Adaptation of fisheries sonar for monitoring schools of large pelagic fish: dependence of schooling behaviour on fish finding efficiency

Abstract

Multibeam omnidirectional sonars allow the monitoring of pelagic fish schools surrounding the platform and are currently used by fishermen. Multibeam processing methods offer improved abilities for raw data storage. For the detection of fish schools associated with drifting Fish Aggregating Devices (FADs), the Simrad SP90 sonar has been used. Digital systems have been developed for the acquisition and processing of volume backscattering echoes and position data. Sampling method was defined according to two modes: during searching periods for FADs and associated schools, and during school monitoring in drifting mode. Validation of several FAD-associated schooling species detection was made by simultaneous visual observations or/and cross-checks with echosounder recordings. The characteristics of the targeted fish species' schooling behaviour are fundamental in order to avoid misleading acoustic data interpretation. The sonar detection threshold is the result of a compromise between fish number, size, species and the nearest neighbour distance (NND) of individuals per dynamic structure (school and shoal). In agreement with the tuna schooling dynamics, their NND can sometimes be too large for detecting their presence notwithstanding their number. The sonar data should be analysed and interpreted in a holistic approach, in combination with the behaviour pattern and the dynamics of all species around the drifting FADs. An autonomous sonar buoy prototype equipped with 360° scanning sonar coupled to video cameras will increase our understanding of tuna behaviour around drifting or anchored objects. A similar methodology can be applied to various platforms, either anchored or in a permanent position, promoting the monitoring of fish schools around artificial reefs or open sea aquaculture farms, across estuaries, channels and straits, which is undoubtedly essential for fisheries management.

Surveying the distribution and abundance of flying fishes and other epipelagics in the northern Gulf of Mexico using airborne lidar

“…Freon (1992) attempted an acoustic survey on a towed body, but was unsuccessful. Brehmer et al (2007) reported the first detection of flying fish aggregations by sonar, but it does not appear that this method was further developed. Studies focusing on tagging have provided general insight into movement and distribution patterns; however, it is difficult to implement on a large scale, and to examine population level abundance (Mulloney 1961, Lewis 1964, Oxenford 1994.…”

Towards an Autonomous Pelagic Observatory: Experiences from Monitoring Fish Communities around Drifting FADs

“…Results obtained with ship-based (C1) multifrequency echosounders and omnidirectional sonar are detailed in Moreno et al (2007a); Brehmer et al (2007) and Trygonis et al (2016), respectively. Corresponding underwater SCUBA visual surveys are described in ).…”

Functional response of fishers in the Isle of Man scallop fishery

“…However, in the scallop fishery, fishing has to be undertaken before CPUE can be determined, unlike other fisheries where sonar can be utilised (e.g. Brehmer et al 2007, Boswell et al 2008. Therefore, knowledge of prey distribution will necessarily be imperfect, limiting the numerical response.…”