Introduction
Various ecological factors negatively affect communication in animals by hindering signal transmission or detection. The ability to communicate effectively with conspecifics is an essential aspect in social interactions because many animals produce and detect sounds during agonistic behaviour, courtship, or foraging. A noisy environment could therefore hinder both signal transmission and signal detection, whereby a reduced signal reception might subsequently influence behavioural responses (Hawkins, 2011; Cole, 2013). Many studies on human and birds have provided important insights on the topic and showed that a single, simple measure can be used to estimate the effect of manmade environmental noises on the perception of communication signals (Brumm, 2004; Bielefeld, 2012); review by Dooling, Leek & Popper (2015). Birds, such as the wild fairy-wrens Malurus cyaneus, showed that background noise affected the response to alarm calls, probably due to acoustic masking rather than distraction or changes in vigilance (Zhou, Radford & Magrath, 2019). Traffic noise has the potential to produce sensory, behavioral, and physiological changes in birds and marine mammals. If the principle holds for species as diverse as birds or humans, it probably also applies for fishes (Dooling, Leek & Popper, 2015; Dooling et al., 2019; Erbe et al., 2016). This calls for assessing the actual impact of anthropogenic noise on sound communication in fishes (Hawkins, 2011; Popper & Hawkins, 2019).
There are many sources of underwater anthropogenic sounds in the oceans, lakes and rivers, and man-made noise is increasingly affecting signaling as well as social behaviour of aquatic animals. Ship or boat traffic, hydrodynamic power plants, seismic exploration and other artificial noise sources have different acoustical characteristics, and their rapidly increasing noise levels constitute a major challenge in the life of animals (Popper & Hawkins, 2019). For example, noise impairs courtship behaviour and breeding in cichlids and gobiids (de Jong et al., 2016; de Jong et al., 2018a; de Jong et al., 2018b; Butler & Maruska, 2020; Butler & Maruska, 2021). Moreover, sound communication is affected in various taxa such as toadfishes (oyster toadfish Opsanus tau: Luczkovich et al., 2016; splendid toadfish: Sanopus splendidus: Pyć et al., 2021; plainfin midshipman Porichthys notatus: Brown et al., 2021, Mackiewicz, Putland & Mensinger, 2021; Lusitanian toadfish Halobatrachus didactylus: Alves, Amorim & Fonseca, 2021), sweepers (captive bigeye Pempheris adspersa: Van Oosterom et al., 2016), gobies (painted goby Pomatoschistus pictus and two-spotted goby Gobiusculus flavescens: de Jong et al., 2016; de Jong et al., 2018b) or labyrinth fishes (croaking gourami Trichopsis vittata: Maiditsch & Ladich, 2022b). Numerous aquatic species rely on acoustic communication for social interaction, and additional studies, reviewed by e.g., Ladich (2019), showed the negative impacts of anthropogenic noise on social behaviour and communication in fishes. Sound detection itself could also be affected by noise because increasing levels result in auditory masking, by which hearing thresholds rise in the presence of another sound (Hamilton, 1957; Tavolga, 1967; Tavolga, 1974; Chapman & Hawkins, 1973; Fay, 1974; Fay & Simmons, 1999; Erbe et al., 2016; Popper & Hawkins, 2019). Such threshold shifts have been reported in representatives of vocal and non-vocal fish families for many noise types including natural ambient, white noise or anthropogenic noise (Ladich, 2019).
Auditory thresholds have been measured in more than 100 fish species from various families covering different hearing sensitivities. These have mostly been conducted under quiet laboratory conditions and, in several of these species, in the presence of different noise types (Fay, 1988; Ladich & Fay, 2013). Masking can occur under relatively quiet conditions such as backwaters of rivers, lakes, ponds, or low-noise aquaria. The Atlantic cod Gadus morhua, for example, shows best hearing sensitivity under the quietest sea conditions, whereas masking occurs with any increase in the level of ambient sea noise (Chapman & Hawkins, 1973). The shifts are much more pronounced at higher noise levels, e.g., in fast-flowing rivers and coastal surf. Masking by various ambient noise types has been investigated in several freshwater fishes (goldfish Carassius auratus: Enger, 1973; Fay, 1974; Gutscher, Wysocki & Ladich, 2011; common carp Cyprinus carpio, the European perch Perca fluviatilis: Amoser & Ladich, 2005; the topmouth minnow Pseudorasbora parva: Scholz & Ladich, 2006; the blacktail shiner Cyprinella venusta: Crovo et al., 2015; Holt & Johnston, 2015). Other studies showed an increase in hearing thresholds in the presence of boat noise and a reduced ability to detect conspecific vocalizations (H. didactylus: Alves, Amorim & Fonseca, 2021; Vasconcelos, Amorim & Ladich, 2007; different Mediterranean fish species: Codarin et al., 2009; meagre Argyrosomus regius: Vieira et al., 2021). White noise was used as a masker in cyprinids, centrarchids, sciaenids and cichlids (C. auratus, the Southern striped raphael catfish Platydoras armatulus and the pumpkinseed sunfish Lepomis gibbosus: Wysocki & Ladich, 2005a; Atlantic croaker Micropogon undulatus and black drum Pogonias chromis: Ramcharitar & Popper, 2004; orange chromide Etroplus maculatus and slender lionhead cichlid Steatocranus tinanti: Ladich & Schulz-Mirbach, 2013).
Importantly, the amount of masking depends not only on the noise level or noise type, but also on the hearing sensitivities. The term sensitivity generally refers to auditory perception of a sound by an individual, and it is likely that all fishes can detect sound (Lucke et al., 2016; Popper & Hawkins, 2019). Importantly, species with enhanced hearing abilities (hearing specialists) such as otophysans or some cichlids exhibit a higher responsiveness in detecting sound. Such species are more affected by noise than those lacking hearing enhancement (Ladich & Schulz-Mirbach, 2013; Ladich, 2019). The anabantoid fish Trichopsis vittata possesses an air-filled suprabranchial chamber for air-breathing laterally to the inner ears; this extends its hearing range (hearing specialists) up to several kHz and lowers the auditory thresholds over the entire frequency range (Schneider, 1964; Ladich & Yan, 1998). Both sexes of T. vittata vocalize loudly when defending territories, and females also vocalize prior to mating (Marshall, 1966; Ladich, 1998; Ladich, 2007). We chose the croaking gourami as a model species to better understand the detection of conspecific sounds in a noisy environment under standardized laboratory conditions.
The aim of the study is twofold: (1) we measured unmasked and masked hearing thresholds to determine the extent to which standardized white noise deteriorates the sound pressure sensitivity in a vocal hearing specialist, the anabantoid T. vittata; (2) we compared unmasked and masked hearing thresholds to the spectra of conspecific sounds. These comparisons will clarify the extent to which noise reduces the ability of T. vittata to detect conspecific sounds and correctly assess opponents and mates (Ladich, 1998; Ladich, 2007).
