Dive Cycle Research Articles

VARIATION IN FORAGING AND PARENTAL BEHAVIOR OF KING CORMORANTS

Abstract We studied sexual and individual differences in foraging and parental behavior of King Cormorants (Phalacrocorax albiventer) during the brood-rearing period at Macquarie Island. King Cormorants exhibit sexual dimorphism in size, with males being 16% heavier than females. Females foraged mainly in the morning and males in the afternoon. Five females were shallow divers (1.9 to 6.8 m), and seven females were deep divers (19.6 to 28.0 m); males dived deeper (15.6 to 44.2 m) than both groups of females. The amount of time spent on the bottom (“bottom time”) relative to the dive cycle was higher for shallow-diving females (x = 40 ± SD of 13%) than for males (x = 26 ± 4%) and deep-diving females (x = 27 ± 3%). Total daily dive time and bottom time per day did not differ significantly among groups because shallow-diving females dived more often (x = 211 ± 81 dives per day) than males (x = 68 ± 21) and deep-diving females (x = 70 ± 7). Provisioning rate, trip duration, and proportion of time at sea...

Diving behaviour of female northern rockhopper penguins, Eudyptes chrysocome moseleyi , during the brooding period at Amsterdam Island (Southern Indian Ocean)

The pattern and characteristics of diving in 14 female northern rockhopper penguins, Eudyptes chrysocome moseleyi, were studied at Amsterdam Island (37°50′S; 77°31′E) during the guard stage, using electronic time–depth recorders. Twenty-nine foraging trips (27 daily foraging trips and two longer trips including one night) with a total of 16 572 dives of ≥3 m were recorded. Females typically left the colony at dawn and returned in the late afternoon, spending an average of 12 h at sea, during which they performed ∼550 dives. They were essentially inshore foragers (mean estimated foraging range 6 km), and mainly preyed upon the pelagic euphausiid Thysanoessa gregaria, fishes and squid being only minor components of the diet. Mean dive depth, dive duration, and post-dive intervals were 18.4 m (max. depth 109 m), 57 s (max. dive duration 168 s), and 21 s (37% of dive duration), respectively. Descent and ascent rates averaged 1.2 and 1.0 ms−1 and were, together with dive duration, significantly correlated with dive depth. Birds spent 18% of their total diving time in dives reaching 15 to 20 m, and the mean maximum diving efficiency (bottom time:dive cycle duration) occurred for dives reaching 15 to 35 m. The most remarkable feature of diving behaviour in northern rockhopper penguins was the high percentage of time spent diving during daily foraging trips (on average, 69% of their time at sea); this was mainly due to a high dive frequency (∼44 dives per hour), which explained the high total vertical distance travelled during one trip (18 km on average). Diving activity at night was greatly reduced, suggesting that, as other penguins, E. chrysocome moseleyi are essentially diurnal, and locate prey using visual cues.

Heart rates and diving behavior of leatherback sea turtles in the eastern pacific ocean

Heart rates and diving behavior of leatherback sea turtles (Dermochelys coriacea) were monitored at sea during the internesting interval. Instruments that recorded the electrocardiogram and the depth and duration of dives were deployed on six female leatherback turtles as they laid eggs at Playa Grande, Costa Rica. Turtles dived continually for the majority of the internesting interval and spent 57-68 % of the time at sea submerged. Mean dive depth was 19+/-1 m (mean +/- s.d.) and the mean dive duration was 7.4+/-0.6 min. Heart rate declined immediately upon submergence and continued to fall during descent. All turtles showed an increase in heart rate before surfacing. The mean heart rate during dives of 17.4+/-0.9 beats min-1 (mean +/- s.d.) was significantly lower than the mean heart rate at the surface of 24.9+/-1.3 beats min-1 (P<0.05). Instantaneous heart rates as low as 1.05 beats min-1 were recorded during a 34 min dive. The mean heart rate over the entire dive cycle (dive + succeeding surface interval; 19.4+/-1.3 beats min-1) was more similar to the heart rate during diving than to the heart rate at the surface. Although dive and surface heart rates were significantly different from each other, heart rates during diving were 70 % of heart rates at the surface, showing that leatherback turtles do not experience a dramatic bradycardia during routine diving.

The optimal allocation of time and respiratory metabolism over the dive cycle

Because anaerobic metabolism is much less efficient than aerobic metabolism in supplying energy, it is widely believed that divers rely predominantly on aerobic metabolism for diving. In this paper, a time budget model, which assumes that the diver can use either completely aerobic or partially aerobic metabolism with additional anaerobic metabolism for diving, is developed and is used to make predictions about patterns in optimal allocation of time and respiratory metabolism during the dive cycle. The results derived from the model are (1) a diver that can vary the ratio of energy supplied anaerobically to total energy spent during dive time is favored by natural selection, but the patterns of time allocation over the dive cycle by the diver do not differ from those of a diver that cannot vary the ratio. (2) Even if it is assumed that divers switch their metabolism for diving, an obvious upturn in the surface time with respect to dive time does not occur at the aerobic dive limit (ADL) but occurs beyond the ADL. (3) Use of additional anaerobic metabolism can be favored for dives shorter than the ADL. These findings provide a useful guide to understanding the factors that limit diving behavior.

Acoustic censusing of sperm whales at Kaikoura, New Zealand—An inexpensive method to count clicks and whales automatically

Special DSP software was developed to automatically count sperm whale clicks recorded along acoustic transects, and to facilitate the calibration of these counts through automated and interactive analyses of dive cycle and range experiments. Minimum pulse width and maximum interpulse interval are used as criteria for automated click detection. In addition to these criteria, interclick intervals and click amplitudes can be used for the tracing of clicks during interactive analyses. Interactive analysis of a dive of a single whale recorded under almost ideal conditions gave the following results: The mean of all interclick intervals measured within bouts of regular clicks was 0.801 s (n=2203 intervals, cv 27.5%), the reciprocal of which corresponds to c. 1.25 clicks per second. Including 10 s between fluke-up and the first click recorded, silences between bouts of regular clicks, and a silence between the last regular click detected and two surface clicks found 3:37 min after the last regular click, the average click rate over a dive time of 42:23 min is 0.88 per second (n=2244 clicks). In several dive cycles analyzed, interclick intervals were found to oscillate periodically during bouts of regular clicks. These oscillations correlated with click amplitude.

Foraging Activity and Performance of Shags Phalacrocorax aristotelis in Relation to Environmental Characteristics

We combined radiotelemetry techniques with an automatic nest balance system to obtain detailed, concurrent data on foraging activity, prey consumption and foraging performance in four pairs of Shags Phalacrocorax aristotelis during chick rearing on the Isle of May, Scotland. During the study period the location of the main feeding area changed abruptly from c. 10 km away from the colony (long-range trips, LRT) with return flight times from the feeding area to the colony of 7-14 min to c. 0.8 km away (short-range trips (SRT), return flight times <3 min). The two feeding areas differed in water depth (LRT dives shallower) and probably also in the size of prey taken. There were significant differences between trip types in almost every aspect of feeding activity recorded. LRT were characterised by high diving efficiency (the proportion of the dive cycle spent foraging) and low feeding rates (mass of prey caught per second foraging). In contrast, diving efficiency was lower and feeding rates were higher for SRT. The gross intake rate (the product of diving efficiency and feeding rate) was significantly higher in SRT than LRT. We suggest that the observed switch in preferred feeding area was caused by the sudden appearance of large numbers of small lesser sandeels Ammodytes marinus in the waters just off the Isle of May. The higher feeding rates attained by birds on SRT could have been due to higher concentrations of prey and/or reduced foraging costs associated with unsuitable habitat for sandeels in this area. Results from this study suggest that the distribution of available resources influences the choice of foraging areas in Shags

The effect of time and digestion constraints in Common Eiders while feeding and diving over Blue Mussel beds

1. To maintain energy intake Common Eiders (Somateria mollissima) in winter should compensate for reduced day-length by increasing both the proportion of time spent feeding and diving efficiency, defined as the proportional duration of a dive bout within a dive cycle. Common Eiders swallowed Blue Mussel (Mytilus edulis) whole with their shells where any behavioural compensation in relation to short days may be limited by digestive processes. 2. Based on time budget studies conducted from mid-December to the end of April in the Gulf of St. Lawrence, Québec, Canada, diving and feeding efficiency were compared for three seasonal periods varying in day-length (from 557 to 890 min). 3. Results showed that eiders were compensating for short days by feeding 56% of the time in mid-winter compared with 33% in spring. However, diving efficiency remained constant through the season and apparently no compensation occurred at this level of their foraging behaviour. Despite this, the daily rate of prey ingestion was much higher in mid-winter than in spring. 4. Ingestion rate values for mid-winter individuals approached or even exceeded the rate at which prey items are defecated and it was concluded that shell crushing performed by the muscular gizzard is physiologically more demanding during that period. On this basis, gizzard mass should be larger in winter when ingestion rates are higher. Data presented support that hypothesis and suggest that compensation in relation to short days can be both behavioural and physiological.

The Optimal Patch Use in Divers: Optimal Time Budget and the Number of Dive Cycles During Bout

Diving animals feeding on patchily distributed prey repeat dives with short surface intervals. For optimal foraging, they must optimally organize both the time budget and the number of dive cycles at a given patch. The cost of time and energy consumption per dive cycle is strongly affected by the respiratory metabolism used during dive because energetically efficient aerobic diving allows only a brief foraging duration during a dive whereas inefficient anaerobic diving allows a long foraging duration. I develop a model that predicts the effects of prey patch type on the optimal combination of time budget and the number of dive cycles by using the “marginal value theorem”. General features of the results are: (1) using anaerobic metabolism increases the range of prey patch types worth using in the habitat. (2) Anaerobic metabolism is favorable when the prey patch quality is high, the prey patch is situated in deep water, and the prey patch is hard to find, although the last one is not a strong condition. (3) When only aerobic diving is favorable, the time budget of the dive cycle is little affected by patch quality, but when anaerobic diving is favorable, it is strongly affected by patch quality. It is affected by patch depth in both cases. (4) When a dive bout is composed of many dive cycles, the dive duration and surface interval of the dive cycle are short. When long dive duration with long surface interval is optimal, the optimal number of dive cycles during the bout is small.

Heart rate and oxygen consumption of northern elephant seals during diving in the laboratory.

Many techniques have been employed to measure metabolic and cardiovascular changes in diving marine mammals. Each of these methods has its advantages, but the methods also have drawbacks when applied to phocid seals. The aim of this study was to investigate heart rate and metabolic responses to diving in juvenile northern elephant seals that are not associated with forced changes in exercise state, and, secondarily, to investigate whether heart rate could be used as an indicator of metabolic rate in this species. Six seals were allowed to dive freely in a metabolic chamber while simultaneous measurements of heart rate and oxygen consumption were made. Within each dive cycle (dive and surface interval), the seals spent an average of 74% of the time submerged. Mean dive duration was 6.43+/-0.6 (SD) min. Mean oxygen consumption during diving was 3.32+/-0.4 mL O2 min-1 kg-1, a decrease of approximately 26% from baseline values. An inverse relationship was observed between oxygen consumption and the percentage of time spent submerged in each dive cycle. The total amount of oxygen consumed during the surface interval increased with increasing dive duration, while the duration of the surface interval itself did not change, indicating that seals alter the rate of O2 uptake rather than the time spent at the surface. Mean heart rate during diving was 34.5+/-6.2 beats min-1, 36% lower than resting values. Mean diving heart rate was independent of dive duration, percent time submerged, and oxygen consumption. Mean surface interval heart rate was 66.6+/-11.1 beats min-1 and was not correlated with oxygen consumption. Average heart rate over the entire dive cycle increased with increasing oxygen consumption in all of the seals, but there was only a significant relationship in two seals, which casts some doubt on the usefulness of heart rate as an indicator of metabolic rate in this species. While providing important information on the changes in heart rate and oxygen consumption during diving in northern elephant seals, a complete understanding of the diving metabolic rate of these animals will require a combination of approaches that can be used in concert with data on freely living animals.

Dive bout organization in the Chinstrap penguin at seal island, antarctica

Diving behavior of 2 breeding Chinstrap penguins (Pygoscelis antarctica) was studied focusing first and primarily on dive bouts rather than dives themselves. Analysis of dive bout organization revealed (1) though there are differences between solitary dives and dive bouts in dive duration and dive depth, the first dives of dive bouts do not differ from solitary dives in the dive parameters, (2) mean dive duration during bout correlates positively to both mean dive depth during bout and mean surface interval during bout, while number of dives during bout negatively correlates to both cost (consumed energy) and duration of a dive cycle during bout. These findings suggest the following possibilities on foraging behavior of penguins: (1) their decision to repeat diving depends on the result of the first dive at a site, and the first dives of bouts would tend to be searching or evaluating dives though they would be also successful foraging dives, (2) they repeat diving at a foraging patch until foraging efficiency decrease to a threshold of diminishing returns.

The optimal allocation of time over the dive cycle: an approach based on aerobic and anaerobic respiration

Houston & Carbone (1992, Behav. Ecol., 3,233–262) developed models that predict the optimal allocation of time during the dive cycle motivated by the assumption that the diver uses aerobic respiration. In this paper this approach is extended using a model based on maximizing the proportion of time spent foraging to allow for the additional use of anaerobic respiration. Two models are presented. One, the ‘switch’ model, assumes the diver uses only one of the two metabolic pathways during a dive. This model predicts the travel time (or water depth) at which the diver switches from aerobic to anaerobic dives. Another, the ‘mixed metabolism’ model, assumes a simultaneous mix of the two metabolic pathways and predicts a shift in the proportional use of aerobic and anaerobic respiration with increasing water depth. In the mixed metabolism model, the predicted time in the food patch can change from a pattern qualitatively similar to the previous models, first increasing and then decreasing with depth, to a novel pattern with two peaks. The second peak in foraging time appears to be largely fuelled by anaerobic respiration. The corresponding time on the surface changes from a pattern similar to the former models with one region with an abrupt upturn to a pattern with two such regions. These models provide insights into issues currently debated among diving physiologists concerning the degree to which and when divers should rely on anaerobic respiration.

Diving performance of male and female Japanese Cormorants

Sexual differences in the diving behavior of the sexually dimorphic Japanese Cormorant, Phalacrocorax capillatus (males are 26% heavier than females), were studied at Teuri Island, Hokkaido, using time–depth recorders. A typical dive cycle involved a rapid descent phase, a bottom phase where they remained for a while, an ascent phase, and a postdive surface phase. Depth and duration across individual birds were greater for males (15.1 ± 3.7 (mean ± SD) m, 37 ± 5 s, respectively) than those for females (7.2 ± 2.4 m, 24 ± 4 s, respectively). While submerged, females spent a similar proportion of time during the bottom phase to males, hence foraging efficiency (proportion of time at the bottom to total dive cycle time) did not differ between the sexes. No sexual differences were found in descent and ascent rates, dive bout duration, or time spent underwater per day. No significant effects of dive duration on postdive surface time were observed for either sex, indicating that birds dived within an aerobic dive limit. However, mean dive durations and maximum dive durations for individual birds were a function of body mass to the power 1.49 and 1.87, respectively, suggesting that body mass partly constrains the diving behavior of this opportunistically feeding cormorant.

Adjustments in the diving time budgets of tufted duck and pochard: is there evidence for a mix of metabolic pathways?

Predictions of models for the optimal allocation of time over the dive cycle for divers using aerobic and anaerobic respiration, were tested experimentally on the tufted duck, Aythya fuligula, and pochard, A. ferina . Patterns in the dive cycle were highly correlated with water depth. In both species, foraging time increased with water depth up to 3 m. In the pochard, foraging time subsequently declined before increasing again at around 4 m. Qualitatively similar patterns in foraging time are predicted by the model based on the use of aerobic and anaerobic respiration, the ‘mixed metabolism’ model. As predicted by this model and previous ‘aerobic’ models, foraging time decreased in response to increasing substrate depth (used to increase foraging costs), while surface time was not affected. Paddle rates, used as an indicator of foraging costs, however, did not increase. In response to lower water temperature (used to increase the energetic costs of both the travel and foraging phases of the dive), surface times increased as predicted, except at the shallowest depth class. The predicted relationship between foraging time and diving costs is complex, being dependent on water depth and a number of other parameters; the observed trends were non-significant. Ascent duration decreased with decreasing temperature, but descent duration did not change significantly. Estimates of paddle rates and travel speeds suggest that foraging is on average more costly than travelling (ascent and descent combined), diving costs decrease with depth and travel speeds change subtly with depth.

SATELLITE‐MONITORED MOVEMENTS AND DIVE BEHAVIOR OF A BOTTLENOSE DOLPHIN (TURSIOPS TRUNCATUS) IN TAMPA BAY, FLORIDA

AbstractAn adult, female bottlenose dolphin (Tursiops trucncatus) was radio tagged and monitored via satellite‐based Argos receivers for 25 d from 28 June to 23 July 1990, in Tampa Bay, Florida. A total of 794 transmissions were obtained during 106 satellite passes. A mean of 3.9 (SE = 0.24) locations/day were determined by Service Argos and showed the animal remained in the bay, usually close to the southeastern shore. The dolphin moved at least 581 km at a minimum mean speed of 1.2 (SE = 0.1) km/h. Data from 63, 922 dives were recorded. The animal spent an average of 87.1 (SE = 0.6)% of the time submerged, with a mean dive duration of 25.8 (SE = 0.5) sec. Mean dive duration differed significantly between four periods of the day, as did the mean percent of time spent submerged. During the early morning the animal spent more time at the surface, averaged shorter dives, and was submerged less than other times of day. This is the first study to demonstrate die1 dive cycles in a bottlenose dolphin. Four months after tag loss, the dolphin was photographed with no evidence of necrosis or disfigurement of the dorsal fin. Satellite telemetry was demonstrated as an effective means of documenting the movements and dive behavior of a small inshore cetacean.

Swimming speed and allocation of time during the dive cycle in Antarctic fur seals

During sustained bouts of diving, predators such as fur seals may adjust the time they spend diving to maximize the time they can spend foraging and minimize the time spent at the surface between dives. To examine this, swimming speeds and the time allocated to different parts of the dive cycle were measured in 10 adult female Antarctic fur seals, Arctocephalus gazella, while they were foraging at sea. Mean swimming speeds during diving ranged from 1·32 to 1·99 m/s and 90% of diving swimming speeds within individuals were between 1 and 2·5 m/s. This represented a narrower range of speed than was exhibited by animals when swimming at the surface. Swimming speeds were fastest during the descent and ascent phases of dives with a reduction in speed near the bottom of the dive, when the seals were assumed to be feeding on krill. Time spent at the surface increased as a curvilinear function of dive duration and was influenced by diving swimming speed. This relationship was, on average, close to that predicted by a model (Houston & Carbone 1992, Behav. Ecol., 3, 255–265) which suggested that the metabolic rate was greatest during the foraging phase of dives (5·9 times predicted BMR) than during the ascent or descent (3·6 times predicted BMR). Deep diving (>30 m) and high swim speeds also incurred costs in terms of reduced foraging time and may explain why mean dive depths for individuals were within the range of 12–33 m. These data broadly support the predictions of models of diving behaviour based on balancing the supply of, and demand for, oxygen and the principle that fur seals are attempting to maximize the time they spend within the foraging area.

Patterns in the diving behaviour of the pochard, Aythya ferina: a test of an optimality model

Abstract. Foraging divers face a trade-off between obtaining oxygen at the surface and food underwater. Houston & Carbone (1992, Behav. Ecol.3, 255-265) developed models to predict the allocation of time over the dive cycle for aerobic divers that maximize gross and net rates of energy intake under a range of conditions. To test some of these predictions diving experiments on the pochard were conducted. The costs and benefits of foraging were manipulated by varying water depth, food concentration and depth of the substrate covering the food. Each stage of the dive cycle was positively correlated with water depth, and the trends were qualitatively similar to the predictions of the models. Food concentration did not significantly affect foraging time: only small effects were predicted. Foraging time (predicted to decrease independently of recovery time, as energy costs during foraging increase) decreased in relation to increasing substrate depth (used to increase foraging costs), while recovery time was not significantly affected. An additional experiment was conducted to determine whether the birds could assess foraging areas that differ in water depth and food concentration. When offered two options, the birds changed their mean preference from a higher to a lower food concentration as the depth of the higher concentration option increased. However, they switched their preference earlier than expected based on estimates of the net rates of gain obtained from these options. This suggests that factors other than energetics influence the choice of foraging areas.

The diving behaviour of Pigeon Guillemots (Cepphus columba) off southern Vancouver Island

The foraging behaviour of Pigeon Guillemots (Cepphus columba) was observed off southern Vancouver Island, British Columbia. Diving bouts comprised 1–24 dives. Birds returned to the surface with prey in 22 of 248 (9%) dives, and mean handling time for prey was 34.7 s. Dives averaged 87 s (ranging from 37 s in water 14 m deep to 144 s at 34 m) and the mean postdive pause lasted 98 s (range 24–232 s). Birds foraged in water depths from 6 to 45 m. The duration of both dives and pauses increased with water depth. Our model of Pigeon Guillemot diving behaviour predicts foraging time at the bottom to be maximized during dives to depths of 22–24 m, while foraging efficiency, (foraging time)/(dive + recovery time), is maximized at 10 m. Calculated work to resist buoyancy and drag during descent and foraging phases of the dive cycle suggest that energetic savings from reduced buoyancy at depth may not explain how birds increase dive duration with increasing depth. Pigeon Guillemots appear to maximize time spent in the foraging patch. In 82% of transects, the most frequently chosen foraging depth was 15–20 m (mode). Model predictions were supported by observations that 43.6% of Pigeon Guillemots preferred water depths of 15–20 m, while 19% preferred water depths of 10–15 m.

Diving depths, diet, and underwater foraging of Rhinoceros Auklets in British Columbia

Rhinoceros Auklets (Cerorhinca monocerata) were studied at Triangle Island, Cleland Island, and Seabird Rocks, British Columbia, in 1986–1989. Epipelagic schooling fish were consistently the most common prey delivered to auklet chicks at all three localities. Pacific sand lance (Ammodytes hexapterus), Pacific herring (Clupea harengus), juvenile salmon (Onchorhyncus spp.), Pacific saury (Cololabis saura), and juvenile rockfish (Sebastes spp.) were important prey. Bite marks on the fish showed that 73% were attacked from below. The mean deepest depth recorded for 16 auklets was 30 m (range 12–60 m). Eleven time-at-depth records showed that the auklets were epipelagic foragers: 90% of the mean underwater time was spent in the upper 10 m, although most birds had a few deeper dives of 20–60 m. Shipboard transects showed that Rhinoceros Auklets usually foraged in water considerably deeper than 15 m. A model of diving efficiency indicated that a relatively large proportion (&gt; 40%) of the average dive cycle was spent foraging rather than travelling or resting, and prolonged dives invoking anaerobic glycolysis were avoided. We discuss the implications of diving limitations on foraging behaviour and the use of Rhinoceros Auklets as indicators of prey availability.

Responses of humpback whales to sonar sounds

Controlled sound playback experiments were used to assess effects of a low‐frequency sonar system on humpback whales, Megaptera novaeangliae, in Hawaiian waters. Focal pods were presented with sounds of a 3.3‐kHz sonar pulse, a sonar frequency sweep from 3.1 to 3.6 kHz, or a control (blank) tape. Behavior, movement, and underwater vocalizations were monitored and compared with baseline periods. While the two types of sonar signals differed in their effects on the whales, both elicited avoidance behaviors. Humpbacks responded to the pulse by increasing their distance from the sound source. The strength of this effect varied directly with time. Responses to the frequency sweep primarily consisted of increased swimming speeds and track linearity. The latter was a direct function of increasing sound intensity. Overall, the sounds did not strongly or consistently affect the whales’ dive cycles or vocalizations. Observed avoidance reactions may have resulted from possible resemblance between the sonar signals and natural sounds in humpbacks’ environment that are associated with biological threats or warnings.

Diving behaviour of the shag Phalacrocorax aristotelis (Aves: Pelecaniformes) in relation to water depth and prey size

The durations of more than 4000 dives and recovery periods on the surface of 31 shags. Phalacrocorax aristotelis, were recorded using radio‐telemetry, during three breeding seasons. Data were also collected on the depth of water where the birds were diving and the size of lesser sandeels. Ammodytes marinus, fed to young each year. On average, shags foraged in water 30 m deep; mean dive and recovery times were 62·0 ± 1·92 sec and 84·4 ± 5·02 sec. respectively. These times were much longer than those recorded in previous studies and also longer than those predicted from allometric relationships for dive and recovery times derived for cormorants. A large proportion (&gt; 50%)) of dives appeared to involve anaerobic metabolism.Dive and recovery times were examined in relation to water depth, time of day and position within the diving sequence (dive number). In each year. dive time was positively correlated with water depth: in 1987 dive number also had a negative effect. The relationship between recovery time and dive time was positively accelerated, suggesting that birds were diving anaerobically on longer dives. The percentage of the dive cycle spent underwater therefore decreased significantly as dive time and water depth increased. Average vertical rates of ascent and descent of shags were calculated to be between 1·46 1·87 m sec−1. Estimated prey capture rates were significantly higher in 1989 (average 6±8 fish dive−1) when adults were feeding their young on small, low energy value sandeels than the two previous years (averages 1·4 and 1·9 fish dive−1, respectively) when birds were feeding on larger, higher quality fish. We calculated that, on average, birds spent 8 10 sec (36 48%) longer on the bottom at any given depth in 1989, which suggested that the higher prey capture ratcs were associated with an increase in duration of the bottom phase of the dive.