Sounds from toadfish Opsanus sp., and 4 other suspected fish sounds were identified in passive acoustic recordings from fixed recorders and autonomous underwater vehicles in the eastern Gulf of Mexico between 2008 and 2011. Data were collected in depths ranging from 4 to 984 m covering approximately 39 000 km 2. The goals of this research were to map the spatial and temporal occurrence of these sounds. Sound production was correlated to environmental parameters (water depth, lunar cycle, and dawn and dusk) to understand the variability in seasonal calling. Toadfish 'boatwhistles' were recorded throughout the diel period, with peaks observed between 15:00 and 04:00 h. Annual peaks coincided with the spawning period in the late spring to early summer. The 4 unknown sounds were termed: '100 Hz Pulsing', '6 kHz Sound', '300 Hz FM Harmonic', and '365 Hz Harmonic'. The 100 Hz Pulsing had the temporal characteristics of a cusk-eel call with frequencies below 500 Hz. Sound production was observed mainly at night with annual peaks in the spring and fall. The 6 kHz Sound was observed exclusively at night between 15 and 50 m bottom depths; occurrence decreased significantly in the winter. The 6 kHz Sound peak frequencies correlated positively to satellite-derived sea surface temperature (SST) and nega tively to chlorophyll concentration. The 300 Hz FM Harmonic was observed largely (89%) at night and appeared offshore (40−200 m depth). The 365 Hz Harmonic was observed 98% of the time at night, inshore (< 40 m depth). The fundamental frequency of the 365 Hz Harmonic was positively correlated with SST, reflecting a temperature-driven increase in sonic muscle contraction rate; conversely, call duration was negatively correlated. The ubiquity of these 4 unknown sounds illustrates how little is known about biological communication in the marine environment.

Dolphins routinely use sound for social purposes, foraging and navigating. These sounds are most commonly classified as whistles (tonal, frequency modulated, typical frequencies 5-10 kHz) or clicks (impulsed and mostly ultrasonic). However, some low frequency sounds have been documented in several species of dolphins. Low frequency sounds produced by bottlenose dolphins (Tursiops truncatus) were recorded in three locations along the Gulf of Mexico. Sounds were characterized as being tonal with low peak frequencies (mean = 990 Hz), short duration (mean = 0.069 s), highly harmonic, and being produced in trains. Sound duration, peak frequency and number of sounds in trains were not significantly different between Mississippi and the two West Florida sites, however, the time interval between sounds within trains in West Florida was significantly shorter than in Mississippi (t = -3.001, p = 0.011). The sounds were significantly correlated with groups engaging in social activity (F=8.323, p=0.005). The peak frequencies of these sounds were below what is normally thought of as the range of good hearing in bottlenose dolphins, and are likely subject to masking by boat noise.

Whistles of bottlenose dolphins (Tursiops truncatus) and Atlantic spotted dolphins (Stenella frontalis) in the eastern Gulf of Mexico were recorded and measured with a calibrated towed hydrophone array. Surveys encountered groups of both bottlenose (N = 10) and spotted dolphins (N = 5). Analysis of those data produced 1695 bottlenose dolphin whistles and 1273 spotted dolphin whistles with a high signal-to-noise ratio. Whistle frequency metrics were lower in bottlenose than spotted dolphins, while whistle duration was longer in spotted dolphins, data that may help inform automatic classification algorithms. Source levels were estimated by determining the range and bearing of an individual dolphin from the array and then adding the predicted transmission loss to the calculated received level. The median bottlenose dolphin source level was 138 dB re 1μPa at 1 m with a range of 114-163 dB re 1μPa at 1 m. The median spotted dolphin source level was 138 dB re 1μPa at 1 m with a range of 115-163 dB re 1μPa at 1 m. These source level measurements, in conjunction with estimates of vocalization rates and transmission loss models, can be used to improve passive acoustically determined dolphin abundance estimates in the Gulf of Mexico.

Trained odontocetes appear to have good control over the timing (pulse rate) of their echolocation clicks; however, there is comparatively little information about how free-ranging odontocetes modify their echolocation in relation to their environment. This study investigates echolocation pulse rate in 14 groups of free-ranging bottlenose dolphins (Tursiops truncatus) at a variety of depths (2.4-30.1 m) in the Gulf of Mexico. Linear regression models indicated a significant decrease in mean pulse rate with mean water depth. Pulse rates for most groups were multi-modal. Distance to target estimates were as high as 91.8 m, assuming that echolocation was produced at a maximal rate for the target distance. A 5.29-ms processing lag time was necessary to explain the pulse rate modes observed. Although echolocation is likely reverberation limited, these results support the hypotheses that free-ranging bottlenose dolphins in this area are adapting their echolocation signals for a variety of target detection and ranging purposes, and that the target distance is a function of water depth.

Deployment of any type of measuring device into the ocean, whether to shallow or deeper depths, is accompanied by the hope that this equipment and associated data will be recovered. The ocean is harsh on gear. Salt water corrodes. Currents, tides, surge, storms, and winds collaborate to increase the severity of the conditions that monitoring devices will endure. All ocean-related research has encountered the situations described in this paper. In collating the details of various deployment and recovery scenarios related to stationary passive acoustic monitoring use in the ocean, it is the intent of this paper to share trouble-shooting successes and failures to guide future work with this gear to monitor marine mammal, fish, and ambient (biologic and anthropogenic) sounds in the ocean-in both coastal and open waters.

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The goals of this project were to determine the daily, seasonal and spatial patterns of red grouper Epinephelus morio sound production on the West Florida Shelf (WFS) using passive acoustics. An 11 month time series of acoustic data from fixed recorders deployed at a known E. morio aggregation site showed that E. morio produce sounds throughout the day and during all months of the year. Increased calling (number of files containing E. morio sound) was correlated to sunrise and sunset, and peaked in late summer (July and August) and early winter (November and December). Due to the ubiquitous production of sound, large-scale spatial mapping across the WFS of E. morio sound production was feasible using recordings from shorter duration-fixed location recorders and autonomous underwater vehicles (AUVs). Epinephelus morio were primarily recorded in waters 15-93 m deep, with increased sound production detected in hard bottom areas and within the Steamboat Lumps Marine Protected Area (Steamboat Lumps). AUV tracks through Steamboat Lumps, an offshore marine reserve where E. morio hole excavations have been previously mapped, showed that hydrophone-integrated AUVs could accurately map the location of soniferous fish over spatial scales of <1 km. The results show that passive acoustics is an effective, non-invasive tool to map the distribution of this species over large spatial scales.

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