Tropical Storm Debby: Soundscape and fish sound production in Tampa Bay and the Gulf of Mexico.

Tropical cyclones have large effects on marine ecosystems through direct (e.g., storm surge) and indirect (e.g., nutrient runoff) effects. Given their intensity, understanding their effects on the marine environment is an important goal for conservation and resource management. In June 2012, Tropical Storm Debby impacted coastal Florida including Tampa Bay. Acoustic recorders were deployed prior to the storm at a shallow water location inside Tampa Bay and a deeper water location in the Gulf of Mexico. Ambient noise levels were significantly higher during the storm, and the highest increases were observed at lower frequencies (≤ 500 Hz). Although the storm did not directly hit the area, mean ambient noise levels were as high as 13.5 dB RMS above levels in non-storm conditions. At both the shallow water and the deep water station, the rate of fish calls showed a variety of patterns over the study period, with some rates decreasing during the storm and others showing no apparent reaction. The rates of fish calls were frequently correlated with storm conditions (storm surge, water temperature), but also with lunar cycle. Reactions to the storm were generally stronger in the inshore station, although fish sounds increased quickly after the storm's passage. Although this was not a major tropical cyclone nor a direct hit on the area, the storm did appear to elicit a behavioral response from the fish community, and ambient noise levels likely limited the abilities of marine species to use sound for activities such as communication. Given the increases in intensity and rainfall predicted for tropical cyclones due to climate change, further studies of the ecological effects of tropical cyclones are needed.

Quantification of Boat Visitation Rates at Artificial and Natural Reefs in the Eastern Gulf of Mexico Using Acoustic Recorders.

Artificial reefs are commonly used as a management tool, in part to provide ecosystem services, including opportunities for recreational fishing and diving. Quantifying the use of artificial reefs by recreational boaters is essential for determining their value as ecosystem services. In this study, four artificial-natural reef pairs in the eastern Gulf of Mexico (off western Florida) were investigated for boat visitation rates using autonomous acoustic recorders. Digital SpectroGram (DSG) recorders were used to collect sound files from April 2013 to March 2015. An automatic detection algorithm was used to identify boat noise in individual files using the harmonic peaks generated by boat engines, and by comparing the sound amplitude of each file with surrounding files. In all four pairs, visitation rates were significantly higher at the artificial reef than the natural reef. This increase in boat visitation was likely due to actual or perceived increased quality of fishing and diving at the artificial reefs, or to lack of knowledge of the presence or locations of the natural reefs. Inshore reefs (<15 m depth) had high variability in monthly visitation rates, which were generally highest in warmer months. However the seasonal signal was dampened on offshore reefs (>25 m depth). This study appears to be the first to use acoustic data to measure participant use of boating destinations, and highlights the utility of acoustic monitoring for the valuation of this important ecosystem service provided by artificial reefs.

A severe red tide (Tampa Bay, 2005) causes an anomalous decrease in biological sound.

Although harmful algal blooms (HABs) are known to cause morbidity and mortality in marine organisms, their sublethal effects are poorly understood. The purpose of this study was to compare ambient noise levels during a severe HAB event in Tampa Bay, Florida, to those during non-HAB periods. Passive acoustic monitoring was conducted using bottom-mounted autonomous acoustic recorders during a severe HAB in summer 2005, and in summers 2006, 2011 and 2012 (non-severe HAB years). Ambient noise levels were significantly higher during the non-HAB years due to an abundance of snapping shrimp (Alpheidae) sounds and fish chorusing. The difference of sound intensity between the study years is most likely attributable to effects of the HAB on the abundance and/or behaviour of fish and snapping shrimp as a result of mortality and stress-induced behavioural modifications.

Whistle source levels of free-ranging bottlenose dolphins and Atlantic spotted dolphins in the Gulf of Mexico.

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.

Low frequency narrow-band calls in bottlenose dolphins (Tursiops truncatus): signal properties, function, and conservation implications.

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.

Trouble-shooting deployment and recovery options for various stationary passive acoustic monitoring devices in both shallow- and deep-water applications.

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.

Depth dependent variation of the echolocation pulse rate of bottlenose dolphins (Tursiops truncatus).

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.