Eels’ travel in the Mediterranean tracked thanks to Argos & goniometer
Eels are still nowadays a very mysterious fish species. To better know their swimming behaviour when leaving the European coasts, accelerometers are used in conjunction with Argos pop-up tags to estimate speed variations during vertical migrations and infer energy expense of eels leaving Europe to reproduce in the Sargasso Sea.
The population of European eel (Anguilla anguilla) have dropped dramatically around the 1980s, without one single reason clearly pinpointed, but a number of possible ones at different stages of their life are examined. Young eels (glass eels) arrive in numbers on the European coasts, including in the numerous Mediterranean lagoons (e.g. Salses-Leucate) from the Sargasso Sea, where they are born. They remain in these coastal areas or go up to fresh water habitats such as rivers for several years, until they metamorphose into silver eels. Then, they change their color and become black on the back, white on the belly. In productive habitats such as lagoons, males (generally under 45-cm long) are 3 years old in average and females (generally over 45-cm long) are older, 5 years old in average (but may reach up to 50 years, as far as we know in less productive habitats). In Autumn, between November and January, these future parents leave the Mediterranean to reproduce in the Sargasso Sea, where they die afterwards.
An international team composed of scientists from the laboratory CEFREM at University of Perpignan, France (Elsa Amilhat, Gaël Simon and Elisabeth Faliex), the DTU in Denmark (Kim Aarestrup), the CEFAS in UK (David Righton) and the SLU in Sweden (Hakan Westerberg) tagged silver eels in autumn 2017 to better understand their swimming behavior at sea. This project was financed by the French Ministry of Agriculture and Food and was conceivable thanks to the tags provided by the CEFAS and the DTU, the collaboration of the scientists from the 4 research institutes cited above and the professional fishermen of the Occitanie Region. Because it is very difficult to follow them once they migrate in the Sea, little is known on their behaviour during this specific period of their life cycle. However, understanding their behaviour on their way back to their reproduction site is a crucial question as the future generation will depend of the success of migration and reproduction of the ultimate silver stage.
Four silver eels leaving Salses-Leucate lagoon, and capture by professional fishermen, were tagged with accelerometers G6 hybrids from CEFAS attached to a pop-up Argos Tag from mrPAT from Wildlife Computer. This system was especially conceived for this study as there is no accelerometer equipped with pop up and geolocalisation system for the moment. The accelerometer needs to be retrieve at sea in order to get the data inside. The accelerometers measure acceleration in 3 dimensions, temperature and depth. The function of the mrPAT was essentially only to get location data for the retrieval. Unfortunately, it is not possible to determine the trajectory of an eel under water since eels swim too deep to allow geolocation from daylight measurements. The procedure is to let the accelerometer and pop-up tag measure during six days (enough time for the eel to reach the deep water under 200 m and starts vertical dial migrations), and detached (programmed released) close enough to the coasts to go and retrieve the instruments with its data stored before the battery of the mrPAT ends after 2 days. The campaign was led with fishermen on their trawling boat. To retrieve the accelerometers, the Argos pop-ups emit when at the water surface. They are then positioned with a goniometer, used to triangulate the position when rounding the emission source on the fishing boats. With that protocol, 3 of the 4 accelerometers were located (the last one was out of range of the boat) and 2 were retrieved (both in less than 20 min!), and their data are being analyzed now at the CEFAS
The purpose of using the accelerometers in the experiment was to shed new light on eel swimming behavior: do they swim at the same rate all day? Is there a difference between ascents and descents? The data are still not fully analyzed. The amount of data is very large, with 3 acceleration measurements 25 times per second during many days. We find the typical diving patterns seen in all eels followed by telemetry.
The next leg of their travel in Eels’ travel in the Atlantic tracked by Argos satellite telemetry
CEFAS: David Righton (Cefas) <firstname.lastname@example.org>
SLU : Håkan Westerberg <email@example.com>
Photo: An eel in the sea with a pop-up tag. Credits G. Simon, Perpignan University