DOI: 10.5061/dryad.xsj3tx9k0
title: Soundscape enrichment enhances recruitment and habitat building on new oyster reef restorations

description: 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.
description: 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 &gt;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 &gt;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 &gt;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.