Effects of noise on acoustic and visual signalling in the Croaking Gourami: differences in adaptation strategies in fish

Numerous fishes produce sounds and their transmission and detection may be hindered by increasing levels of anthropogenic noise. We investigated acoustic communication during dyadic contests between male croaking gouramis (Trichopsis vittata, Osphronemidae) in the presence and absence of white noise. We hypothesised that fish modify acoustic signalling in the presence of noise in order to maintain intraspecific communication. Under controlled laboratory settings we compared agonistic behaviour, visual and acoustic signalling between noise and no-noise conditions. Trichopsis vittata produced sounds that were significantly lower in level and higher in dominant frequency under noise treatments. No difference was found in visual signalling or temporal sound characteristics. This study indicates that noise does not affect the amount of signalling during agonistic behaviour in a highly vocal fish. No increase in sound level was observed in croaking gouramis, indicating that a Lombard effect is not present in all vocalising fish. The lack of a Lombard effect shows that sound communication is potentially hindered by (human-made) noise in fish, which may affect territory maintenance and reproduction.