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The foraging activity and provisioning strategies of Antarctic fur seals in relation to changes in oceanographic conditions at the Kerguelen Archipelago

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Lea, M-A (2002) The foraging activity and provisioning strategies of Antarctic fur seals in relation to changes in oceanographic conditions at the Kerguelen Archipelago. PhD thesis, University of Tasmania.

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Abstract

The study examined how the foraging activity, foraging efficiency and the pup provisioning
of a top marine predator, the Antarctic fur seal (Arctocephalus gazella), were affected by
inter-annual variability in oceanographic conditions and prey availability in the Antarctic
Polar Frontal Zone (APFZ), in February 1998, 1999 and 2000. It formed part of a larger
multidisciplinary study examining predator-prey relationships in relation to oceanographic
features between Antarctic fur seals, king penguins (Aptenodytes patagonicus) and their
mesopelagic, fish prey, myctophids. The Kerguelen Archipelago (49°07'S, 70°45'E), which
is located in a region of the southern Indian Ocean characterised by considerable
hydrographic complexity, was chosen as the study site.
Seals were fitted with time-depth recorders and/or satellite transmitters to study their diving
behaviour and at-sea distribution. Intra-population variability in diving behaviour was
studied for female fur seals from two study sites at Cap Noir and lies Nuageuses during the
three years. Dietary composition was determined and compared over the three years by (1)
faecal analysis and (2) fatty acid signature analysis of milk samples, to identify both short
and longer-term dietary preferences. Nocturnal transect surveys were conducted by a
dedicated research vessel to estimate fish availability in the foraging zones of seals, as
determined from satellite tracking of seals. The fatty acid profiles and calorific content of
fish prey captured during transect surveys conducted in the fur seal foraging zones were also
determined. The spatial distribution of the diving effort of the fur seals was analysed in
relation to oceanographic variables (sea surface temperature, chlorophyll-a concentration,
and bathymetry) using spatial analysis techniques for a range of spatial scales. The foraging
parameters of female seals were subsequently related to the growth performance of their
pups on a per trip basis and throughout lactation.
Inter-annual differences in foraging behaviour were identified for all parameters examined.
The diving activity of seals varied inter-annually and considerably between individuals
within a season. The diving behaviour of seals during one foraging trip (n=117) was
categorised by cluster analysis to one of four behavioural dive groups: (1) deep divers; (2)
shallow active divers; (3) shallow-divers and (4) daytime divers. The proportion of seals
displaying the deep diving behaviour declined from 82% in 1998 to around 50% in 1999 and
2000, indcating changes in prey availability and accessibility. Seals at lies Kerguelen dived
consistently to greater mean depths than seals than previously recorded for this species.
Energy-rich mesopelagic myctophid fish formed the majority of estimated dietary biomass
consumed by seals in all years, with Gymnoscopelus piabilis, G. nicholsi and Electrona
subaspera always comprising the three main species. However, in 1998 fur seals also preyed heavily on icefish (Champsocephalus gunnari) and other fish families. Simultaneous shipbased
transect surveys in the foraging areas of seals confirmed that 1998 was a year of low
prey availability in the foraging areas of seals in comparison to 1999. Species caught during
the surveys correlated only wealdy to those species identified in seal scats, indicating a high
degree of prey selectivity by seals was occurring. Fatty acid analyses of fur seal milk
samples confirmed the finding of the inter-annual differences observed in dietary intake for
1999 and 2000. Levels of fat within milk samples were also significantly higher in 2000
(~53%), the year of highest apparent prey availability and female body condition, than 1999
(~43%).
Seals foraged relatively close to the colony, on the edge of the continental shelf, in 1998,
with foraging zones becoming more dispersed in 1999 and 2000 as females increased their
median foraging range over the three years from 111Icm to 215km. This behaviour coincided
with a lower average chlorophyll-a concentration (mg n-3 ) over the foraging zone of fur
seals in 1998, than in the two following seasons. Anomalous warm sea surface temperatures
(SST) were present within the same region in 1998, and were cooler in 1999 and 2000,
indicating a migration further north of Polar Frontal waters during the study. There is also
evidence to suggest that the warm SST anomaly recorded in the Kerguelen region in 1997/98
coincided with one of the largest ENSO events recorded in the central Pacific Ocean in this
year. A preferred foraging area to the northeast of the archipelago was identified, although in years
of higher marine productivity foraging strategies were more diverse. In 2000, the at-sea
distribution and diving behaviour of the fur seals on a fine scale reflected specific
combinations of environmental variables. Seals foraging to the southeast in cool SST over
the continental slope dived to shallow depths whilst those foraging to the NE of the island in
warmer SST over deeper water spent greater amounts of time diving to deeper depths. The
foraging efficiency of mothers (daily allocation of energy to the pup) was similar between
seals foraging in the different ecoregions, in this year of high prey availability.
Despite changes in the diving behaviour and dietary intake of the fur seal females, it appears
that the seals were unable to fully compensate for the changes in prey availability and prey
composition in this way, as rates of pup growth was much reduced in 1998 compared to
1999 and 2000. Both maternal and pup body composition were significantly poorer in 1998
than in other years, as were rates of foraging efficiency measured as the daily mass gain of
pups over a foraging cycle (combined duration of a foraging trip and subsequent shore bout).
Sex-based differences in mean mass and body length prior to weaning (March) were evident for pups, however mean growth rates during the second half of lactation in all years were
similar between sexes.
The study highlights the flexible foraging strategies displayed by at top marine predator in
response to changing oceanographic conditions and the importance of energy-rich
myctophids in the diet of seals foraging in the Antarctic Polar Frontal Zone. Female seals
adjusted their foraging behaviour, including the location of foraging zones, their diving
activity and diet, and foraging efficiency, concurrently with arinual changes in SST and prey
availability. Parameters such as foraging efficiency over a foraging cycle and variability in
mean provisioning rates (trip duration), were identified as proxies of prey availability within
the foraging range of seals in 1998, 1999 and 2000, emphasising the effectiveness of using
Antarctic fur seal foraging behaviour as an indicator of both food and oceanographic
variability.

Item Type: Thesis (PhD)
Keywords: Southern fur seals
Copyright Information:

Copyright 2002 the author

Additional Information:

Chapter 2 appears to be the equivalent of a post-print version of an article published as: Lea, M-A., Hindell, M.A., Guinet, C., Goldsworthy, S.D. (2002), Variability in the diving activity of Antarctic fur seals, Arctocephalus gazelle, at Iles Kerguelen. Polar Biology 25(4), 269-279. The final publication is available at Springer via http://dx.doi.org/10.1007/s00300-001-0339-6

Chapter 3 appears to be the equivalent of a post-print version of an article published as: Lea, M.-A., Nichols, P.D., Wilson, G. (2002), Fatty acid composition of lipid-rich myctophids and mackerel icefish (Champsocephalus gunnari) - Southern Ocean food-web implications. Polar Biology 25(11), 843-854. The final publication is available at Springer via http://dx.doi.org/10.1007/s00300-002-0428-1

Chapter 4 appears to be the equivalent of a post-print version of an article published as: Lea, M.-A., Cherel, Y., Guinet, C., Nichols, P.D., (2002), Antarctic fur seals foraging in the Polar Frontal Zone: inter-annual shifts in diet as shown from faecal and fatty acid analyses. Marine ecology progress series, 245, 281-297

Date Deposited: 19 Dec 2014 02:43
Last Modified: 18 Oct 2016 22:38
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