Acoustic techniques have the potential to assess marine mammal populations in areas of naval interest such as within the Southern California Offshore Range (SCORE). We report on a multi-year study of marine mammal presence within the SCORE range using both acoustic and visual techniques. The primary objective of this SERDP project was to develop methods for acoustic monitoring of marine mammals. We compared acoustic methods against conventional ship based and aerial visual methods for estimating marine mammal seasonality and relative abundance.

Environmental assessments are made to determine the potential impacts to marine mammals from Naval activities. The problem that the Navy confronts in preparing these assessments is a lack of scientific information on marine mammal population dynamics and behavioral ecology. To date, marine mammal assessments have relied on visual surveys. The difficulty and expense of visual surveys, and the low numbers of sightings that they generate has impeded their usefulness for Navy environmental assessment. The technology of passive acoustics has recently been advanced to allow these methods to make a contribution to studies of marine mammal seasonality and relative abundance.

We studied marine mammal populations within the SCORE range using four techniques: (1) aerial surveys, (2) ship-based transect surveys, (3) sonobuoy-based mobile acoustic surveys, and (4) continuous fixed-site acoustic surveys. By combining visual and acoustic methods, we have gained a more complete temporal and spatial coverage than is possible by use of a single approach.

Project milestones were: (1) conduct continuous acoustic recording with the SCORE range, (2) conduct quarterly shipboard visual and acoustic surveys within the SCORE range, (3) conduct quarterly aerial surveys within SCORE, (4) develop new technology and algorithms for the detection and classification of whale calls, (5) estimate whale calling statistics, and (6) conduct technology transfer to the Navy. Cascadia Research Collective was our collaborator for the marine mammal visual surveys, and the National Marine Fisheries Service was our collaborator for the aerial surveys.

Long-term acoustic recording and shipboard visual and acoustic surveys, were accomplished during quarterly shipboard expeditions to the SCORE region. During each of these expeditions visual surveys were conducted, sonobuoy acoustic data were collected, and autonomous acoustic recording packages were serviced. Aerial surveys were conducted quarterly in the SCORE region from spring 2001 to spring 2003. Following a comprehensive review, we discontinued the SCORE aerial surveys, owing to flight dangers, and low numbers of marine mammal sightings.

Two new technology developments from this SERDP project were high capacity (1.2 Tbyte) autonomous acoustic recording packages and acoustic recording whale-attached tags. The autonomous acoustic recording packages allowed long-term marine mammal monitoring. With these instruments we were able to select environmental settings appropriate for study of whales within SCORE. Through collaboration with Greeneridge Sciences Inc., we developed and applied a suction cup deployed acoustic recording tag for study of blue whale calling behavior. In collaboration with SAIC (Michael Porter) we developed new algorithms for automatic detection and localization of calling whales.

We assessed the behavioral context of acoustic calls produced by Northeast Pacific blue whales using visual and acoustic observations along the California coast. We deployed acoustic recording tags, and employed photo-ID, tissue sampling, and acoustic monitoring with sonobuoys to evaluate the relationship between blue whale call occurrence and behavior. Only a small proportion of monitored blue whales produced calls, and these fell into three categories, 1) repeated low frequency pulsed A and tonal B calls or song, 2) irregularly patterned A and B calls, and 3) downswept D calls and highly variable amplitude or frequency modulated calls. A and B calls are produced only by male blue whales, while D calls are produced by both sexes. Song calls are heard from lone, traveling male blue whales, while D calls are heard during foraging, commonly within groups of animals. The precise function of each call type is not understood; however, information on the sex-bias in call production, and the non-acoustic behaviors associated with each call type help us to understand the context of call production and allow more effective use of certain call types to assess population status and habitat associations.

We developed algorithms for automated tracking of whales using arrays of sensors. Pair-wise time-differences in arrival of whale calls as measured by a phase-only correlation process were compared to time-lags predicted by an acoustic propagation model. Differences between measured and modeled time-lags defined an ambiguity surface that identifies the most probable whale location in a horizontal plane around an array. We applied this technique to tracking calling blue whales off the coast of California. The algorithm performed extremely well, providing the capability for real-time, automated monitoring

Diel and seasonal calling patterns for blue whales were observed in coastal waters off southern California using seafloor-mounted autonomous acoustic recording packages. Automated call counting from spectrogram cross-correlation showed peak seasonal calling in late summer/early fall. When call counts were organized by daily time intervals, calling peaks were observed during twilight periods, just after sunset and before sunrise. Minimum calling was observed during the day. Night time calling was greater than day time calling, but also showed a minimum between the dusk and dawn calling peaks. These peaks correlate with vertical migration times of krill, the blue whales’ primary prey. One hypothesis to explain these diel variations is that blue whale calling and foraging may be mutually exclusive activities. Fewer calls are produced during the day while preys are aggregated at depth and foraging is efficient. More calls are produced during the twilight time periods when prey are vertically migrating and at night when preys are dispersed near the sea surface and foraging is less efficient.

Blue and fin whale call patterns were monitored at Cortez and Tanner Banks in the Southern California Bight from August 2000 to February 2004. Seasonal and diel variation of calls may indicate changing environmental conditions and whale behavior. Three types of blue whale calls were monitored, 1) low frequency (type B) calls occurring in stereotyped sequences, or songs, 2) low frequency (type B) calls occurring in irregular temporal patterns, and 3) downswept (type D) calls. Fin whale downswept calls were also monitored. Blue whale calls were produced seasonally from April to January each year, with type D calls present predominantly early in the season, from April to November, and song and singular B calls predominantly present later in the season from June to January. Fin whale calls were heard year-round, with a seasonal peak from August to December. Fin whale song character was variable among individuals and over the four year monitoring effort. The peak of acoustic detection for blue whale song calls and fin whale calls is delayed by one to two months from their peak presence observed during visual surveys in the Southern California Bight, suggesting a bias in the visual detection of vocal whales and/or an increase in individual calling late in the season, changes that may be related to reproductive activities. Blue whale D calls precede visual detections, though they have a common seasonal peak and both are thought to connote feeding whales.

Blue whale songs provide a measure for characterizing worldwide blue whale population structure. These songs are divided into nine regional types, which maintain a stable character. Five of the nine song types have been recorded over time spans greater than 30 years showing no significant change in character. The nine song types can be divided into those containing only simple tonal components (high latitude North Pacific, North Atlantic and Southern Ocean song types), those comprised of complex pulsed units in addition to the tonal components (Pacific Ocean margin song types from California, Chile and New Zealand), and those which have the greatest complexity of all and the longest cycling times (Indian Ocean song types from Sri Lanka, Fremantle and Diego Garcia). We suggest that temporally stable differences in song provide another characteristic for comparison with genetic and morphological data when defining blue whale populations. Furthermore, we recommend that when there is a lack of other data or lack of clarity in other data sets, evidence of distinct differences in songs between areas be used as a provisional hypothesis about population structure when making management decisions.

Using acoustic and satellite remote sensing, we have continuously monitored the acoustic activity and habitat of blue whales during 1994-2000. Calling blue whales primarily aggregate off the coast of southern and central California in the late summer, coinciding with the timing of the peak euphausiid biomass, their preferred prey. The northbound movements of the blue whales and primary production are apparent in the acoustic and satellite records, with the calling blue whales moving north along the Oregon and Washington coasts, to a secondary foraging area with high primary productivity off Vancouver Island in the late fall. El Nino conditions, indicated by higher sea surface temperature and lower chlorophyll-a concentrations, are apparent in the satellite record, particularly in the Southern California Bight. These conditions disrupt calling blue whale habitat and alter their presence in primary feeding locations. Remote sensing using acoustics is well suited to characterizing the seasonal movements and relative abundance of the northeast Pacific blue whale, and remote sensing using satellites allows for monitoring their habitat. These technologies are invaluable because of their ability to provide continuous large scale spatial and temporal coverage of the blue whale migration.

There are several ways that the data and results of our SERDP project are being transitioned to the Navy and to the SCORE range in particular. First, the raw visual and acoustic marine mammal data that were generated by this project are being included in a marine resources assessment that is being preparing for the Navy’s Southern California Operating Area (SOCAL) (Julie Rivers, NAVFAC PAC, personal communication). Our dataset provides the most intensive and comprehensive set of marine mammal observations to date within the SCORE range and surrounding areas. Second, we have developed new technology and algorithms that are finding application in other Naval ranges. For instance, we are now conducting long term acoustic monitoring, using the tools developed by this project, in the Navy’s Quinault Range, located off the coast of Washington. Likewise, we are involved in plans to conduct similar acoustic and visual monitoring in the Navy’s proposed East Coast shallow water test range. As part of this SERDP project we collaborated with the Naval Post-Graduate School in collecting data from the SCORE underwater tracking range hydrophones, as a means for assessing how the installed hydrophones of the SOAR tracking range could be used for marine mammal monitoring. In addition, we helped to raise awareness of the potential for marine mammal tracking with Navy range hydrophones, so that consideration could be given to the potential for future hydrophone installations in the SCORE region, such as the future shallow water test range planned for theTanner/Cortez Banks region, to be used for tracking marine mammals as part of Navy mitigation and/or monitoring efforts.