Dive Efficiency Research Articles

High dive efficiency in shallow water

Dive studies across mammals, birds, reptiles and fish often focus on deep dives, and shallow water diving has tended to be overlooked. For air-breathers, foraging in shallow water poses challenges since the lungs generate buoyancy, and shallow divers must trade off the extent of inhalation against the negative buoyancy needed to avoid floating to the surface. Using high-resolution depth loggers, we addressed this knowledge gap around the ecology of shallow water diving at a foraging site for hawksbill turtles (Eretmochelys imbricata) where depth was typically < 3 m. Contrary to predictions, dive durations were long, particularly at night (mean dive duration per turtle: 17–61 min, n = 12 turtles, n = 2576 nocturnal dives), despite warm water temperatures (24–37 °C). Dive efficiency (% time submerged) for hawksbills was 98%, the highest recorded for any air-breathing marine vertebrate including penguins (60–78%), seals (51–91%), cetaceans (68–87%), and other sea turtle species (68–95%). Hawksbills usually dive for much longer (42–286% increase) than green and loggerhead turtles when depth and temperature are accounted for. Hawksbill turtles likely forage in very shallow water to reduce predation risk from sharks: of 423 hawksbills captured by hand, none had any evidence of shark attack, although large sharks were present in nearby deeper water. Our results challenge the prediction that shallow water dives by air-breathers will usually be short and open the way for comparative studies of the ecology of shallow water diving in a range of other taxa. Our work emphasises the likely importance of predation risk in shaping patterns of habitat utilisation.

The dive performance of immature king penguins following their annual molt suggests physiological constraints.

Like all birds, penguins undergo periodic molt, during which they replace old feathers. However, unlike other birds, penguins replace their entire plumage within a short period while fasting ashore. During molt, king penguins (Aptenodytes patagonicus) lose half of their initial body mass, most importantly their insulating subcutaneous fat and half of their pectoral muscle mass. The latter might challenge their capacity to generate and sustain a sufficient mechanical power output to swim to distant food sources and propel themselves to great depth for successful prey capture. To investigate the effects of the annual molt fast on their dive/foraging performance, we studied various dive/foraging parameters and peripheral temperature patterns in immature king penguins across two molt cycles, after birds had spent their first and second year at sea, using implanted data-loggers. We found that the dive/foraging performance of immature king penguins was significantly reduced during post-molt foraging trips. Dive and bottom duration for a given depth were shorter during post-molt and post-dive surface interval duration was longer, reducing overall dive efficiency and underwater foraging time. We attribute this decline to the severe physiological changes that birds undergo during their annual molt. Peripheral temperature patterns differed greatly between pre- and post-molt trips, indicating the loss of the insulating subcutaneous fat layer during molt. Peripheral perfusion, as inferred from peripheral temperature, was restricted to short periods at night during pre-molt but occurred throughout extended periods during post-molt, reflecting the need to rapidly deposit an insulating fat layer during the latter period.

How Elephant Seals (Mirounga leonina) Adjust Their Fine Scale Horizontal Movement and Diving Behaviour in Relation to Prey Encounter Rate.

Understanding the diving behaviour of diving predators in relation to concomitant prey distribution could have major practical applications in conservation biology by allowing the assessment of how changes in fine scale prey distribution impact foraging efficiency and ultimately population dynamics. The southern elephant seal (Mirounga leonina, hereafter SES), the largest phocid, is a major predator of the southern ocean feeding on myctophids and cephalopods. Because of its large size it can carry bio-loggers with minimal disturbance. Moreover, it has great diving abilities and a wide foraging habitat. Thus, the SES is a well suited model species to study predator diving behaviour and the distribution of ecologically important prey species in the Southern Ocean. In this study, we examined how SESs adjust their diving behaviour and horizontal movements in response to fine scale prey encounter densities using high resolution accelerometers, magnetometers, pressure sensors and GPS loggers. When high prey encounter rates were encountered, animals responded by (1) diving and returning to the surface with steeper angles, reducing the duration of transit dive phases (thus improving dive efficiency), and (2) exhibiting more horizontally and vertically sinuous bottom phases. In these cases, the distance travelled horizontally at the surface was reduced. This behaviour is likely to counteract horizontal displacement from water currents, as they try to remain within favourable prey patches. The prey encounter rate at the bottom of dives decreased with increasing diving depth, suggesting a combined effect of decreased accessibility and prey density with increasing depth. Prey encounter rate also decreased when the bottom phases of dives were spread across larger vertical extents of the water column. This result suggests that the vertical aggregation of prey can regulate prey density, and as a consequence impact the foraging success of SESs. To our knowledge, this is one of only a handful of studies showing how the vertical distributions and structure of prey fields influence the prey encounter rates of a diving predator.

Examining the metabolic cost of otariid foraging under varying conditions

In order to understand the effects of changing prey availability on dive behavior and energetic costs, behavior and metabolism must be measured simultaneously. Video dive analysis and open-flow respirometry were used to assess changes in dive behavior and energetics of two captive trained California sea lions (Zalophus californianus) in response to high and low prey densities and increased cost of swimming. Differences in dive duration, foraging duration, dive efficiency and foraging efficiency were examined between high and low prey encounter rates, and between standard and cost-increased dives. Changes in foraging metabolic rate (FVO2) and foraging carbon dioxide elimination (FVCO2) were also assessed across all four conditions. Both animals had longer dive durations, greater dive efficiency and lower FVO2 and FVCO2 levels at high prey encounter rates. Foraging metabolic rate and FVCO2 showed a slight increasing trend in cost-increased bouts for both animals, and surface interval durations were shorter. The results suggest that at high prey encounter rates animals minimize their metabolic rate to extend dive duration. These results also suggest that although FVCO2 decreases under high prey encounter rates, increasing the cost of transport keeps VCO2 levels elevated and increases the animal's oxygen debt. The authors hypothesize that as the level of work increases, the ability of the body to sequester CO2 into bicarbonate diminishes due to an inability of the proton buffering process within the blood and muscles to keep pace with increasing CO2 production. Therefore, CO2 may be a more sensitive physiological marker than O2 when examining the metabolic cost of foraging.

Diel diving behavior of breeding gentoo penguins on King George Island in Antarctica

Many marine birds dive to catch prey in water. The gentoo penguin (Pygoscelis papua), a specialized diving seabird that preys on krill and fish, is distributed from the sub-Antarctic islands to Antarctic regions. Here, we observed the diving behavior of breeding gentoo penguins on King George Island, South Shetland Islands, Antarctica. Using a time–depth recorder, we collected diving depths every second from seven gentoo breeders for 5–21 days during the chick-rearing period and analyzed their diving characteristics. Most dives occurred in shallow water, although the dive efficiency (=bottom duration time/[dive duration + post-dive surface time]) was highest at depths of 30–35 m and decreased as the penguins dove deeper. Gentoo penguins did not dive more frequently during the day than at night, but during nighttime, most dives occurred in shallow water (<20 m) and the dive efficiency was also higher at this time. As penguins repeated their foraging trips, the number of dives, depth of dives, and trip duration did not change significantly. Our results suggest that the diel dive patterns of gentoo penguins might be related to the vertical migration of krill (upward to the surface at night). In addition, we observed that gentoo penguins could perform active diving behavior even at night, possibly aided by civil twilight during the chick-rearing period in Antarctic regions.

Dive efficiency versus depth in foraging emperor penguins

Dive duration generally decreases with dive depth in air-breathing vertebrates. In most penguin species, this occurs due to increasing transit (descent and ascent) durations. However, in U-shaped dives, this is also because the duration of the bottom phase of the dive increases with increasing depth. We considered the time-based efficiency with which birds can use different diving depths by using depth recorders on 9 foraging emperor penguins Aptenodytes forsteri, studied dur- ing the early and late chick-rearing period in Adelie Land, Antarctica. Dive and post-dive duration positively correlated with diving depth, but particularly long recovery durations were apparent for dives longer than 456 s. This inflection point (i.e. 456 s) corresponded to a theorized maximum rate of gain of prey per dive cycle. By using the number of undulations in the bottom phase of the dive as a proxy for prey capture success, we conclude that the most lucrative dive depths for the birds studied were between 50 and 225 m. Since these depths were also those most often visited, we think that foraging emperor penguins focus on depths where profitability is highest.

Working beneath the surface: interplay of biomechanics, physiology and behavioural ecology in diving seabirds

ABSTRACT: This Theme Section looks at new approaches in the study of seabird foraging ecology,with particular emphasis on the use of recently developed recording devices attached to birds. It in-cludes studies with energy as a theme, such as the energetics of foraging in Magellanic penguins,how dive efficiency varies with depth in emperor penguin, optimality of stroke frequency in divingseabirds, and how using energy to counteract depth-dependent buoyancy may limit pursuit speedsin hunting imperial shags. A more physiological approach is adopted in two studies, one on the divecapacity of emperor penguins and one on planktivorous auklets. Foraging ecology is examined morebroadly in a study on the foraging behaviour of breeding thick-billed murres while a modellingapproach is used to describe foraging profitability in aucklets (the smallest marine endotherms) feed-ing on pelagic patches. Finally, consideration is given to bird-attached instrument artefacts in pathreconstruction of diving birds using dead-reckoning.KEY WORDS: Diving seabirds · Biomechanics · Energy efficiency · Foraging ecology · Recordingdevices

Pushed for time or saving on fuel: fine-scale energy budgets shed light on currencies in a diving bird.

Animals may forage using different currencies depending on whether time minimization or energy maximization is more pertinent at the time. Assessment of net energy acquisition requires detailed information on instantaneous activity-specific power use, which varies according to animal performance, being influenced, for example, by speed and prey loading, and which has not been measured before in wild animals. We used a new proxy for instantaneous energy expenditure (overall dynamic body acceleration), to quantify foraging effort in a model species, the imperial shag Phalacrocorax atriceps, during diving. Power costs varied nonlinearly with depth exploited owing to depth-related buoyancy. Consequently, solutions for maximizing the gross rate of gain and energetic efficiency differed for dives to any given depth. Dive effort in free-ranging imperial shags measured during the breeding season was consistent with a strategy to maximize the gross rate of energy gain. We suggest that the divergence of time and energy costs with dive depth has implications for the measurement of dive efficiency across diverse diving taxa.

Diving in shallow water: the foraging ecology of darters (Aves: Anhingidae)

Diving birds have to overcome buoyancy, especially when diving in shallow water. Darters and anhingas (Anhingidae) are specialist shallow‐water divers, with adaptations for reducing their buoyancy. Compared to closely‐related cormorants (Phalacrocoracidae), darters have fully wettable plumage, smaller air sacs and denser bones. A previous study of darter diving behaviour reported no relationship between dive duration and water depth, contrary to optimal dive models. In this study I provide more extensive observations of African darters Anhinga melanogaster rufa diving in water&lt;5 m deep at two sites. Dive duration increases with water depth at both sites, but the relationship is weak. Dives were longer than dives by cormorants in water of similar depth (max 108 s in water 2.5 m deep), with dives of up to 68 s observed in water&lt;0.5 m deep. Initial dives in a bout were shorter than expected, possibly because their plumage was not fully saturated. Dive efficiency (dive:rest ratio) was 5–6, greater than cormorants (2.7±0.4 for 18 species) and other families of diving birds (average 0.2–4.3). Post‐dive recovery periods increased with dive duration, but only slowly, resulting in a strong increase in efficiency with dive duration. All dives are likely to fall within the theoretical anaerobic dive limit. Foraging bouts were short (17.8±4.3 min) compared to cormorants, with birds spending 80±5% of time underwater. Darters take advantage of their low buoyancy to forage efficiently in shallow water, and their slow, stealthy dives are qualitatively different from those of other diving birds. However, they are forced to limit the duration of foraging bouts by increased thermoregulatory costs associated with wettable plumage.

Distribution, Diet and Dive Behavior of Barrow’s and Common Goldeneyes during Spring and Autumn in the St. Lawrence Estuary

Barrow’s (Bucephala islandica) and Common (B. clangula) goldeneyes winter in large numbers in the St. Lawrence estuary and little is known of their distribution and diet. The objective of the study was to characterize how these two similar species, co-existed in the St. Lawrence estuary during the non-breeding season. Their local distribution, diet, and dive efficiency were compared. There was little overlap in habitat use by both goldeneyes. Their distribution was not correlated in the autumn (r = 0.04) but was in the spring (0.68). Autumn and spring distributions were more correlated in Common Goldeneyes (r = 0.82) than in Barrow’s Goldeneyes (r = 0.49). Both species moved to the north shore of the estuary during January and February 1999 as south shore intertidal areas froze. Barrow’s Goldeneyes foraged in larger flocks than Common Goldeneyes (x̄ = 20.3 vs 9.8 birds) and flocks were larger in autumn than spring. Both species fed on amphipods but differed in their use of gastropods (Barrow’s Goldeneye) and polychaetes (Common Goldeneye). Dive duration was similar in both species but varied between areas. Pause duration was shorter in Common Goldeneyes than in Barrow’s Goldeneyes but was not affected by area. Dive efficiency was higher in Common Goldeneyes than Barrow’s Goldeneyes and varied between areas. The predominance of polychaetes in the diet of Common Goldeneyes and of amphipods in the diet of Barrow’s Goldeneyes had not been highlighted before. These species provide a good example of niche differentiation between closely related species.

Modelling the daily food requirements of wintering great cormorants: a bioenergetics tool for wildlife management

Summary Great cormorants Phalacrocorax carbo, Linnaeus are large piscivorous birds which occur in Asia, Australia, Africa, Europe and North America. Their European breeding population has increased by at least 15% per annum over the last 15 years, reaching a total of c. 200 000 pairs in the late 1990s. There are concerns that this increase is adversely affecting freshwater fish populations throughout Europe, but real assessment requires a detailed knowledge of cormorant food requirements. The daily food intake (DFI) of great cormorants has been measured during the breeding season, but little is known about DFI in winter when these poorly insulated birds experience consistently low temperatures. DFI is likely to vary widely according to abiotic and biotic conditions, making predictions about impact particularly difficult. We modelled DFI for great cormorants wintering at Loch Leven, Scotland, using behavioural data recorded via radio‐tracking of free‐ranging individuals, metabolic measurements obtained from captive birds, and published data. DFI was estimated to be 672 g day−1 (predicted maximum range 441–1095 g day−1), values similar to DFI of great cormorants breeding under temperate conditions and of other aquatic bird species. During winter great cormorants at Loch Leven decreased their average dive time and increased dive efficiency (higher proportion of time spent underwater). They nonetheless spent 130 min day−1 in the water and allocated more than a third of their daily energy budget to diving. Synthesis and applications: In view of the need for the sound management of cormorant populations, we present a general bioenergetics model, based on simple behavioural and dietary inputs, that computes an estimate of DFI outside the breeding season for a range environmental conditions and habitats. An interactive computer programme for this model is available (http://www.cepe.c‐strasbourg.fr) to help scientists and managers estimate local values for average, minimum and maximum DFI.

Diving Behavior of Rock Shags at a Patagonian Colony of Argentina

-The diving behavior of ten adult Rock Shags (Phalacrocorax magellanicus) foraging at Punta Loma, Patagonia, Argentina was studied, using radiotransmitters deployed during two chick-rearing periods. Shags dived continuously during foraging trips, spending 75% of their time at sea underwater. Mean dive duration was 47 ? 14 sec while mean subsequent surface interval was 14 + 6 sec (N = 11,212 dives). During both seasons, there was a weak linear relationship between dive duration and recovery time. Only four of the birds showed a positive and significant linear function between these diving parameters. Recovery time at the surface was independent of the previous dive time only during low tides. Dive efficiency (dive time/recovery time) was the highest reported for a foot-propelled diver (mean = 3.7 and 4.2 for 1996 and 1997, respectively). Food resources located in shallow waters at Punta Loma allowed the shags to optimize their searching period underwater by decreasing the energetic costs associated with diving. Received 11June 1999, accepted 24July 1999.

Diving of Otters (Lutra lutra) in a Marine Habitat: Use of Depths by a Single-Prey Loader

1. We test the hypotheses that otters Lutra lutra, diving in a marine habitat, prefer shallow water because (a) breathing is more efficient, (b) searching is more efficient, or (c) body heat is lost less rapidly. 2. Otters were observed along the coast in Shetland over a 3-year period, and dives were plotted on a depth-contour map. The duration of the dives and, when possible, the hunting bouts were timed. Success rate was scored. Swimming and diving speed were derived from film-sequences of a captive otter. 3. Otters strongly preferred diving in shallow water 0-3 m deep. The recovery time was linearly related to underwater time. Dive efficiency decreased from 0 to 7 m deep, but tended to increase again at larger depths. Although (in unsuccessful dives) the time spent at the bottom initially increased with depth, searching efficiency decreased with depth because of a rapid increase of the travel time. The maximum duration of hunting bouts was shorter for those in which more deep dives were included. 4. Our data support hypothesis (c), but are not in full agreement with the hypotheses (a) and (b) which were developed for multiple-prey loaders unlike the otter. The otters' preference for shallow water is possibly a result of the lower thermoregulatory costs, and the shorter travel time.