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.