature of the object, for the scientist it is an FAD or Fish Aggregating Device, acting as a magnet by attracting fish in sometimes impressive numbers.
|The first mission by the Fadio project to the Seychelles – fitting an echo sounder to the exterior of the vessel.© IRD/C. Girard|
The phenomenon has not escaped the attention of fishermen who, after making use of natural FADs, decided to manufacture their own. There are many such buoys anchored around the islands of the Pacific, the Atlantic and the Indian Oceans, generally used by local populations for subsistence. But there are also hundreds of 'drifting' buoys deposited in the open sea by seiners, huge industrial fishing vessels using nets some 1 600 metres long and 200 metres deep in which they try to entrap the schools of tuna fish. Today, more than half of tropical tuna fish caught worldwide (3.8 million tonnes a year) are caught with the aid of an FAD.
The primary aim of the Fadio researchers is to manage and conserve the natural environment – and that means understanding how it works. They are also looking at possible ways of using these FADs as windows through which to observe the pelagic ecosystem. Attracting concentrations of fish in the way they do, they provide ideal 'observatories' for studying this otherwise very elusive fauna. But to do so means having the equipment to be able to observe what is happening below the surface.
High-tech and customised FADs
Fadio therefore set about designing and making special FADs, packed full of instruments so as to collect the maximum information. Buoys with simple vertical sounders already exist. 'This type of device, which is quite easy to use, has one drawback,' explains Laurent Dagorn, project coordinator. 'It gives little information about the tuna fish themselves because they have the unfortunate habit of not liking to swim beneath the FAD, preferring to remain alongside it.' The sounder is nevertheless essential for understanding the fauna composition, especially the smaller fish which are the tuna's natural prey.
|Example of a Fish Aggregating Device (FAD). Made from wood and bamboo, this 'floating object' is able to attract fish in impressive numbers sometimes. © IMR/L.Nottestad|
Future buoys will be equipped with an omni-directional sonar. This will make it possible to know what is going on around the buoy, and thus details of the number of tuna fish present, the depth at which they are swimming and perhaps even the specific species and size. 'With a horizontal beam, you collect more complex data requiring more sophisticated software to interpret them,' continues Dagorn. 'There are also setting-up problems to be solved. The engineers who prepare the equipment must know, for example, whether it is better to observe at 300, 500 or 800 metres to capture 80% of the biomass. Compromises must be made, as the further you look, the more energy is needed.'
These buoys, which are genuine technological marvels, will also be equipped with hydrophones to record sounds (an important source of information) and a listening station with which to read the data supplied by the 'tags' that the researchers have fitted to some fish.
Benefits of tagging
Tags are small transmitters either attached to the tuna fish or surgically placed in its abdominal cavity. They give information on a particular fish and are the logical complement to the sonar which provides information on group behaviour. These tags not only indicate the presence of a fish of a particular origin, but also the depth and temperature of the water it is swimming in. Fadio is also trying to develop new sensors able to indicate if the tagged fish has an empty or full stomach, is alone or is a member of a group.
As one of the pioneers of this system, Kim Holland, of the University of Hawaii, has unrivalled know-how. Holland's university is closely involved in the Fadio project and, together with the Institut de Recherche pour le Développement (FR), was the only institution for many years to show an interest in tuna fish behaviour around FADs. It has its own large pool where it is possible to study these animals in captivity.
Together, all these instruments should make it possible to build up a valuable fund of information on tuna fish behaviour. The Fadio project is concentrating on instrumentation for drifting buoys, while the University of Hawaii is working with anchored buoys. Collecting the data is not the only problem, however. The information must be stored for subsequent retrieval, while the various instruments also need a power supply. Whatever the solution finally adopted from among the many possibilities, it would no doubt be preferable not to have all the instruments operating continuously but only at the time of the most significant events. It will be for the researchers to try and identify these periods. We know, for example, that many small pelagic fish form schools during the day and then disperse at night. If tuna fish do the same, nocturnal data collection would not yield a result.
Co-operating with the fishermen
Fadio's first sea mission was designed to travel to the study zone off the Seychelles, find the FADS, test and calibrate the equipment and establish contact with the fishermen. 'We did not exactly expect a hostile reception,' points out the coordinator, 'but we were surprised by the readiness with which the fishermen were prepared to help us. They told us where to find their buoys which, in their business, is obviously information that must be very jealously guarded. It augurs well for the future as we will no doubt soon be asking anyone who finds one of our buoys to collect it so we can retrieve the information.'
|Top right, the Skipjack tuna (Katsuwonus pelamis, 2-5 kg, 40-50 cm), one of the most common species found around FADs. Bottom right, the Yellowfin tuna (Thunnus albacares, 4-5 kg, 50 cm although sometimes measuring over a metre), which can also be found around floating objects.© IRD/P.Opic|
The contact was helped by the presence of French and Spanish fishermen in the team – the two principal nationalities of fishermen in this zone. Greek, Belgian and Norwegian scientists are also working on the project, each contributing specific skills. 'We are working, for example, with a team from the University of Las Palmas, in the Canary Islands, that is specialised in remote detection,' explains Dagorn. 'In fact, we need precise satellite information on the zones where our FADs will be located. We need as much data as possible on the environment around the buoys (temperature, quantity of plankton, etc.) because if any one of them indicates a change in behaviour we must try and find out if there is an environmental explanation.'
Understanding how and at what distance fish locate FADs (perhaps from 10 km), why and when they form schools, and how their diet influences their behaviour, are all of scientific interest. Beyond that, ultimately Fadio could help provide for the better management of fish stocks and thus pave the way towards a more sustainable fishing. At present, the only real source of information on fish populations is the fishermen themselves. This information is often incomplete and subject to bias. Acoustic data therefore provide an independent and complementary source of information when compiling fish statistics, to enable study of the populations of species fished.
In the longer term, researchers hope that – as the tuna fish reveal their secrets and more instrument buoys are launched – it will be possible to obtain details of population status, their spatial dynamic and other elements, all of which are vital to protecting a bountiful but fragile resource.
(1)Located in surface waters as opposed to the benthic ecosystem on the seabed.