Sonar and selectivity: new frontiers for acoustic fish size estimation – Acoustic estimation of fish size before capture is a strategic priority to improve fishing selectivity and maximise the economic value of the catch. These are the findings of the Norwegian project “Acoustic Estimation of Fish Size during Pre-catch in Commercial Fishing of Pelagic Species”, led by the Institute of Marine Research (IMR) and funded by the Fisheries and Aquaculture Industry Research Fund (FHF). The initiative tested the possibility of using broadband echosounders to obtain reliable size estimates of fish prior to the catch operation.
Objective: to improve selectivity and efficiency
In the context of commercial pelagic fishing, knowing the size of fish before netting can avoid unwanted catches and ensure compliance with regulatory limits. For this reason, the project involved installing an EK80 echosounder on board the FV Libas vessel, equipped with a 3° directional transducer and an operating frequency between 160-260 kHz. The device, positioned under the retractable keel, enabled individual mackerel to be detected up to 120 metres away while fishing in UK waters in 2024, without interfering with normal operations.
Advanced technology, results to be consolidated
The original method for estimating size, based on detecting two distinct peaks in the broadband acoustic signal, did not provide the desired results due to the complexity of the fish’s actual acoustic response. As an alternative, the researchers adopted an approach based on Target Strength (TS) and the angle of incidence of the fish relative to the acoustic beam. The data showed a good match between the maximum TS and the estimated length, with an average deviation of about 45 mm from the real biological samples.
The project also confirmed the validity of the theoretical approach using FEM (Finite Element Method) modelling, simulating the physical structure of the fish, including the skeleton. However, automatic tracking algorithms and standardised methodologies for broadband TS analysis remain to be developed, which have not yet been universally adopted by the scientific community.
Operational Perspectives and Future Developments
In addition to the experimental results, the project highlighted important practical benefits for industry. Having a reliable tool to estimate the size of fish in real time can turn into a competitive advantage for the entire industry: improving the quality of the catch, reducing unwanted catches and making the business more sustainable.
The researchers propose to continue testing in controlled environments, such as rearing cages or coastal areas frequented by free-ranging mackerel, to further refine estimation models and facilitate the collection of data that can be compared with results in the open sea. This is a key step in transforming the research into an operational product for the commercial fleet.
The Norwegian project demonstrates that advanced acoustic technologies represent one of the most promising avenues for the evolution towards selective, efficient and sustainable fishing. There is no shortage of challenges, but the results already obtained open up new operational and industrial possibilities for European and international fleets.
Sonar and selectivity: new frontiers for acoustic fish size estimation
