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Fish acoustic receiver network

As part of the Belgian LifeWatch observatory, a permanent acoustic receiver network for fish tracking is established in the Belgian Part of the North Sea and the Western Scheldt estuary. Using acoustic telemetry distribution, movements and habitat use of tagged migratory fish can be tracked. Tags or transmitters are surgically implanted in the belly of individual fish; and acoustic receivers are installed on buoys, ship wrecks and river banks. When a fish swims in the vicinity of an acoustic receiver, the information of the unique ID-code of the transmitter is stored on the receiver together with an exact timing of the event. When signals are simultaneously received by multiple receivers, an exact positioning of the tagged individuals can be derived from the raw data.

 

 
 
Left: location of acoustic receivers in the Western Scheldt estuary and the Belgian coastal area - Top right: acoustic receivers (©VLIZ) - Bottom right: mooring of the acoustic receivers on buoys (©Jan Reubens)

In the framework of LifeWatch the permanent acoustic receiver network will be maintained as a research infrastructure that allows studies on migration patterns of several migratory fish species. For marine, coastal and estuarine systems, Atlantic cod (Gadus morhua Linnaeus, 1758), will be the model species, while in Belgian rivers European eel (Anguilla anguilla (Linnaeus, 1758)) is representative for catadromous downstream migration. In a later phase, other species, such as European sea bass (Dicentrarchus labrax (Linnaeus, 1758)), twaite shad (Alosa fallax (Lacepède, 1803)), river lamprey (Lampetra fluviatilis (Linnaeus, 1758)) and sea trout (Salmo trutta Linnaeus, 1758) can be investigated.

Setting up the fish acoustic receiver network is a cooperation between the Flanders Marine Institute (VLIZ), the Research Institute for Nature and Forest (INBO), and the Ghent University - Marine Biology Research Group (UGent-MARBIOL).

Currently, (situation end 2016), a total of 87 receivers are installed and 163 individuals have been tagged.

 

Receivers

Currently (situation end 2016), a total of 87 receivers are installed on buoys, ship wrecks and river banks in the Belgian tidal areas: 26 in the Belgian Part of the North Sea, 39 in the Western Scheldt estuary (Westerschelde) (outer belt: 12, middle belt: 21, inner belt: 6), and 22 in the Sea Scheldt (Zeeschelde) from Antwerp to Ghent.

Prior to 2016, the LifeWatch Acoustic Fish Tracking Network was part of the Belgian Fish Tracking Network, which spanned a larger area, included more acoustic receivers and tags, and tracked additional fish species. This overarching fish tracking network, which was initiated by the INBO in 2012, is currently tracking over 716 fishes from 12 different species with 190 active receivers. In 2016, the entire Belgian Fish Tracking Network was included within LifeWatch.

Several types of receivers are being used:

  • Type VR2W: “classic” receiver: This receiver type is installed on buoys, and needs to be read out manually in the field to obtain the data, which is done on a seasonal basis. This is the case for the majority of the receivers within the LifeWatch network.
  • Type VR2AR: receiver with acoustic release: This receiver type is placed at the sea floor. With device deck unit (VR100) you can make contact with the receiver from the ship, check if the receiver detected any fish, and release the receiver to the surface if this is indeed the case. The advantages of this receiver type is that you are not dependent on surface floats to deploy the receiver, and that no diving is needed to recover the devices. When the receiver did not detect any fish, you can leave it in place at the sea floor, and move on to the next receiver, which saves a lot of time.
  • Type VR2TX: This receiver type is similar to type VR2AR, but does not have the acoustic release function, so diving is required to recover the devices. You can also use the VR100 device to check if the receiver detected any fish. If not, the receiver can be left at the sea floor.
  • Type VR2C: receiver with real-time data flow.

 

Atlantic cod

Atlantic cod (Gadus morhua Linnaeus, 1758) is a species with high commercial value. However, as fishing pressure has been very high for decades, population levels dropped in the 80s and have ever since been at very low levels. Although fishing pressure is strongly reduced nowadays, the population in the North Sea has not yet recovered. Information on ecology, migration routes and habitat preferences is crucial to support and give advice on efficient, long-term management plans for the recovery of Atlantic cod. This telemetry study in the framework of LifeWatch will help to gather the information needed.

So far 78 individual cods were tagged. Some data visualizations can be seen here.

 

    

Left: fish tag (©Jan Reubens) - Middle: tagging of a cod (©Karl Vanginderdeuren) - Right: tagging of a cod (©Jan Reubens)

European eel

In order to stop the decline of the endangered European eel (Anguilla anguilla (Linnaeus, 1758)), all European countries are obliged to set up an eel management plan and support this with scientific data. Particularly downstream eel migration, when maturing eels migrate from their freshwater habitat to the open ocean, is thought to be a critical phase in the eel's life cycle. On their way to the ocean, eels have to face several challenges such as sluices and pumping stations. We need to understand how these barriers affect the chances of eels to reach the sea, to reveal whether constructions such as fish passages indeed have a positive impact and to create innovative solutions to facilitate migration. Therefore it's crucial to understand the behavior, habitat use and migration patterns of the eel. The INBO initiated an acoustic telemetry network in 2012 and the LifeWatch fish tracking network will build on that expertise.

So far 85 European eels have been tagged in the LifeWatch network.

 

    

Left: measurement of an eel before tag implantation (©INBO) - Middle: tagging of an eel (©Pieterjan Verhelst) - Right: installment of an acoustic receiver (©INBO)

 

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