Ambient underwater sound : understanding its origins, variations and biological role

Abstract

Underwater sound may act as an orientation cue for guiding the long range movements of the larvae of some fishes and crabs toward suitable settlement habitats on the coast. Experimental evidence for this claim has been hard to secure due to the virtual impossibility of controlling sound in experimental aquaria, and difficulties of conducting field experiments. However, a number of different approaches, including the use of diver observation, have found behavioural evidence in support of orientation to underwater sound. The present study examined the use of in situ SCUBA observations as a research tool, and found it produced high intensity noise in the frequency range to which fish and decapods are most sensitive. These findings raised questions about the appropriateness of diver observation as a tool. Consequently, an in situ behavioural binary choice chamber was used to demonstrate that crab post-larvae of five species showed an orientation response toward a sound cue. These results add to earlier behavioural data indicating that acoustic orientation could be of considerable ecological importance in influencing the settlement success of coastal fish and crustaceans. Evaluation of the importance of sound as an orientation cue also requires a better characterisation of reef sound. A systematic investigation into the temporal and spatial (scales< 2 km) variation of ambient underwater sound in a shallow coastal environment found there were remarkable daily, lunar and seasonal variation in ambient underwater noise power levels. Dusk was the loudest time of day, the new moon was significantly louder than the full moon, and summer was the loudest season. Snapping shrimp are a ubiquitous source of ambient underwater noise between 5 - 20 kHz, with the number of snaps produced having the same significant daily, lunar and seasonal variations as the power levels. Snapping shrimp activity also varied between habitat types. Additional to snapping shrimp sounds, there was a significant rise in intensity between 500 - 3000 Hz caused by an unknown source. Laboratory experiments provided conclusive evidence that Evechinus chloroticus produce feeding noises in this frequency band, while field recordings from different reefs indicated power level in this frequency band reflected the size frequency and density of E. chloroticus populations. These results highlight the potential importance of sea urchins as a significant source of biological noise in temperate shallow coastal waters. Overall, the results have provided additional behavioural evidence for the orientation of crustacean to reef sound, and significantly extended our understanding of reef noise characteristics and origins. It is argued that the temporal, spatial and biological characteristics of ambient underwater sound could play a significant role in structuring coastal communities.