The classification and identification of economic fish is of great significance to the development and utilization of marine fishery resources. It is a feasible method to identify fish through the characteristics of fish vocal spectrum (Chen et al., 2021). The vocalization of fish usually associates with specific behaviors, so monitoring this vocalization can help identify different behavioral characteristics (Myrberg et al., 1965; Luh and Mok, 1986; Lobel and Kerr, 1999; Bertucci et al., 2010). Fish can make different sounds by various methods, such as the sound of whirlpool or spoondrift produced by swinging fins or tails, the sound of chewing baits, the sound of moving vertebrae, and the sound from the resonance and squeeze of swim bladders, of which the last one is the loudest. There are two types of fish: physostomi and physoclisti. The former refers to the fish whose swim bladders are connected to the esophagus through a pneumatic duct. It is categorized as the lower-grade Osteichthyes, such as carps. The latter refers to the fish without a pneumatic duct. It is categorized as the higher-grade Osteichthyes, such as perches (Fish and Mowbray, 1971; Sprague, 2000a; Sprague et al., 2000b). Vocation is an important part of fish behavior. Different behaviors correspond to different sounds. Fish’s courtship, breeding, feeding, swimming and other behaviors will produce different sounds (Holt and Johnston, 2014; Picciulin et al., 2020; Mackiewicz et al., 2021). In addition, some vocal fish make sounds when a predator is approaching or when it is caught (Ladich, 2022).The characteristics of sound intensity and frequency produced by different behaviors of fish are different (Qu et al., 2021). Thus, much work is needed to adequately characterize vocal species, acoustic features, and behavioral contexts, and to build sound repertoires, which will contribute to researching the soundscape of fish shoals and provide important vocal library data for identifying the distribution of fish populations (Tricas and Boyle, 2014).

The passive acoustic monitoring (PAM) technology is also known as passive sonar. Without a special sound emission system, this technology detects the underwater targets and determines their states and natures by receiving the radiated noise made by themselves, thus locating, tracking, and identifying them. It is a non-intrusive and non-invasive observation method (noiseless input) (Cato et al., 2005) used to evaluate the biodiversity, monitor the habitat environment, and locate the spawning fish (Lindseth and Lobel, 2018). In 2008, Anderson et al. first investigated the vocalization of fish in freshwater areas using this method and found that Oyster Toadfish, Ophidion marginatum, Ameiurus nebulosus, and Ietalurus Punetaus could produce sounds (Anderson et al., 2008). In 2008, Aalbers and Drawbridge first combined the underwater video surveillance with this method, with a finding that the size and gender of Atractoscion nobilis could be distinguished by its vocalization(Aalbers and Drawbridge, 2008).

Many scholars carried out time-domain and frequency analyses on the acoustic spectra of fish and proposed that the acoustic frequency was significantly different in diverse types of fish. The frequency of Anguilli Formes is 350 Hz, and that of Stromateus can reach 2000 Hz (Shen et al., 2014). The main frequency range of Bahaba taipingensis vocalization is 0~1000 Hz The main frequency segment of Sebasticus marmoratus’ vocalization ranges from 68 to 175 Hz. (Zhang et al., 2013).

In marine environments, the energy in the forms of light, heat, or electromagnetic wave will attenuate quickly, but sound signals can be transmitted over long distances, which differs from the situation in air environments on land. For this reason, the PAM technology has become the best method to research fish vocalization by recording and analyzing underwater sound signals. In addition, PAM technology can not only monitor fish, but also be used to locate the spawning grounds of some fish based on the identified sound frequency (Picciulin et al., 2020). However, this technology can only detect the underwater noises in marine environments and the sounds produced by marine species, which merely reflects a part of the sonic characteristics of these species.In some cases, the acoustic spectra of vocal species are unidentified or misidentified (Sprague and Luczkovich, 2001; Mann and Jarvis, 2004; Anderson et al., 2008). Under such circumstances, the soundscape in fish habitats can be monitored by combining passive acoustics and optics to match the collected acoustic spectra with corresponding species and behaviors, thus exploring the relationship between the soundscape and these behaviors. This lays a foundation for researching the fish vocalization and the impact of man-made noise on this soundscape (Tricas and Boyle, 2014).

Korean rockfish (Sebastes schlegelii), belonging to the Sebastes of Scorpaenidae in the Scorpaeniformes (Xu et al., 2018), mainly lives in the northwest Pacific Ocean. It is an important commercial fish species in China, primarily distributed in the Bohai Sea, Yellow Sea, and East China Sea (Feng et al., 2021). In 1974, Hallacher studied the swim bladder muscle of nearly 30 types of fish in the Scorpaeniformes and speculated that the fish of Sebastes with this muscle could produce sounds (Hallacher, 1974). However, the fish of Scorpaenidae are rarely reported in respect of vocalization (Nichols, 2005; Širović and Demer, 2009).

S. schlegelii is a major breeding and releasing fish species in typical marine ranches of North China. There is also vocal behavior in the wild population of S.schlegelii, but there are few studies on using acoustic technology to monitor the wild population of it. This work seeks to analyze the relationship between biological noises and behaviors of S. schlegelii. PAM was used to obtain the acoustic spectrum characteristics of the indoor S.schlegelii, aiming to provide data support for identifying the S.schlegelii population and studying its cluster behavior in the marine ranch demonstration area. At the same time, it provides scientific basis for controlling fish population activities, establishing a modern marine fishery production model of ecological health, environmental friendliness and resource protection, and optimizing and improving the construction of marine ranch with S.schlegelii as the target in the future.

Under the equipment shutdown, the low-frequency noise (<1 kHz) is mainly caused by ground vibration and water flow (Bart et al., 2001), which may mute the feeding and swimming noises at low frequencies (Takemura, 1988; Fujieda et al., 1993). S. schlegelii eats fast when baits are cast into the water tank, with the following manifestations: it swims faster to ingest baits, producing the sounds caused by the splashing of water and the swinging of tail. This feeding mode indicates that the test fish are vivacious (Phillips, 1989). The feeding noise is primarily influenced by the properties of baits: the peak frequency of noise made by chewing varies with the hardness of baits.Furthermore, the duration of feeding is determined by the amount and particle distribution of baits, which refer to the settling granular baits in this experiment. During bait casting, the test fish swim faster to the surface of the water tank for feeding. The “click” sound produced by S. schlegelii during chewing and swallowing is detected at the main frequency band of 2~4.5 kHz. By contrast, the main frequency band of the feeding noise made by Oncorhynchus mykiss and Scophthalmus maximus is measured to be 4~6 kHz when they swim faster to the surface of the water tank to eat granular baits (Lagardere et al., 2004), which is similar to that of the feeding noise made by S. schlegelii within 6 kHz.

However, Scophthalmus maximus has a special feeding frequency of 7~9 kHz, which is attributed to its exclusive suction feeding mode (Lagardere et al., 2004). This mode is also observed in Cyprinus carpio, with the maximum acoustic energy of 7~9 kHz (Takemura, 1988). However, S. schlegelii does not eat in this mode. The bony fish have the most diverse vocalization mechanisms in all vertebrates (Ladich and Fine, 2006). In 1974, Hallacher studied nearly 30 types of fish in the Scorpaeniformes and speculated that the fish of Sebastes with a swim bladder muscle could produce sounds (Hallacher, 1974). This hypothesis is supported by the observations of Zhang et al. (2013) made during the study on morphological structures of the acoustic system and acoustic features of Sebasticus marmoratus(Zhang et al., 2013). The “glouglou” sound with continuous pulses produced by this fish species is monitored at the main frequency band of 68~175 Hz. Based on the morphological and behavioral analyses, it is deduced that Sebasticus marmoratus makes a sound by contracting its swim bladder muscle.

In this study, S. schlegelii can produce a “coo” sound with continuous pulses, and the primary peak of this sound at the low frequency band is basically consistent with that of Sebasticus marmoratus. The reason may be that both S. schlegelii and Sebasticus marmoratus belong to the Scorpaenidae, with similar swim bladders and vocal muscles. The sounds produced by them are caused by the vibration of swim bladders and retractor muscles. However, the fish in different families and genera have distinct acoustic frequencies, and there are also certain differences in their vocalization mechanisms. For example, the pharyngeal jaw apparatus also involves in sound production in the Cichlidae and Pomacentridae (Rice and Lobel, 2003). The Doradidae, Pimelodidae, Mochokidae, and Ariidae have two kinds of sound-production mechanisms (Ladich and Fine, 2006). They can make a stridulation through pectoral spines and produce a low-frequency sound by vibrating swim bladders (Fine and Ladich, 2003).

The Sciaenidae can also make a sound, with the main frequency band of 300~800 Hz (Chen et al., 2021). The main frequency bands of sounds produced by Nibea albiflora and cultured Larimichthys crocea are 300~800 Hz and 550~800 Hz, respectively (Ren et al., 2016). In addition to the difference in vocalization mechanisms, the vocalization modes also vary with different fish species or with different behaviors of the same species (Akamatsu et al., 2002; Zhang et al., 2013; Lindseth and Lobel, 2018; Chen et al., 2021). For instance, the longsnout seahorse can produce two types of “click” sounds (50~800 Hz) during feeding and courting. Zhang et al. (2013) believed that the sounds with single and continuous pulses made by Sebasticus marmoratus in a territorial invasion test could reflect its different states in the face of invasion (Zhang et al., 2013).

Our study shows that S. schlegelii can produce sounds both with single and continuous pulses, which are speculated to be made under different behaviors. By matching the video and audio data, we obtained that the two kinds of sounds were produced during the normal feeding and the scrambling for baits, which verified the above conjecture. The sound signals emitted by S. schlegelii are mostly presented as single pulses during normal habitation and ingestion but presented as continuous pulses during attacking. Fish can produce sounds when they are interfered or threatened. In the same species, fish usually vocalize during attacking and fighting, but rarely make a sound during defending and escaping (Ladich, 1997). This phenomenon is also verified in this study.

The PAM technology can be used to monitor the underwater soundscape and collect the vocalization of underwater organisms in indoor and outdoor environments, including fish, marine mammals, crustaceans, etc (Kikuchi et al., 2015; Putland et al., 2018; Emmons et al., 2021; Muñoz-Duque et al., 2021). Aalbers and Drawbridge (2008) proposed that this technology could be combined with behavioral surveillance to make up for its deficiency of non-visualization (Aalbers and Drawbridge, 2008). The combination was performed in this study to research the feeding behavior and biological noises of S. schlegelii by matching sounds with behaviors, which solved the problem of non-visualization during acoustic monitoring. However, these sounds and behaviors were only monitored during feeding. Therefore, the behavioral surveillance shall be combined with PAM technology in future studies to explore the acoustic features of S. schlegelii in the face of territorial invasion. Meanwhile, the vocalization mechanisms of this fish species shall be further explored using morphological and behavioral methods, thus providing better data support for the research on the sounds and behaviors of S. schlegelii.

In summary, taking S. schlegelii as an example, analyzing the relationship between its biological noise information and its behavior can provide scientific basis for controlling the action of fish stocks in the future, creating a modern marine fishery production mode of ecological health, environmental friendliness and resource conservation, and optimizing and improving the construction of marine pasture with S. schlegelii as the target fish species.

The original contributions presented in the study are included in the article/supplementary material. Further inquiries can be directed to the corresponding authors.

The animal study was reviewed and approved by Dalian Ocean University. Written informed consent was obtained from the owners for the participation of their animals in this study.

MS and DG conceived and designed the experiment. BX and LY conducted the experiment. XC, XY and PX are involved in data analysis. MS and DG wrote the manuscript. MS and DG contributed equally to this work and share first authorship. All authors contributed to the article and approved the submitted version.