SUMMARYSound-source localization behavior was studied in the plainfin midshipman fish (Porichthys notatus) by making use of the naturally occurring phonotaxis response of gravid females to playback of the maleʼs advertisement call. The observations took place outdoors in a circular concrete tank. A dipole sound projector was placed at the center of the tank and an 80-90Hz tone (the approximate fundamental frequency to the maleʼs advertisement call) was broadcast to gravid females that were released from alternative sites approximately 100cm from the source. The phonotaxic responses of females to the source were recorded, analyzed and compared with the sound field. One release site was approximately along the vibratory axis of the dipole source, and the other was approximately orthogonal to the vibratory axis. The sound field in the tank was fully characterized through measurements of the sound pressure field using hydrophones and acoustic particle motion using an accelerometer. These measurements confirmed that the sound field was a nearly ideal dipole. When released along the dipole vibratory axis, the responding female fish took essentially straight paths to the source. However, when released approximately 90deg to the sourceʼs vibratory axis, the responding females took highly curved paths to the source that were approximately in line with the local particle motion axes. These results indicate that the acoustic cues used by fish during sound-source localization include the axes of particle motion of the local sound field.

Morphometric changes in body condition, liver, sonic muscle and gonadal development associated with the annual reproductive cycle and behaviour of the intertidal-nesting male plainfin midshipman Porichthys notatus were investigated. Body condition of type I males rapidly increased during the pre-nesting (PN) period, peaked at the beginning of the summer nesting cycle and then gradually declined to lowest levels during the non-reproductive (NR) period. The gonado-somatic index of type I males peaked during PN and then declined during the summer nesting cycle to lowest levels at the end of the nest cycle and during NR. Indices of sonic muscle and liver of type I males were lowest during NR, gradually increased during PN and then peaked during the summer nesting cycle. Results indicate that body condition and fecundity of type I males were positively correlated with body mass at the end of the nest cycle. These findings as they relate to the annual reproductive cycle and behaviour of the type I male P. notatus are discussed.

The aim of this study was to use plainfin midshipman fish (Porichthys notatus) as a general model to explore how fishes localize an underwater sound source in the relatively simple geometry of a monopole sound field. The robust phonotaxic responses displayed by gravid females toward a monopole sound projector (J-9) broadcasting a low-frequency (90 Hz) tone similar to the fundamental frequency of the male's advertisement call were examined. The projector's sound field was mapped at 5 cm resolution azimuth using an eight-hydrophone array. Acoustic pressure was measured with the array and acoustic particle motion was calculated from pressure gradients between hydrophones. The response pathways of the fish were analyzed from video recordings and compared to the sound field. Gravid females at initial release were directed toward the sound source, and the majority (73%) swam to the playback projector with straight to slightly curved tracks in the direction of the source and in line with local particle motion vectors. In contrast, the initial direction of the control (sound-off) group did not differ from random. This paper reports on a comparison of fish localization behavior with directional cues available in the form of local particle motion vectors.

We investigated the roles of the swim bladder and the lateral line system in sound localization behavior by the plainfin midshipman fish (Porichthys notatus). Reproductive female midshipman underwent either surgical deflation of the swim bladder or cryoablation of the lateral line and were then tested in a monopolar sound source localization task. Fish with nominally 'deflated' swim bladders performed similar to sham-deflated controls; however, postexperiment evaluation of swim bladder deflation revealed that a majority of 'deflated' fish (88%, seven of the eight fish) that exhibited positive phonotaxis had partially inflated swim bladders. In total, 95% (21/22) of fish that localized the source had at least partially inflated swim bladders, indicating that pressure reception is likely required for sound source localization. In lateral line experiments, no difference was observed in the proportion of females exhibiting positive phonotaxis with ablated (37%) versus sham-ablated (47%) lateral line systems. These data suggest that the lateral line system is likely not required for sound source localization, although this system may be important for fine-tuning the approach to the sound source. We found that midshipman can solve the 180 deg ambiguity of source direction in the shallow water of our test tank, which is similar to their nesting environment. We also found that the potential directional cues (phase relationship between pressure and particle motion) in shallow water differs from a theoretical free-field. Therefore, the general question of how fish use acoustic pressure cues to solve the 180 deg ambiguity of source direction from the particle motion vector remains unresolved.

The auditory system of the plainfin midshipman fish, Porichthys notatus, is an important sensory receiver system used to encode intraspecific social communication signals in adults, but the response properties and function of this receiver system in pre-adult stages are less known. In this study we examined the response properties of auditory-evoked potentials from the midshipman saccule, the main organ of hearing in this species, to determine whether the frequency response and auditory threshold of saccular hair cells to behaviorally relevant single tone stimuli change during ontogeny. Saccular potentials were recorded from three relative sizes of midshipman fish: small juveniles [1.9-3.1 cm standard length (SL), large juveniles (6.8-8.0 cm SL) and non-reproductive adults (9.0-22.6 cm SL)]. The auditory evoked potentials were recorded from the rostral, middle and caudal regions of the saccule while single tone stimuli (75-1,025 Hz) were presented via an underwater speaker. We show that the frequency response and auditory threshold of the midshipman saccule is established early in development and retained throughout ontogeny. We also show that saccular sensitivity to frequencies greater than 385 Hz increases with age/size and that the midshipman saccule of small and large juveniles, like that of non-reproductive adults, is best suited to detect low frequency sounds (<105 Hz) in their natural acoustic environment.