Male Antarctic fur seals: neglected food competitors of bioindicator species in the context of an increasing Antarctic krill fishery

Abstract

The fishery for Antarctic krill is currently managed using a precautionary, ecosystem-based approach to limiting catch, with performance indices from a long-term monitoring program focused on several krill-dependent predators that are used to track ecosystem health. Concerns over increased fishing in concentrated areas and ongoing efforts to establish a Marine Protected Area along the Peninsula, a key fishing region, is driving the development of an adaptive management system for the fishery. The cumulative effects of fishing effort and interactions among krill-dependent predators and their performance is at present neglected in the CCAMLR Ecosystem Monitoring Program. However, we show considerable overlap between male Antarctic fur seals and the krill fishery in a complex mosaic, suggesting potential for cumulative impacts on other krill dependent predators. A holistic view is required as part of future efforts to manage the krill fishery that incorporates various sources of potential impacts on the performance of bioindicator species, including the fishery and its interactions with various krill dependent predators.

Winter distribution of juvenile and sub-adult male Antarctic fur seals (Arctocephalus gazella) along the western Antarctic Peninsula

“…According to our results, the South Shetland Islands and the Bransfield Strait represent the core of the distribution area of juvenile and sub-adult male Antarctic fur seals in the western Antarctic Peninsula in winter. This conclusion is in accordance with previous ship surveys 6 , 74 and satellite telemetry data 11 , although previous ship surveys did not asses the abundance of Antarctic fur seals south to the South Shetland Islands and did not assess either sex or age class of the spotted individuals. The Bransfield Strait and the South Shetland Islands were indeed included in the list of Antarctic areas of ecological significance in a recent study 58 and are also a hotspot for Antarctic krill year-round 4 – 6 , 67 .…”

“…However, thermoregulation is not necessarily the main reason why females do not overwinter in maritime Antarctica. Young males, which overwinter in maritime Antarctica (this study and previous studies 11 , 21 , 25 ), have a blubber thickness similar to that of adult females 26 and the sea surface temperature of the summer foraging grounds of the females breeding at the South Shetland Islands 17 is not different from the sea surface temperature of the foraging grounds used throughout the present study by juvenile and subadult males.…”

“…Younger juveniles often exhibit distinct stable isotope ratios in their vibrissae 21 , 25 , either because they use foraging grounds at much lower latitude due to poorer thermoregulatory skills in very cold water 76 , they fast for a long period after weaning 77 , 78 or both. There are no published stable isotope data from males born at the South Shetland Islands to date, but the data reported here and a recent study 11 suggest that males present in the South Orkney and the South Shetland Islands immediately after the breeding season remain in maritime Antarctica throughout winter and likely occur in large numbers in the region year-round.…”

“…Only penguins, crabeater seals ( Lobodon carcinophaga ) and Antarctic fur seals ( Arctocephalus gazella ) remain in large numbers during winter in maritime Antarctica (i.e. the part of the Southern Ocean closer to the Antarctic continent and limited to the sea ice) 6 , 8 – 11 .…”

“…However, ship surveys have identified sea ice concentration as one of the major determinants of male Antarctic fur seal distribution off the South Shetland Islands and the Bransfield strait in winter 6 . Furthermore, males occuring at the South Orkney immediately after the breeding season remain in maritime Antarctica throughout winter 11 and males breeding at islands close to the Antarctic Polar Front overwinter in maritime Antarctica 11 , 13 – 16 , 21 , 23 , 24 , which they do first from 2 to 3 years old 21 , 25 . Antarctic fur seals are highly sexually dimorphic 26 – 30 and differences between sexes in winter habitat use could be related to differences in body mass, which in turn is a major determinant of their thermoregulatory skills and diving performance 31 , 32 .…”