Michael A. Fedak

Foraging ecology of southern elephant seals in relation to the bathymetry and productivity of the Southern Ocean

Southern elephant seals ( Mirounga leonina ) are among the most proficient of mammalian divers and are a major component of the Antarctic food web. Yet little is known of their movements or interaction with their oceanic environment. Specially designed satellite-link data loggers allowed us to visualize the 3-D movements of elephant seals as they swam rapidly from South Georgia to distant (up to 2650 km) areas of Antarctic continental shelf. One seal dived continuously to the sea bed in one small area for a month, implying consumption of benthic prey. Dives here were shorter even though average swimming velocity was lower. It is suggested that the physiological requirements of feeding and digestion reduced the aerobic dive limit. Long distance travel to relocatable hydrographic or topographical features, such as shelf breaks, may allow large predators to locate prey more consistently than from mid-ocean searches.

Variations in behavior and condition of a Southern Ocean top predator in relation to in situ oceanographic conditions

Responses by marine top predators to environmental variability have previously been almost impossible to observe directly. By using animal-mounted instruments simultaneously recording movements, diving behavior, and in situ oceanographic properties, we studied the behavioral and physiological responses of southern elephant seals to spatial environmental variability throughout their circumpolar range. Improved body condition of seals in the Atlantic sector was associated with Circumpolar Deep Water upwelling regions within the Antarctic Circumpolar Current, whereas High-Salinity Shelf Waters or temperature/salinity gradients under winter pack ice were important in the Indian and Pacific sectors. Energetic consequences of these variations could help explain recently observed population trends, showing the usefulness of this approach in examining the sensitivity of top predators to global and regional-scale climate variability.

How long should a dive last? A simple model of foraging decisions by breath-hold divers in a patchy environment

Abstract Although diving birds and mammals can withstand extended periods under water, field studies show that most perform mainly short, aerobic dives. Theoretical studies of diving have implicitly assumed that prey acquisition increases linearly with time spent searching and have examined strategies that maximize time spent foraging. We present a simple model of diving in seals, where dive durations are influenced by the seals assessment of patch quality, but are ultimately constrained by oxygen balance. Prey encounters within a dive are assumed to be Poisson distributed and the scale of the patches is such that a predator will encounter a constant prey density during a dive. We investigated the effects of a simple giving-up rule, using recent prey encounter rate to assess patch quality. The model predicts that, for shallow dives, there should always be a net benefit from terminating dives early if no prey are encountered early in the dive. The magnitude of the benefit was highest at low patch densities. The relative gain depended on the magnitude of the travel time and the time taken to assess patch quality and the effect was reduced in deeper dives. As the time taken to decide decreased, the relative benefit increased, up to a maximum, but fell as decision time was reduced further. Frequency distributions of different aspects of dive durations from three deep-diving and two shallow-diving seal species are presented and compared with the models predictions.

SEASONAL AND SEX-RELATED VARIATION IN THE ACTIVITY PATTERNS OF COMMON SEALS (PHOCA VITULINA)

SUMMARY (1) Radio-telemetry was used to study common seal activity patterns around Orkney, and to assess the degree of individual, seasonal and sex-related variation in haul-out behaviour in 1984-86. (2) During the summer, both males and females hauled-out regularly, in a marked diurnal pattern, but there were differences in other aspects of their haul-out behaviour. (3) Males showed a marked change in their haul-out behaviour at the beginning of moult. At this time, they hauled-out every day and, consequently, male behaviour became more synchronous. The change in behaviour of the male followed in 1985 was closely related to changes in abundance at the haul-out site that he used over this period. (4) In contrast, females showed no sign of a similar change in behaviour at the beginning of moult, but appeared to spend more of their time at sea after lactation. (5) During the winter, seals appeared to spend more time in offshore waters, although they regularly returned to the inshore study area to haul-out. At this time of year there was no marked diurnal haul-out pattern. (6) There was a high degree of individual variation in the effect of the tidal cycle on activity patterns, which appeared to be related to tide-related changes in food and habitat availability. Overall, however, the tidal cycle had less effect on haul-out behaviour in summer, when strong circadian patterns were more important. (7) The variations in activity patterns found in this study are discussed in relation to seasonal changes in feeding activity and the demands of breeding and moult.

Overcoming the Constraints of Long Range Radio Telemetry from Animals: Getting More Useful Data from Smaller Packages

Abstract Many species carry out their most interesting activities where they cannot readily be observed or monitored. Marine mammals are extreme among this group, accomplishing their most astounding activities both distant from land and deep in the sea. Collection, storage and transmission of data about these activities are constrained by the energy requirements and size of the recording loggers and transmitters. The more bits of information collected, stored and transmitted, the more battery is required and the larger the tag must be. We therefore need to be selective about the information we collect, while maintaining detail and fidelity. To accomplish this in the study of marine mammals, we have designed “intelligent” data logger/transmitters that provide context-driven data compression, data relay, and automated data base storage. We later combine these data with remotely sensed environmental information and other oceanographic data sets to recreate the environmental context for the animals activity, and we display the combined data using computer animation techniques. In this way, the system can provide near real time “observation” of animal behavior and physiology from the remotest parts of the globe.

Southern Ocean frontal structure and sea-ice formation rates revealed by elephant seals

Polar regions are particularly sensitive to climate change, with the potential for significant feedbacks between ocean circulation, sea ice, and the ocean carbon cycle. However, the difficulty in obtaining in situ data means that our ability to detect and interpret change is very limited, especially in the Southern Ocean, where the ocean beneath the sea ice remains almost entirely unobserved and the rate of sea-ice formation is poorly known. Here, we show that southern elephant seals (Mirounga leonina) equipped with oceanographic sensors can measure ocean structure and water mass changes in regions and seasons rarely observed with traditional oceanographic platforms. In particular, seals provided a 30-fold increase in hydrographic profiles from the sea-ice zone, allowing the major fronts to be mapped south of 60°S and sea-ice formation rates to be inferred from changes in upper ocean salinity. Sea-ice production rates peaked in early winter (April–May) during the rapid northward expansion of the pack ice and declined by a factor of 2 to 3 between May and August, in agreement with a three-dimensional coupled ocean–sea-ice model. By measuring the high-latitude ocean during winter, elephant seals fill a “blind spot” in our sampling coverage, enabling the establishment of a truly global ocean-observing system.

Grey seal males: energetic and behavioural links between size and sexual success

Abstract The costs of breeding in male grey seals (Halichoerus grypus) have been estimated by repeated weighings of animals on the breeding grounds. Individual variations in male size and rate of weight loss were positively correlated with measures of sexual behaviour. Grey seal males did not form a linear dominance hierarchy, nor were they of equal status, but large males lost few encounters. Finite resources set a limit on the time that can be spent ashore being sexually active. These limits can be predicted using the collected data on size and energy use, and the results emphasize the significance of large size in relation to breeding success in grey seal males. The largest males may on average sire 10 times as many offspring as the smallest breeding males.

FACTORS AFFECTING MATERNAL EXPENDITURE IN SOUTHERN ELEPHANT SEALS DURING LACTATION

Southern elephant seals Mirounga leonina provide a unique opportunity for examination of parental investment because postpartum pup growth is fueled exclusively by energy from stored reserves in fasting mothers, and the seals are extremely sexually dimorphic as adults. We examined the influence of pup sex, maternal size, and other factors on the variation in postpartum maternal mass change and pup growth. Elephant seals (178 mothers and 445 pups) were weighed during four breeding periods at South Georgia Island. Maternal mass change during lactation increased markedly with the mass of the mother at parturition. Postpartum maternal mass accounted for 75% of the variation in mass loss and 62% of the variation of pup mass at weaning. Size of the pup at birth explained <4% of this variation, and the sex of the pup explained virtually none (<0.1%). The duration of lactation was positively correlated with the pstpartumo mass of mothers, but negatively correlated with the rate of maternal mass loss when corrected for the effect of maternal postpartum mass. Mothers giving birth late in the season had shorter lactation periods than those that gave birth early but seemed to compensate for this by increasing the rate of mass transfer. Average transfer efficiency (pup mass gain/maternal mass loss) was 46±0.5%. Mothers lost, on average, 35% of their postpartum mass during lactation and 40% during the whole breeding period. Females whose postpartum mass increased between seasons increased their expenditure on their pups; females whose postpartum mass decreased, decreased their expenditure. These data from mothers with single pups do not clarify whether differences in investment were controlled by mothers or their offspring. However on three occasions, study females raised two pups in a season. Despite the increased demand, these females did not increase their expenditure, suggesting that levels of investment are maternally controlled. These results show that levels of expenditure in southern elephant seals appear to be determined largely by a single variable: female mass at parturition.

Salinity and temperature structure of a freezing Arctic fjord: monitored by white whales (Delphinapterus leucas)

Received 10 May 2002; accepted 29 August 2002; published 11 December 2002. [1] In this study we report results from satellite-linked conductivity-temperature-depth (CTD) loggers that were deployed on wild, free-ranging white whales to study the oceanographic structure of an Arctic fjord, Storfjorden, Svalbard. The whales dove to the bottom of the fjord routinely during the study and occupied areas with up to 90% ice-cover, where performance of conventional ship-based CTD-castswouldhavebeendifficult.Duringtheinitialperiod of freezing in the fjord, over a period of approximately 2 weeks, 540 CTD profiles were successfully transmitted. The dataindicatethatStorfjordenhasasubstantialinflowofwarm NorthAtlanticWater;thisiscontrarytoconventionalwisdom thathassuggestedthatitcontainsonlycoldArcticwater.This study confirms that marine-mammal-based CTDs have enormous potential for cost-effective, future oceanographic studies; many different marine mammal species target oceanographic discontinuities for foraging and thus may be good ‘adaptive samplers’ that naturally seek areas of high oceanographic interest. INDEX TERMS: 4294 Oceanography: General:Instrumentsandtechniques;4536Oceanography:Physical: Hydrography; 4219 Oceanography: General: Continental shelf processes; 1635 Global Change: Oceans (4203); KEYWORDS: CTD-measurements, Arctic oceanography, marine mammals, satellite telemetry. Citation: Lydersen, C., O. A. Nost, P. Lovell, B. J. McConnell, T. Gammelsrod, C. Hunter, M. A. Fedak, and K. M. Kovacs, Salinity and temperature structure of a freezing Arctic fjord—monitored by white whales (Delphinapterus leucas), Geophys. Res. Lett. , 29(23), 2119, doi:10.1029/2002GL015462, 2002.

Using GPS data to evaluate the accuracy of state–space methods for correction of Argos satellite telemetry error

Recent studies have applied state-space models to satellite telemetry data in order to remove noise from raw location estimates and infer the true tracks of animals. However, while the resulting tracks may appear plausible, it is difficult to determine the accuracy of the estimated positions, especially for position estimates interpolated to times between satellite locations. In this study, we use data from two gray seals (Halichoerus grypus) carrying tags that transmitted Fastloc GPS positions via Argos satellites. This combination of Service Argos data and highly accurate GPS data allowed examination of the accuracy of state-space position estimates and their uncertainty derived from satellite telemetry data. After applying a speed filter to remove aberrant satellite telemetry locations, we fit a continuous-time Kalman filter to estimate the parameters of a random walk, used Kalman smoothing to infer positions at the times of the GPS measurements, and then compared the filtered telemetry estimates with the actual GPS measurements. We investigated the effect of varying maximum speed thresholds in the speed-filtering algorithm on the root mean-square error (RMSE) estimates and used minimum RMSE as a criterion to guide the final choice of speed threshold. The optimal speed thresholds differed between the two animals (1.1 m/s and 2.5 m/s) and retained 50% and 65% of the data for each seal. However, using a speed filter of 1.1 m/s resulted in very similar RMSE for both animals. For the two seals, the RMSE of the Kalman-filtered estimates of location were 5.9 and 12.76 km, respectively, and 75% of the modeled positions had errors less than 6.25 km and 11.7 km for each seal. Confidence interval coverage was close to correct at typical levels (80-95%), although it tended to be overly generous at smaller sizes. The reliability of uncertainty estimates was also affected by the chosen speed threshold. The combination of speed and Kalman filtering allows for effective calculation of location and also indicates the limits of accuracy when correcting service Argos locations and linking satellite telemetry data to spatial covariate and habitat data.