Bigeyes or bigears? The nature of bigeye vocalisations

Abstract

A large number of fishes are known to produce sound, but research into soniferous fishes in New Zealand is surprisingly limited. The New Zealand bigeye, Pempheris adspersa, are nocturnal planktivorous reef fish that feed in loose shoals and have recently been discovered to be soniferous. They produce low frequency ‘pops’ both day and night, however the purpose of their vocalisations remains unknown. The aim of this thesis was to determine the temporal variation in vocalisation rates of wild bigeyes, and to gain experimental evidence for the nature of bigeye vocalisations. Visual and acoustic techniques were used to determine the nocturnal behaviour of bigeyes and temporal patterns in vocalisation rates. Bigeyes were found to carry out daily migrations to and from their daytime refuge at dusk and dawn, respectively, leaving their cave just before sunrise and returning just after sunset. While travelling, bigeyes maintained loose shoal structure. The timing of daily migrations also corresponded with the highest vocalisation rates, which peaked during dusk and dawn, and during full moon and summer. This suggests that bigeye vocalisations are important during daily migrations. Ambient sound also varied temporally with the highest mean levels detected during the summer new moon. Laboratory experiments investigating the effect of ambient sound playback on bigeye shoals show that they significantly increased their group cohesion when exposed to recordings of ambient reef sound at three sound levels (125, 130 and 135 dB re 1 μPa), while also decreasing vocalisation rates – patterns of behaviour consistent with acoustic masking. When exposed to playback of conspecific vocalisations, group cohesion and vocalisation rates of bigeyes both significantly increased. These results, along with previously reported behaviours of wild bigyes provide evidence that their vocalisations are used as contact calls to maintain group cohesion. This appears to be the first experimental evidence of contact calls being used for group cohesion in fishes, making fish the evolutionarily oldest vertebrate group in which this phenomenon has been observed.