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.     

Shallow food for deep divers: Dynamic foraging behavior of male sperm whales in a high latitude habitat

Groups of female and immature sperm whales live at low latitudes and show a stereotypical diving and foraging behavior with dives lasting about 45 min to depths of between 400 and 1200 m. In comparison, physically mature male sperm whales migrate to high latitudes where little is known about their foraging behavior and ecology. Here we use acoustic recording tags to study the diving and acoustic behavior of male sperm whales foraging off northern Norway. Sixty-five hours of tag data provide detailed information about the movements and sound repertoire of four male sperm whales performing 83 dives lasting between 6 and 60 min. Dives ranged in depth between 14 and 1860 m, with a median depth of 175 m, and 92% of the surfacings lasted less than 15 min. The four whales clicked for an average 91% (SD = 10) of the dive duration, where the first usual click was produced at depths ranging between 4 and 218 m and the last usual click at depths ranging between 1 and 1114 m. Echolocation buzzes, which are used as an indication of prey capture attempts, were emitted at depths between 17 and 1860 m, during both the descent and ascent phase of deep dives. The foraging behavior varied markedly with depth, with the timing and duration of prey capture attempts during shallow dives suggesting that the whales target more sparsely distributed prey. In contrast, deep dives involve frequent prey capture attempts and seem to target more dense food layers. The evidence of exploitation of different food layers, including epipelagic prey, is consistent with the hypothesis that male sperm whales may migrate to high latitudes to access a productive, multi-layered foraging habitat.     

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.     

Following a foraging fish-finder: diel habitat use of Blainville's beaked whales revealed by echolocation

Simultaneous high resolution sampling of predator behavior and habitat characteristics is often difficult to achieve despite its importance in understanding the foraging decisions and habitat use of predators. Here we tap into the biosonar system of Blainville''s beaked whales, Mesoplodon densirostris, using sound and orientation recording tags to uncover prey-finding cues available to echolocating predators in the deep-sea. Echolocation sounds indicate where whales search and encounter prey, as well as the altitude of whales above the sea-floor and the density of organisms around them, providing a link between foraging activity and the bio-physical environment. Tagged whales (n = 9) hunted exclusively at depth, investing most of their search time either in the lower part of the deep scattering layer (DSL) or near the sea-floor with little diel change. At least 43% (420/974) of recorded prey-capture attempts were performed within the benthic boundary layer despite a wide range of dive depths, and many dives included both meso- and bentho-pelagic foraging. Blainville''s beaked whales only initiate searching when already deep in the descent and encounter prey suitable for capture within 2 min of the start of echolocation, suggesting that these whales are accessing prey in reliable vertical strata. Moreover, these prey resources are sufficiently dense to feed the animals in what is effectively four hours of hunting per day enabling a strategy in which long dives to exploit numerous deep-prey with low nutritional value require protracted recovery periods (average 1.5 h) between dives. This apparent searching efficiency maybe aided by inhabiting steep undersea slopes with access to both the DSL and the sea-floor over small spatial scales. Aggregations of prey in these biotopes are located using biosonar-derived landmarks and represent stable and abundant resources for Blainville''s beaked whales in the otherwise food-limited deep-ocean.     

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.     

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.     

The diving behavior of blue and fin whales: is dive duration shorter than expected based on oxygen stores?

Many diving seabirds and marine mammals have been found to regularly exceed their theoretical aerobic dive limit (TADL). No animals have been found to dive for durations that are consistently shorter than their TADL. We attached time-depth recorders to 7 blue whales and 15 fin whales (family Balaenopteridae). The diving behavior of both species was similar, and we distinguished between foraging and traveling dives. Foraging dives in both species were deeper, longer in duration and distinguished by a series of vertical excursions where lunge feeding presumably occurred. Foraging blue whales lunged 2.4 (+/-1.13) times per dive, with a maximum of six times and average vertical excursion of 30.2 (+/-10.04) m. Foraging fin whales lunged 1.7 (+/-0.88) times per dive, with a maximum of eight times and average vertical excursion of 21.2 (+/-4.35) m. The maximum rate of ascent of lunges was higher than the maximum rate of descent in both species, indicating that feeding lunges occurred on ascent. Foraging dives were deeper and longer than non-feeding dives in both species. On average, blue whales dived to 140.0 (+/-46.01) m and 7.8 (+/-1.89) min when foraging, and 67.6 (+/-51.46) m and 4.9 (+/-2.53) min when not foraging. Fin whales dived to 97.9 (+/-32.59) m and 6.3 (+/-1.53) min when foraging and to 59.3 (+/-29.67) m and 4.2 (+/-1.67) min when not foraging. The longest dives recorded for both species, 14.7 min for blue whales and 16.9 min for fin whales, were considerably shorter than the TADL of 31.2 and 28.6 min, respectively. An allometric comparison of seven families diving to an average depth of 80-150 m showed a significant relationship between body mass and dive duration once Balaenopteridae whales, with a mean dive duration of 6.8 min, were excluded from the analysis. Thus, the short dive durations of blue whales and fin whales cannot be explained by the shallow distribution of their prey. We propose instead that short duration diving in large whales results from either: (1) dispersal behavior of prey; or (2) a high energetic cost of foraging.     

FORAGING DEPTHS OF SEA OTTERS AND IMPLICATIONS TO COASTAL MARINE COMMUNITIES

We visually observed 1,251 dives, of 14 sea otters instrumented with TDRs in southeast Alaska, and used attribute values from observed dives to classify 180,848 recorded dives as foraging (0.64), or traveling (0.36). Foraging dives were significantly deeper, with longer durations, bottom times, and postdive surface intervals, and greater descent and ascent rates, compared to traveling dives. Most foraging occurred in depths between 2 and 30 m (0.84), although 0.16 of all foraging was between 30 and 100 m. Nine animals, including all five males, demonstrated bimodal patterns in foraging depths, with peaks between 5 and 15 m and 30 and 60 m, whereas five of nine females foraged at an average depth of 10 m. Mean shallow foraging depth was 8 m, and mean deep foraging depth was 44 m. Maximum foraging depths averaged 61 m (54 and 82 for females and males, respectively) and ranged from 35 to 100 m. Female sea otters dove to depths ≤20 m on 0.85 of their foraging dives while male sea otters dove to depths ≥45 m on 0.50 of their foraging dives. Less than 0.02 of all foraging dives were >55 m, suggesting that effects of sea otter foraging on nearshore marine communities should diminish at greater depths. However, recolonization of vacant habitat by high densities of adult male sea otters may result in initial reductions of some prey species at depths >55 m.     

中国大陆海域沿岸搁浅和误捕的喙鲸类调查

从2009 年1 月至2010 年2 月,我们对中国大陆海域沿岸搁浅和误捕的喙鲸类头骨标本进行了测量和鉴
定。目前中国大陆海域搁浅和误捕的喙鲸类有5 种,分别为柯氏喙鲸、贝氏喙鲸、朗氏喙鲸、柏氏中喙鲸和小
中喙鲸,其中小中喙鲸为中国海域(包括台湾)的首次报道,也是在西北太平洋区的首次记录。小中喙鲸的主
要形态鉴别特征为有1 对纺锤形牙齿着生在下颌骨联合后面。经对已报道过的大陆海域搁浅和误捕的3 个雌性
银杏齿中喙鲸头骨标本和台湾省搁浅银杏齿中喙鲸的颅骨、鼻骨、下颌骨以及牙齿特征对比重新鉴定,订正大
陆的3 个样本为雌性柏氏中喙鲸。本文报道了大陆沿岸海域搁浅和误捕的喙鲸类最新调查研究结果。     

SPERM WHALES TAGGED WITH TRANSPONDERS AND TRACKED UNDERWATER BY SONAR

Abstract: Two sperm whales tagged with acoustic transponder tags were tracked by sonar during a cruise from 16 to 30 October 1991 in the southeast Caribbean west of Dominica Island. The whales dove to depths of 400–600 m and more, including a dive to 1,185 m and one possibly to 2,000 m. They were tracked for periods of 3–14 h, over distances of 8.5–40 km. The tagged whales were found together four and eight days after tagging, and were tracked simultaneously for 13 h, over 31 km. Whale movements on different days at the surface averaged from 0.68 to 0.82 m/set, with dive descent rates from 0.82 to 1.13 m/set, ascent rates from 0.74 to 1.16 m/set, and horizontal movement during dives from 0.76 to 1.29 m/set. Dives lasted from 18 min to 1 h and 13 min, averaging 33 and 41 min on different days. Every track ended when tag signals became obscured at night by dense biological scatterers concentrated in offshore areas where the whales were diving. Both tagged whales appear to have been males of 15 and 11m, each dominant in different groups; but when together the larger whale was dominant, as evidenced by chases and agonistic vocalizations. The whales did not appear to react to the tags or to the sounds associated with tracking (30, 32, and 36 kHz).     

Cheetahs of the deep sea: deep foraging sprints in short-finned pilot whales off Tenerife (Canary Islands)

1. Empirical testing of optimal foraging models for breath-hold divers has been difficult. Here we report data from sound and movement recording DTags placed on 23 short-finned pilot whales off Tenerife to study the foraging strategies used to catch deep-water prey. 2. Day and night foraging dives had a maximum depth and duration of 1018 m and 21 min. Vocal behaviour during dives was consistent with biosonar-based foraging, with long series of echolocation clicks interspersed with buzzes. Similar buzzes have been associated with prey capture attempts in other echolocating species. 3. Foraging dives seemed to adapt to circadian rhythms. Deep dives during the day were deeper, but contained fewer buzzes (median 1), than night-time deep dives (median 5 buzzes). 4. In most deep (540-1019 m) daytime dives with buzzes, a downward directed sprint reaching up to 9 m s(-1) occurred just prior to a buzz and coincided with the deepest point in the dive, suggestive of a chase after escaping prey. 5. A large percentage (10-36%) of the drag-related locomotion cost of these dives (15 min long) is spent in sprinting (19-79 s). This energetic foraging tactic focused on a single or few prey items has not been observed previously in deep-diving mammals but resembles the high-risk/high-gain strategy of some terrestrial hunters such as cheetahs. 6. Deep sprints contrast with the expectation that deep-diving mammals will swim at moderate speeds optimized to reduce oxygen consumption and maximize foraging time at depth. Pilot whales may have developed this tactic to target a deep-water niche formed by large/calorific/fast moving prey such as giant squid.     

Stejneger's beaked whale strandings in Alaska, 1995–2020

Presented here is the first comprehensive and updated compilation of history, distribution, and findings of Stejneger's beaked whales (Mesoplodon stejnegeri) in Alaska. Stejneger's beaked whales are a poorly understood, elusive, deep-diving cetacean species found in the North Pacific Ocean. Since Stejneger's beaked whale strandings data in Alaska through 1994 were last published, 35 additional strandings have been documented. Twenty-seven animals stranded in the Aleutian Islands, seven stranded in Southcentral Alaska, and one animal stranded on St. Lawrence Island. Twenty-two carcasses were necropsied, but only four were fresh. Seventeen of the 22 died during mass stranding events and cause of death could not be definitively determined. Barotrauma was suspected in three cases and infectious disease possibly complicated by barotrauma occurred in two cases. We documented an expansion of strandings into the northern Bering Sea, characterized a sex bias, examined stomach contents that included macroplastic, and identified parasites not previously associated with Stejneger's beaked whales. Also included are data on the largest known mass stranding of Stejneger's beaked whales, which occurred on Adak Island in 2018. The history, distribution, and findings presented here are central to further our understanding of this species.     

Behavioral responses of satellite tracked Blainville's beaked whales (Mesoplodon densirostris) to mid-frequency active sonar

The vulnerability of beaked whales (Family: Ziphiidae) to intense sound exposure has led to interest in their behavioral responses to mid-frequency active sonar (MFAS, 3–8 kHz). Here we present satellite-transmitting tag movement and dive behavior records from Blainville's beaked whales (Mesoplodon densirostris) tagged in advance of naval sonar exercises at the Atlantic Undersea Test and Evaluation Center (AUTEC) in the Bahamas. This represents one of the largest samples of beaked whales individually tracked during sonar operations (n = 7). The majority of individuals (five of seven) were displaced 28–68 km after the onset of sonar exposure and returned to the AUTEC range 2–4 days after exercises ended. Modeled sound pressure received levels were available during the tracking of four individuals and three of those individuals showed declines from initial maxima of 145–172 dB re 1 μPa to maxima of 70–150 dB re 1 μPa following displacements. Dive behavior data from tags showed a continuation of deep diving activity consistent with foraging during MFAS exposure periods, but also suggested reductions in time spent on deep dives during initial exposure periods. These data provide new insights into behavioral responses to MFAS and have important implications for modeling the population consequences of disturbance.     

Time/depth usage of Adélie penguins: an approach based on dive angles

Data on the swim speed, dive depth and feeding rates of three Adélie penguins (Pygoscelis adeliae) foraging in summer 1998/1999 in Adélie Land, Antarctica were collected using dorsally-mounted loggers, in tandem with oesophageal temperature sensors. Swim speed could be integrated, together with the rate of change of depth, to determine dive and return-to-surface angles. Overall, birds increased rates of change of depth during commuting phases so that dive angles were steeper in dives terminating at greater depths. Angles of descent and ascent during feeding dives were greater than during non-feeding dives. Variation in the descent angle over time of particular dives was generally less than 10°, but the angles of the ascent phases varied more widely. The importance of selecting the optimum descent and ascent angles with respect to prey exploitation, oxygen stores and time gained in the feeding area over the course of a dive by diving at a steeper angle is discussed.     

Satellite tracking and diving behaviour of sub-adult narwhals (<Emphasis Type="Italic">Monodon monoceros</Emphasis>) in Svalbard,Norway

Three juvenile narwhals captured during August 1998 in the northeast of Svalbard, Norway, were equipped with satellite-relayed data loggers (SRDLs) that transmitted diving and swim-speed data, in addition to location, for up to 46 days. A total of 1,354 complete dive cycles were recorded. Most of the diving was shallow and of short duration. Maximum recorded dive depth was 546 m, maximum recorded dive duration was 24.8 min, and maximum recorded swim-speed was 4.7 ms⁻¹. Ascent speed, vertical ascent speed, descent speed and vertical descent speed were all significantly higher during deep dives (>200 m) than for shallow dives (<200 m). In addition both ascent and descent angles were much steeper for deep dives than during shallow dives. Most of the shallow diving seemed to be associated with travelling, with the animal shifting between various locations, while the deep diving (often to the bottom) for extended periods in some specific areas might have been associated with foraging. Even though the sample size in this study is small, the data are the first information available for movements and diving behaviour of narwhals near Svalbard.     

A NEW SPECIES OF BEAKED WHALE MESOPLODON PERRINI SP. N. (CETACEA: ZIPHIIDAE) DISCOVERED THROUGH PHYLOGENETIC ANALYSES OF MITOCHONDRIAL DNA SEQUENCES

Mesoplodon perrini , a new species of beaked whale is described on the basis of five animals stranded on the coast of California (between 32°55'N, 117°15'W and 36°37'N, 121°55'W) from May 1975 to September 1997. Four of these animals were initially identified as Hector's beaked whales M. hectori based on cranial morphology (Mead 1981). A fifth specimen was initially identified as a neonate Cuvier's beaked whale Ziphius cavirostris based on external features. These specimens were first recognized as representatives of an undescribed species through phylogenetic analysis of mitochondrial (mt) DNA control region and cytochrome b sequence data. Although similar morphologically, the genetic data do not support a close evolutionary relationship between M. perrini and M. hectori . Instead, these data suggest a possible sister species relationship with the lesser beaked whale M. peruvianus . Sightings of two small beaked whales off California in the 1970s which were tentatively identified as M. hectori are also likely to be M. perrini . We suggest that M. hectori is confined to the Southern Hemisphere, while M. perrini is known to date only from the North Pacific.     

Odontocete occurrence in relation to changes in oceanography at a remote equatorial Pacific seamount

Seamounts are considered hot spots of biodiversity and can aggregate pelagic predators and their prey. Passive acoustic monitoring was conducted over 3 mo in 2012 to document the occurrence of odontocetes near a seamount chain in the central equatorial Pacific in relation to oceanographic changes over time. Beaked whale echolocation signals were most frequently encountered. The main beaked whale signal was an unknown type, BW38, which resembled signals produced by Blainville's beaked whales. It had high occurrence during high sea surface temperature and low sea surface salinity. Cuvier's beaked whales were the second most detected. They had an opposite pattern and were encountered more often when sea surface temperature was low and net primary productivity was high. Risso's dolphins and short‐finned pilot whales had high acoustic densities, and echolocated predominantly at night. Risso's dolphins occurred more often during low sea surface height deviation. False killer whales were less frequently detected and mostly occurred during the day. Sperm whale detections were fewer than expected and associated with high chlorophyll a. Short duration Kogiidae encounters occurred on average every third day. These types of long‐term site studies are an informative tool to comparatively assess species composition, relative abundance, and relationship to oceanographic changes.     

The relationship among oceanography, prey fields, and beaked whale foraging habitat in the Tongue of the Ocean

Beaked whales, specifically Blainville's (Mesoplodon densirostris) and Cuvier's (Ziphius cavirostris), are known to feed in the Tongue of the Ocean, Bahamas. These whales can be reliably detected and often localized within the Atlantic Undersea Test and Evaluation Center (AUTEC) acoustic sensor system. The AUTEC range is a regularly spaced bottom mounted hydrophone array covering >350 nm(2) providing a valuable network to record anthropogenic noise and marine mammal vocalizations. Assessments of the potential risks of noise exposure to beaked whales have historically occurred in the absence of information about the physical and biological environments in which these animals are distributed. In the fall of 2008, we used a downward looking 38 kHz SIMRAD EK60 echosounder to measure prey scattering layers concurrent with fine scale turbulence measurements from an autonomous turbulence profiler. Using an 8 km, 4-leaf clover sampling pattern, we completed a total of 7.5 repeat surveys with concurrently measured physical and biological oceanographic parameters, so as to examine the spatiotemporal scales and relationships among turbulence levels, biological scattering layers, and beaked whale foraging activity. We found a strong correlation among increased prey density and ocean vertical structure relative to increased click densities. Understanding the habitats of these whales and their utilization patterns will improve future models of beaked whale habitat as well as allowing more comprehensive assessments of exposure risk to anthropogenic sound.     

SITE FIDELITY, ASSOCIATIONS, AND MOVEMENTS OF CUVIER'S (ZIPHIUS CAVIROSTRIS) AND BLAINVILLE'S (MESOPLODON DENSIROSTRIS) BEAKED WHALES OFF THE ISLAND OF HAWAI'I

Although the Ziphiidae are the second-most speciose family of cetaceans, information on beaked whale species and populations has been limited by the difficulties in finding and approaching free-ranging individuals. Site fidelity, patterns of association, and movements of two species, Cuvier's ( Ziphius cavirostris ) and Blainville's ( Mesoplodon densirostris ) beaked whales, were assessed using a 21-yr photographic data set from the west coast of the island of Hawai i. Resightings of individuals of both species spanned 15 yr, suggesting long-term site fidelity to the area. Long-term resightings were documented primarily from adult females of both species. Group sizes for both species were small and most groups had only a single adult male present. For Blainville's beaked whales, repeated associations between adult females and adult males were documented for all resightings of adult males over periods from 1 to 154 d. Among adult females, although repeated associations occurred up to 9 yr apart, individuals were seen separately in intervening years. Individuals of both species seen on multiple occasions were typically documented in multiple months/seasons, suggesting they may use the study area throughout the year. Such long-term site fidelity has implications both for potential population structure and for susceptibility of beaked whale populations to anthropogenic impacts.     

Diving pattern and performance in relation to foraging ecology in the gentoo penguin, Pygoscelis papua

We present data on diving pattern and performance (dive depth, duration, frequency and organization during the foraging trip) in gentoo penguins Pygoscelis papua , obtained using time-depth recorders ( n = 9 birds, 99 foraging trips). These data are used to estimate various parameters of foraging activity, e.g. foraging range, prey capture rates, and are compared in relation to breeding chronology. Foraging trip duration was 6 h and 10 h, and trip frequency 1.0/day and 0.96/day, during the brooding and creche periods, respectively. Birds spent on average 52%of each foraging trip diving. Dive depth and duration were highly bimodal: shallow dives (< 21 m) averaged 4 m and 0.23 min, and deep dives (> 30 m) 80 m and 2.5 min, respectively. Birds spent on average 71%and 25%of total diving time in deep and shallow dives, respectively. For deep dives, dive duration exceeded the subsequent surface interval, but shallow dives were followed by surface intervals 2–3 times dive duration. We suggest that most shallow dives are searching/exploratory dives and most deep dives are feeding dives. Deep dives showed clear diel patterns averaging 40 m at dawn and dusk and 80–90 m at midday. Estimated foraging ranges were 2.3 km and 4.1 km during the brood and creche period, respectively. Foraging trip duration increased by 4 h between the brood and creche periods but total time spent in deep dives (i.e. time spent feeding) was the same (3 h). Of 99 foraging trips, 56%consisted of only one dive bout and 44%of 2–4 bouts delimited by extended surface intervals > 10 min. We suggest that this pattern of diving activity reflects variation in spatial distribution of prey rather than the effect of physiological constraints on diving ability.