The foraging activity and provisioning strategies of Antarctic fur seals in relation to changes in oceanographic conditions at the Kerguelen Archipelago
Creators: name: Lea, M-A
Publisher: name: University Of Tasmania
Year: 2023

description: 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¬¨‚àû07'S, 70¬¨‚àû45'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.