Detecting derelict fishing gear with the help of side-scan sonar and sonar expert

WWF Germany has been working with methods to find and retrieve lost and derelict fishing gear. The latest experiments used acoustic detecting and the findings were verified by divers. According to WWF Germany, the results are promising.

In the MARELITT Baltic and Ghostnet projects, WWF Germany, along with other partners, tries to understand where derelict fishing gear (DFG) can be found and retrieved in the most efficient way. The first crucial point in DFG searches is the knowledge where to find lost gear. After working together with fishermen and divers with limited area knowledge, the desire of gaining knowledge for an extended area increased.

Havbunn
Side-scan sonar image of a gillnet target.
Copyright: WWF Germany, Submaris

In April 2018, WWF Germany invited sonar expert Crayton Fenn of Fenn Enterprises and Innerspace Exploration Team based in Seattle Washington to a workshop to learn and proof how side-scan techniques can help to search for derelict fishing gear (DFG) in the Baltic Sea. Until now the Derelict Fishing Gear Program of the Northwest Straits Foundation retrieved 6.000 lost gillnets and several 10.000s of crab pots in the Puget Sound.

Crayton Fenn detected all of these lost items with the aid of side scan sonar, and WWF Germany was happy to proof that this technique also works in the Baltic Sea marine environment. Crayton Fenn visited Germany again in August 2018 and April this year to deepen the knowledge about side scan imaging and the handling of the sonar device with the aim to scan extended areas off the German coast. The evaluation of the gained data just started – but this start is a promising one.

In an area of approximately 2 square km, which was scanned for 1 hour, 5 targets were localized. Ground truth with divers proved 3 lost gillnets of a length of 100m each and 2 lines. On the 14th of May WWF Germany retrieved the nets by the help of commercial and scientific divers and a fishermen. By this  best practice method of detecting nets by sonar, visual ground verification by divers and retrieving with the help of fishermen was proven and WWF Germany will go on to establish knowledge and skills.

Vertification by divers
Ground truth by divers Retrieving with the help of fishermen
Copyright: WWF Germany, Submaris

Convinced by these promising results, WWF Germany purchased the sonar system and is optimistic to get a bigger scale overview of the DFG pollution in the German Baltic Sea. Moreover an international project together with WWF Hong Kong and WWF Peru and Crayton Fenn is planned to transfer the knowledge and technique to places where DFG comprises the biggest proportion of marine litter. This knowledge will help to plan and realise retrieving actions at sea in a very efficient way and to communicate that DFG is not hidden – we just have to search in the right way.

submaris WWF Germany
Copyright: WWF Germany, Submaris

Author: Gabriele Dederer, WWF Germany

Published: 15.5.2019