SEASONAL OCCURRENCE OF SPERM WHALE (PHYSETER MACROCEPHALUS) SOUNDS IN THE GULF OF ALASKA, 1999–2001

An acoustic survey for sperm whales was conducted in the Gulf of Alaska. Six autonomous hydrophones continuously recorded sound signals below 500 Hz from October 1999 to May 2001. After recovery, recordings were processed using an automatic process to detect usual clicks of sperm whales. The detection algorithm equalized background noise, summed the data in a frequency band, and then used autocorrelation to detect the whales' highly regular clicks. Detections were checked manually, revealing that 98% of detections did contain clicks. Results indicate that sperm whales are present in the Gulf of Alaska year-round; this result extends what is known from whaling data, which were gathered principally in summer. Sperm whales were more common in summer than winter by a factor of roughly two, and occurred less often at the westernmost site surveyed (52°N, 157°W) than elsewhere in the Gulf. This is the first study of sperm whales based exclusively on remote acoustic sensing. This methodology is feasible because sperm whale clicks extend to frequencies (∼100 Hz) low enough to be recorded by low-sample-rate instruments that operate continuously, and because the detection algorithm has a low false-detection rate. The methodology may be replicated to facilitate comparisons between different time periods and geographic regions.     

Sperm whale behaviour indicates the use of echolocation click buzzes "creaks" in prey capture

During foraging dives, sperm whales (Physeter macrocephalus) produce long series of regular clicks at 0.5-2 s intervals interspersed with rapid-click buzzes called "creaks". Sound, depth and orientation recording Dtags were attached to 23 whales in the Ligurian Sea and Gulf of Mexico to test whether the behaviour of diving sperm whales supports the hypothesis that creaks are produced during prey capture. Sperm whales spent most of their bottom time within one or two depth bands, apparently feeding in vertically stratified prey layers. Creak rates were highest during the bottom phase: 99.8% of creaks were produced in the deepest 50% of dives, 57% in the deepest 15% of dives. Whales swam actively during the bottom phase, producing a mean of 12.5 depth inflections per dive. A mean of 32% of creaks produced during the bottom phase occurred within 10 s of an inflection (13x more than chance). Sperm whales actively altered their body orientation throughout the bottom phase with significantly increased rates of change during creaks, reflecting increased manoeuvring. Sperm whales increased their bottom foraging time when creak rates were higher. These results all strongly support the hypothesis that creaks are an echolocation signal adapted for foraging, analogous to terminal buzzes in taxonomically diverse echolocating species.     

Deep-diving foraging behaviour of sperm whales (Physeter macrocephalus)

1. Digital tags were used to describe diving and vocal behaviour of sperm whales during 198 complete and partial foraging dives made by 37 individual sperm whales in the Atlantic Ocean, the Gulf of Mexico and the Ligurian Sea. 2. The maximum depth of dive averaged by individual differed across the three regions and was 985 m (SD = 124.3), 644 m (123.4) and 827 m (60.3), respectively. An average dive cycle consisted of a 45 min (6.3) dive with a 9 min (3.0) surface interval, with no significant differences among regions. On average, whales spent greater than 72% of their time in foraging dive cycles. 3. Whales produced regular clicks for 81% (4.1) of a dive and 64% (14.6) of the descent phase. The occurrence of buzz vocalizations (also called 'creaks') as an indicator of the foraging phase of a dive showed no difference in mean prey capture attempts per dive between regions [18 buzzes/dive (7.6)]. Sperm whales descended a mean of 392 m (144) from the start of regular clicking to the first buzz, which supports the hypothesis that regular clicks function as a long-range biosonar. 4. There were no significant differences in the duration of the foraging phase [28 min (6.0)] or percentage of the dive duration in the foraging phase [62% (7.3)] between the three regions, with an overall average proportion of time spent actively encountering prey during dive cycles of 0.53 (0.05). Whales maintained their time in the foraging phase by decreasing transit time for deeper foraging dives. 5. Similarity in foraging behaviour in the three regions and high diving efficiencies suggest that the success of sperm whales as mesopelagic predators is due in part to long-range echolocation of deep prey patches, efficient locomotion and a large aerobic capacity during diving.     

The influence of habitat and time of day on the occurrence of odontocete vocalizations in Onslow Bay,North Carolina

To determine whether the occurrence and duration of odontocete vocal events varied by depth or time of day in Onslow Bay, North Carolina, we analyzed acoustic data collected by five underwater recorders. These recorders were deployed in July 2008 at three depths: two in shallow (64–73 m), one in medium (236 m), and two in deep (~366 m) water. We found that habitat influenced the occurrence of odontocete vocalizations, with significantly greater daily vocal activity from delphinids on recorders in deeper waters and sperm whale clicks recorded only on the medium and deep recorders. These findings suggest that a greater diversity and occurrence of animals are located in waters beyond the shelf break in this area, a conclusion supported by visual surveys. We also found an increase in the occurrence of delphinid clicks at night on the shallow and deep recorders, likely reflecting nocturnal foraging activity, and a regular nocturnal occurrence of sperm whale clicks on the medium‐depth recorder located near the shelf break, suggesting that one or more sperm whales moved into that area to feed at night. These observations improve our understanding of the occurrence and behavior of odontocetes in this region of the U.S. Atlantic seaboard.     

SPERM WHALES IN THE GULF OF CALIFORNIA: RESIDENCY, MOVEMENTS, BEHAVIOR, AND THE POSSIBLE INFLUENCE OF VARIATION IN FOOD SUPPLY

Sperm whale movements, residency, population structure, and behavior were investigated in the Gulf of California in 1998 and 1999. Variations in sperm whale movement patterns and behavior were related to changes in prey abundance (jumbo squid, Dosidicus gigas ) determined by fishery statistics. Photo-identification data revealed that seven female sperm whales moved into the Gulf of California from the Galapagos Islands, traveling up to 3,803 km. These are among the longest documented movements for female sperm whales. There were significant differences in speed and distance traveled during a dive cycle between 1998 and 1999 (low and high squid abundance). In 1999 there were also significant differences in small-scale movements and behavior between the northern and the southern part of the study area (high and low prey abundance). These results suggest that when food resources are low, sperm whales travel in straighter lines, dive for longer periods, travel larger distances during dive cycles, and at higher speed. In 1999 there were significant differences in time spent socializing in areas of high prey abundance versus areas of low abundance. All of these changes in behavior were consistent with increased foraging effort when squid abundance was low. A high proportion of mature males and first-year calves were observed in the Gulf of California, suggesting that it is an important sperm whale breeding ground.     

Using Models of Social Transmission to Examine the Spread of Longline Depredation Behavior among Sperm Whales in the Gulf of Alaska

Fishing, farming and ranching provide opportunities for predators to prey on resources concentrated by humans, a behavior termed depredation. In the Gulf of Alaska, observations of sperm whales depredating on fish caught on demersal longline gear dates back to the 1970s, with reported incidents increasing in the mid-1990s. Sperm whale depredation provides an opportunity to study the spread of a novel foraging behavior within a population. Data were collected during National Marine Fisheries Service longline surveys using demersal longline gear in waters off Alaska from 1998 to 2010. We evaluated whether observations of depredation fit predictions of social transmission by fitting the temporal and spatial spread of new observations of depredation to the Wave of Advance model. We found a significant, positive relationship between time and the distance of new observations from the diffusion center (r² = 0.55, p-value  = 0.003). The data provide circumstantial evidence for social transmission of depredation. We discuss how changes in human activities in the region (fishing methods and regulations) have created a situation in which there is spatial-temporal overlap with foraging sperm whales, likely influencing when and how the behavior spread among the population.     

Geographic distribution of the Cross Seamount beaked whale based on acoustic detections

Beaked whales produce frequency-modulated echolocation pulses that appear to be species-specific, allowing passive acoustic monitoring to play a role in understanding spatio-temporal patterns. The Cross Seamount beaked whale is known only from its unique echolocation signal (BWC) with no confirmed species identification. This beaked whale spans the Pacific Ocean from the Mariana Archipelago to Baja California, Mexico, south to the equator, but only as far north as latitude 29°N. Within these warm waters, 92% of BWC detections occurred at night, 6% during crepuscular periods, and only 2% during daylight hours. Detections of BWC signals on drifting recorders with a vertical hydrophone array at 150 m depth demonstrated that foraging often occurred shallow in the water column (<150 m). No other species of beaked whale to date has been documented foraging in waters this shallow. Given their nocturnal, shallow foraging dives, this species appears to prefer prey that may be available in the water column only during those hours. The foraging behavior of Cross Seamount beaked whales appears to be unique among all beaked whales, and these findings contribute additional ecological and acoustic information which can help guide future efforts to identify this cryptic whale.     

Beaked whales respond to simulated and actual navy sonar

Beaked whales have mass stranded during some naval sonar exercises, but the cause is unknown. They are difficult to sight but can reliably be detected by listening for echolocation clicks produced during deep foraging dives. Listening for these clicks, we documented Blainville's beaked whales, Mesoplodon densirostris, in a naval underwater range where sonars are in regular use near Andros Island, Bahamas. An array of bottom-mounted hydrophones can detect beaked whales when they click anywhere within the range. We used two complementary methods to investigate behavioral responses of beaked whales to sonar: an opportunistic approach that monitored whale responses to multi-day naval exercises involving tactical mid-frequency sonars, and an experimental approach using playbacks of simulated sonar and control sounds to whales tagged with a device that records sound, movement, and orientation. Here we show that in both exposure conditions beaked whales stopped echolocating during deep foraging dives and moved away. During actual sonar exercises, beaked whales were primarily detected near the periphery of the range, on average 16 km away from the sonar transmissions. Once the exercise stopped, beaked whales gradually filled in the center of the range over 2-3 days. A satellite tagged whale moved outside the range during an exercise, returning over 2-3 days post-exercise. The experimental approach used tags to measure acoustic exposure and behavioral reactions of beaked whales to one controlled exposure each of simulated military sonar, killer whale calls, and band-limited noise. The beaked whales reacted to these three sound playbacks at sound pressure levels below 142 dB re 1 μPa by stopping echolocation followed by unusually long and slow ascents from their foraging dives. The combined results indicate similar disruption of foraging behavior and avoidance by beaked whales in the two different contexts, at exposures well below those used by regulators to define disturbance.     

ESTIMATES OF SPERM WHALE ABUNDANCE IN THE NORTHEASTERN TEMPERATE PACIFIC FROM A COMBINED ACOUSTIC AND VISUAL SURVEY

We estimate the abundance of sperm whales in a 7.8 million km² study area in the eastern temperate North Pacific using data from a ship-based acoustic and visual line-transect survey in spring 1997. Sperm whales were detected acoustically using a hydrophone array towed at 15 km/h and 100 m depth. The hydrophone array was towed for 14,500 km, and locations were estimated acoustically for 45 distinct sperm whale groups. Whales producing slow clicks (>2-s period) were detected at greater distance (up to 37 km), and the estimation of effective strip widths was stratified based on initial click period. Visual survey effort (using 25° binoculars and naked eyes) covered 8,100 km in Beaufort sea states 0–5 and resulted in only eight sightings. The effective strip width for visual detections was estimated from previous surveys conducted using the same methods and similar vessels in the eastern Pacific. Estimated sperm whale abundance in the study area was not significantly different between acoustic (32,100, CV = 0.36) and visual (26,300, CV = 0.81) detection methods. Acoustic techniques substantially increased the number of sperm whales detected on this line-transect survey by increasing the range of detection and allowing nighttime surveys; however, visual observations were necessary for estimating group size.     

Vocal repertoires and insights into social structure of sperm whales (Physeter macrocephalus) in Mauritius,southwestern Indian Ocean

Sperm whales communicate using codas (stereotyped click sequences). Females and juveniles live in long-term social units, and units with similar coda repertoires share vocal clan membership. Vocal clans exhibit culturally defined differences in their multilevel social structure. Here, we aimed to identify different social units among sperm whales in Mauritius in the southwestern Indian Ocean, and to describe and compare their coda repertoires to investigate the presence of different vocal clans. We conducted six boat-based surveys between 2008 and 2013, during which 101 different individuals were photo-identified. Analysis of associations between 22 resighted individuals divided them into four candidate social units, but a lack of resightings impedes solid delineation of social units. Based on number and rhythm of clicks, at least 24 discrete coda types were detected among 4,767 analyzed codas using two different classification methods. Comparison of coda repertoires recorded from seven sperm whale groupings revealed the possible existence of two sympatric vocal clans, but the size differences of recorded repertoires warrant caution of interpretation. To further evaluate social structuring and the presence of different vocal clans in this region, future surveys should aim at obtaining long-term sighting and acoustic data, and cover a more extensive area.     

Sperm whale depredation of sablefish longline gear in the northeast Pacific Ocean

Interactions between marine mammals and fisheries include competition for prey (catch), marine mammal entanglement in fishing gear, and catch removal off fishing gear (depredation). We estimated the magnitude of sperm whale depredation on a major North Pacific longline fishery (sablefish) using data collected during annual longline surveys. Sperm whale depredation occurs while the longline gear is off‐bottom during retrieval. Sperm whales were observed on 16% of longline survey sampling days, mostly (95% of sightings) over the continental slope. Sightings were most common in the central and eastern Gulf of Alaska (98% of sightings), occasional in the western Gulf of Alaska and Aleutian Islands, and absent in the Bering Sea. Longline survey catches were commonly preyed upon when sperm whales were present (65% of sightings), as evidenced by damaged fish. Neither sperm whale presence (P = 0.71) nor depredation rate (P = 0.78) increased significantly from 1998 to 2004. Longline survey catch rates were about 2% less at locations where depredation was observed, but the effect was not significant (P = 0.34). Estimated sperm whale depredation was <1% of the annual sablefish longline fishery catch off Alaska during 1998 to 2004.     

Vocal clans in sperm whales (Physeter macrocephalus)

Cultural transmission may be a significant source of variation in the behaviour of whales and dolphins, especially as regards their vocal signals. We studied variation in the vocal output of 'codas' by sperm whale social groups. Codas are patterns of clicks used by female sperm whales in social circumstances. The coda repertoires of all known social units (n = 18, each consisting of about 11 females and immatures with long-term relationships) and 61 out of 64 groups (about two social units moving together for periods of days) that were recorded in the South Pacific and Caribbean between 1985 and 2000 can be reliably allocated into six acoustic 'clans', five in the Pacific and one in the Caribbean. Clans have ranges that span thousands of kilometres, are sympatric, contain many thousands of whales and most probably result from cultural transmission of vocal patterns. Units seem to form groups preferentially with other units of their own clan. We suggest that this is a rare example of sympatric cultural variation on an oceanic scale. Culture may thus be a more important determinant of sperm whale population structure than genes or geography, a finding that has major implications for our understanding of the species' behavioural and population biology.     

Deep-diving beaked whales dive together but forage apart

Echolocating animals that forage in social groups can potentially benefit from eavesdropping on other group members, cooperative foraging or social defence, but may also face problems of acoustic interference and intra-group competition for prey. Here, we investigate these potential trade-offs of sociality for extreme deep-diving Blainville′s and Cuvier''s beaked whales. These species perform highly synchronous group dives as a presumed predator-avoidance behaviour, but the benefits and costs of this on foraging have not been investigated. We show that group members could hear their companions for a median of at least 91% of the vocal foraging phase of their dives. This enables whales to coordinate their mean travel direction despite differing individual headings as they pursue prey on a minute-by-minute basis. While beaked whales coordinate their echolocation-based foraging periods tightly, individual click and buzz rates are both independent of the number of whales in the group. Thus, their foraging performance is not affected by intra-group competition or interference from group members, and they do not seem to capitalize directly on eavesdropping on the echoes produced by the echolocation clicks of their companions. We conclude that the close diving and vocal synchronization of beaked whale groups that quantitatively reduces predation risk has little impact on foraging performance.     

SPERM WHALE DISTRIBUTIONS AND COMMUNITY ECOLOGY ASSOCIATED WITH A WARM-CORE RING OFF GEORGES BANK

Sperm whale (Physeter macrocephalus) sighting rate in the vicinity of a thermal front on the eastern boundary of a Gulf Stream warm-core ring was five times greater than the pooled rate in the remainder of the study area. No sperm whales were sighted within the ring away from frontal boundaries. Sperm whale distribution patterns associated with the ring were examined using hydrographic and zooplankton data. Hydrographic structure and zooplankton densities and community composition differed between the high-use region and the remainder of the study area, suggesting that mesoscale patterns in the biological and physical environments regulate sperm whale habitat usage. Satellite thermal images indicated cool water associated with the eastern front was shelf or slope water entrained from the Scotian Shelf to the north. Interaction of the warm-core ring with the shelf or slope water is probably indirectly responsible for greater sperm whale presence on the eastern boundary of the ring, providing suitable habitat for their squid prey. Entrainment of shelf water by warm-core rings occurs frequently, providing potential sperm whale habitat in the vicinity of rings. Surveys of warm-core rings and other Gulf Stream features are needed to improve accuracy of sperm whale population estimates.     

How does social behavior differ among sperm whale clans?

When individuals primarily associate with and learn from those who behave similarly, society and culture become closely tied. Sperm whales (Physeter macrocephalus) exhibit multilevel social structure, the levels of which are differentiated in part by characteristic cultural behaviors. Sperm whales are organized into sympatric clans, with distinctive vocal repertoires that are socially learned. Other behaviors, such as movement patterns and foraging, also differ among clans. Here we ask whether the clan partition also includes divergences in social behavior. Off the Galápagos Islands, members of two clans differed consistently in diving synchrony, heterogeneity, and temporal stability of social relationships. While number of associates (indicated by social unit, group, and cluster sizes) were similar between clans, Regular clan members dived more synchronously and had more homogeneous relationships than the Plus‐One clan members. Plus‐One social units had generally longer associations than those of the Regular clan. Differences in surface‐time coordination and quality of social relationships are likely byproducts of the clan segregation, which could affect alloparental care giving, therefore scaling up to differential calf survival rates between clans. This new dimension of behavioral divergence between sperm whale clans indicates that sympatric, socio‐cultural entities of nonhumans can also display characteristic social behavior.     

No shallow talk: Cryptic strategy in the vocal communication of Blainville's beaked whales

Communicating animals must balance fitness benefits against the costs of signaling, such as increased predation risk. Cetaceans communicate mainly with sound and near‐surface vocalizations can place signalers at risk from shallow‐diving top‐predators with acute hearing such as killer whales. Beaked whales are deep divers living in small cohesive groups with little social defense from predation. Little if anything is known about their acoustic communication. Here, eight Blainville's beaked whales were studied with suction cup attached DTags to provide the first report on social communication as a function of diving behavior for any of the 21 ziphiid species. Tagged whales produced two previously unrecorded signals with apparent communicative functions: (1) fast series of ultrasonic frequency modulated clicks (rasps) were recorded from six individuals, and (2) harmonically rich short whistles with a mean fundamental frequency of 12 kHz were recorded from one whale at up to 900 m depth, the deepest whistles recorded from a marine mammal. Blainville's were silent 80% of the time, whenever shallower than 170 m depth and during the prolonged (19 min) silent ascents from vocal dives. This behavior limits the ability of shallow‐diving predators to track Blainville's acoustically and may provide a striking example of the evolutionary influence of the risk of predation on animal communication.     

Vessel noise affects beaked whale behavior: results of a dedicated acoustic response study

Some beaked whale species are susceptible to the detrimental effects of anthropogenic noise. Most studies have concentrated on the effects of military sonar, but other forms of acoustic disturbance (e.g. shipping noise) may disrupt behavior. An experiment involving the exposure of target whale groups to intense vessel-generated noise tested how these exposures influenced the foraging behavior of Blainville's beaked whales (Mesoplodon densirostris) in the Tongue of the Ocean (Bahamas). A military array of bottom-mounted hydrophones was used to measure the response based upon changes in the spatial and temporal pattern of vocalizations. The archived acoustic data were used to compute metrics of the echolocation-based foraging behavior for 16 targeted groups, 10 groups further away on the range, and 26 non-exposed groups. The duration of foraging bouts was not significantly affected by the exposure. Changes in the hydrophone over which the group was most frequently detected occurred as the animals moved around within a foraging bout, and their number was significantly less the closer the whales were to the sound source. Non-exposed groups also had significantly more changes in the primary hydrophone than exposed groups irrespective of distance. Our results suggested that broadband ship noise caused a significant change in beaked whale behavior up to at least 5.2 kilometers away from the vessel. The observed change could potentially correspond to a restriction in the movement of groups, a period of more directional travel, a reduction in the number of individuals clicking within the group, or a response to changes in prey movement.     

Abundance and Distribution of Sperm Whales in the Canary Islands: Can Sperm Whales in the Archipelago Sustain the Current Level of Ship-Strike Mortalities?

Sperm whales are present in the Canary Islands year-round, suggesting that the archipelago is an important area for this species in the North Atlantic. However, the area experiences one of the highest reported rates of sperm whale ship-strike in the world. Here we investigate if the number of sperm whales found in the archipelago can sustain the current rate of ship-strike mortality. The results of this study may also have implications for offshore areas where concentrations of sperm whales may coincide with high densities of ship traffic, but where ship-strikes may be undocumented. The absolute abundance of sperm whales in an area of 52933 km², covering the territorial waters of the Canary Islands, was estimated from 2668 km of acoustic line-transect survey using Distance sampling analysis. Data on sperm whale diving and acoustic behaviour, obtained from bio-logging, were used to calculate g(0) = 0.92, this is less than one because of occasional extended periods when whales do not echolocate. This resulted in an absolute abundance estimate of 224 sperm whales (95% log-normal CI 120–418) within the survey area. The recruitment capability of this number of whales, some 2.5 whales per year, is likely to be exceeded by the current ship-strike mortality rate. Furthermore, we found areas of higher whale density within the archipelago, many coincident with those previously described, suggesting that these are important habitats for females and immature animals inhabiting the archipelago. Some of these areas are crossed by active shipping lanes increasing the risk of ship-strikes. Given the philopatry in female sperm whales, replacement of impacted whales might be limited. Therefore, the application of mitigation measures to reduce the ship-strike mortality rate seems essential for the conservation of sperm whales in the Canary Islands.     

'Megapclicks': acoustic click trains and buzzes produced during night-time foraging of humpback whales (Megaptera novaeangliae)

Humpback whales (Megaptera novaeangliae) exhibit a variety of foraging behaviours, but neither they nor any baleen whale are known to produce broadband clicks in association with feeding, as do many odontocetes. We recorded underwater behaviour of humpback whales in a northwest Atlantic feeding area using suction-cup attached, multi-sensor, acoustic tags (DTAGs). Here we describe the first recordings of click production associated with underwater lunges from baleen whales. Recordings of over 34000 'megapclicks' from two whales indicated relatively low received levels at the tag (between 143 and 154dB re 1 microPa pp), most energy below 2kHz, and interclick intervals often decreasing towards the end of click trains to form a buzz. All clicks were recorded during night-time hours. Sharp body rolls also occurred at the end of click bouts containing buzzes, suggesting feeding events. This acoustic behaviour seems to form part of a night-time feeding tactic for humpbacks and also expands the known acoustic repertoire of baleen whales in general.     

Echolocation behaviour adapted to prey in foraging Blainville's beaked whale (Mesoplodon densirostris)

Toothed whales echolocating in the wild generate clicks with low repetition rates to locate prey but then produce rapid sequences of clicks, called buzzes, when attempting to capture prey. However, little is known about the factors that determine clicking rates or how prey type and behaviour influence echolocation-based foraging. Here we study Blainville's beaked whales foraging in deep water using a multi-sensor DTAG that records both outgoing echolocation clicks and echoes returning from mesopelagic prey. We demonstrate that the clicking rate at the beginning of buzzes is related to the distance between whale and prey, supporting the presumption that whales focus on a specific prey target during the buzz. One whale showed a bimodal relationship between target range and clicking rate producing abnormally slow buzz clicks while attempting to capture large echoic targets, probably schooling prey, with echo duration indicating a school diameter of up to 4.3m. These targets were only found when the whale performed tight circling manoeuvres spending up to five times longer in water volumes with large targets than with small targets. The result indicates that toothed whales in the wild can adjust their echolocation behaviour and movement for capture of different prey on the basis of structural echo information.