Echogram at 200 kHz showing three days of acoustic data from the sea surface (top) to the seabed (undulating red line at bottom) recorded by Lyra. Note the clear diurnal (day-night) cycle of vertically migrating zooplankton. (Image courtesy Cefas)

A state-of-the-art technique for mapping and counting schools of fish has been tested on an autonomous marine platform in the North Sea, offering a new method for collecting data on fish stocks.

Cefas (Centre for Environment, Fisheries and Aquaculture Science, UK) and Liquid Robotics have successfully deployed, tested and recovered a remotely piloted Wave Glider, which was adapted to allow scientists to collect high-quality broadband fisheries acoustics data.

As part of a multi-vehicle mission under the UK NERC/Defra funded AlterEco project, the Wave Glider called Lyra was deployed in the central North Sea about 270 km north-east of Newcastle-upon-Tyne, U.K. by RV Cefas Endeavour. The glider spent 41 days at sea, repeatedly covering a 64-km long transect along which data were collected under different conditions, before finally being recovered 20 km off Lowestoft. While deployed, the Wave Glider covered over 1,700 km at sea and was remotely piloted by the Liquid Robotics operations team from their California headquarters in Silicon Valley almost 9,000 km away.

For this mission, Lyra was fitted with the Simrad EK80 broadband echosounder with two transducers (200 kHz and 70 kHz), utilizing a version of the electronics that has been specifically designed for autonomous surface vehicles called the WBT Mini. The system was self-contained with its own dedicated battery and used in standalone mode. Lyra collected acoustic data continuously for 23 days on the organisms in the water column at very high resolution, which scientists are now using to learn more about zooplankton and fish populations around the U.K.

Preliminary results suggest that evidence has been captured of diurnal (day-night) vertical migrations of zooplankton and other organisms in the water column, as well as schools of fish. At the start of the deployment, net samples of zooplankton were collected aboard RV Cefas Endeavour, which will be used to help identify the organisms that are acoustically detected.

A second AlterEco mission is planned to start in mid-August. For this mission, Cefas engineers are assessing the feasibility of integrating the echosounder into Lyra’s computer system so it can transmit summary data over the satellite communications link and offer the ability to change pre-planned mission setups when required by Cefas scientists. —Cefas

Cefas’ Wave Glider Lyra sailing away on the start of the 41-day mission after being deployed from RV Cefas Endeavour. (Image courtesy Cefas)