Katherine Cameron, SIO
Coastal fish communities are an important natural resource that are often challenging to manage due to limitations of traditional methods. However, with over 800 species of fish known to produce sound, passive acoustic methods including localization may be used to overcome these limitations and monitor fish communities. Data from passive acoustic arrays deployed at two different locations between 2015 and 2017 in Little Cayman, Cayman Islands during the spawning of Nassau grouper (Epinephelus striatus) have been used to study the temporal and spatial dynamics of this spawning aggregation. 2D and 3D hyperbolic localization methods were evaluated and applied to the data collected in 2017 to look at fine-scale movements of the spawning aggregation at nighttime when visual observations are not possible. These data revealed significant differences in the calling trends of Nassau grouper between sites and years and movement of the aggregation around sunset in 2017 that are related to spawning activity. I will discuss these methods and results further in this presentation and the benefit these methods may have for fisheries management.
Estimating the sediment sound speed from the horizontal coherence of the head wave excited by a Robinson R44 helicopter
A series of shallow-water acoustic experiments has been conducted off the coast of southern California using a Robinson R44 helicopter as a low-frequency (≈ 13 – 3500 Hz) sound source. The aim of the experiments was to recover the sound speed of a fine to very-fine sand sediment from the horizontal coherence of the head wave excited in the water column by the helicopter. Two hydrophones, separated horizontally by approximately 15 m and situated 0.5 m above the seabed, received the head-wave signals, allowing the coherence function to be formed over the bandwidth of the airborne source. The sediment sound speed was recovered by matching the zero crossings of the measured coherence function to those predicted from a recently developed theory of head-wave generation in shallow water. Using this technique, the sediment sound speed can be estimated over a frequency range extending between 500 Hz, the lowest zero crossing of the coherence function, and 2.5 kHz, the bandwidth of the source. In the middle of the frequency band, the sound speed of the sediment was estimated to be 1682 ± 16 m/s, consistent with the known sediment type. [Research supported by ONR, SMART(DOD), NAVAIR and SIO].