No abstract

In order to study the long-term stability of fin whale (Balaenoptera physalus) singing behavior, the frequency and inter-pulse interval of fin whale 20 Hz vocalizations were observed over 10 years from 2003–2013 from bottom mounted hydrophones and seismometers in the northeast Pacific Ocean. The instrument locations extended from 40°N to 48°N and 130°W to 125°W with water depths ranging from 1500–4000 m. The inter-pulse interval (IPI) of fin whale song sequences was observed to increase at a rate of 0.54 seconds/year over the decade of observation. During the same time period, peak frequency decreased at a rate of 0.17 Hz/year. Two primary call patterns were observed. During the earlier years, the more commonly observed pattern had a single frequency and single IPI. In later years, a doublet pattern emerged, with two dominant frequencies and IPIs. Many call sequences in the intervening years appeared to represent a transitional state between the two patterns. The overall trend was consistent across the entire geographical span, although some regional differences exist. Understanding changes in acoustic behavior over long time periods is needed to help establish whether acoustic characteristics can be used to help determine population identity in a widely distributed, difficult to study species such as the fin whale.

Source levels of fin whale calls can be used to determine range to recorded vocalizations and to model maximum communication range between animals. In this study, source levels of fin whale calls were estimated using data collected on a network of eight ocean bottom seismometers in the Northeast Pacific Ocean. The acoustic pressure levels measured at the instruments were adjusted for the propagation path between the calling whales and the instruments using the call location and estimating losses along the acoustic travel path. A total of 1241 calls were used to estimate an average source level of 189 ± 5.8 dB re 1μPa at 1 m. This variability is largely attributed to uncertainties in the horizontal and vertical position of the fin whale at the time of each call and the effect of these uncertainties on subsequent calculations. Variability may also arise from station to station differences within the network. For call sequences produced by a single vocalizing whale, no consistent increase or decrease in source level was observed over the duration of a dive. Calls within these sequences that immediately followed gaps of 27 s or longer were classified as backbeat calls and were consistently lower in both frequency and amplitude.

A large number of fin whale calls have been observed in a 3-year ocean bottom seismometer dataset (2003–2006) over the Endeavor Ridge (48°N/129°W), a hydrothermally active area in the Northeast Pacific Ocean. Most of the vocalizations were detected during the winter months. Because zooplankton constitute an important part of fin whales’ diets, and enhanced populations of zooplankton have been observed at all depths above the Endeavor hydrothermal vents, it has been hypothesized that the fin whales could be near the vents specifically for feeding. As part of the analysis of the Endeavor vent field data set, algorithms have been developed, which utilize the absolute and relative spectral energy levels in the frequency band of the whale vocalizations. In order to test whether the concentrations of whale vocalizations are unusually high over the hydrothermally active area, the detection algorithm is being applied to data from individual ocean bottom seismometers at other nearby locations including the center of Explorer Plate (49.5°N, 129°W), and the base of the continental slope off Nootka Sound (49.3°N, 127.6°W).

Portsmouth Harbor, New Hampshire, is frequently used as a testing area for multibeam and sidescan sonars, and is the location of numerous ground-truthing studies. Having the ability to accurately position underwater sensors is an important aspect of this type of work. However, underwater positioning in Portsmouth Harbor is challenging. It is relatively shallow, approximately one kilometer wide with depths of less than 25 meters. There is mixing between fresh river water and seawater, which is intensified by high currents and strong tides. This causes a very complicated spatial and temporal sound speed structure. Solutions that use the time-of-arrival of an acoustic pulse to estimate range will require very precise knowledge of the travel paths of the signal in order to separate out issues of multipath arrivals. An alternative solution is to use the phase measurements between closely spaced hydrophones to measure the bearing of an acoustic pinger. By using two bearing measurement devices that are widely separated, the intersection of the two bearings can be used to position the pinger. The advantage of this approach is that the sound speed only needs to be known at the location of the phase measurements. Both time-of-arrival and phase difference systems may encounter difficulties arising from horizontal refraction due to spatially varying sound speed. To ascertain which solution would be optimal in Portsmouth Harbor, the time-of-arrival and phase measurement approaches are being examined individually. Initial field tests have been conducted using a 40 kHz signal to look at bearing accuracy. Using hydrophones that are spaced 2/3 wavelengths apart, the bearing accuracy was found to be 1.25°for angles up to 20°from broadside with signal to noise ratios (SNR) greater than 15dB. The results from the closely spaced hydrophones were used to resolve phase ambiguities, allowing finer bearing measurements to be made between hydrophones spaced 5 wavelengths apart. The fine bearing measurements resulted in a bearing accuracy of 0.3°for angles up to 20°from broadside with SNR greater than 15dB. Field tests planned for summer 2007 will include a more detailed investigation of how the environmental influences affect each of the measurement types including range, signal to noise ratio, currents, and sound speed structure.