Thomas F. Norris

Acoustic detections of singing humpback whales (Megaptera novaeangliae) in the eastern North Pacific during their northbound migration

Numerous (84) acoustic detections of singing humpback whales were made during a spring (08 March-09 June 1997) research cruise to study sperm whales in the central and eastern North Pacific. Over 15,000 km of track-line was surveyed acoustically using a towed hydrophone array. Additionally, 83 sonobuoys were deployed throughout the study area. Detection rates were greatest in late March, near the Hawaiian Islands, and in early April, northeast of the islands. Only one detection was made after April. Detection rates for sonobuoys were unequal in three equally divided longitudinal regions of the study area. Two high density clusters of detections occurred approximately 1200-2000 km northeast of the Hawaiian Islands and were attributed to a large aggregation of migrating animals. The distribution of these detections corroborates findings of previous studies. It is possible that these animals were maintaining acoustic contact during migration. Two unexpected clusters of singing whales were detected approximately 900 to 1000 km west of central and southern California. The location of these detections may indicate a previously undocumented migration route between an offshore breeding area, such as the Revillagigedo Islands, Mexico, and possible feeding areas in the western North Pacific or Bering Sea.

A Review and Inventory of Fixed Autonomous Recorders for Passive Acoustic Monitoring of Marine Mammals

Fixed autonomous acoustic recording devices(autonomous recorders [ARs]) are defined as any electronic recording system that acquires and stores acoustic data internally (i.e., without a cable or radio link to transmit data to a receiving station), is deployed semi-permanently underwater (via a mooring, buoy, or attached to the sea floor), and must be retrieved to access the data. More than 40 ARs were reviewed. They varied greatly in capabilities and costs, from small, hand-deployable units for detecting dolphin and porpoise clicks in shallow water to larger units that can be deployed in deep water and can record at high-frequency bandwidths for over a year. The capabilities and limitations of the systems reviewed herein are discussed in terms of their effectiveness in monitoring and studying marine mammals.

Frequency beamforming of dolphin whistles using a sparse three-element towed array

Acoustic bearings are obtained from dolphin whistles using frequency-domain (FD) beamforming techniques on signals recorded on a three-element 9-m aperture towed array. Due to the wide element separation, the high-frequency (kHz range) signals generate numerous grating lobes, but these lobes shift bearing with beamformed frequency, allowing identification of the true bearing whenever the whistles have over 1 kHz bandwidth. This method was validated by matching a sighting of a compact group of dolphins with acoustic bearing estimates. The system was subsequently used to detect and determine bearings from animals at least 3 km away and in Beaufort 5+ conditions. Frequency-domain beamforming has advantages over temporal cross correlation when the signals are faint and/or overlapping.

Effects of boat noise on the acoustic behavior of humpback whales

The effects of boat noise on cetacean acoustic behavior are not well understood. To examine these, real sources of boat noise were experimentally introduced to singing humpback whales (Megaptera novaeangliae). Humpback whales were chosen as subjects because they sing long songs that are easy to record. Also, they are often distributed in nearshore environments with heavy boat traffic. Songs from nine animals were analyzed (n=9). Ten variables describing time and frequency characteristics of humpback song signals and the structure of song patterns were compared before and during exposure to boat noise. Means of two variables (unit duration and phrase duration) were significantly less during boat passes than during control periods. Means of eight other variables were not significantly different. The statistical power of detecting a difference between the means was ≳90% for all variables describing frequency characteristics of songs. Because the durations of some variables were shortened, these results indic...

Acoustic identification of nine delphinid species in the eastern tropical Pacific Ocean

Acoustic techniques have the potential to increase the reliability of cetacean species identification during shipboard surveys. The whistles of nine odontocete species were compared using data collected from a towed array and sonobuoys deployed during dolphin abundance surveys in the eastern tropical Pacific. Twelve variables were measured manually from spectrographic displays of each whistle (n=912). Multivariate discriminant function analysis (DFA) resulted in 49.9% of whistles being classified to the correct species. It was hypothesized that some whistles carry less species‐specific information than others, therefore, groups of five whistles were averaged to reduce the effect of these ambiguous whistles. Correct classification increased to 65.4% when DFA was run on the averaged data set. A species identification decision tree that used 7 of the 12 whistle variables was constructed using nonparametric techniques (classification and regression trees) and resulted in 53.1% correct classification when applied to the original data set. Prior probabilities were added to the decision tree based on sighting rates for each species in the study area, resulting in 56.7% correct classification. The species identification decision tree provides a relatively simple acoustic method that can be used to augment conventional visual techniques.

Acoustic Ecology and Behavior of Minke Whales in the Hawaiian and Marianas Islands: Localization, Abundance Estimation, and Characterization of Minke Whale “Boings”

Balaenoptera acutorostrata (minke whale) is a small and elusive baleen whale that is rarely sighted in the tropical waters of the North Pacific Ocean. During winter and spring, complex sounds called ”boings” are frequently detected around the Hawaiian Islands and other Pacific island regions (Norris et al. 2009; Thompson and Friedl 1982). Although boings were described over 45 years ago (Wenz 1964), they were not attributed to minke whales until very recently (Rankin and Barlow 2005). Sightings of Balaenoptera acutorostrata are uncommon in tropical and subtropical waters; however, boings are frequently detected around the Hawaiian Islands using seafloor hydrophones and from hydrophone arrays towed from research vessels. Even today, very little is known about acoustic behaviors and ecology of Balaenoptera acutorostrata. The long-term objective of this research effort is to improve our understanding of the acoustic ecology and behavior of Balaenoptera acutorostrata in their breeding habitat.

Short duration sounds recorded from blue whales (Balaenopteramusculus) off Peru

Recordings of low‐frequency sounds were made in the presence of blue whales (Balaenoptera musculus) in 1998 and 1999 in waters off northern Peru during a marine mammal research cruise. Recordings were made on DAT from sonobuoys deployed near vocalizing whales. Photographic identifications and biopsy samples were collected from several animals that were recorded. Over 20 signals were extracted and analyzed from DAT segments that contained the best signal quality. All signals from the 1998 encounter consisted of rapid, frequency modulated (FM) downsweeps (begin frequency x=94, S.D.=13.6 Hz; end frequency x=59, S.D.=7.8 Hz; frequency range x=41.7, S.D.=14.9) of relatively short duration (x=1.2, S.D.=0.3 s). Approximately half of the signals exhibited a characteristic s‐shape pattern when examined spectrographically. Long duration sweeps and pulses, typical of blue whales sounds from the eastern North and Central Pacific, were not detected. The vocalizations recorded in this study are compared to recordin...

Patterns of Occurrence and Marine Mammal Acoustic Behavior in Relation to Navy Sonar Activity Off Jacksonville, Florida

Passive acoustic data collected from marine autonomous recording units deployed off Jacksonville, FL (from 13 September to 8 October 2009 and 3 December 2009 to 8 January 2010), were analyzed for detection of cetaceans and Navy sonar. Cetaceans detected included Balaenoptera acutorostrata, Eubalaena glacialis, B. borealis, Physeter macrocephalus, blackfish, and delphinids. E. glacialis were detected at shallow and, somewhat unexpectedly, deep sites. P. macrocephalus were characterized by a strong diel pattern. B. acutorostrata showed the strongest relationship between sonar activity and vocal behavior. These results provide a preliminary assessment of cetacean occurrence off Jacksonville and new insights on vocal responses to sonar.

Automatic localization of individual Hawaiian minke whales from boing vocalizations.

A method is described to automatically localize Hawaiian minke whales from their boing vocalizations. Recorded passive acoustic data from 15 deep water seafloor mounted hydrophones at the Pacific Missile Range Facility is utilized. A critical step is the automatic association of the same vocalization as received by the widely spaced hydrophones. The peak frequency of the vocalization in the detection bandwidth is shown to aid in the association process. Temporal integration of standard time difference of arrival localizations reduces erroneous automatic localizations, which occur for a variety of reasons. A case study of a 2009 minke visual sighting by a field team, which was facilitated by radioing near real‐time location information from shore is described. The peak frequency feature (PFF) has unexpectedly been observed to be very stable for what is believed to be the sighted individual over a 6 hour time period (n = 57, PF¯F=1384.0 Hz, σ = 1.78 Hz). When the minke ceased vocalizing at 13:44 HST, no voc...

Temporal and geographic patterns in the occurrence and distribution of minke whale (Balaenoptera acutorostrata) boings in the central and western North Pacific.

Minke whales are elusive and difficult to study using visual methods. The source of the “boing” sound was recently linked to North Pacific minke whales, allowing passive acoustics to be used to study this species. The seasonal occurrence of minke whales was examined using data collected at the Station ALOHA Cabled Observatory, an ocean bottom hydrophone 100 km north of Oahu. Preliminary analysis of data collected between February and June 2007 indicates that boings occur during all of these months, peaking in early April. No diurnal variation was evident. Towed hydrophone‐array surveys were conducted in the offshore waters of the islands of Oahu, Kauai and Ni’ihau (February 2005) and off Guam and the Northern Mariana Islands (January–April 2007). Although rarely observed visually, the prevalence of boings detected in these areas indicates that minke whales are common. Distribution patterns from both studies suggest that minke whales prefer deep but not the deepest waters. Boings recorded from Guam and the Northern Mariana Islands appear to be more similar to the “central” boing (which includes the Hawaiian Islands) than the “eastern” boing [which includes those recorded east of 138°W, Rankin and Barlow (2005)]. This has important implications for North Pacific minke whale stock structure.