Dive Rate Research Articles

Dive characteristics of Cross Seamount beaked whales from long‐term passive acoustic monitoring at the Pacific Missile Range Facility, Kauaʻi

AbstractBeaked whale foraging pulses were detected on the Pacific Missile Range Facility (PMRF) off Kauaʻi, Hawaiʻi, via long term passive acoustic monitoring. The unidentified pulses do not match foraging pulses of known species on the range but are similar to the unidentified beaked whale first detected at Cross Seamount, Hawaiʻi. Although there has not been a visual confirmation of the unidentified beaked whale species, analysis of data collected from 2007 to 2019 has identified beaked whale foraging dive characteristics from echolocation pulses. From the 13 years of data, the most distinct patterns were that all foraging dives occurred at night and the nighttime foraging dive rate was 0.11 group vocal periods (GVP) per hour, with most detections on shallow hydrophones (625–1,000 m deep) over steep bathymetric slopes. Data collected during U.S. Navy training events were used to compare dive behavior during mid‐frequency active sonar (MFAS) activity against baseline periods; it was determined that the whales reduced GVPs during sonar and these remained low for at least 3 days after the training events. These results are the first long‐term record of acoustic signals from the Cross Seamount beaked whale and provide important insights into their habitat use and occurrence patterns.

Underwater visibility constrains the foraging behaviour of a diving pelagic seabird.

Understanding the sensory ecology of species is vital if we are to predict how they will function in a changing environment. Visual cues are fundamentally important for many predators when detecting and capturing prey. However, many marine areas have become more turbid through processes influenced by climate change, potentially affecting the ability of marine predators to detect prey. We performed the first study that directly relates a pelagic seabird species's foraging behaviour to oceanic turbidity. We collected biologging data from 79 foraging trips and 5472 dives of a visually dependent, pursuit-diving seabird, the Manx shearwater (Puffinus puffinus). Foraging behaviour was modelled against environmental variables affecting underwater visibility, including water turbidity, cloud cover and solar angle. Shearwaters were more likely to initiate area-restricted search and foraging dives in clearer waters. Underwater visibility also strongly predicted dive rate and depth, suggesting that fine-scale prey capture was constrained by the detectability of prey underwater. Our novel use of dynamic descriptors of underwater visibility suggests that visual cues are vital for underwater foraging. Our data indicate that climate change could negatively impact seabird populations by making prey more difficult to detect, compounded by the widely reported effects of reduced prey populations.

Diel and lunar variation in diving behavior of rough‐toothed dolphins (Steno bredanensis) off Kauaʻi, Hawaiʻi

AbstractObservational studies describe rough‐toothed dolphins (Steno bredanensis) actively foraging during the day on epipelagic species. Using data from depth‐transmitting satellite tags deployed on nine individuals off Kauaʻi, we investigated diving behavior and the effects of lunar phase and solar light levels on vertical movements. Overall, tagged rough‐toothed dolphins primarily used near‐surface waters, spending between 83.6% and 93.7% of their time in the top 30 m of the water column. When diving, grand mean, median, and maximum dive depths were 76.9 m, 67.5 m, and 399.5 m, although individuals were in water with depths from approximately 700–1,450 m. Dive rates varied by time of day, being lowest during the day and at dawn and highest at dusk and night. Dives were deepest (M = 133.7 m, SD = 52.6 m, median = 106.5 m) and longest (M = 4.0 min, SD = 0.4 min, median = 4.0 min) at dusk, suggesting dolphins were taking advantage of prey rising to the surface in response to reduced light levels. Lunar phase indirectly affected diving, with deeper and longer dives occurring with increasing illumination. The variations in dive behavior across solar and lunar cycles indicate diving patterns shift based on the distribution of prey.

Intertrip consistency in hunting behavior improves foraging success and efficiency in a marine top predator.

Substantial variation in foraging strategies can exist within populations, even those typically regarded as generalists. Specializations arise from the consistent exploitation of a narrow behavioral, spatial or dietary niche over time, which may reduce intraspecific competition and influence adaptability to environmental change. However, few studies have investigated whether behavioral consistency confers benefits at the individual and/or population level. While still recovering from commercial sealing overexploitation, Australian fur seals (AUFS; Arctocephalus pusillus doriferus) represent the largest marine predator biomass in south‐eastern Australia. During lactation, female AUFS adopt a central‐place foraging strategy and are, thus, vulnerable to changes in prey availability. The present study investigated the population‐level repeatability and individual consistency in foraging behavior of 34 lactating female AUFS at a south‐east Australian breeding colony between 2006 and 2019. Additionally, the influence of individual‐level behavioral consistency on indices of foraging success and efficiency during benthic diving was determined. Low to moderate population‐level repeatability was observed across foraging behaviors, with the greatest repeatability in the mean bearing and modal dive depth. Individual‐level consistency was greatest for the proportion of benthic diving, total distance travelled, and trip duration. Indices of benthic foraging success and efficiency were positively influenced by consistency in the proportion of benthic diving, trip duration and dive rate but not influenced by consistency in bearing to most distal point, dive depth or foraging site fidelity. The results of the present study provide evidence of the benefits of consistency for individuals, which may have flow‐on effects at the population level.

Effect of Long-Term Diving Exposure on Sleep of Male Occupational Divers in Southern Taiwan: A Cross-Sectional Study.

Divers with a history of decompression sickness may be at high risk for sleep problems. However, limited studies have investigated the relationship between diving exposure and sleep problems of occupational divers. This study investigated the association between diving exposure and sleep quality and quantity among male occupational divers in southern Taiwan. This descriptive, cross-sectional study included 52 occupational divers and 121 non-divers recruited from southern Taiwan in 2018. Survey data were collected using the Taiwanese version of the Hospital Anxiety and Depression Scale, Pittsburgh Sleep Quality Index, Epworth Sleepiness Scale, and a self-report questionnaire that included demographic variables, diving exposure/protocols, and factors associated with sleep quality. Among all participants examined, occupational divers were significantly more likely to have both poor sleep quality (adjusted odds ratio [OR] = 3.00; 95% confidence interval [CI] = 1.48-6.06; P = 0.002) and excessive daytime sleepiness (adjusted OR = 4.49; 95% CI = 2.12-9.52; P < 0.001). The diving exposure time, depth, ascent rate, and decompression table use showed no significant associations between poor and good sleepers in the divers group. However, a history of decompression sickness was associated with poor sleep quality among divers (adjusted OR = 2.20; 95% CI = 1.07-4.54; P = 0.032). Our results showed that occupational divers had poor sleep quality and more excessive sleepiness than non-divers. Decompression sickness likely contributes to poor sleep quality. Prevention and early detection of decompression sickness-related sleep problems should be an occupational health priority.

Long-term foraging dive characteristics of Cross Seamount beaked whales at the Pacific Missile Range Facility

Beaked whale foraging dive clicks similar to those detected at Cross Seamount were detected during passive acoustic monitoring of marine mammals at the Pacific Missile Range Facility (PMRF) off Kauai, Hawaii using bottom-mounted hydrophones from January 2007 through August 2018. The Cross Seamount beaked whale (BWC) foraging dive click frequency sweeps from approximately 18 kHz to above the recording bandwidth of the PMRF sensors of 48 kHz at inter-click-intervals of 0.14 s (std 0.06 s). These unique clicks have only been detected at Cross Seamount and around the Hawaiian Islands, and because they have only been detected at night there has been no visual detections or species confirmation. However, they are distinctive enough to automatically detect, and can be used to describe basic presence at PMRF. Detections averaged 0.047 dives/h (in a 24-h cycle), and occurred almost entirely on the southern part of the range (water depth <1000 m). 702 BWC foraging dives were used to determine a foraging dive baseline over a 12-year period, to compare foraging dive rates during US Naval exercise events and baseline periods, and to contrast BWC foraging dive behavior from Blainville’s beaked whale foraging dive behavior at PMRF.Beaked whale foraging dive clicks similar to those detected at Cross Seamount were detected during passive acoustic monitoring of marine mammals at the Pacific Missile Range Facility (PMRF) off Kauai, Hawaii using bottom-mounted hydrophones from January 2007 through August 2018. The Cross Seamount beaked whale (BWC) foraging dive click frequency sweeps from approximately 18 kHz to above the recording bandwidth of the PMRF sensors of 48 kHz at inter-click-intervals of 0.14 s (std 0.06 s). These unique clicks have only been detected at Cross Seamount and around the Hawaiian Islands, and because they have only been detected at night there has been no visual detections or species confirmation. However, they are distinctive enough to automatically detect, and can be used to describe basic presence at PMRF. Detections averaged 0.047 dives/h (in a 24-h cycle), and occurred almost entirely on the southern part of the range (water depth <1000 m). 702 BWC foraging dives were used to determine a foraging dive baseli...

Rapid descent flight by a molossid bat (Chaerephon plicatus) returning to its cave

In Southeast Asia, wrinkle-lipped bats (Chaerephon plicatus) roost in colonies that may count several million individuals. Birds of prey frequently hunt these bats when they emerge from or return to their colonies. Here, we report on an extreme anti-predator behavior of this species during return flights at dawn. Based on data logger, we documented that bats may ascend to several hundred meters altitude above the cave shortly before diving at high speed (maximum 20 m/s) towards the entrance. Dive rates were 40 times higher than those reported for other open-space foraging bats performing flapping descending flights. Based on high speed video recordings, we show that C. plicatus brake the rapid descents close to the cave entrance using the tail wing membrane and partly extended wings. Maximum gravitational forces involved 2.4 G, probably when adjusting abruptly the speed close to ground. We argue that these descending maneuvers may constitute an anti-predator strategy of C. plicatus, since such descents may be too fast and complex for birds of prey at the cave entrance, particularly when performed by thousands of individuals at the same time.

Diving behavior of the Atlantic walrus in high Arctic Greenland and Canada

Investigations of diving behavior of the Atlantic walrus (Odobenus rosmarus rosmarus) in the high Arctic Greenland and Canada are important for understanding behavioral adaptations and area utilization of this Arctic benthic feeder. Furthermore, such information along with estimations of annual consumption and carrying capacity of walruses are needed in management decisions of this utilized species. Satellite-linked transmitters deployed on 27 walruses from 2010 to 2013 provided data for investigations of diving behavior in three predefined main areas: NW Greenland, Smith Sound and NE Canada. Sub-areas within each main area were also compared. Depth of dives, dive rates, time at depth of dives, haul-out periods and vertical speeds were estimated. Majority of dives targeted depths from 10 to 100m, which corresponds to the distribution of walrus preferred food items. Four dives to depths >500m occurred and are the deepest ever documented for a walrus. Dive rates and time at depth of dives were significantly different between sub-areas (p<0.0001), whereas haul-out periods were not (p=0.072). Mean vertical speeds to destination depths ranged from 1.0ms−1 (95% CI: 0.8–1.2) to 1.8ms−1 (95% CI: 1.0–2.6). Based on dive rates, time at depth, haul-out and percentage of feeding dives Alexandra Fjord and Princess Mary Bay in NE Canada and Carey Island in NW Greenland were identified as the most important areas for walrus feeding during summer. Walrus predation on the standing bivalve biomass in NW Greenland (within 5–100m of depth) was estimated to 3.2% annually based on assessments of mean biomass of walrus preferred prey items. From a simple relationship between available shallow water habitat, current population size (n=2544) and walrus pre-exploitation population sizes it is furthermore proposed that the carrying capacity in the Smith Sound region does not exceed 5000 walruses.

Identification of Potential Foraging Areas for Bowhead Whales in Baffin Bay and Adjacent Waters

The bowhead whale (Balaena mysticetus) is the Arctic’s largest and most dependent predator on zooplankton; however, knowledge about its important foraging areas in Baffin Bay and adjacent waters is limited. Data on movement, horizontal velocity (ms-1), dive depth (m), and dive rate (dives h-1) were obtained from 39 bowhead whales (31 females, 6 males, and 2 of undetermined sex) instrumented with satellite-linked time-depth recorders (SLTDRs) in spring 2009 and 2010 in Disko Bay, West Greenland. Thirty-eight whales provided information on dive rates and movement, and potential foraging areas were identified on the basis of low dive rates and stationary behaviour. Nine potential foraging areas were identified: Disko Bay and adjacent region, Clyde Inlet, Isabella Bay, Broughton Island, Cumberland Sound, Frobisher Bay, Hudson Strait, southern Foxe Basin, and northern Foxe Basin. Two females returned to Disko Bay the following spring (duration of tags &gt; 420 days). Their diving behavior indicated that all whales exhibited a large degree of flexibility in their use of potential feeding areas in Baffin Bay and adjacent waters. The variability of habitat selection may buffer against climate-induced changes in the preferred habitats of bowhead whales.

Fine-scale foraging ecology of leatherback turtles

Remote tracking of migratory species and statistical modeling of behaviors have enabled identification of areas that are of high ecological value to these widely distributed taxa. However, direct observations at fine spatio-temporal scales are often needed to correctly interpret behaviors. In this study, we combined GPS-derived locations and archival dive records (1 sec sampling rate) with animal-borne video footage from foraging leatherback turtles (Dermochelys coriacea) in Nova Scotia, Canada (Northwest Atlantic Ocean) to generate the most highly detailed description of natural leatherback behavior presented to date. Turtles traveled shorter distances at slower rates and increased diving rates in areas of high prey abundance, which resulted in higher prey capture rates. Increased foraging effort (e.g., dive rate, dive duration, prey handling time, number of bites) was not associated with increased time at the surface breathing to replenish oxygen stores. Instead, leatherbacks generally performed short, shallow dives in the photic zone to or above the thermocline, where they disproportionately captured prey at bottoms of dives and during ascents. This foraging strategy supports visual prey detection, allows leatherbacks to exploit physically structured prey at relatively shallow depths (typically <30m), and increases time turtles spend in warmer water temperatures, thus optimizing net energy acquisition. Our results demonstrate that leatherbacks appear to be continuously foraging during daylight hours while in continental shelf waters of Nova Scotia, and that leatherback foraging behavior is driven by prey availability, not by whether or not a turtle is in a resource patch characterized by a particular size or prey density. Our study demonstrates the fundamental importance of obtaining field-based, direct observations of true behaviors at fine spatial and temporal scales to enhance our efforts to both study and manage migratory species.

Testing assumptions of central place foraging theory: a study of Adélie penguins Pygoscelis adeliae in the Ross Sea

We investigated central place foraging (CPF) in the context of optimal foraging theory in Adélie penguins Pygoscelis adeliae of the southern Ross Sea by using satellite tracking and time‐depth recorders to explore foraging at two spatio‐temporal scales: within the day‐to‐day (sub‐mesoscale: single foraging trip, 10s of km2) and the entire breeding season (mesoscale: trips by multiple individuals across the collective foraging area, 100s of km2). Specifically, we examine whether three basic assumptions of the Orians–Pearson CPF model, shown to occur in other CPF species, are met: 1) within a patch, the rate of prey acquisition declines with time spent in that patch; 2) food is distributed in discrete patches and is not available between those patches; and 3) CPF species have knowledge of the potential (or average, at least) feeding rate within their universe of patches, and use this knowledge to determine their foraging strategy when planning or engaging in a foraging trip. We found that prey consumption rates did not decline with time spent in patches, and penguins foraged to some degree most of the time when at sea. Food availability, as measured by foraging dive rate, appeared to be predictable within the same day at the same location, but predictability broke down after 2 d at distances &gt; 10 km away. We conclude that the assumptions of the Orians–Pearson CPF model are not a good fit to the circumstances of Ross Sea penguins, which clearly are central place foragers.

Diving behaviour of white-chinned petrels and its relevance for mitigating longline bycatch

The white-chinned petrel (Procellaria aequinoctialis) is the seabird species most commonly killed by Southern Hemisphere longline fisheries. Despite the importance of diving ability for mitigating longline bycatch, little is known of this species’ diving behaviour. We obtained data from temperature–depth recorders from nine white-chinned petrels breeding on Marion Island, southwestern Indian Ocean, during the late incubation and chick-rearing period. Maximum dive depth (16 m) was slightly deeper than the previous estimate (13 m), but varied considerably among individuals (range 2–16 m). Males dived deeper than females, and birds feeding chicks dived deeper than incubating birds, but dive rate did not differ between the sexes. Time of day had no significant effect on dive depth or rate. Our findings will help to improve the design and performance of mitigation measures aimed at reducing seabird bycatch in longline fisheries, such as the calculation of minimum line sink rates and optimum aerial coverage of bird-scaring lines.

Sensitivity to hypercapnia and elimination of CO2 following diving in Steller sea lions (Eumetopias jubatus)

The diving ability of marine mammals is a function of how they use and store oxygen and the physiological control of ventilation, which is in turn dependent on the accumulation of CO2. To assess the influence of CO2 on physiological control of dive behaviour, we tested how increasing levels of inspired CO2 (hypercarbia) and decreasing inspired O2 (hypoxia) affected the diving metabolic rate, submergence times, and dive recovery times (time to replenish O2 stores and eliminate CO2) of freely diving Steller sea lions. We also measured changes in breathing frequency of diving and non-diving individuals. Our findings show that hypercarbia increased breathing frequency (as low as 2% CO2), but did not affect metabolic rate, or the duration of dives or surface intervals (up to 3% CO2). Changes in breathing rates indicated respiratory drive was altered by hypercarbia at rest, but blood CO2 levels remained below the threshold that would alter normal dive behaviour. It took the sea lions longer to remove accumulated CO2 than it did for them to replenish their O2 stores following dives (whether breathing ambient air, hypercarbia, or hypoxia). This difference between O2 and CO2 recovery times grew with increasing dive durations, increasing hypercarbia, and was greater for bout dives, suggesting there could be a build-up of CO2 load with repeated dives. Although we saw no evidence of CO2 limiting dive behaviour, the longer time required to remove CO2 may eventually exhibit control over the overall time they can spend in apnoea and overall foraging duration.

The individual counts: within sex differences in foraging strategies are as important as sex‐specific differences in masked boobies Sula dactylatra

Understanding how animals allocate their foraging time is a central question in behavioural ecology. Intrinsic factors, such as body mass and size differences between sexes or species, influence animals’ foraging behaviour, but studies investigating the effects of individual differences in body mass and size within the same sex are scarce. We investigated this in chick‐rearing masked boobies Sula dactylatra, a species with reversed sexual dimorphism, through the simultaneous deployment of GPS and depth‐acceleration loggers to obtain information on foraging movements and activity patterns. Heavier females performed shorter trips closer to the colony than lighter females. During these shorter trips, heavier females spent higher proportions of their flight time flapping and less time resting on the water than lighter females did during longer trips. In contrast, body mass did not affect trip duration of males, however heavier males spent less time flapping and more time resting on the water than lighter males. This may occur as a result of higher flight costs associated with body mass and allow conservation of energy during locomotion. Body size (i.e. wing length) had no effect on any of the foraging parameters. Dive depths and dive rates (dives h−1) were not affected by body mass, but females dived significantly deeper than males, suggesting that other factors are important. Other studies demonstrated that females are the parent in charge of provisioning the chick, and maintain a flexible investment under regulation of their own body mass. Variation in trip length therefore seems to be triggered by body condition in females, but not in males. Consequently, shorter trips are presumably used to provision the chick, while longer trips are for self‐maintenance. Our findings underline the importance of accounting for the effects of body mass differences within the same sex, if sex‐specific foraging parameters in dimorphic species are being investigated.

Foraging Parameters Influencing the Detection and Interpretation of Area-Restricted Search Behaviour in Marine Predators: A Case Study with the Masked Booby

Identification of Area-restricted search (ARS) behaviour is used to better understand foraging movements and strategies of marine predators. Track-based descriptive analyses are commonly used to detect ARS behaviour, but they may be biased by factors such as foraging trip duration or non-foraging behaviours (i.e. resting on the water). Using first-passage time analysis we tested if (I) daylight resting at the sea surface positions falsely increase the detection of ARS behaviour and (II) short foraging trips are less likely to include ARS behaviour in Masked Boobies Sula dactylatra. We further analysed whether ARS behaviour may be used as a proxy to identify important feeding areas. Depth-acceleration and GPS-loggers were simultaneously deployed on chick-rearing adults to obtain (1) location data every 4 minutes and (2) detailed foraging activity such as diving rates, time spent sitting on the water surface and in flight. In 82% of 50 foraging trips, birds adopted ARS behaviour. In 19.3% of 57 detected ARS zones, birds spent more than 70% of total ARS duration resting on the water, suggesting that these ARS zones were falsely detected. Based on generalized linear mixed models, the probability of detecting false ARS zones was 80%. False ARS zones mostly occurred during short trips in close proximity to the colony, with low or no diving activity. This demonstrates the need to account for resting on the water surface positions in marine animals when determining ARS behaviour based on foraging locations. Dive rates were positively correlated with trip duration and the probability of ARS behaviour increased with increasing number of dives, suggesting that the adoption of ARS behaviour in Masked Boobies is linked to enhanced foraging activity. We conclude that ARS behaviour may be used as a proxy to identify important feeding areas in this species.

A Rapid Assessment Technique to Assist Management of the White Shark (Carcharodon Carcharias) Cage Dive Industry, South Australia

Port Lincoln, South Australia is the departure port for the only white shark, Carcharodon carcharias, cage dive industry in Australia. Established in the early 1960’s as a niche tourism venture, the industry has recently undergone a rapid expansion to accommodate greater passenger numbers, more tourism operators, and additional infrastructure aimed at capturing a greater proportion of the tourist dollar. However, to date, there has been no assessment of growth in the industry. We have used the operator logbook system, introduced in 2000, as the basis for a rapid assessment of the maturity as well as a conservative estimate of the economic value of the industry, with a focus on 2011. From the logbook system the number of days on-site has increased from an average of 67 days per year prior to 2007 to 287 days on-site in 2011. In 2011 the industry accommodated approximately 5200 passengers with a direct domestic expenditure estimated to be in excess of 6 million AUD. Changes in shark behaviour have been observed following the increase in days on-site. The white shark cage dive industry has reached a stage in its development where increased management is required in order to ensure a viable industry into the future. The rapid assessment technique described herein will allow managers to track changes in cage dive participation rates and quickly respond to changes in the industry.

Mass and bathymetry influences on the foraging behaviour of dependent yearling New Zealand sea lions (Phocarctos hookeri)

Juvenile foraging ability is limited due to physiological and morphological constraints. Restricted foraging skills can result in reduced juvenile survival, which can lead to population decline. Given the important role mass plays in influencing foraging behaviour and survival, we deployed satellite-linked time-depth recorders (TDRs) on seven dependent yearling New Zealand sea lions (Phocarctos hookeri) over three seasons from 2008–2010 at the New Zealand subantarctic Auckland Islands to study how mass influenced at sea movements and diving behaviour. Mass was significantly negatively related to dive depth and descent rate, indicating yearling diving behaviour may be influenced by their mass. The yearlings demonstrated inter-individual variation in foraging locations and strategies. This individual variation was likely attributed to the bathymetry of different foraging areas dictating possible dive depths. Thus, when studying foraging behaviour, in addition to physiological and morphological influences, researchers must examine external factors such as the physical characteristics of the foraging habitat.

Prey capture success and chick diet of Damara terns Sterna balaenarum in Namibia

Feeding terns are affected by a variety of environmental conditions. We studied prey capture success of Damara terns Sterna balaenarum in relation to six variables at two breeding colonies in southern Namibia: tidal phase, wind speed, water clarity, cloud cover, water depth and locality. Damara terns dived most successfully at high tide and least successfully at low tide. Prey capture success improved whereas total dive rate decreased with increasing wind speed. Prey capture success was highest in the least turbid water, but was not significantly affected by cloud cover or water depth, and there was no significant difference in dive success between localities. The diet of Damara tern chicks were studied throughout Namibia. Of 55 prey items collected, Cape silverside Atherina breviceps, southern mullet Liza richardsonii and Cape anchovy Engraulis encrasicolus were among the most abundant prey species. At least nine prey species were found in the diet of Damara tern chicks that had not been previously reported from Namibian waters

Intra-seasonal variation in foraging behavior among Adélie penguins (Pygocelis adeliae) breeding at Cape Hallett, Ross Sea, Antarctica

We investigated intra-seasonal variation in foraging behavior of chick-rearing Adelie penguins, Pygoscelis adeliae, during two consecutive summers at Cape Hallett, northwestern Ross Sea. Although foraging behavior of this species has been extensively studied throughout the broad continental shelf region of the Ross Sea, this is the first study to report foraging behaviors and habitat affiliations among birds occupying continental slope waters. Continental slope habitat supports the greatest abundances of this species throughout its range, but we lack information about how intra-specific competition for prey might affect foraging and at-sea distribution and how these attributes compare with previous Ross Sea studies. Foraging trips increased in both distance and duration as breeding advanced from guard to creche stage, but foraging dive depth, dive rates, and vertical dive distances travelled per hour decreased. Consistent with previous studies within slope habitats elsewhere in Antarctic waters, Antarctic krill (Euphausia superba) dominated chick meal composition, but fish increased four-fold from guard to creche stages. Foraging-, focal-, and core areas all doubled during the creche stage as individuals shifted distribution in a southeasterly direction away from the coast while simultaneously becoming more widely dispersed (i.e., less spatial overlap among individuals). Intra-specific competition for prey among Adelie penguins appears to influence foraging behavior of this species, even in food webs dominated by Antarctic krill.

A dive counting density estimation method for Blainville’s beaked whale ( Mesoplodon densirostris) using a bottom-mounted hydrophone field as applied to a Mid-Frequency Active (MFA) sonar operation

We present a passive acoustic method for estimating the density of echolocating cetaceans that dive synchronously, based on isolation of dive starts using a field of distributed bottom-mounted hydrophones. The method assumes that all dive starts of the target species within a defined area are detected, and that independent estimates of dive rate and group size are available. We apply the method to estimate the density of Blainville’s beaked whales ( Mesoplodon densirostris) at the Atlantic Undersea Test and Evaluation Center (AUTEC) in the Bahamas during the time of a multi-ship active sonar exercise. Estimated densities for the 65 h before the exercise, 68 h during, 65 h after, and the final 43 h monitored were 16.99 (95% CI 13.47–21.43), 4.76 (3.78–6.00), 8.67 (6.87–10.94), and 24.75 (19.62–31.23) respectively, illustrating a possible avoidance reaction. Results for the 65 h before were compared with those from the click count density estimation algorithm developed by Marques et al. [Marques T, Thomas L, Ward J, DiMarzio N, Tyack P. Estimating cetacean population density using fixed passive acoustic sensors. An example with Blainville’s beaked whales. J Acoust Soc Am 2009;125(4):1982–1994]. The click count-based estimate was 19.23 animals/1000 km 2 (95% CI 12.69–29.13)—similar (13% higher), but with higher variance (CV 21% for click count method versus 12% for the dive count method). We discuss potential reasons for the differences, and compare the utility of the two methods. For both, obtaining reliable estimates of the factors that scale the measured quantity (dive starts or detected clicks) to density is the key hurdle. Defining the area monitored in the dive count method can also be problematic, particularly if the array is small.