Chris T. Tindle

Sound as an Orientation Cue for the Pelagic Larvae of Reef Fishes and Decapod Crustaceans

The pelagic life history phase of reef fishes and decapod crustaceans is complex, and the evolutionary drivers and ecological consequences of this life history strategy remain largely speculative. There is no doubt, however, that this life history phase is very significant in the demographics of reef populations. Here, we initially discuss the ecology and evolution of the pelagic life histories as a context to our review of the role of acoustics in the latter part of the pelagic phase as the larvae transit back onto a reef. Evidence is reviewed showing that larvae are actively involved in this transition. They are capable swimmers and can locate reefs from hundreds of metres if not kilometres away. Evidence also shows that sound is available as an orientation cue, and that fishes and crustaceans hear sound and orient to sound in a manner that is consistent with their use of sound to guide settlement onto reefs. Comparing particle motion sound strengths in the field (8 x 10(-11) m at 5 km from a reef) with the measured behavioural and electrophysiological threshold of fishes of (3 x 10(-11) m and 10 x 10(-11), respectively) provides evidence that sound may be a useful orientation cue at a range of kilometres rather than hundreds of metres. These threshold levels are for adult fishes and we conclude that better data are needed for larval fishes and crustaceans at the time of settlement. Measurements of field strengths in the region of reefs and threshold levels are suitable for showing that sound could be used; however, field experiments are the only effective tool to demonstrate the actual use of underwater sound for orientation purposes. A diverse series of field experiments including light-trap catches enhanced by replayed reef sound, in situ observations of behaviour and sound-enhanced settlement rate on patch reefs collectively provide a compelling case that sound is used as an orientation and settlement cue for these late larval stages.

How do spiny lobster post‐larvae find the coast?

Abstract The larvae or phyllosomes of many species of spiny lobsters (Palinuridae) are known to complete their development in offshore oceanic waters. Phyllosomes metamorphose to non‐feeding, nektonic post‐larvae or pueruli, which move into shallow coastal waters where they settle to become benthic dwelling juveniles. There is growing evidence that the movement of pueruli is directed toward the coast rather than a process of random dispersal. The migration inshore by the non‐feeding pueruli is likely to be one of the more extreme examples of onshore orientation among marine organisms, but is still poorly understood. This article provides a synthesis of the current state of knowledge of the possible cues and sensory mechanisms that might be used by pueruli of spiny lobsters for orienting toward the coast from offshore waters. The review is used to identify the potential cues that would benefit from future research efforts.

Reflection of underwater sound from surface waves

A tank experiment has been conducted to measure reflection of underwater sound from surface waves. Reflection from a wave crest leads to focusing and caustics and results in rapid variation in the received waveform as the surface wave moves. Theoretical results from wavefront modeling show that interference of three surface reflected eigenrays for each wave crest produces complicated interference waveforms. There is good agreement between theory and experiment even on the shadow side of caustics where there are two surface reflected arrivals but only one eigenray.

Bubbled waters: The noise generated by underwater breathing apparatus

Underwater breathing apparatus (UBA) has played a vital role in the study of aquatic environments, and is commonly used in visual census of mobile aquatic animals. The possibility of artifacts arising from diver presence and from the noise produced by UBA have long been recognised but not systematically studied. Here we analyse the noise produced by the three types of UBA used for research; self-contained underwater breathing apparatus (SCUBA), semi-enclosed circuit re-breather (SECR), and fully enclosed circuit re-breather (FECR) systems. There were significant differences in the source levels (SL) produced by the different UBA for both mean SL (p < 0.001) and mean peak SL (p < 0.001). SCUBA produced the most noise followed by SECR and FECR (161 ± 1, 131 ± 2, and 108 ± 1 dB re 1 μPa at 1 m, ±S.E.). Much of the sound produced by all three UBA was at low frequencies (<200 Hz), the range in which the hearing organs of fish and decapod crustaceans are most sensitive. Calculations indicated that the UBA are likely to be detectable by fishes at considerable distances depending on natural ambient noise levels.

Shallow water sound propagation with surface waves

The theory of wavefront modeling in underwater acoustics is extended to allow rapid range dependence of the boundaries such as occurs in shallow water with surface waves. The theory allows for multiple reflections at surface and bottom as well as focusing and defocusing due to reflection from surface waves. The phase and amplitude of the field are calculated directly and used to model pulse propagation in the time domain. Pulse waveforms are obtained directly for all wavefront arrivals including both insonified and shadow regions near caustics. Calculated waveforms agree well with a reference solution and data obtained in a near-shore shallow water experiment with surface waves over a sloping bottom.

Deterministic forward scatter from surface gravity waves

Deterministic structures in sound reflected by gravity waves, such as focused arrivals and Doppler shifts, have implications for underwater acoustics and sonar, and the performance of underwater acoustic communications systems. A stationary phase analysis of the Helmholtz-Kirchhoff scattering integral yields the trajectory of focused arrivals and their relationship to the curvature of the surface wave field. Deterministic effects along paths up to 70 water depths long are observed in shallow water measurements of surface-scattered sound at the Marthas Vineyard Coastal Observatory. The arrival time and amplitude of surface-scattered pulses are reconciled with model calculations using measurements of surface waves made with an upward-looking sonar mounted mid-way along the propagation path. The root mean square difference between the modeled and observed pulse arrival amplitude and delay, respectively, normalized by the maximum range of amplitudes and delays, is found to be 0.2 or less for the observation periods analyzed. Cross-correlation coefficients for modeled and observed pulse arrival delays varied from 0.83 to 0.16 depending on surface conditions. Cross-correlation coefficients for normalized pulse energy for the same conditions were small and varied from 0.16 to 0.06. In contrast, the modeled and observed pulse arrival delay and amplitude statistics were in good agreement.

Wavefronts and waveforms in deep-water sound propagation

A new method of calculating waveforms in underwater sound propagation is presented. The method is based on a Hankel transform-generalized Wentzel-Kramers-Brillouin (WKB) solution of the wave equation. The resulting integral leads to a new form of ray theory which is valid at relatively low frequencies and allows evaluation of the acoustic field on both the illuminated and shadow sides of caustics and at cusps where two caustics meet to form a focus. The integral is evaluated by stationary phase methods for the appropriate number of stationary points. Rays of nearby launch angle which have a travel time difference less than a quarter period must be considered together. The description of all other ray arrivals corresponds to simple ray theory. The phase, amplitude, and travel time of broadband acoustic pulses are obtainable directly from a simple graph of ray travel time as a function of depth at a given range. The method can handle range dependence but is illustrated here in long-distance propagation in deep water where the ray paths do not pass close to surface or bottom. The method is fast and gives close agreement with normal-mode calculations. The field on the shadow side of a caustic is properly given in terms of rays with complex launch angles, but good approximations can be obtained without the need to find complex rays.

Microseisms and ocean wave measurements

Measurements of microseisms in Auckland, New Zealand, are compared with ocean wave data taken on the west coast of New Zealand, about 150 km southwest of Auckland. There is strong correlation at most times. Exceptions are when the Auckland area is subject to strong winds from the easterly quarter. Microseism activity in Auckland in the 0.05-1-Hz band appears to be entirely due to ocean waves.

Coupled mode perturbation theory of range dependence

The conventional coupled mode solution is combined with perturbation theory to give a fast, accurate range-dependent normal mode solution for deep water acoustic propagation. Perturbation theory is used to calculate the new normal modes at each range step. The new modes are obtained as a linear combination of the modes for the previous step without requiring a numerical solution of the depth-separated wave equation. The process may be repeated for many steps and yields normal modes and eigenvalues which are sufficiently accurate for solution of practical problems in deep water. The method is applied to long-range propagation through oceanic fronts.

A three-dimensional analysis of acoustic propagation in a penetrable wedge slice

The method of source images is used to develop an analytical solution for the three‐dimensional acoustic field in a penetrable wedge. The acoustic field for a harmonic, point source is computed assuming isovelocity sound field profiles in the water and sediment. The solution for the field consists of a sum of contributions due to source images and gives the sound field at any point in the water or sediment of the wedge. This analysis differs from the earlier work of Deane and Tindle (submitted to JASA, 10 December 1991), which is restricted to propagation in a plane perpendicular to the wedge apex. The present work uses a Bessel function expansion that results in a computationally efficient solution to the field which is valid throughout the wedge.