Dive behaviour, foraging locations and maternal-attendance patterns in Australian fur seals (Arctocephalus pusillus doriferus)

The dive behaviour, foraging locations, and colony-attendance patterns of female Australian fur seals (Arctocephalus pusillus doriferus) from Kanowna Island (39°10'S, 146°18'E) in Bass Strait, southeastern Australia, were determined throughout lactation during 1997–1999. Foraging-trip durations increased as lactation progressed, being shortest in summer (3.71 ± 0.24 days; mean ± 1 SE) and longest in winter (6.77 ± 0.57 days, P < 0.05), but maternal-attendance periods did not differ in duration (1.70 ± 0.10 days, P > 0.5). Individual mean attendance periods and trip durations were positively correlated (r² = 0.21, P < 0.005). Diving commenced shortly after seals left the colony (2.6 ± 0.4 h), was continuous for long periods (up to 36 h), occurred mostly during daylight hours, and lacked regular diel variation in depth. The majority of dives (78%) were typically U-shaped and reached depths corresponding to the prevailing depths in Bass Strait (65–85 m), indicating that these animals forage mostly on the benthos of the shallow continental shelf in this region. Such behaviour is unusual for fur seals but is reminiscent of that of some sea lion species. Mean dive durations varied between 2.0 and 3.7 min (maximum 8.9 min) and the theoretical aerobic dive limit (3.91–4.26 min) was exceeded on 17.3% of dives. Dive frequency (8.3 ± 0.6/h) and the proportion of time at sea spent diving (40.7 ± 2.1%) were weakly negatively related to the duration of the foraging trip (r²= 0.07, P < 0.004, and r² = 0.13, P < 0.0001, respectively). Data from at-sea locations showed that lactating females forage almost exclusively within Bass Strait during all seasons.

When surfacers do not dive : multiple significance of extended surface times in marine turtles

Marine turtles spend more than 90% of their life underwater and have been termed surfacers as opposed to divers. Nonetheless turtles have been reported occasionally to float motionless at the surface but the reasons for this behaviour are not clear. We investigated the location, timing and duration of extended surface times (ESTs) in 10 free-ranging loggerhead turtles (Caretta caretta) and the possible relationship to water temperature and diving activity recorded via satellite relay data loggers for 101–450 days. For one turtle that dived only in offshore areas, ESTs contributed 12% of the time whereas for the other turtles ESTs contributed 0.4–1.8% of the time. ESTs lasted on average 90 min but were mostly infrequent and irregular, excluding the involvement of a fundamental regulatory function. However, 82% of the ESTs occurred during daylight, mostly around noon, suggesting a dependence on solar radiation. For three turtles, there was an appreciable (7°C to 10.5°C) temperature decrease with depth for dives during periods when ESTs occurred frequently, suggesting a re-warming function of EST to compensate for decreased body temperatures, possibly to enhance digestive efficiency. A positive correlation between body mass and EST duration supported this explanation. By contrast, night-active turtles that exceeded their calculated aerobic dive limits in 7.6–16% of the dives engaged in nocturnal ESTs, probably for lactate clearance. This is the first evidence that loggerhead turtles may refrain from diving for at least two reasons, either to absorb solar radiation or to recover from anaerobic activity.

Sex-specific dive characteristics in a sexually size dimorphic duck

Dive duration generally increases with body size in animals including wildfowl. Therefore, diving behaviour may vary between the sexes in sexually size dimorphic species, such as the extremely sexually size dimorphic Musk Duck Biziura lobata. However, a previous study reports longer dives in the smaller sex (females) when breeding. In this study, non-breeding male Musk Ducks dived for significantly longer periods than females and tended to have longer inter-dive intervals, conforming to the general patterns described for other species. The differences in dive behaviour we describe may be explained by niche partitioning or differential oxygen requirements or uptake rates by the sexes.

Influence of intrinsic variation on foraging behaviour of adult female Australian fur seals

Phenotypic variation and individual experience can create behavioural and/or dietary variation within a population. This may reduce intra-specific competition, creating a buffer to environmental change. This study examined how intrinsic variation affects foraging behaviour of Australian fur seals. Foraging movements of 29 female Australian fur seals were recorded using FastLoc GPS and dive behaviour recorders. For each individual, body mass, flipper length and axis length were recorded, a tooth was sampled to determine age, and milk was collected for diet analysis. Clustering of fatty acid dietary analysis revealed 5 distinct groups in the population. Behaviour was described using 19 indices, which were then reduced to 7 principal components (>80% of the behavioural variation). Bayesian mixed effect models were developed to describe the relationship between these components and intrinsic variation. No association was found between diet and age or body shape; however, age had a negative relationship with component 1 (27% of variation). Older females spent less time at-sea and foraged nearer to the colony. Age had an effect on component 5 (7% of variation), which represented haul-outs and dive depth; older females made fewer visits to haul-out sites and dived deeper to the benthos. This suggests that as animals age they are able to utilise prior knowledge to exploit nearby foraging sites that younger animals are either unaware of, or have yet to gain the experience required to efficiently utilise. Mass had a negative effect on components representing the directedness of a foraging trip, suggesting heavier individuals were more likely to travel directly to a foraging site.

Dive depth and dive duration data of subadult and adult, male and female southern elephant seals from Marion Island between 2004 and 2008 with links to datasets

Although numerous studies have addressed the migration and dive behaviour of southern elephant seals (Mirounga leonina), questions remain about their habitat use in the marine environment. We report on the vertical use of the water column in the species and the potential lifetime implications for southern elephant seals from Marion Island. Long-term mark-resight data were used to complement vertical habitat use for 35 known individuals tagged with satellite-relay data loggers, resulting in cumulative depth use extrapolated for each individual over its estimated lifespan. Seals spent on average 77.59% of their lives diving at sea, 7.06% at the sea surface, and 15.35% hauled out on land. Some segregation was observed in maximum dive depths and depth use between male and female animals-males evidently being physiologically more capable of exploiting increased depths. Females and males spent 86.98 and 80.89% of their lives at sea, respectively. While at sea, all animals spent more time between 300 and 400 m depth, than any other depth category. Males and females spent comparable percentages of their lifetimes below 100 m depth (males: 65.54%; females: 68.92%), though males spent 8.98% of their lives at depths in excess of 700 m, compared to females' 1.84% at such depths. Adult males often performed benthic dives in excess of 2,000 m, including the deepest known recorded dive of any air-breathing vertebrate (>2,133 m). Our results provide a close approximation of vertical habitat use by southern elephant seals, extrapolated over their lifespans, and we discuss some physiological and developmental implications of their variable depth use.

Dive depth and dive duration data of subadult and adult, male and female southern elephant seals from Marion Island between 2004 and 2008 with links to datasets

Sexual segregation in habitat use occurs in a number of animal species, including southern elephant seals, where differences in migration localities and dive behaviour between sexes have been recorded. Due to the extreme sexual size dimorphism exhibited by southern elephant seals, it is unclear whether observed differences in dive behaviour are due to increased physiological capacity of males, compared to females, or differences in activity budgets and foraging behaviour. Here we use a mixed-effects modelling approach to investigate the effects of sex, size, age and individual variation on a number of dive parameters measured on southern elephant seals from Marion Island. Although individual variation accounted for substantial portions of total model variance for many response variables, differences in maximum and targeted dive depths were always influenced by sex, and only partly by body length. Conversely, dive durations were always influenced by body length, while sex was not identified as a significant influence. These results support hypotheses that physiological capability associated with body size is a limiting factor on dive durations. However, differences in vertical depth use appear to be the result of differences in forage selection between sexes, rather than a by-product of the size dimorphism displayed by this species. This provides further support for resource partitioning and possible avoidance of inter-sexual competition in southern elephant seals.

Dive depth frequency of 23 adult male southern elephant seals (Mirounga leonina) from Marion Island and King George Island

Access to different environments may lead to inter-population behavioural changes within a species that allow populations to exploit their immediate environments. Elephant seals from Marion Island (MI) and King George Island (KGI) (Isla 25 de Mayo) forage in different oceanic environments and evidently employ different foraging strategies. This study elucidates some of the factors influencing the diving behaviour of male southern elephant seals from these populations tracked between 1999 and 2002. Mixed-effects models were used to determine the influence of bathymetry, population of origin, body length (as a proxy for size) and individual variation on the diving behaviour of adult male elephant seals from the two populations. Males from KGI and MI showed differences in all dive parameters. MI males dived deeper and longer (median: 652.0 m and 34.00 min) than KGI males (median: 359.1 m and 25.50 min). KGI males appeared to forage both benthically and pelagically while MI males in this study rarely reached depths close to the seafloor and appeared to forage pelagically. Model outputs indicate that males from the two populations showed substantial differences in their dive depths, even when foraging in areas of similar water depth. Whereas dive depths were not significantly influenced by the size of the animals, size played a significant role in dive durations, though this was also influenced by the population that elephant seals originated from. This study provides some support for inter-population differences in dive behaviour of male southern elephant seals.

Leatherback turtles satellite-tagged in European waters

The North Atlantic is considered a stronghold for the critically endangered leatherback sea turtle. However, limited information exists regarding the movements of individuals to and from the seas off Europe's northwesterly fringe, an area where featherbacks have been historically sighted for the past 200 yr. Here, we used satellite telemetry to record the movements and behaviour of 2 individuals bycaught in fisheries off the southwest coast of Ireland. The turtle T1 (tagged 1 September 2005; female; tracked 375 d) immediately travelled south via Madeira and the Canaries, before residing in West African waters for 3 mo. In spring, T1 migrated north towards Newfoundland where transmissions ceased. T2 (29 June 2006; male; 233 d) travelled south for a short period before spending 66 d west of the Bay of Biscay, an area previously asserted as a high-use area for leatherbacks. This prolonged high latitude summer residence corresponded with a mesoscale feature evident from satellite imagery, with the implication that this turtle had found a rich feeding site. A marked change in dive behaviour was apparent as the turtle exited this feature and provided useful insights on leatherback diving behaviour. T2 headed south in October 2006, and performed the deepest-ever dive recorded by a reptile (1280 m) southwest of Cape Verde. Unlike T1, T2 swam southwest towards Brazil before approaching the major nesting beaches of French Guiana and Surinam. Importantly, these tracks document the movement of leatherbacks from one of the remotest foraging grounds in the North Atlantic. © Inter-Research 2008.

Leatherback turtles satellite-tagged in European waters

The North Atlantic is considered a stronghold for the critically endangered leatherback sea turtle. However, limited information exists regarding the movements of individuals to and from the seas off Europe’s northwesterly fringe, an area where leatherbacks have been historically sighted for the past 200 yr. Here, we used satellite telemetry to record the movements and behaviour of 2 individuals bycaught in fisheries off the southwest coast of Ireland. The turtle T1 (tagged 1 September 2005; female; tracked 375 d) immediately travelled south via Madeira and the Canaries, before residing in West African waters for 3 mo. In spring, T1 migrated north towards Newfoundland where transmissions ceased. T2 (29 June 2006; male; 233 d) travelled south for a short period before spending 66 d west of the Bay of Biscay, an area previously asserted as a high-use area for leatherbacks. This prolonged high latitude summer residence corresponded with a mesoscale feature evident from satellite imagery, with the implication that this turtle had found a rich feeding site. A marked change in dive behaviour was apparent as the turtle exited this feature and provided useful insights on leatherback diving behaviour. T2 headed south in October 2006, and performed the deepest-ever dive recorded by a reptile (1280 m) southwest of Cape Verde. Unlike T1, T2 swam southwest towards Brazil before approaching the major nesting beaches of French Guiana and Surinam. Importantly, these tracks document the movement of leatherbacks from one of the remotest foraging grounds in the North Atlantic.

Utilisation of intensive foraging zones by female Australian fur seals

Within a heterogeneous environment, animals must efficiently locate and utilise foraging patches. One way animals can achieve this is by increasing residency times in areas where foraging success is highest (area-restricted search). For air-breathing diving predators, increased patch residency times can be achieved by altering both surface movements and diving patterns. The current study aimed to spatially identify the areas where female Australian fur seals allocated the most foraging effort, while simultaneously determining the behavioural changes that occur when they increase their foraging intensity. To achieve this, foraging behaviour was successfully recorded with a FastLoc GPS logger and dive behaviour recorder from 29 individual females provisioning pups. Females travelled an average of 118 ± 50 km from their colony during foraging trips that lasted 7.3 ± 3.4 days. Comparison of two methods for calculating foraging intensity (first-passage time and first-passage time modified to include diving behaviour) determined that, due to extended surface intervals where individuals did not travel, inclusion of diving behaviour into foraging analyses was important for this species. Foraging intensity 'hot spots' were found to exist in a mosaic of patches within the Bass Basin, primarily to the south-west of the colony. However, the composition of benthic habitat being targeted remains unclear. When increasing their foraging intensity, individuals tended to perform dives around 148 s or greater, with descent/ascent rates of approximately 1.9 m•s-1 or greater and reduced postdive durations. This suggests individuals were maximising their time within the benthic foraging zone. Furthermore, individuals increased tortuosity and decreased travel speeds while at the surface to maximise their time within a foraging location. These results suggest Australian fur seals will modify both surface movements and diving behaviour to maximise their time within a foraging patch.