Can noisy reefs attract more fish and crustaceans?
A study on sound recordings of reef noise from different habitats has revealed that the highest quality reefs are also the noisiest, potentially attracting more larval recruits using sound to orient towards reefs.
Nearly all fish and decapod crustaceans associated with reefs spend their larval stage in the open ocean after being broadcast from the reefs as eggs or hatchlings. They soon develop strong swimming abilities which allow them to counter the effect of sea currents and choose the direction in which to swim and eventually return to the reef.
The precise reason why the larval stage is spent in the open ocean is still under debate, but generally it is agreed that this strategy ensures that the larvae are far from the many reef associated predators during this vulnerable stage.
However, this strategy can only be beneficial if some of the larvae are able to return to the reef – not an easy task in the vast expanse of the ocean. Over recent years it has become clear that larvae use their sensory abilities to home in on a reef and two senses in particular have emerged as the most likely candidates.
Experiments have shown that larvae can be attracted to the odour and the sound of a reef, both of which have the potential to be detected over distances up to 20 kilometres. Despite the importance of this phenomenon in determining population dynamics across reefs, there is still very little known about the sensory cues produced at the reefs, how they propagate through the environment and the actual sensory abilities of the larvae.
The sound of a reef
Like cities, reefs concentrate a lot of life in a small area and this, again like cities, makes them very noisy places.
The background ‘crackling’ sound is produced by hundreds of snapping shrimp while the ‘grunts’ and ‘croaks’ are from vocalising fish.
Each reef also has its own signature sound and our recent work using recordings of reefs of similar size in the Philippines has found that the reefs within three different well managed Marine Protected Area (MPA) for the previous 10 years had significantly higher sound levels at the source (average sound intensity of 133.1 ± 2.2 dB re 1 µPa) compared to three overfished macroalgal and urchin dominated reefs (average sound intensity 122.0 ± 1.2 dB re 1 µPa).
The clear difference between recordings from different habitats may empower the fish and crustacean larvae not only to detect the location of the reef but to discriminate between good and bad reefs.
This finding is important for the way we manage Marine Protected Areas (MPAs), underlining how the acoustic signature of the reef will also need to be considered if we want to improve the efficacy of an MPA. It also opens up the possibility of surveying and monitoring reef quality rapidly and cost effectively in the future.
Our future work on Hoga Island aims to identify if the difference in sound levels with habitat quality can be detected on smaller spatial scales to refine reef quality assessment surveys.
This will form part of a larger project which aims to develop a detailed map of the soundscape around Hoga Island up to 5 km away from the reefs, combined with behavioural experiments on fish larvae to determine how they respond to different reef sounds and over what distance they can detect reef noise.