Dive Duration Research Articles

Movements and diving behavior of Risso’s dolphins in the Southern California Bight

Risso’s dolphins (Grampus griseus), uncommon prior to the 1970’s, are now regularly observed within the Southern California Bight. During long-term cetacean monitoring programs on United States Navy range areas in the Southern California Bight from 2009–2019, we deployed 16 Argos-linked satellite tags on Risso’s to acquire objective, detailed depictions of their movements and behaviors. Individuals were tracked for a median of 10.7 days (range = 0.8 – 19.7). Kernel density estimation suggested individuals utilized the entire Southern California Bight with the 50% core use area centered around San Clemente and Santa Catalina Islands where most of the tag deployments occurred. Grand median dive depth was 101 m (max = 528) and dive duration was 5.6 min (max = 11.1). We used generalized mixed models to assess seasonal and environmental effects on distribution and diving behavior including month, distance to shore, time of day, lunar phase, sea surface temperature, and chlorophyll-aresiduals. Animals were further from shore (including islands) during a full versus new moon and from the mainland during the last versus first quarter moon. Animals also tended to be closer to land in the fall and early winter months. Dives were deeper yet shorter during the night, during a full moon, and when animals were further offshore. Animals conducted nearly twice as many dives at night compared to day, though deep dives (> 500 m) occurred at all times of day. This study provides insights into Risso’s distribution and behavioral trends while identifying priorities for future research.

Application of Reverse Electrodialysis Power for Oxygen Generation in Undersea Diving

With continued growth of marine exploration and underwater missions, there is a need to design new diving technologies. Currently, dive time is limited by scuba tank weight and volume. The use of an electrolysis system, powered by a battery, for driving chemical reactions could be introduced to produce a steady supply of oxygen during dives. However, batteries have a limited capacity, are heavy, and are detrimental to the environment when disposed. Developing an electrochemical technology that utilizes resources abundantly available to a diver (salt water) to drive the oxygen evolution reaction would expand the duration and sustainability of ocean dives. Reverse electrodialysis (RED) is a multi-membrane system that aims to convert salinity gradient energy into electrical energy. Redox mediators are typically used to convert ionic mixing into electricity at electrodes placed adjacent to a stack of membranes. However, replacing redox mediators with water splitting electrodes can enable the sustainable production of fuel (H2) and/or oxygen (O2). We aim to examine if a RED oxygen generation system could meet oxygen requirements for scuba diving. Evolving oxygen from water would enable the development of artificial gills, allowing for an inexhaustible supply of oxygen thus displacing the need to carry oxygen tanks. Here we propose a system where instead of an air tank, a diver carries a RED cell, a 2 L volume of dilute solution, and a cylinder of other compressed gases for breathing (Fig. 1a). A RED cell is comprised of a stack of alternating anion and cation exchange membranes separated by channels of high and low concentration solutions (HC and LC) (Fig. 1b). A stack is made up of repeating units, which consist of a pair of selective membranes and a pair of solution channels, between the anode and cathode chambers. Salt ions in solution migrate due to the concentration gradient and two redox based electrodes convert this ionic flux into electric flux. We show here that the oxygen evolution activation overpotentials are a significant fraction of internal resistance (77%) in a RED system with a small number of cell pairs (N=5). However, this resistance is nearly negligible (3.2%) as the number of cell pairs increases (N=500). We further compare the RED systems to four different battery-electrolysis systems (Li-ion, Ni-MH, Ni-Cd, and lead acid) to contextualize the performance of the RED oxygen generation system with competing technologies. For large (N=100+) systems and long (10 hr) dives, RED is comparable in size to a battery-electrolysis system. With 500 membrane pairs, a RED powered diving system is 10.8 L and weighs 29 kg. Typical 12 L scuba tanks weigh 16 kg, and the size of a Li-ion battery powered electrolysis system for a 1 hour dive is, 2.2 L and 3.5 kg (19.2 L and 32.6 kg for a 10 hour dive).Figure 1. (a) Configuration of a RED cell for scuba divers using the surrounding seawater, a fixed dilute volume, and a fixed volume of additional gases for breathing carried by the diver. (b) Schematic of RED cell. Oxygen and hydrogen are produced at the anode and cathode by redox reactions. Figure 1

First Suction Cup Tagging on a Small and Coastal Form Bryde’s Whale (Balaenoptera edeni edeni) in China to Investigate Its Dive Profiles and Foraging Behaviours

Small-and-coastal-form Bryde’s whales (Balaenoptera edeni edeni) are known to inhabit coastal and continental shelf waters. However, little is known on their ethological activities, especially underwater behaviours. Here, we deployed a suction cup tag on a small-and-coastal-form Bryde’s whale to study its ethological activities in Dapeng Bay, a coastal water of Shenzhen off the east Pearl River Estuary, China. The whale was tagged for 6 hours in total and displayed a head-lifting feeding behaviour 77 times during two tag deployments. The swim speed, dive duration and depth and bottom duration were collected by the tag to describe detailed information on dive profiles. The rate of feeding behaviours was observed decreasing with the presence of anthropogenic whale-approaching (AWA) boat activities occurring close (within 300 m) to the animal. Our study, for the first time, investigated behaviours of Bryde’s whales using suction cup tagging in Chinese waters. The results reveal the dive and feeding patterns of a small-and-coastal-form Bryde’s whale with short-term monitoring and provide a fundamental advancement in the knowledge of both the diving behaviour and the behavioural response of this subspecies to human activities. The findings are valuable for possible rescue of the investigated whale and for policy and management regarding conservation of this subspecies as well.

A satellite-linked tag for the long-term monitoring of diving behavior in large whales

Despite spending most time underwater, the technology in use to track whales over large geographic ranges via satellite has been largely limited to locational data, with most applications focusing on characterizing their horizontal movements. We describe the development of the RDW tag, a new Argos-based satellite telemetry device that incorporates sensors for monitoring the movements and dive behavior of large whales over several months without requiring recovery. Based on an implantable design, the tag features a saltwater conductivity switch, a tri-axial accelerometer, and an optional pressure transducer, along with onboard software for data processing and detection of behavioral events or activities of interest for transmission. We configured the software to detect dives and create per-dive summaries describing behavioral events associated with feeding activities in rorqual whales. We conducted a validation by proxy of the dive summary and event detection algorithms using field data from a medium-duration archival tag. We also conducted a simulation exercise to examine how the expected data recovery would vary under different dive behavior scenarios and compared those results to empirical values from field deployments of the RDW tag on blue (Balaenoptera musculus) and humpback (Megaptera novaeangliae) whales. The dive summary algorithm accurately reported dive depth and duration, while the accuracy of the lunge-feeding event detection algorithm was dependent on the precision of the accelerometer data that was used, with a predicted accuracy of 0.74 for correctly classifying feeding dives from 1/64-G precision data and 0.95 from 1-mG precision data. Simulated data recovery was lower with sparser transmission schedules, shorter mean dive durations, and lower rates of successfully received transmissions. Empirical data recovery was lower than expected from the simulation, suggesting the effect of additional factors, such as data gaps. By measuring key aspects of the per-dive behavior of large whales over multi-month timescales of movement, the RDW tags provide the ability to monitor previously unobservable behaviors across entire geographic ranges, extending the applications of satellite telemetry devices to new areas of whale physiology, behavior, ecology, and conservation.

Iron mobilization during lactation reduces oxygen stores in a diving mammal

The profound impacts that maternal provisioning of finite energy resources has on offspring survival have been extensively studied across mammals. This study shows that in addition to calories, high hemoprotein concentrations in diving mammals necessitates exceptional female-to-pup iron transfer. Numerous indices of iron mobilization (ferritin, serum iron, total-iron-binding-capacity, transferrin saturation) were significantly elevated during lactation in adult female Weddell seals (Leptonychotes weddellii), but not in skip-breeders. Iron was mobilized from endogenous stores for incorporation into the Weddell seal’s milk at concentrations up to 100× higher than terrestrial mammals. Such high rates of iron offload to offspring drew from the female’s own heme stores and led to compromised physiologic dive capacities (hemoglobin, myoglobin, and total body oxygen stores) after weaning their pups, which was further reflected in shorter dive durations. We demonstrate that lactational iron transfer shapes physiologic dive thresholds, identifying a cost of reproduction to a marine mammal.

Practical Approaches for Providing Empirical Data on Seabird Behavior and Prey Assemblages in Tidal Channels

Persisting knowledge gaps relating to the ecological context and potential environmental impacts of marine renewable energy (MRE) devices continue to add substantial costs and uncertainty to MRE projects globally. Increasingly sophisticated technological approaches to environmental monitoring can have fundamental non-trivial shortcomings for environmental impact assessment (EIA), whilst substantial practical and financial costs of deployments can prevent their application. For the tidal stream energy (TSE) sector, there is a need for practical and cost-effective methods that can provide site-specific information on predator behavior and associated prey assemblages. Considering existing knowledge of animal communities in tidal channel environments, a proportional approach using shore-based observation and baited fish trap methods was developed. During April 2021, a trial of these methods was conducted in tidal channel environments in Shetland, UK. The practical application of the proposed approach is demonstrated here with results from three tidal channels including Bluemull Sound, the site of active TSE installations (the Shetland Tidal Array). Observation of predator behavior across the study sites are reported for great cormorant Phalacrocorax carbo and European shag Phalacrocorax aristotelis. Seabird diving data calculated from the shore-based observations provided metrics relevant to informing EIA and collision risk modelling including dive duration, dives per minute, and % time underwater. Fish trap deployments targeted the benthic and demersal prey of these predators in the three study sites and across a range of depths and hydrodynamic conditions. A variety of fish and invertebrate species known to be important components of benthic-foraging seabird diet were successfully captured by the traps, providing basic biological information on the prey assemblages observed in each site. The fish species observed in the highest abundance were saithe Pollachius virens and cod Gadus morhua. Benefits, limitations, and applications of this approach are discussed along with various factors relating to the performance of both methods. The shore-based observations rapidly identified relevant patterns in predator foraging activity which informed the targeted deployment of fish traps to provide complementary prey data. This novel combined approach has potential to reduce costs and uncertainty in EIA and for supporting the responsible development of the MRE industry.

A validation of abstracted dive profiles relayed via the Argos satellite system: a case study of a loggerhead turtle

Satellite telemetry devices can record movement data of animals along with the environmental data. Such data are relayed remotely via satellite systems, but are constrained by the limited bandwidth availability. A satellite relay data logger (SRDL) that can abstract dive profiles and compress the data for transmission using a broken stick model (BSM) has been widely used in studies on dive behavior and physiology of marine animals. However, there is still uncertainty in the abstracted dive profiles. Here, we aimed to evaluate the certainty of abstracted dive profiles (via satellite communication) in terms of dive performance (dive depth, duration, and dive type) by comparing it with the actual dive data (from the retrieved tag) in a loggerhead turtle deployed with the SRDL throughout a 1.4-year foraging period. There was no significant difference in the maximum dive depth between the retrieved and satellite transmission data; however, there was a slight but significant difference in the dive duration. The dives from both datasets were classified into five types. Inconsistent dive classifications occurred in 1.7% of the data. There was no significant difference in the proportion of time spent diving between the retrieved and satellite transmission data for each type during the common recording period. In monthly scale comparisons, however, a significant difference was detected when the amount of data via satellite transmission was the smallest. Our results demonstrated that the dive data abstracted using BSM almost reconstructed the actual dive profiles with certainty in a loggerhead turtle, although slight inconsistencies were observed.

Dive Performance and Aquatic Thermoregulation of the World's Smallest Mammalian Diver, the American Water Shrew (Sorex palustris).

Allometry predicts that the 12-17-g American water shrew (Sorex palustris)-the world's smallest mammalian diver-will have the highest diving metabolic rate coupled with the lowest total body oxygen storage capacity, skeletal muscle buffering capacity, and glycolytic potential of any endothermic diver. Consistent with expectations, and potentially owing to their low thermal inertia, water shrews had a significantly higher diving metabolic rate in 10°C water (8.77 mL O2 g-1 h-1) compared with 30°C water (6.57 mL O2 g-1 h-1). Unlike larger-bodied divers, muscle myoglobin contributed minimally (7.7%-12.4%) to total onboard O2 stores of juvenile and adult water shrews, respectively, but was offset by high blood O2 carrying capacities (26.4%-26.9% v/v). Diving was predominantly aerobic, as only 1.2%-2.3% of dives in 10°C and 30°C water, respectively, exceeded the calculated aerobic dive limits at these temperatures (10.8-14.4 s). The mean voluntary dive time of water shrews during 20-min trials in 3°C-30°C water was s (, ), with a mean maximum dive time of s. However, the average dive duration ( s, ) of radio-telemetered shrews exclusively foraging in a simulated riparian environment (3°C water) for 12-28 h suggests that mean (but not maximum) dive times of water shrews in the wild may be longer. Mean dive duration, duration of the longest dive, and total time in water all decreased significantly as water temperature declined, suggesting that shrews employed behavioral thermoregulation to defend against immersion hypothermia. Additionally, free-diving shrews in the 24-h trials consistently elevated core body temperature by ∼1°C immediately before initiating aquatic foraging bouts and ended these bouts when body temperature was still at or above normal resting levels (∼37.8°C). We suggest that this observed predive hyperthermia aids to heighten the impressive somatosensory physiology, and hence foraging efficiency, of this diminutive predator while submerged.

Profound hypercapnia but only moderate hypoxia found during underwater rugby play

Background: Underwater rugby is a team sport where players try to score points with a negatively buoyant ball while submerged in a swimming pool. Reports of syncope incidents at the Swedish Championships led to us to investigate end-tidal oxygen and carbon dioxide levels during simulated match play. Methods: Eight male underwater rugby club players of varying experience participated. Repetitive measurements were made while players were defending during simulated match play. Each time a player surfaced they exhaled through a mouthpiece connected to a flow meter and a gas analyzer to measure tidal volume, PETO2 and PETCO2. Results: Measurements were made over 12 dives, with an average dive duration of 18.5 seconds. The mean maximal PETCO2 across the eight participants was 10.0 kPa (~75 mmHg) (range, 9.1–11.7 [~68–88]). The corresponding mean minimum PETO2 was 7.6 kPa (~57 mmHg) (6.3–10.4 [~47–78)). PETCO2 drifted upward, with the mean upward change from the first to last dive for each participant being +1.8 (~13.5 mmHg) (SD 1.74) kPa. A similar trend for PETO2 was not detected, with a mean change of -0.1 (~0.75 mmHg) (SD 3.79) kPa. Conclusion: Despite high PETCO2 values that were close to narcotic being recorded, these players seemed to regulate their urge to breathe based on hypoxia rather than hypercapnia.

Examining the Plasticity of the Dive Response in Relation to Dive Behavior of Northern Elephant Seals

Searching and capturing prey are extraordinary challenges for marine mammals, as they must actively hunt during breath‐hold dives. This challenge is conquered by the dive response (the decreased heart rate and increased peripheral vasoconstriction associated with a breath hold), which lowers the rate of oxygen consumption and prolongs the duration of aerobic metabolism. The dive response is dynamic and potentially influenced by dive duration, depth, anticipation, and exercise. However, it is still unknown how these influences interact with each other to facilitate oxygen management pre‐ and post‐dive, during different dive phases, and at fine scale. We studied the dive response of one of the deepest diving marine mammals, the northern elephant seal (Mirounga angustirostris) across three years (2018, 2019, 2021) to determine interactions between these behavioral and physiological factors. Using a biologger that records electrocardiogram data (50‐100 Hz) and a Daily Diary time‐depth‐recorder that records acceleration and depth, we calculated heart rate, stroke rate, and minimum specific acceleration (MSA) in 7 seals in dives as deep as 460m. We examined the influence of dive depth, duration, and activity (MSA) on dive heart rate, pre‐ and post‐dive heart rate, and heart rate during different dive phases and at fine scale. We recorded heart rates as low as 20 bpm during natural dives, with the average heart rate at depth being 48.91 ± 25.93 bpm. The heart rate rapidly drops at the start of dives from ~100‐120 bpm to ~40‐50 bpm and continues to decline as the seal passively drifts to depth. At the bottom seals tend to stroke and glide while maintaining a relatively stable heart rate. On ascent heart rate gradually increases as the seals stroke and glide to the surface. We see a drastic increase in heart rate just before seals surface, likely to maximize offloading of carbon dioxide and onloading of oxygen upon surfacing. Our preliminary results reveal that both dive depth and MSA influence the dive heart rate at fine scale intervals of 20 s within the dive. These results suggest that the seals actively regulate their oxygen during dives. Our results will connect dive behavior and the dive response to understand how marine mammals perform deep‐dives while actively searching and capturing prey.

Sowerby's beaked whale biosonar and movement strategy indicate deep-sea foraging niche differentiation in mesoplodont whales.

ABSTRACTClosely related species are expected to diverge in foraging strategy, reflecting the evolutionary drive to optimize foraging performance. The most speciose cetacean genus, Mesoplodon, comprises beaked whales with little diversity in external morphology or diet, and overlapping distributions. Moreover, the few studied species of beaked whales (Ziphiidae) show very similar foraging styles with slow, energy-conserving movement during long, deep foraging dives. This raises the question of what factors drive their speciation. Using data from animal-attached tags and aerial imagery, we tested the hypothesis that two similar-sized mesoplodonts, Sowerby's (Mesoplodon bidens) and Blainville's (Mesoplodon densirostris) beaked whales, exploit a similar low-energy niche. We show that, compared with the low-energy strategist Blainville's beaked whale, Sowerby's beaked whale lives in the fast lane. While targeting a similar mesopelagic/bathypelagic foraging zone, they consistently swim and hunt faster, perform shorter deep dives, and echolocate at a faster rate with higher frequency clicks. Further, extensive near-surface travel between deep dives challenges the interpretation of beaked whale shallow inter-foraging dives as a management strategy for decompression sickness. The distinctively higher frequency echolocation clicks do not hold apparent foraging benefits. Instead, we argue that a high-speed foraging style influences dive duration and echolocation behaviour, enabling access to a distinct prey population. Our results demonstrate that beaked whales exploit a broader diversity of deep-sea foraging and energetic niches than hitherto suspected. The marked deviation of Sowerby's beaked whales from the typical ziphiid foraging strategy has potential implications for their response to anthropogenic sounds, which appears to be strongly behaviourally driven in other ziphiids.

Estimating the complex patterns of survey availability for loggerhead turtles

AbstractSuccessful management strategies are important for conservation and allow accurate surveying and monitoring of populations for presence, abundance, and trend. This becomes challenging for cryptic, low‐density species, and for animals that have complicated life histories where not every stage of the life cycle can be surveyed effectively. We used information from animal‐borne data loggers to characterize the dive‐surfacing behavior of cryptic loggerhead turtles (Caretta caretta) in the northwest Atlantic from 2009–2018. Our data covered a large geographic area off the east coast of North America, and allowed us to present estimates for and variation in 3 metrics that can be used to assess availability bias affecting visual surveys: average dive duration, average surface duration, and the proportion of time at the surface. We used a stochastic partial differential equation approach to construct spatiotemporal regression models for the availability bias metrics. Model predictions showed pronounced individual, spatial, and spatiotemporal (seasonal) variation among the 245 turtles. Overall, we estimated an average dive duration of 14.5 ± 1.36 minutes (SE), an average surface duration of 15.1 ± 2.77 minutes, and an average proportion of time at the surface of 0.50 (95% CI = 0.41–0.59). We made predictions of the 3 availability bias metrics on a 20‐km × 20‐km grid and further used predictions to explore seasonal variations. Our results contribute new insights into loggerhead turtle behavior and provide information that enables survey counts to be translated into more accurate abundance estimates.

Hyperbaric Exposure of Scuba Divers Affects the Urinary Excretion of Nucleic Acid Oxidation Products and Hypoxanthine.

In recent studies, oxidative stress after scuba diving has been explored by measuring urinary biomarkers in volunteers under controlled conditions. Dive depth and duration, water temperature, and workload are all variables that can elicit metabolic responses. A controlled diving experiment was performed in an indoor pool at 20, 30, and 40 m depths at a water temperature of 32 °C, on three different days. Samples of urine from five male scuba divers were taken before diving and at four time points after diving, and then tested for their concentration of five different oxidative stress biomarkers by means of liquid chromatography tandem mass spectrometry and by 1H nuclear magnetic resonance metabolomics analysis. The results showed no variation in the five biomarkers after diving, but a decreasing trend was observed over the three days, with no differences among the three depths. The lack of effect on oxidative stress biomarkers has been attributed to the comfortable water temperature and to the absence of exercise in the divers during the experiment. Instead, an increase in hypoxanthine excretion, which can be considered a biomarker sensitive to hyperbaric exposure, was found after diving. Finally, the results suggest a physiological mechanism of metabolic adaptation to a new condition.

Ontogeny and synchrony of diving behavior in Humpback whale mothers and calves on their breeding ground

Abstract For humpback whales, the mother–calf pair is the only stable social unit with calves following their mother after birth and staying in close proximity. This following strategy ensures the maintenance of such close proximity between the mother and her calf, with calves benefiting from maternal protection and care. Using multi-sensor tags, we recorded the diving behavior of calves at three different age-classes (C1, C2, C3) to assess how calves developed in their natural environment at an early stage of their life. From 29 deployments on calves, we extracted the diving metrics from two C1 neonate calves, eight C2 calves, and 19 C3 calves, and we found that some diving metrics (dive duration, time at bottom, maximal depth, or maximal dive duration) differed among calves’ age-classes. On 23 tagged mothers, we analyzed if their diving profiles also varied depending on calf’s age-class. We showed that only two dive metrics of mothers varied with the age of their own calves (time spent at the bottom, and time interval between dives), but all others were not reliant on the calf’s age. Simultaneous deployments on seven mother–calf pairs in 2016 and 2017 revealed highly synchronized dives, with mothers leading the diving pattern. This work represents an extensive study investigating the diving behavior in humpback whale mother–calf pairs on their breeding ground.

Humpback whale adult females and calves balance acoustic contact with vocal crypsis during periods of increased separation.

Acoustic communication is important for animals with dependent young, particularly when they are spatially separated. Maternal humpback whales (Megaptera novaeangliae) use acoustic calling to help minimize the risk of separation from their young calves during migration. These pairs also use acoustic crypsis to minimize detection by males. How they balance a restricted active space with the need to maintain acoustic contact during periods of separation is not yet understood. Here, we analyzed movement metrics of tagged adult female–calf pairs during migration to identify two behavioral states, “resting/milling” and “travelling.” When travelling, these pairs dived synchronously and exhibited little to no spatial separation. Alternatively, adult females had significantly longer dive durations (p < .01) when resting, and while they spent prolonged times at depth, calves would surface several times independently. This demonstrated that these pairs are frequently separated during periods of rest. We then determined whether the call rates and acoustic levels of these pairs increased with more frequent separation, finding that both adult females and calves significantly increased their call rates, but not levels, when resting. We also found that adult female–calf pairs have a restricted active space, with less than 15% of calls estimated to be detectable beyond 2 km. However, as with call level, detection distance did not differ significantly between the two behavioral states. In summary, adult female–calf pairs maintain successful communication during periods of separation by calling more frequently rather than by producing louder calls. This strategy aids in maintaining acoustic contact while simultaneously limiting detectability by conspecifics.

Behavioral Pattern of Risso’s Dolphin (Grampus griseus) in the Gulf of Taranto (Northern Ionian Sea, Central-Eastern Mediterranean Sea)

Relatively scant information is available on the Risso’s dolphin in comparison to the other species regularly present in the Mediterranean Sea. Recently, its conservation status has been updated to Endangered by the International Union for Conservation of Nature (IUCN) in this Sea. Therefore, the need to increase information on its biology and ecology is even more urgent. This study reports the first preliminary information on the behavioral traits of the species occurring in the Gulf of Taranto (Northern Ionian Sea). Data on predominant behavioral activity states and on a set of group composition variables (group formation, cruising speed, dive duration and interaction between individuals) were collected from April 2019 to September 2021, applying the focal-group protocol with instantaneous scan sampling. Group size, depth and group composition variables were compared between activity states. Results highlight that both the group size and the several variables considered varied significantly depending on activity state. The group size was significantly smaller during feeding than resting and traveling and a characterization in terms of group formation, cruise speed, dive duration and interaction between animals is provided for the different activity states. Moreover, a list of behavioral events which occurred, as well as their relative frequency of distribution among activity states, is reported. Finally, details on the sympatric occurrences between Risso’s and striped dolphins, as well as the repetitive interaction observed between adult individuals and plastic bags floating on the sea surface, are reported and discussed.

Intra- and inter-annual shifts in foraging tactics by parental northern gannets Morus bassanus indicate changing prey fields

Seabirds are constrained by central-place foraging during breeding, when the energy obtained from prey must outweigh the costs of travel, search, capture and transport. The distribution and phenology of the cold-blooded marine fishes they exploit are heavily influenced by oceanic climate. Northern gannets, the largest breeding seabird in the North Atlantic, use a generalist foraging strategy, preying on a wide array of pelagic fishes. They employ different foraging tactics for different prey types, with rapid, shallow V-shaped dives used for large, powerful prey such as mackerel, and U-shaped dives for smaller forage fishes like capelin. Here we assess intra- and inter-annual differences in foraging effort and influences of prey availability at the southernmost colony of the species at Cape St. Mary’s, Newfoundland, Canada. We compared foraging trip characteristics (total and maximum distance, directness, duration and number of dives) of parental gannets during the breeding seasons of 2019 (n = 10) and 2020 (n = 7) using GPS/time-depth recorders. Individual gannets shifted away from using U-shaped dives in early chick-rearing to primarily V-shaped dives in late chick-rearing. Shifts were abrupt and occurred in mid-August in 2019 and 2020. Maximum and total foraging trip distance and duration were significantly greater during early chick-rearing in 2020 than in 2019. Kernel density 50% utilization distributions were larger and expanded further from the colony during early chick-rearing in 2020 (7297 ± 1419 km2; mean ± SE) than 2019 (2382 ±797 km2). Increased foraging effort during early chick-rearing in 2020 was likely due to decreased capelin availability, resulting from earlier spawning, and greater variation in the timing of spawning among sites, which may have been influenced by warmer waters.

Cuvier’s beaked whale foraging dives identified via machine learning using depth and triaxial acceleration

Knowledge of Cuvier’s beaked whale Ziphius cavirostris behavior has expanded through the utilization of animal-borne tags. However, many tag types do not record sound—thus preventing echolocation click detections to identify foraging—or have short deployments that sample a limited range of behaviors. As the quantity of such non-acoustic tag data grows, so too does the need for robust methods of detecting foraging from non-acoustic data. We used 692 dives from 5 sound-recording tags on Cuvier’s beaked whales in southern California, USA, to develop extreme gradient boosting tree models to detect foraging based on 1 Hz depth and 16 Hz triaxial acceleration data. We performed repeated 10-fold cross validation using classification accuracy to tune 500 models with randomly partitioned training and testing datasets. An average of 99.9 and 99.2% of training and testing dataset dives, respectively, were correctly classified across the 500 models. Dives without associated sound recordings (n = 2069 from 7 whales including 4 non-acoustic tags) were classified via a model that maximized training information using dive depth and duration, ascent and descent rates, bottom-phase average vertical speed, and roll circular variance during dive descents and bottom phases. Of all long, deep dives (conventionally assumed to include foraging), 2.4% were classified as non-foraging dives, while 0.3% of short, shallow dives were classified as foraging dives. Results confirm that conventional depth and/or duration classifiers provide reasonable estimates of longer-term foraging patterns. However, additional variables previously listed enhance foraging detections for unusual dives (notably non-foraging deep dives) for southern California Cuvier’s beaked whales.

Wintering Common Loons (Gavia immer) exhibit daily social behavior on Lake Jocassee Reservoir, South Carolina

We investigated both social and foraging plasticity in a population of Common Loons (Gavia immer) that overwinter inland on Lake Jocassee Reservoir, South Carolina, using transects and time-activity budgets from 4 January to 10 March 2017. Loons were observed as either solitary individuals (65%) or in groups (35%, i.e., within ≤5 body lengths of another loon). Loons were more social (i.e., in groups) on transects with a river mouth (40.9% ± 7.1) than on transects without a river mouth (24.2% ± 5.4). The mean (± SE) dive duration for solitary loons was 71.3 ± 7.7 s (n = 46, range 15–219 s) and several dives exceeded 2 min (22.2%, 10/46). In contrast, 15 foraging flocks were characterized by numerous shallow dives of <20 s. The mean group size (± SE) was 6.9 ± 0.8 individuals and group foraging bouts lasted 8.3 ± 1.2 min. In addition, Horned Grebes (Podiceps auritus) and Ring-billed Gulls (Larus delawarensis) joined loons forming multispecies foraging flocks, 26.7% and 13.3% of the time, respectively. These data indicate Common Loons wintering at Lake Jocassee are social and exhibit foraging plasticity.

Optimal diving and oxygen use

Animals that hunt for food underwater and return to the surface for air (‘divers’) are limited by their ability to store oxygen. The aerobic dive limit (ADL) is the dive duration at which all the possible oxygen stores have been used. It has been claimed that models of optimal diving predict that divers will be close to this limit and that the data do not match this prediction. I characterize closeness to the ADL by the fraction ψ of the maximum amount of stored oxygen that is used during a dive. At the ADL this fraction is 1. I show that an optimal diver does not necessarily have a ψ close to 1 and point out that the key feature predicted by the model is that the diver should have no oxygen stores left when it returns to the surface. The data cited against optimal diving do not address this issue. • Animals that dive for food and return to surface for air are limited by oxygen stores. • Models claim that optimal divers should use nearly all their maximum oxygen stores. • I develop a measure of oxygen use for divers and illustrate it with data from whales. • I show that optimal diving does not predict that the maximum amount of oxygen is used. • I argue that the data cited against optimal diving do not address this issue.