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Uncoupling oceanographic and life-history drivers to alternate foraging strategies in Long-nosed fur sealsDahlia Foo1, Simon Goldsworthy2, Mark Hindell1, Jayson Semmens1, Clive McMahon1, Fred Bailleul2, Robert Harcourt3

1 Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, TAS, Australia2 South Australian Research and Development InstituteeAquatic Sciences, West Beach, SA, Australia3 Marine Mammal Research Group, Graduate School of the Environment, Macquarie University, North Ryde, NSW, Australia

Introduction

Methods

Aims• Long-nosed fur seals (LNFS, formerly New Zealand fur seals,

Arctocephalus forsteri) breeding in South Australia are the most abundant seal species in Australia.

• During lactation, mothers alternate periods of foraging at sea and provisioning their offspring ashore.

• Early in lactation (austral summer), females make short foraging trips (~4 days duration) to near continental shelf waters, associated with seasonal localised upwelling.

• However, during winter, most females switch to foraging in distant oceanic waters (foraging trips ~ 2 weeks duration) associated with the substopical front (permanent oceanic frontal zone). They forage in oceanic waters up till pups are 10 months old when they are weaned

• It is unclear if this major shift in foraging distribution occurs in response to: 1) reduced prey availability as a consequence of cessation in coastal upwelling; 2) life-history/pup growth factors that enable females to switch to more reliable but distant foraging grounds once their pup reaches a size and condition threshold where they can sustain longer fasts; or 3) a combination of factors.

• Changes in summer/local oceanographic features associated with fur seal foraging areas will be monitored by simultaneously fitting biologging (Global Positioning System/Time-Depth recorders; GPS/TDR) tags to fur seals and oceanographic (Conductivity-Temperature-Depth-Flurometer/Irradiance; CTD-F) tags onto Australian sea lion males. Sea lions from nearby nearby colonies (Fig. 1) forage over shelf waters year round and sample the entire water column, providing a means to monitor changes in local oceanographic conditions from the summer to winter period.

• Winter GPS/TDR tags fitted to fur seasls will collect oceanographic data from the subtropical front, to be analysed in combination with remote sensed oceanographic data.

• Stable isotope analyses (SIA), from whisker/blood samples of adult females and their pups, will be used to (i) contrast and identify shelf vs. oceanic foraging; (ii) examine the extent and degree of transition and timing to alternate strategies; and (iii) their life-history consequences within and between fur seal colonies. Pup growth rates will be monitored throughout the year in association with ontogenic changes in stable isotope as a function of maternal foraging area and diet.

1. Identify the key oceanographic features that lactating fur seals target in shelf and oceanic waters;

2. determine the combination of factors that drive the transition and timing from shelf to oceanic foraging;

3. assess the consequences of trade-offs and optimisation in the allocation of time in shelf and oceanic foraging areas, and the timing of transition between shelf and oceanic foraging; and

4. quantify how variability in oceanographic conditions in shelf and oceanic environments impacts fur seal population dynamics.

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Figure 1. Annual foraging distribution of female long-nosed fur seals from one of the primary colonies at Cape Gantheaume. Australian sea lions from a nearby colony forage on the shelf year round.

(SIA)

Example of upwelling region

Kangaroo Is.

Cape Gantheaume

200 m2000 m

Oceanic waters – winter foraging area

Continental shelf – summer foraging area

Fem

ale l

ong-n

osed fur seal

Male

Australian sea lion

0 100

km

GPS tag

TDR �ipper tag

CTD-F tag

Summer and winter data collection:

+ + +δ13C / δ15N

pup