Diet Of Antarctic Fur Seals Research Articles

Wintertime overlaps between female Antarctic fur seals (Arctocephalus gazella) and the krill fishery at South Georgia, South Atlantic.

The diet of Antarctic fur seals (Arctocephalus gazella) at South Georgia is dominated by Antarctic krill (Euphausia superba). During the breeding season, foraging trips by lactating female fur seals are constrained by their need to return to land to provision their pups. Post-breeding, seals disperse in order to feed and recover condition; estimates indicate c.70% of females remain near to South Georgia, whilst others head west towards the Patagonian Shelf or south to the ice-edge. The krill fishery at South Georgia operates only during the winter, providing the potential for fur seal: fishery interaction during these months. Here we use available winter (May to September) tracking data from Platform Terminal Transmitter (PTT) tags deployed on female fur seals at Bird Island, South Georgia. We develop habitat models describing their distribution during the winters of 1999 and 2003 with the aim of visualising and quantifying the degree of spatial overlap between female fur seals and krill harvesting in South Georgia waters. We show that spatial distribution of fur seals around South Georgia is extensive, and that the krill fishery overlaps with small, highly localised areas of available fur seal habitat. From these findings we discuss the implications for management, and future work.

Squid in the diet of Antarctic fur seals: potential links to oceanographic conditions and Antarctic krill abundance

Squid in the diet of Antarctic fur seals: potential links to oceanographic conditions and Antarctic krill abundance

Seasonal variation in the diet of Antarctic fur seals, Arctocephalus gazella, at 25 de Mayo/King George Island, South Shetland Islands, Antarctica

Assessing the trophic ecology of fur seals is essential for defining their roles as a top predator in Antarctic marine ecosystems. To this end, we examined the diet of Antarctic fur seals, Arctocephalus gazella, in the South Shetlands. While previous studies have considered the diet of this species at this location in summer, to the best of our knowledge, this is the first study to report on the diet of this population in winter, thus adding essential information on the extent of seasonal variation. A total of 224 scats were collected in the winter 2004 (n = 118) and summer 2004/2005 (n = 106) on the coasts of Stranger Point, Isla 25 de Mayo/King George. For the total study period krill (Euphausia superba) was the main prey taxon, followed by fish, cephalopods and penguins. During winter Pleuragramma antarcticum was dominant, while myctophids were almost absent. Conversely, in summer Electrona antarctica and Gymnoscopelus nicholsi, followed by P. antarcticum, dominated the fish diet. It is concluded that fur seals centred their foraging activity on a krill community and fish associated with krill aggregations.

Intraspecific differences in the diet of Antarctic fur seals at Nyrøysa, Bouvetøya

Intraspecific differences in the diets of many species of pinnipeds are to be expected in view of the great differences in morphology, life history and foraging behaviour between the sexes of many species. We examined the diet of the Antarctic fur seal population at Bouvetoya, Southern Ocean, to assess intersexual differences. This was made possible by the analysis of prey remains extracted from scats and regurgitations collected in areas used primarily by one or the other sex. The results indicate that both males and females feed primarily on Antarctic krill Euphausia superba with several species of fish and squid being taken, likely opportunistically given their prevalence. Significant differences were identified in the frequency of occurrence of otoliths in scats and the percentage numerical abundance of the major fish prey species in the diet. Adult males ate a smaller quantity of fish overall, but ate significantly more of the larger fish species. The greater diving capabilities of males and the fact that they are not limited in the extent of their foraging area by having to return regularly to feed dependant offspring may play a role in the differences found between the diets of males and females. Additionally, females might be more selective, favouring myctophids because they are richer in energy than krill. The absence of major differences in the diet between the sexes at this location is likely due to the high overall abundance of prey at Bouvetoya.

Spatial and temporal variability in the fish diet of Antarctic fur seal (Arctocephalus gazella) in the Atlantic sector of the Southern Ocean

The spatial and temporal variability in the fish component of the diet of Antarctic fur seals (Arctocephalus gazella (Peters, 1875)) in the Atlantic sector of the Southern Ocean was examined using diet data from 10 sites in the region including a 13-year time series from South Georgia. The fish species composition in the diet at each site showed a strong relationship with the local marine habitat / topography. The absence of formerly harvested fish species indicates a lack of recovery of stocks of Notothenia rossii Richardson, 1844 at South Georgia and Champsocephalus gunnari Lönnberg, 1905 at the South Orkney Islands. At South Georgia, Protomyctophum choriodon Hulley, 1981, Lepidonotothen larseni (Lönnberg, 1905), and C. gunnari were the most important species in the diet between 1991 and 2004. Variability in the occurrence of C. gunnari was driven mainly by annual scale processes, particularly those that influence the availability of Antarctic krill (Euphausia superba (Dana, 1852)). The occurrence of the pelagic P. choriodon was primarily influenced by shorter-term water mass changes within the foraging range of the seals. The fish composition in the diet reflects differences in marine habitat / topography, as well as variability, at a range of time scales that reflect environmental variability and harvesting.

The diet of Antarctic fur seals, Arctocephalus gazella, at King George Island, during the summer?autumn period

The diet of non-breeding male Antarctic fur seals, Arctocephalus gazella, was investigated at Stranger Point, King George Island, by scat analysis from February to April 1996. Overall, krill and fish were the most frequent prey, occurring in an average of 97% and 69% of samples (n=128), followed by cephalopods (12%). Myctophids constituted almost 90% of the fish predated, with Electrona antarctica and Gymnoscopelus nicholsi being the most abundant and frequent species consumed. All fish taxa identified were krill-feeding species suggesting that seals foraged primarily on krill and opportunistically on fish species associated with krill swarms. A seasonal change observed in the relative proportions of the different fish prey taxa indicates that fur seals spent more time foraging over the shelf in summer and off the shelf in autumn. During the study period, commercial fishing in the area was not based upon any of the fish identified in this study.

Mackerel icefish Champsocephalus gunnari in the diet of upper trophic level predators at South Georgia: implications for fisheries management

The mackerel icefish Champsocephalus gunnari Lonnberg is an important component of the ecosystem at South Georgia. Its diet is dominated by Antarctic krill Euphausia superba; in turn, it is also an important prey of a number of upper trophic level predators, as well as being the target of commercial fisheries. Data on the frequency and size structure of mackerel icefish in the diet of Antarctic fur seals Arctocephalus gazella and gentoo penguins Pygoscelis papua from 1991 to 2002 were used to examine trophic interactions involving this species and to evaluate the potential impact of predators on its population. Mackerel icefish occurred in 10 to 20% of scats from Antarctic fur seals and comprised 48% of the diet by mass of gentoo penguins. The contribution of mackerel icefish to the diet of predators was in inverse proportion to the contribution of Antarctic krill. The length- frequency distributions of mackerel icefish indicated a dominant mode at 130 to 180 mm total length (the 1+ age class), with apparent strong cohorts entering the population in 1993, 1996, 1999 and 2001. Extrapolation of diet data to produce a consumption estimate for the South Georgia population of Antarctic fur seals and gentoo penguins suggests that scientific trawl surveys may underestimate the standing stock of mackerel icefish by 1 order of magnitude. Changes in the South Georgia ecosystem over the past 2 decades may have increased the level of consumption of mackerel icefish by predators, providing a potential ecosystem-based explanation for the lack of a recovery to the pre- exploitation population size.

Widening the net: spatio-temporal variability in the krill population structure across the Scotia Sea

Resolving the spatial variability in the population structure of Antarctic krill (Euphausia superba) requires a synoptic sample, as in the design of the CCAMLR 2000 Survey. However, this approach is not appropriate for assessing temporal variability. The size of krill in the diet of Antarctic fur seals (Arctocephalus gazella) has been shown to provide a good representation of the temporal changes in the krill population structure from within their foraging area. At Cape Shirreff, South Shetland Islands, krill in nets had modal size classes of 46–48 mm and 52–54 mm in length and appeared to reflect the presence of larger krill offshore and smaller krill inshore; krill in the diet of fur seals contained both modes, indicating that the foraging area of fur seals integrated the spatial variability in the krill population. At Signy Island, South Orkney Islands, krill in nets and fur seal diets had a modal size class of 52 mm when sampled simultaneously; however, krill in the diet of seals showed a decline in size later in the season with an overall mode of 48 mm. At Bird Island, South Georgia, there was little overlap between net samples, with a modal size class of 30–40 mm, and fur seal diets, with distinct modes of 44 and 54 mm; and there was also much greater spatial variability in the size of krill in these net samples than in those from the other two locations. Extending the comparison of krill size in the diet of seals, to include spatially congruent net samples collected immediately prior to the CCAMLR 2000 Survey, showed almost complete overlap and indicated an even greater spatial variability in the krill population structure at South Georgia. Interactions between the oceanographic transport and enhanced growth rates of krill at South Georgia may combine to produce a higher degree of spatial variability in the krill population compared to other locations and this may limit the use of differences in krill length as an indicator of their provenance. This study underlines the importance of using data from multiple sources when considering large-scale krill population dynamics; information that is crucial to the effective management of the commercial exploitation of krill.

Widening the net: spatio-temporal variability in the krill population structure across the Scotia Sea

Resolving the spatial variability in the population structure of Antarctic krill (Euphausia superba) requires a synoptic sample, as in the design of the CCAMLR 2000 Survey. However, this approach is not appropriate for assessing temporal variability. The size of krill in the diet of Antarctic fur seals (Arctocephalus gazella) has been shown to provide a good representation of the temporal changes in the krill population structure from within their foraging area. At Cape Shirreff, South Shetland Islands, krill in nets had modal size classes of 46-48 mm and 52-54 mm in length and appeared to reflect the presence of larger krill offshore and smaller krill inshore; krill in the diet of fur seals contained both modes, indicating that the foraging area of fur seals integrated the spatial variability in the krill population. At Signy Island, South Orkney Islands, krill in nets and fur seal diets had a modal size class of 52 mm when sampled simultaneously; however, krill in the diet of seals showed a decline in size later in the season with an overall mode of 48 mm. At Bird Island, South Georgia, there was little overlap between net samples, with a modal size class of 30-40 mm, and fur seal diets, with distinct modes of 44 and 54 mm; and there was also much greater spatial variability in the size of krill in these net samples than in those from the other two locations. Extending the comparison of krill size in the diet of seals, to include spatially congruent net samples collected immediately prior to the CCAMLR 2000 Survey, showed almost complete overlap and indicated an even greater spatial variability in the krill population structure at South Georgia. Interactions between the oceanographic transport and enhanced growth rates of krill at South Georgia may combine to produce a higher degree of spatial variability in the krill population compared to other locations and this may limit the use of differences in krill length as an indicator of their provenance. This study underlines the importance of using data from multiple sources when considering large-scale krill population dynamics; information that is crucial to the effective management of the commercial exploitation of krill.

The diet of the Antarctic fur seal Arctocephalus gazella at Harmony Point, South Shetland Islands: evidence of opportunistic foraging on penguins?

The analysis of 523 scats collected at Harmony Point, Nelson Island, South Shetland Islands, from January to March in 2001 and 2002, indicated that the diet of non-breeding male Antarctic fur seals Arctocephalus gazella was diverse and composed of both pelagic and benthic-demersal prey. Overall, the Antarctic krill Euphausia superba and fish were the most frequent and numerous prey, followed by penguins, cephalopods (mainly squid) and gastropods. Myctophids represented 86.5% and 65.8% of the fish mass in 2001 and 2002 respectively, with Gymnoscopelus nicholsi being the main prey. Interestingly, penguin remains were present in 39.0% and 31.9% of the samples in 2001 and 2002, respectively and these birds were the main prey by reconstituted mass (74.0% and 76.1% in both seasons). The occurrence of penguins in the diet of A. gazella at Harmony Point is discussed in terms of the foraging strategy employed by seals and the temporal availability of prey.

Feeding ecology of Antarctic fur seals at Cape Shirreff, South Shetlands, Antarctica

This study examined the diet of Antarctic fur seals, Arctocephalus gazella, from an active breeding colony at Cape Shirreff (62°28′S, 60°48′W), Livingston Island, South Shetland Archipelago, Antarctica. It analysed faecal samples from five consecutive years (1997–2001) and length distribution of krill taken by trawl nets in the vicinity of Livingston Island. Antarctic krill, Euphausia superba, was the most frequent prey item, followed by several myctophid species (Gymnoscopelus nicholsi, Electrona antarctica and Electrona carlsbergi), squid and penguin remains. From 1998 to 2001, a modal progression in krill size was evident, suggesting that A. gazella was depending on a strong krill cohort, at least over the study period. Analysis of size distribution and size selectivity of krill preyed upon by fur seals suggests a preference for larger krill (>34 mm), despite the broader size range of preys items available.

Geographical variation in the diet of the Antarctic fur seal Arctocephalus gazella

The diet of non-breeding male Antarctic fur seals Arctocephalus gazella was investigated at different localities of the Antarctic Peninsula (Cierva Point and Hope Bay), South Shetland Islands (Deception Island and Potter Peninsula) and the South Orkney Islands (Laurie Island), by the analysis of 438 scats collected from January to March 2000. The composition of the diet was diverse, with both pelagic and benthic-demersal prey represented in the samples. Antarctic krill Euphausia superba was the most frequent and numerous prey at all the study sites except at Cierva Point, followed by fish, penguins and cephalopods. Antarctic krill also predominated by mass, followed by either fish or penguins. Fish were the second most important prey by mass at the Antarctic Peninsula whereas penguins were the second most important prey by mass at the South Shetland and South Orkney Islands. Among fish, Pleuragramma antarcticum was the most important species in the diet of the Antarctic fur seals at the Antarctic Peninsula whereas Gymnoscopelus nicholsi predominated at the South Shetland and South Orkney Islands. The results are compared with previous studies, and the possibility of implementing monitoring studies on the distribution/abundance of myctophids and P. antarcticum based on the analysis of the diet of the Antarctic fur seal is considered.

Diet of Antarctic fur seals Arctocephalus gazella at the Danco Coast, Antarctic Peninsula

The diet of non-breeding male Antarctic fur seals, Arctocephalus gazella, was investigated at the Danco Coast, Antarctic Peninsula, by the analysis of 31 and 149 scats collected from January to March 1998 and 2000, respectively. Overall, fish and krill, followed by penguins and squids, were the most frequent prey and constituted the bulk of the diet. The importance of the remaining taxa represented in the samples (octopods, gastropods, bivalves, isopods, polychaetes and poriferans) was negligible. Among fish, channichthyids constituted the bulk of the diet, with Chionodraco rastrospinosus and Chaenodraco wilsoni, followed by the nototheniid, Pleuragramma antarcticum, being the main prey. The myctophid, Electrona antarctica, was the most frequent and numerous fish prey. The results are discussed and compared with those reported for the South Shetland Islands, the closest area for which similar information is available.

Antarctic fur seals foraging in the Polar Frontal Zone: inter-annual shifts in diet as shown from fecal and fatty acid analyses

We studied the dietary preferences of Antarctic fur seals Arctocephalus gazella from Cap Noir, Iles Kerguelen, foraging in the Antarctic Polar Frontal Zone (PFZ) in February of 1998, 1999 and 2000. Scats were collected and analyzed for remaining prey hard parts in each of the 3 years, and in 1999 and 2000, the fatty acid (FA) composition of fur seal milk samples was also examined for longer- term dietary preferences. Scat analyses revealed that seals foraged primarily on fish and some squid in all 3 years with 25 species of fish being taken during the study. Myctophid fish accounted for an aver- age of 94% by number of all fish consumed with 3 species, Gymnoscopelus nicholsi, G. piabilis and Electrona subaspera, forming the core diet. Inter-annual differences in dietary species composition were apparent, however, with the presence of the mackerel icefish Champsocephalus gunnari in 1998 and the myctophid Protomyctophum tenisoni in 1999 accounting primarily for the differences ob- served between years. While reconstituted prey biomass per scat was similar between years, scats from 1998 represented less energy per gram than those from 1999 and 2000. This study highlights the usefulness of using FA signature analysis to confirm longer-term shifts in dietary intake of fur seals us- ing milk samples. Polyunsaturated fatty acids (PUFAs) were significantly more prevalent in the 1999 milk samples, which were also lower in overall lipid content (43% vs 53%). G. nicholsi, a particularly oily fish, occurred in higher proportions in the diet in 2000, perhaps explaining the higher incidence of monounsaturated fatty acids (MUFAs) in this year and the generally higher lipid levels present in milk samples. The inter-annual variation in the diet of Antarctic fur seals confirmed by these 2 techniques lends support to the hypothesis that previously identified variations in oceanographic conditions surrounding Iles Kerguelen in 1998, 1999 and 2000 affect the availability of fur seal prey resources.

Growth of Antarctic krill Euphausia superba at South Georgia

Antarctic krill Euphausia superba has a central role in the ecosystem of the Southern Ocean and knowledge of its growth rate is central to determining the factors influencing population dynamics. The length of Antarctic krill in the diet of Antarctic fur seals Arctocephalus gazella at South Georgia revealed a consistent increase in size between ca. 42 and ca. 54 mm over the period October–March, indicating growth rates much higher than predicted by existing models. Geographical variation in growth rate may result in 2-year-old krill at South Georgia attaining the same size as 3-year-old krill in the Antarctic Peninsula region. The effect of geographical variation in growth rate on the population structure of krill has important implications for comparing the fate of individual cohorts over large scales and in the interpretation of krill life-cycles.

The diet of the Antarctic fur seal Arctocephalus gazella at Harmony Point, Nelson Island, South Shetland Islands

The diet of non-breeding male Antarctic fur seals (Arctocephalus gazella) was investigated at Harmony Point, Nelson Island, South Shetland Islands, by the analysis of 18 and 33 scats collected during February 1996 and 1997 respectively. Overall, fish were the most frequent prey (74.5%) and predominated by mass (54.4%), whereas krill predominated by number (94.2%). This coincides well with the pattern observed in 1997, but in 1996 krill was the most important prey by number and mass (50.2%). The importance of the remaining taxa represented in the samples (octopods, hyperiids and bivalves) was negligible. Among fish, myctophids represented 85.2% of the fish mass, with Gymnoscopelus nicholsi and Electrona antarctica being the main prey. These two species predominated in 1997, whereas the channichthyid Cryodraco antarcticus and the nototheniid Gobionotothen gibberifrons were dominant in 1996. The importance of the myctophids as prey of the Antarctic fur seal is discussed.

Fish in the diet of Antarctic fur seals (Arctocephalus gazella) at South Georgia during winter and spring

The occurrence of fish in the diet of the Antarctic fur seal (Arctocephalus gazella) at Bird Island, South Georgia was investigated by analysis of fish otoliths in scats (faeces) collected during late May to early November 1983. Of the 55 scats examined, 49 contained fish remains, and 45 contained fish otoliths. Ten fish species were represented by 415 otoliths, and 33 otoliths were too digested to be identified unequivocally. Fish size was estimated from otolith size based on published allometric equations. Four coastal notothenioid fishes dominated the fish component of the diet: Champsocephalus gunnari and Gobionotothen gibberifrons each comprised about 40% of the total fish mass; Chaenocephalus aceratus was ranked third by mass and the smaller Lepidonotothen larseni occurred in one quarter of the scats but was of lower importance in terms of mass. The length-frequency distribution of C. gunnari landed by the commercial fishery in October 1982 to June 1983 is similar to that which comprised the bulk of the diet in the present study. Compared with recent studies on the fish component of the diet in the literature, the dominance of C. gunnari is generally similar, however, there was a greater proportion of G. gibberifrons during the 1983 winter and spring than reported for recent winters.

The diet of Antarctic fur sealsArctocephalus gazella during the breeding season at South Georgia

The diet of lactating female Antarctic fur sealsArctocephalus gazella at South Georgia was investigated during the pup-rearing period (January/March) of 1991–1994. Antarctic krillEuphausia superba was the main prey item, occurring in 88% of all scats (n=497), whereas fish occurred in 47% and squid in 5% of all scats. There was considerable intra- and inter-annual variation in the characteristics of krill taken by fur seals. The distribution of krill sizes taken suggests that fur seals are not actively selecting particular sizes of krill and, therefore, that the krill in the diet reflects the krill available around South Georgia. The absence of group 3 krill (44–48 mm in length) in the South Georgia area, as indicated by their absence in the diet of seals, is suggested as a possible reason for low availability of krill and the subsequent reproductive failure among krill predators. The frequency of occurrence of fish was much higher than in previous studies; the pattern of fish consumption showed a consistent seasonal pattern in 3 of the 4 years studied. Of the total number of the myctophidProtomyctophum choriodon, the most numerous fish taxon, 98% were taken between early February and the middle of March.Champsocephalus gunnari andLepidonotothen larseni agg., which both feed on krill, dominated the fish component of the diet outside this period and together constituted 94% of the total estimated biomass of fish consumed. The intra- and inter-annual variability in the diet of Antarctic fur seals emphasise the need for diet studies to be conducted during the entire pup-rearing periods of several years.

The diet of Antarctic fur seals Arctocephalus gazella during the breeding season at South Georgia

The diet of Antarctic fur seals Arctocephalus gazella during the breeding season at South Georgia

The diet of Antarctic fur seals (Arctocephalus gazella Peters 1875) during winter at South Georgia

The diet of mainly adult and sub-adult male Antarctic fur seals (Arctocephalus gazella) was investigated during the austral winters of 1992 and 1993 using faecal (scat) analysis. Of samples containing identifiable prey remains (n = 376), c. 28% contained krill alone, c. 37% contained fish alone and c. 35% contained both krill and fish. The mean size of krill taken was smaller than in summer due to the absence of large adult krill of 55–60 mm total length. Of the 25 fish taxa taken, krill-feeding species predominated, especially in scats that also contained krill. Non-krill feeding species were more abundant in scats containing fish alone. The mackerel icefish (Champsocephalus gunnari Lönnberg 1905), which feeds mainly on krill, was the most important fish species in terms of absolute frequency (53%), frequency of occurrence (69%) and estimated biomass (47%); fur seals may have a significant impact on local stocks of this species. Cephalopods and pelagic fish (myctophids) were of minor importance in the diet (<2%). The importance of krill and krill-feeding fish species suggests that the male fur seals, which are present around South Georgia in winter, are targeting their foraging on a krill and fish community, probably associated with krill aggregations.