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. Received: 7 October 1997 / Accepted: 24 May 1998     

Geographical variation in the diet of the Antarctic fur seal<Emphasis Type="Italic"> Arctocephalus gazella</Emphasis>

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.     

Fish diet of the Antarctic fur seal,Arctocephalus gazella,in summer,at Stranger Point,King George Island,South Shetland Islands

 In February 1992, 34 faecal samples from non-breeding male Antarctic fur seals, Arctocephalus gazella, were collected at Stranger Point, King George Island, South Shetlands. Fish constituted an important part of the diet, occurring in 90% of those scats containing prey remains. From 1162 otoliths found in the faeces, 1086 were identified to at least family level. Myctophids and nototheniids represented together almost 90% of the fish eaten. The dominant species were Gymnoscopelus nicholsi, Pleuragramma antarcticum and Electrona antarctica, contributing 33.3%, 30.8% and 12.0% of the otoliths respectively. The standard length of these three species was estimated from otoliths with little or no signs of erosion. This study showed that fur seals fed mainly on pelagic fish species that are often associated with krill. These findings are corroborated by fur seal diving patterns. Received: 7 October 1994/Accepted: 5 October 1995     

Fish prey of Antarctic fur seals,Arctocephalus gazella,during the summer-autumn period at Laurie Island,South Orkney Islands

The fish component in the diet of Antarctic fur seals, Arctocephalus gazella, was investigated at Laurie Island, South Orkney Islands from mid January to April 1988. Fish otoliths occurred in 78.8% of faecal droppings. Most of the otoliths extracted from scats belonged to Myctophid fish (93.4%), mainly Electrona antarctica and Gymnoscopelus nicholsi. These two fish species thus constituted the bulk of the diet. A greater percentage of the otoliths from G. nicholsi (31.1%) in this study were considered suitable for measurement in comparison with 10.3% from scats at Heard Island in 1990 and 11.4% at Macquarie Island in 1988/89. This lesser degree of erosion would suggest that foraging areas of fur seals during this study were closer to land than during previous studies. During the period studied, the commercial fishery around the South Orkneys was not based on Myctophid fish so there was no direct competition for fish between the fur seals and fisheries.     

Interannual variation in the diet of non-breeding male Antarctic fur seals, <Emphasis Type="Italic">Arctocephalus gazella</Emphasis>, at Isla 25 de Mayo/King George Island

The diet of non-breeding male Antarctic fur seals, Arctocephalus gazella, was investigated at Stranger Point, King George Island, through the analysis of scats during three consecutive summer seasons (1996, 1997, 1998). Overall, fish and krill were the most frequent prey occurring, respectively, in an average of 82.9% and 78.8% of samples (n = 131), followed by penguins (22.8%) and cephalopods (17.8%). 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 mainly on a krill and a fish community associated with krill aggregations. However, a seasonal change was observed in the relative proportions of the different prey taxa, with a progressive decrease with time in the occurrence of krill and a concomitant increase of fish, penguins and squid. Possible influence of the strong 1997/98 ENSO event is discussed.     

The diet of Antarctic fur seals,Arctocephalus gazella,during the summer-autumn period at Mossman Peninsula,Laurie Island (South Orkneys)

Summary From mid January to April 1988 a total of 105 faecal samples of non breeding male fur seals, Arctocephalus gazella, was collected at Mossman Peninsula, Laurie Island. Examination of scats containing prey remains showed that fur seals fed mainly on krill, occurring in 100% of samples but fish were also a common food item (78.8%). Myctophids constituted more than 90% of fish taken by fur seals. Squid beaks occurred in 34.3% of scats, being more common in the autumn period.     

Penguins, fur seals, and fishing: prey requirements and potential competition in the South Shetland Islands, Antarctica

Antarctic and sub-Antarctic seabirds, marine mammals, and human fisheries concentrate their foraging efforts on a single species, Antarctic krill (Euphausiasuperba). Because these predators may have a significant effect on krill abundance, we estimated the energy and prey requirements of Adelie (Pygoscelisadeliae), chinstrap (Pygoscelisantarctica), and gentoo (Pygoscelispapua) penguins and female Antarctic fur seals (Arctocephalusgazella) breeding on the South Shetland Islands, Antarctica and compared these estimates with catch statistics from the Antarctic krill fishery. Published data on field metabolic rate, population size, diet, prey energy content, and metabolic efficiency were used to estimate prey requirements of these breeding, adult, land-based predators and their dependent offspring. Due to their large population size, chinstrap penguins were the most significant krill predators during the period examined, consuming an estimated 7.8 × 10⁸ kg krill, followed by Adelie penguins (3.1 × 10⁷ kg), gentoo penguins (1.2 × 10⁷ kg), and Antarctic fur seals (3.6 × 10⁶ kg). Total consumption of all land-based predators on the South Shetland Islands was estimated at 8.3 × 10⁸ kg krill. The commercial krill fishery harvest in the South Shetland Island region (1.0 × 10⁸ kg) was approximately 12% of this. Commercial harvest coincides seasonally and spatially with peak penguin and fur seal prey demands, and may affect prey availability to penguins and fur seals. This differs from the conclusions of Ichii et al. who asserted that the potential for competition between South Shetland predators and the commercial krill fishery is low. Received: 26 August 1997 / Accepted: 16 December 1997     

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.     

First documentation of leopard seal predation of South Georgia pintail duck

Leopard seals are regular winter visitors to Bird Island, South Georgia, where they mostly prey on fur seals and penguins, and to a lesser extent on Antarctic krill and fish. Leopard seals can exploit many different species, but there are no records of predation on flying shorebirds in the wild. On 4 October 2008, an individually identified juvenile leopard seal female was observed killing and eating a South Georgia Pintail duck. It also preyed on Antarctic fur seals and gentoo and macaroni penguins during its 2-month temporary residency around the island. The varied diet of this seal exemplifies the generalist prey utilization typical of its species. Long-term diet studies at Bird Island and the published record suggest that predation on ducks is a rather exceptional finding; individual ducks are more likely to escape leopard seal attacks than penguins and provide a far less substantial ration. This note documents the first observation of this species of duck in the diet of leopard seals.     

Fish prey of Antarctic fur seals Arctocephalus gazella at Ile de Croy,Kerguelen

 The composition of Antarctic fur seal prey was assessed through analysis of scats collected in March 1994 on Ile de Croy, Iles Nuageuses. Fish remains predominated in samples, occurring in 95% of droppings. A total of 968 otoliths allowed the identification of 16 fish species. Myctophid fish (12 species) dominated the diet both by number (94% of the otoliths) and by fish reconstituted mass (76%). Three fish species constituted together 87% of the reconstituted mass: the myctophids Gymnoscopelus nicholsi (52%) and G. piabilis (12%), and the channichthyid Champsocephalus gunnari (23%). Prey distribution suggests that during late summer seals forage in upper slope waters in the northeast of the Kerguelen Archipelago. Received:1 March 1996/Accepted: 20 May 1996     

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.     

Ecological drivers of change at South Georgia: the krill surplus,or climate variability

Macaroni penguins Eudyptes chrysolophus are thought to be one of the most important mesopredators in the Southern Ocean having a greater impact on prey availability and abundance than any other seabird species. Their population centre has long been held to be South Georgia where populations were thought to comprise many million animals. Here we report the results of a recent census of the macaroni population at South Georgia undertaken using aerial survey methods. We report dramatic declines in numbers (～1.0 million breeding pairs) compared to numbers observed in the late 1970s (～5.4 million pairs), but show that these reductions have occurred principally at sites where numbers had previously been very large. During the breeding season, the main foraging grounds of birds from these sites overlap with the foraging grounds of Antarctic fur seals Arctocephalus gazella, a major competitor for their principal prey, Antarctic krill Euphausia superba. We suggest that the redistribution of the macaroni penguin population at South Georgia reflects the recent recovery of fur seal populations and thus the ongoing consequences of human intervention at South Georgia, a process which started more than 2 centuries previously. The implied resource competition and the observed population changes may also be exacerbated by recent reductions in Antarctic krill abundance which have been linked with reductions in seasonal sea ice following recent, rapid, regional warming in the Antarctic; however, the recovery of fur seal populations, and the ongoing recovery of krill‐eating whale populations argues that tropho‐dynamic interactions may be sufficient to explain the observed changes.     

The diet of the Antarctic fur seal<Emphasis Type="Italic"> Arctocephalus gazella</Emphasis> 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°28S, 60°48W), 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.     

Feeding habits of three seal species at the Danco Coast, Antarctica: a re-assessment

The analysis of prey overlap among Weddell, Antarctic fur and leopard seals was conducted using fecal samples collected at the Danco Coast, Antarctic Peninsula, in 1998 and 2000. The re-occurrence of prey species was moderate in samples collected in 1998, and low in 2000, and reflects resource partitioning among seal species. Prey species that mostly co-occurred in seals’ diet were the Antarctic krill Euphausia superba, bivalves, and the myctophids Gymnoscopelus nicholsi and Electrona antarctica. A dietary similarity index of prey overlap has been calculated and demonstrates evident fluctuations in pairwise comparisons between the seal species. The highest and lowest values of prey overlap were observed between Antarctic fur seals and leopard seals, and between Weddell seals and leopard seals, respectively. Prey overlap between Antarctic fur seals and Weddell seals was moderate in both seasons.     

The diet of the Antarctic fur seal <Emphasis Type="Italic">Arctocephalus gazella</Emphasis> at the South Orkney Islands in ten consecutive years

To improve the knowledge on the Antarctic fur seal foraging behavior, evaluate whether changes in its diet are associated with changes in prey availability, and evaluate whether fisheries had negative impacts on Antarctic fur seal populations, a total of 1359 scats of non-breeding males were collected in ten consecutive sampling periods between 1994 and 2003 at Laurie Island, South Orkney Islands. Antarctic krill was the most numerous prey throughout the sampling period followed in importance by fish prey. Antarctic krill also predominated by reconstructed mass, except during 1994 and 1998 when penguins were the most important prey, and during 1995 when fish dominated. Among fish, demersal-benthic species (mainly Gobionotothen gibberifrons, Chaenocephalus aceratus and Chionodraco rastrospinosus) dominated the diet except in 1997, 1998, 1999 and 2002 when myctophids (mainly Gymnoscopelus nicholsi and Electrona antarctica) were the most important fish prey. In seasons when fish dominated the diet, the diet was primarily comprised of demersal species. Although our results are in overall agreement with previous studies, the contribution to the diet of the main preys changed throughout the sampling period, probably in parallel with changes in prey abundance. According to historical results on fish consumption, the fisheries carried out in the past have had negative impacts on seal populations from the South Orkney Islands.     

Identifying patterns in the diet of mackerel icefish (<Emphasis Type="Italic">Champsocephalus gunnari)</Emphasis> at South Georgia using bootstrapped confidence intervals of a dietary index

Ontogenetic, inter-annual and regional variations in diet were investigated for mackerel icefish, Champsocephalus gunnari, in three successive summer seasons around South Georgia. Stomach contents from 2239 C. gunnari (130–560 mm total length) were examined. A bootstrapping technique was used to calculate confidence intervals for an index of relative importance of prey categories (% IRI_DC). Diet varied significantly between years and age classes but there was little regional difference in diet. In general, diet was dominated by krill, Euphausia superba and by the amphipod Themisto gaudichaudii. Smaller (younger) fish tended to prey on a higher proportion of T. gaudichaudii and small euphausiids such as Thysanoessa sp. and took smaller quantities of E. superba. In a season of poor krill availability (summer of 2003–2004) the proportion of krill in the diet, stomach fullness and fish condition (indicated by length–weight relationships) were significantly lower than in the other summer seasons. A large reduction (>80%) in the estimated annual (2005) biomass of the C. gunnari stock directly followed the season of poor krill availability. This decline was largely because of mortality of 2+ and 3+ fish, which were more krill dependent than 1+ fish. Younger fish appear to have survived, leading to an increase in the estimated population biomass in 2006.     

Summer diet of leopard seals (<Emphasis Type="Italic">Hydrurga leptonyx</Emphasis>) in Prydz Bay,Eastern Antarctica

The diet of male and female leopard seals (Hydrurga leptonyx) was investigated in Prydz Bay, Eastern Antarctica. A total of 70 scats, 1 regurgitate and 3 stomach contents were collected, during the austral summer, between November 1999 and March 2002. Eight prey species were identified, including birds, mammals, fish and invertebrates. Adelie penguins (Pygoscelis adeliae) were the main prey item and crabeater seals (Lobodon carcinophagus), benthic and pelagic fish, amphipods and krill were found to supplement the diet. Cephalopods did not occur in the diet. Crabeater seals were still being captured well after weaning, and were found in the diet of both male and female leopard seals.     

Diet of the Antarctic starry skate Amblyraja georgiana (Rajidae, Chondrichthyes) at South Georgia (Southern Ocean)

Stomach contents were identified from 206 Antarctic starry skate (Amblyraja georgiana) that were collected during three groundfish surveys (September 2007, April 2008 and January 2009) at South Georgia, Southern Ocean. The diet of A. georgiana varied with skate size and between years. Preferred prey included fish (particularly for larger individuals) and Antarctic krill, Euphausia superba, as well as amphipods, polychaetes and other benthic fauna. The skate A. georgiana appears to be an opportunistic predator, and the clear presence of Antarctic krill in this demersal predator’s diet may indicate a benthic habit of this euphausiid species, which has hitherto mainly been considered as occupying a purely pelagic niche.