Crabeater Seals Lobodon Carcinophaga Research Articles

In-air acoustic repertoire and associated behaviour of wild juvenile crabeater seals during rehabilitation

ABSTRACT In-air sounds of pinnipeds are important for interactions with conspecifics and threat displays. However, little is known about the in-air acoustic repertoire and associated behaviour of crabeater seals Lobodon carcinophaga. We investigated the in-air acoustic repertoire and associated behaviour of two male and one female wild juvenile crabeater seals that beached separately, and were rehabilitated in Durban and Gqeberha, South Africa. In-air sounds were visually identified and categorised into five classes validated via random forest model classification: brief, intermediate, and long moan calls, croaks, and hisses. Hisses were common (n = 25,105 sounds from 136 hrs of acoustic recordings) and detected during heightened arousal states and interactions with animal care staff and a conspecific. Furthermore, hisses were also recorded in low arousal states during exhalation. Moan calls (n = 241) were only detected from two of the three seals. During rehabilitation, the female seal ceased producing moan calls and later produced a series of croaks (n = 204). Acoustic characteristics of in-air moan calls differed from published underwater moan calls according to duration, minimum, and maximum frequencies, and pulse repetition rate. Our study shows that the in-air acoustic repertoire and associated behaviour of these Antarctic seals are dynamic, vary inter-individually and are context dependent.

Habitat-based density models of pack-ice seal distribution in the southern Weddell Sea, Antarctica

Climate variability and changes in sea ice dynamics have caused several ice-obligate or krill-dependent populations of marine predators to decline, eliciting concern about their demographic persistence and the indirect ecological consequences that predator depletions may have on marine ecosystems. Pack-ice seals are dominant ice-obligate predators in the Antarctic marine ecosystem, but there is considerable uncertainty about their abundance and population trends. We modelled the density and distribution of pack-ice seals as a function of environmental covariates in the southern Weddell Sea, Antarctica. Our density surface modelling approach used data from aerial surveys of pack-ice seals collected in the 2013/14 austral summer. Crabeater seals Lobodon carcinophaga, the most numerous pack-ice seal we observed, occurred at the highest densities in areas with extensive sea ice near the continental shelf break, but were almost absent in areas of similar sea ice concentration in the southern extent of the Weddell Sea. The highest densities of Weddell seals Leptonychotes weddelli, which were less abundant than crabeater seals within the pack-ice habitat, were predicted to occur over the continental shelf, near the shelf break. The distribution of both seal species broadly corresponded with the distribution and relative abundance of their main prey (Antarctic krill Euphausia superba and Antarctic silverfish Pleuragramma antarctica) obtained from concurrent ecosystem surveys. Ross seals Ommatophoca rossii and leopard seals Hydrurga leptonyx were not detected at all and are apparently rare within the southern Weddell Sea. These results can contribute to biodiversity assessments in the context of marine protected area planning in this region of the Southern Ocean.

Distribution, density and abundance of Antarctic ice seals off Queen Maud Land and the eastern Weddell Sea

The Antarctic Pack Ice Seal (APIS) Program was initiated in 1994 to estimate the abundance of four species of Antarctic phocids: the crabeater seal Lobodon carcinophaga, Weddell seal Leptonychotes weddellii, Ross seal Ommatophoca rossii and leopard seal Hydrurga leptonyx and to identify ecological relationships and habitat use patterns. The Atlantic sector of the Southern Ocean (the eastern sector of the Weddell Sea) was surveyed by research teams from Germany, Norway and South Africa using a range of aerial methods over five austral summers between 1996–1997 and 2000–2001. We used these observations to model densities of seals in the area, taking into account haul-out probabilities, survey-specific sighting probabilities and covariates derived from satellite-based ice concentrations and bathymetry. These models predicted the total abundance over the area bounded by the surveys (30°W and 10°E). In this sector of the coast, we estimated seal abundances of: 514 (95 % CI 337–886) × 103 crabeater seals, 60.0 (43.2–94.4) × 103 Weddell seals and 13.2 (5.50–39.7) × 103 leopard seals. The crabeater seal densities, approximately 14,000 seals per degree longitude, are similar to estimates obtained by surveys in the Pacific and Indian sectors by other APIS researchers. Very few Ross seals were observed (24 total), leading to a conservative estimate of 830 (119–2894) individuals over the study area. These results provide an important baseline against which to compare future changes in seal distribution and abundance.

Summer social structure of crabeater seal Lobodon carcinophaga in the Amundsen Sea, Antarctica

During the PS75 expedition of ice-breaking RV Polarstern in the Amundsen Sea, in February–March 2010, we studied the at-sea quantitative distribution of the “upper trophic levels,” seabirds and marine mammals. In the Amundsen Sea Embayment, 14,200 pinnipeds belonging to four species were recorded (2100 during 670 half-hour transect counts from the bridge and 12,100 during 50 h of helicopter flight). Crabeater seal Lobodon carcinophaga represented more than 97 % of the total. Two types of major aggregations of crabeaters were noted: on the one hand, very large groups hauled out on the pack ice, composed mainly of adults/bachelors and very few calves. On the other hand, swimming pods of calves were accompanied by one or two leading adults around icebergs. Our interpretation is that, after the beginning of the reproduction period with trios (cow, calf and future father) scattered on pack ice, such pods can be considered the last step of the breeding cycle.

Aerial surveys for pack-ice seals along the Ingrid Christensen and Princess Astrid Coasts, East Antarctica

We conducted aerial surveys in the austral summer of 2009-2010 to count and record the spatial distribution patterns of pack-ice seals hauled-out along the Ingrid Christensen and Princess Astrid coast of East Antarctica. A total of 3,601 hauled-out seals were counted from six aerial surveys totalling a length of approx. 1,200km, with each survey lasting about two hours. Weddell Seal Leptonychotes weddellii was the most commonly sighted species in both the areas surveyed (98.2%), and had an encounter rate of 2.9 seals/km. The other species encountered during the survey were Crabeater Seal Lobodon carcinophaga (1.7%) and Leopard Seal Hydrurga leptonyx (0.03%). Group size of hauled-out Weddell Seals varied considerably and ranged from solitary to maximum of 42 individuals. The median group size of Weddell seals hauled-out along the Ingrid Christenson Coast was found to be significantly different between the December 2009 and January 2010 survey. Further, along this coast Weddell Seals were found hauled-out mainly close to the ice shelf and their spatial distribution appeared to be influenced by the extent of sea ice in the area.

Diet of a specialist in a changing environment: the crabeater seal along the western Antarctic Peninsula

Although crabeater seals Lobodon carcinophaga are among the most abundant con- sumers of Antarctic krill Euphausia superba, their diet has rarely been studied throughout most of the species' range. Using δ 13 C and δ 15 N values in vibrissae from 53 seals, we examined the trophic ecology of crabeater seals from the western Antarctic Peninsula (wAP) in 2001, 2002 and 2007. We observed a wide variability in individual seal mean δ 13 C values, which ranged from −19.8‰ to −24.9‰ , whereas mean δ 15 N value varied from 5.4‰ to 7.9‰. We identified a positive significant effect of seal mass on δ 13 C values, as well as a significant seasonal effect (higher δ 13 C values in austral winter), which likely resulted from changes in the composition of the community of pri- mary producers. δ 15 N values for crabeater seals, on the other hand, were affected by year, with individuals in 2002 having higher δ 15 N values. The median (with range) contribution of Antarctic krill to the diet of crabeater seals, as estimated using the Bayesian mixing model MixSIR, was 87.9% (81.2 to 94.8%). During 2002, krill biomass in the wAP was at one of its lowest levels during the last 2 decades, coinciding with a slight reduction in the importance of krill for the diet of the seals that year, which reached 84.5% (75.1 to 92.4%). Despite the relative plasticity observed in the diet of crabeater seals, it is unknown to what extent, and at what rate, crabeater seals might be able to switch to a more generalized diet, which might impact their fitness, given the ongoing environmental change along the wAP.

Acoustic ecology of Antarctic pinnipeds

In aquatic-mating pinnipeds, acoustic communication plays an important role in male competition and mate attraction. Vocal repertoire size and composition during the breeding season varies between species and is presumed to be a product of interspecific differences in sexual selec- tion. In this study, we examine seasonal and diel patterns in acoustic repertoire size, composition and call activity of 4 Antarctic pinniped species: Weddell seal Leptonychotes weddellii, leopard seal Hydrurga leptonyx, Ross seal Ommatophoca rossii and crabeater seal Lobodon carcinophaga. An 11 mo (Jan 2006 - Jan 2007, no recordings Jul and Nov 2006) near-continuous dataset was collected from the Perennial Acoustic Observatory in the Antarctic Ocean (PALAOA) located on the Ekstrom Iceshelf. The Weddell seal vocal repertoire consisted of 14 call types. Calls were present throughout the year except in February (11 mo), while repertoire composition varied considerably between months. The leopard seal vocal repertoire consisted of 7 call types. Calls were present between Octo- ber and January (4 mo). All call types were used in a uniform manner throughout the entire call period. The Ross seal vocal repertoire consisted of 5 call types. Ross seal vocalizations were present from December until February (3 mo). Repertoire composition varied little between months. Crab- eater seals produced one vocalization type, present from August to December (5 mo). Vocalizations in these species are likely produced in a breeding context. Inter-specific differences in behavioral ecology and interactions with abiotic and biotic environmental factors shape vocal behavior resulting in each species filling its own acoustic ecological niche.

Density of pack-ice seals and penguins in the western Weddell Sea in relation to ice thickness and ocean depth

Aerial band transect censuses were carried out parallel with ice thickness profiling surveys in the pack ice of the western Weddell Sea during the ISPOL (Ice Station POLarstern) expedition of R.V. Polarstern from November 2004 to January 2005. Three regions were surveyed: the deep sea of the Weddell Sea, a western continental shelf/slope region where R.V. Polarstern passively drifted with an ice floe (ISPOL), and a northern region (N). Animal densities were compared among regions and in relation to bathymetry and ice thickness distribution. Crabeater seals Lobodon carcinophaga were the most abundant species in all three regions. Their density was significantly lower in the deep sea (0.50 km � 2 ) than in the ISPOL (1.00 km � 2 ) and northern regions (1.21 km � 2 ). Weddell seals Leptonychotes weddellii were not sighted in the deep-sea region, their density elsewhere ranging from 0.03 (N) to 0.08 km � 2 (ISPOL). Leopard seals Hydrurga leptonyx were observed in all three areas, but could only be quantified in the deep-sea (0.05 km � 2 ) and northern regions (0.06 km � 2 ). The abundance of emperor penguins Aptenodytes forsteri was markedly higher in the northern (0.75 km � 2 ) than in the ISPOL (0.13 km � 2 )

Taking account of dependent species in management of the Southern Ocean krill fishery: estimating crabeater seal abundance off east Antarctica

SummaryThe crabeater sealLobodon carcinophagais considered to be a key species in the krill‐based food‐web of the Southern Ocean. Reliable estimates of the abundance of this species are necessary to allow the development of multispecies, predator–prey models as a basis for management of the krill fishery in the Southern Ocean.A survey of crabeater seal abundance was undertaken in 1500 000 km2of pack‐ice off east Antarctica between longitudes 64–150° E during the austral summer of 1999/2000. Sighting surveys, using double observer line transect methods, were conducted from an icebreaker and two helicopters to estimate the density of seals hauled out on the ice in survey strips. Satellite‐linked dive recorders were deployed on a sample of seals to estimate the probability of seals being hauled out on the ice at the times of day when sighting surveys were conducted. Model‐based inference, involving fitting a density surface, was used to infer densities in the entire survey region from estimates in the surveyed areas.Crabeater seal abundance was estimated to be between 0·7 and 1·4 million animals (with 95% confidence), with the most likely estimate slightly less than 1 million.Synthesis and applications.The estimation of crabeater seal abundance in Convention for the Conservation of Antarctic Marine Living Resources (CCAMLR) management areas off east Antarctic where krill biomass has also been estimated recently provides the data necessary to begin extending from single‐species to multispecies management of the krill fishery. Incorporation of all major sources of uncertainty allows a precautionary interpretation of crabeater abundance and demand for krill in keeping with CCAMLR's precautionary approach to management. While this study focuses on the crabeater seal and management of living resources in the Southern Ocean, it has also led to technical and theoretical developments in survey methodology that have widespread potential application in ecological and resource management studies, and will contribute to a more fundamental understanding of the structure and function of the Southern Ocean ecosystem.

Predicting the distribution of crabeater seals Lobodon carcinophaga off east Antarctica during the breeding season

Predictive species-environment models have numerous ecological and management applications, and can provide insight into the functional relationships between a species and its envi- ronment. Generalised linear models were used to develop predictive models of crabeater seal Lobo- don carcinophaga distribution during the breeding season in relation to physical-oceanographic and ice-cover variables. Models were constructed from data collected on shipboard surveys in the pack- ice off east Antarctica between 50 and 115° E from 1985 to 1987, and were tested from surveys in the same region from 1995 to 1997. Models of presence selected ocean depth as the only significant phys- ical-oceanographic predictor variable for both breeding and non-breeding seals. Seals were most likely to be present on the ice at moderate ocean depths of approximately 2500 m. Unlike non-breed- ing seals, breeding seals were all but absent on the ice associated with depths of >4000 m. The pre- dicted distribution of presence comprised a band delineated by the shelf-break to the south and extending northwards for 1.5 to 5° latitude, with non-breeding seals having a more northerly limit than breeding seals. This band is coincident with the known distribution of the crabeater seals' pri- mary food source (Antarctic krill), and also coincides with frontal features such as the Antarctic slope front and the southern boundary of the Antarctic Circumpolar Current, which are considered to be areas of enhanced primary and secondary productivity. The presence-depth models were improved by the inclusion of ice-cover variables. Breeding seals may avoid areas with a high proportion of all ice as floes <20 m in size, because the chance of successfully weaning a pup in such areas is low.