Biogenic marine soundscapes provide important navigational cues to dispersing larvae in search of suitable habitat. Yet, widespread habitat loss has degraded marine soundscapes and their functional role in recruitment. Habitat restorations can provide suitable substrate for habitat regeneration, such as reefs constructed to facilitate recruitment and habitat growth by oysters, but typically occur where soundscapes are degraded and recruitment limited. Enhancing marine soundscapes on newly constructed reefs using speaker technology may ensure sufficient recruitment to establish a trajectory of recovery for the desired habitat.

Across two of the largest oyster reef restorations in Australia, we deployed speakers at four sites and at three times throughout the recruitment season to test whether soundscape enhancement could boost recruitment and habitat building by oysters. In the presence and absence of soundscape playback, we compared oyster recruitment rates to settlement panels across space and time, and oyster habitat formation on newly constructed boulder reefs.

On the settlement panels deployed across the two reef restorations, soundscape playback significantly increased oyster recruitment at 8 of the 10 sites by an average (±1SE) 5.1 ± 1.9 times (5,281 ± 1,384 more larvae per m2), and by as much as 18 times.

On boulders atop newly constructed reefs, where the restoration goal is for oysters to form three-dimensional habitat, the surface area covered by oysters after 5 months did not differ between speaker and control treatments. However, soundscape playback appeared to influence the earlier recruitment of oysters, resulting in significantly more large oysters per boulder that formed significantly more three-dimensional habitat building by an average 4.3 ± 1.2 times relative to non-speaker controls.

Synthesis and applications. Our results show that using speakers to enhance marine soundscapes boosts the number of oyster recruits, resulting in more larger oysters that form more three-dimensional habitat atop reef restorations. In accelerating the formation of these vertical growth forms, which provide the ecological functions that motivate restoration efforts, the early application of speaker technology on new reef restorations may help steer ecological succession on a trajectory of desired habitat recovery, potentially reducing the substantial cost of ongoing intervention.

This study was conducted across two large oyster reef restorations in Gulf St. Vincent, South Australia: Windara Reef (34°30.604″ S, 137°53.949″ E), a 20-hectare reef constructed in 2017–2018, and Glenelg Reef (34°58.314″ S, 138°29.787″ E), a 3-hectare reef constructed in 2020. These two restoration sites, which are approximately 80 km apart on opposite sides of Gulf St. Vincent, are each located ~1 km offshore in 7–10 m of water.

To locally enrich marine soundscapes at multiple sites across each restoration, we deployed underwater speakers at two sites across each of Windara Reef and Glenelg Reef. Speaker treatments enriched soundscapes by continuously playing a looped recording of a healthy reef soundscape recorded from a rocky reef habitat located 20 km south of Glenelg Reef (Port Noarlunga Reef). This rocky reef was selected because no flat oyster reefs remain in mainland Australia, and because previous soundscape monitoring throughout Gulf St. Vincent showed this site to be among the most bio-acoustically active.

To test the impact of soundscape enrichment on oyster settlement and habitat formation, we assessed oyster recruitment to settlement panels and oyster habitat formation on limestone boulders in the presence and absence of speaker playback. At each site, six plastic crates (40 × 40 × 40 cm) were positioned 2 m apart and 2 m from a speaker (or dummy control) such that they encircled the speaker. These crates provided attachment points for vertical settlement panels and to house limestone boulders. To assess oyster recruitment in space and time, we deployed standardised settlement panels (15 × 15 cm fibreboard) at each site for 1 month to avoid over-saturation by recruits (observed during longer deployments), and repeated these deployments three times throughout the recruitment season. For each time period, divers attached two vertical settlement panels to the outside of each crate, securing them 30 cm above the seafloor using cable ties. After 1 month, settlement panels were removed, and the number of recruited oysters counted from the central 7 × 7 cm area (an area shown to be representative of the entire panel) of the outer surface of the settlement panel under dissection microscope. The number of larvae per tile was calculated per m2 and averaged between the two tiles per crate to provide n = 6 replicate crates per treatment, per site, for each time. At Windara Reef, storms prevented the exchange of speakers to maintain our sound treatments through March, and therefore these data were excluded from the analysis.

To assess how soundscape enrichment influences habitat formation on new boulder reefs, we quantified attributes of the habitat formed by oysters on boulders 5 months after the construction of Glenelg Reef. This component was only run at Glenelg Reef for logistical reasons (see manuscript for details). Within a week of reef construction, we placed eight boulders (diameter: 15–30 cm) inside each of the n = 6 crates per site to form independent replicate reefs that reached 30 cm above the seafloor. After 5 months of continual exposure to either speaker or non-speaker control treatments, the top three boulders were removed per crate for analysis in the laboratory. On the exposed upper surface of each boulder, we measured the (1) percentage cover of oyster habitat on each boulder, (2) the number of oysters that were >25 mm in height (the largest size class) as an indication of the earliest recruits to reef boulders and (3) the percentage of early three-dimensional habitat growth (hereafter ‘habitat building’) that was >5 mm above the boulder surface (a height above which no solitary oyster grew, but represented habitat formed by the converging growth of multiple oysters). Boulder surface area and percentage cover was measured in ImageJ from photos taken in the plane of boulder's upper surface. Three-dimensional habitat over >5 mm was manually measured (using a measuring probe) and marked on the boulder surface, after which the percentage cover was measured. Data were averaged across the three boulders per crate (n = 6 per treatment, per site).

Full methods available in the published article.