Underwater Acoustic Technologies for Sustainable Fisheries

Underwater acoustic technologies have emerged as indispensable tools for advancing research, management, and conservation across aquatic ecosystems. Their capacity to reveal critical information on animal behavior, migration patterns, population dynamics, and habitat use underscores their value not only for sustainable fisheries but also for the protection of threatened and endangered species. In the face of growing pressures from anthropogenic impacts and climate change, such technologies represent essential means of supporting resilient and sustainable aquatic systems. This Special Issue aims to showcase cutting-edge applications of underwater acoustic technologies, from supporting sustainable fisheries to improving acoustic monitoring precision and efficiency and advancing broader ecosystem research, highlighting their growing importance in aquatic science and conservation.

This Special Issue brings together nine significant contributions that collectively highlight the potential of underwater acoustic technologies to advance sustainable fisheries and inform aquatic ecosystem studies. Through case studies and comprehensive reviews, these studies enhance our understanding of aquatic organisms and provide practical tools and methodologies for fisheries’ resource assessment and species-level ecological research. By illustrating applications ranging from target strength estimation and broadband acoustic techniques to behavioral analysis, these contributions demonstrate the versatility and growing importance of acoustic approaches in aquatic science and conservation.

Target strength (TS) is a fundamental parameter in fisheries’ acoustic assessments, as it directly influences the species identification and the estimation of biomass. Accurate TS measurements are therefore essential for reliable resource assessment and the effective application of acoustic techniques in fisheries research. In this Special Issue, three studies focus specifically on improving TS estimation and understanding the factors that affect acoustic scattering. These contributions collectively address technical challenges in broadband quantification, validate theoretical models such as the Kirchhoff–Ray Mode (KRM) against empirical data, and provide specific TS–body length relationships essential for accurate biomass assessment. Ai et al. (article 1) developed a broadband scattering quantification method for single fish using split-beam echosounders, integrating echo field theory, transducer equivalent circuits, and signal processing to enhance the generalizability across sonar systems and reduce the reliance on calibration standards. Zhu et al. (article 2) examined the TS in Japanese mackerel (Scomber japonicus), assessing the effects of freezing on swimbladder morphology and TS and demonstrating strong agreement between ex situ measurements and KRM model simulations, while providing empirical broadband TS–length relationships. Meng et al. (article 3) analyzed the TS in large yellow croaker (Larmichthys crocea), revealing its dependence on the tilt angle, frequency, and body length, and establishing least-squares fitted TS–length equations superior to conventional b20 values for accurate acoustic assessment. Collectively, these studies highlight how biological and environmental factors—including swimbladder integrity, tilt angle, and sample preservation—affect acoustic scattering, providing a robust foundation for species-level identification and quantitative fisheries resource evaluation.

Building on the foundational role of TS in quantitative assessments, underwater acoustics also provide powerful tools for investigating fish distributions and monitoring ecological change at broader ecosystem scales. Recognizing the crucial intermediary role of Antarctic silverfish (Pleuragramma antarcticum) in the Antarctic food web, Lee et al. (article 4) investigated their spatiotemporal distribution in the Ross Sea, using the KRM backscattering model to estimate the TS and mean volume backscattering strength (MVBS) to reveal vertical and horizontal patterns. Their findings showed that most juveniles concentrated around 100 m depth, near sea ice and polynya waters, while also highlighting the need for refined algorithms to separate silverfish from krill. Li et al. (article 5) explored the temporal and spatial dynamics of fish resources in the confluence of Poyang Lake and the Yangtze River during the early stage of China’s ten-year fishing ban. Acoustic surveys indicated higher fish densities in the confluence zone during the high-water period and aggregation in deeper river channels during the low-water period. The study revealed increases in both fish density and body length, providing strong evidence for the policy’s effectiveness in stock recovery. They also pointed out the ongoing challenge in accurately estimating the fish total length from the TS, emphasizing the need for incorporating diverse equations that consider the fish shape, swimbladder size, and transducer–fish positional relationships for enhanced accuracy in future research. The review by González-Máynez et al. (article 6) strongly supports the relevance of such studies in fresh and shallow waters. It indicates that inland waters, including lakes, rivers, streams, and reservoirs, play a significant role in sustaining riverine fish abundance and serve as crucial reservoirs of biodiversity. The review further highlights the practical benefits of acoustic methods in these environments, noting that echosounders can be readily installed on small boats, thereby presenting new opportunities for evaluation. Additionally, it underscores the value of acoustic methods for providing non-invasive long-term observations in sensitive regions such as Marine Protected Areas (MPAs). This characteristic makes acoustic approaches particularly applicable for assessing the impact of conservation measures such as fishing bans.

The studies discussed above primarily employed scientific echosounders. In addition, acoustic cameras represent another important technological approach in fishery acoustics. Wang et al. (article 7) assessed the use of an acoustic camera named Adaptive Resolution Imaging Sonar (ARIS) for monitoring large jellyfish in Liaodong Bay. Their results demonstrated the ability of acoustic imaging to identify species based on the size, shape, and movement, while yielding abundance estimates substantially higher than those from net sampling. The method also proved advantageous in shallow, turbid, or nighttime conditions, despite some limitations related to blind zones and the minimum detectable size.

Extending the application of acoustic camera technology, Shen et al. (article 8) combined artificial intelligence with another type of acoustic camera, the Dual-frequency Identification Sonar (DIDSON), to develop a fish target identification and counting method. This approach, utilizing the YOLOv5 deep learning model combined with the DeepSort tracking algorithm, achieved high identification accuracies and significantly reduced the processing time compared to manual or traditional software analysis. By minimizing human effort and bias, this study highlights the potential of combining acoustic imaging with AI to enhance the precision, efficiency, and objectivity of aquatic ecosystem monitoring and management.

Passive acoustic monitoring is another acoustic technology that significantly contributes to aquatic ecosystem research, expanding beyond active fishery acoustics. This approach, which involves listening to the sounds produced by aquatic animals, provides invaluable insights into their presence, behavior, and the health of their environment without direct physical intervention. Chen et al. (article 9) provide a compelling example of passive acoustic monitoring through their study on the vocalization patterns and echolocation signals of the endangered Yangtze Finless Porpoise in captivity. They showed that the signal characteristics vary with behavioral states, enabling precise target discrimination. These insights are valuable for conservation, guiding strategies to reduce human disturbance and assess interactions with fishing gear, and demonstrate the broader potential of passive acoustics for monitoring endangered species and supporting ecosystem management.

Collectively, the studies presented in this Special Issue highlight the pivotal role of underwater acoustic technologies in supporting sustainable fisheries. From precise biomass estimation using echosounders to species identification and behavioral monitoring via acoustic cameras and non-invasive assessment of endangered species through passive acoustic monitoring, these contributions demonstrate how acoustic approaches can enhance both resource management and ecological understanding. Looking ahead, continued advances in acoustic instrumentation, modeling, and artificial intelligence will further strengthen the capacity for the effective long-term monitoring of fish populations and aquatic ecosystems. By improving species discrimination and enabling automated data processing, these technological developments pave the way for broader applications. Coupling underwater acoustics with emerging observation platforms—including aquaculture net–cage monitoring systems, autonomous aerial, surface, and underwater vehicles, and moored or drifting buoys—will enable more comprehensive and adaptive monitoring of aquatic ecosystems. Such developments have the potential to advance ecosystem-based management and reinforce the role of acoustics in achieving sustainable fisheries under changing environmental conditions.