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A systematic study of the ambient noise in the shallow coastal waters of north-eastern New Zealand shows large temporal variability in acoustic power levels between seasons, moon phase and the time of day. Ambient noise levels were highest during the new moon and the lowest during the full moon. Ambient noise levels were also significantly higher during summer and lower during winter. Bandpass filtering (700-2,000 Hz and 2-15 kHz), combined with snap counts and data from other studies show that the majority of the sound intensity increases could be attributed to two organisms: the sea urchin and the snapping shrimp. The increased intensity of biologically produced sound during dusk, new moon and summer could enhance the biological signature of a reef and transmit it further offshore. Ambient noise generated from the coast, especially reefs, has been implicated as playing a role in guiding pelagic post-larval fish and crustaceans to settlement habitats. Determining a causal link between temporal increases in ambient noise and higher rates of settlement of reef fish and crustaceans would provide support for the importance of ambient underwater sound in guiding the settlement of these organisms.

Ambient sound intensity in coastal waters typically increases by as much as 2 to 3 orders of magnitude (20 to 30 dB) immediately after sunset and before sunrise in what is known as a dawn and evening chorus. The dominant feature of the chorus is most often a dramatic increase in spectrum level usually in a narrow frequency range of around 400 to 4000 Hz. While the sources of some choruses have been identified, the sources of many choruses remain unidentified. Here we confirm that in New Zealand, the sound is the feeding noises of sea urchins for which frequencies in the range of 800 to 2800 Hz are amplified by the ovoid calcareous skeleton, or 'test', of urchins acting as a Helmholtz resonator. Furthermore, the timing of the dawn and dusk choruses is related to the crepuscular feeding activity of sea urchins. Underwater sound recordings from individual sea urchins of a range of sizes confirm earlier speculation that the urchin test acts as a Helmholtz resonance chamber capable of generating sufficient acoustic power to create these choruses. These results indicate the potential importance of coastal urchin populations as a major contributor to the underwater choruses, which appear to be important in assisting the larvae of key reef species, such as fishes, crabs, and lobsters, to locate suitable settlement sites. KEY WORDS: Sea urchins · Dusk chorus · Helmholtz resonator · Ambient underwater sound · Orientation cue · Evechinus chloroticus Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 362: [37][38][39][40][41][42][43] 2008 Many of the organisms responsible for the dramatic increase in ambient underwater sound immediately after dusk and before dawn, which is a feature of coastal reef ecosystems in many parts of the world (Fish 1964), remain unidentified. For example, the source of underwater dusk choruses, which increase ambient sound levels by as much as 20 to 30 dB re 1 µPa in temperate waters such as in Australia and New Zealand, has never been reliably confirmed, although responsibility has been assigned to a range of potential candidate species, including fishes and urchins (Cummings et al. 1964, Fish 1964, Cato 1978. The feeding activities of sea urchins have been suggested as a possible source of the major component of the chorus due to their ubiquitous presence in most temperate reef systems (Tait 1964, Cato 1978. In addition, the possibility of the feeding noises being amplified by the ovoid calcareous skeleton (or 'test') of urchins acting as a resonance chamber was invoked in an attempt to reconcile the large overall intensity level of the dusk chorus versus the relatively small size of the urchins (Castle 1974, Castle & Kibblewhite 1975, Cato 1977. Although an urchin test would appear to have suitable attributes to operate as a Helmholtz resonator, i.e. an enclosed volume of fluid with an external aperture allowing oscillations of fluid through the aperture to entrain the enclosed fluid to oscillate in sympathy (Cato 1977), there is no expe...

Underwater sound emanating from reefs has been shown to be attractive to pre-settlement larval stages of fish and crustaceans, but its ecological importance depends on the range at which this cue can be detected by these larvae. Here we show, through field measurement and modelling, that the spatially extended sound source of a reef creates a surrounding zone, which extends for a distance offshore equal to the length of the reef, within which there is almost no loss in the sound level. Beyond this zone, the sound level decreases with cylindrical spreading plus any seafloor attenuation. This 'reef effect' means that the sound from a reef would be detectable at a much greater distance from the reef than would be estimated from a spot measurement near the reef or by using theoretical models of sound spreading from a point source. The greater reach for sound emanating from a reef means that reef noise could play a greater role in directing larval reef fishes and crabs to suitable settlement habitats than previously estimated.

A normal mode method for propagation modeling in acousto-elastic ocean waveguides is described. The compressional (p-) and shear (s-) wave propagation speeds in the multilayer environment may be constant or have a gradient (1/c2 linear) in each layer. Mode eigenvalues are found by analytically computing the downward- and upward-looking plane wave reflection coefficients R1 and R2 at a reference depth in the fluid and searching the complex k plane for points where the product R1R2=1. The complex k-plane search is greatly simplified by following the path along which |R1R2|=1. Modes are found as points on the path where the phase of R1R2 is a multiple of 2π. The direction of the path is found by computing the derivatives d(R1R2)/dk analytically. Leaky modes are found, allowing the mode solution to be accurate at short ranges. Seismic interface modes such as the Scholte and Stonely modes are also found. Multiple ducts in the sound speed profile are handled by employing multiple reference depths. Use of Airy function solutions to the wave equation in each layer when computing R1 and R2 results in computation times that increase only linearly with frequency.