Mia Wege

Return Customers: Foraging Site Fidelity and the Effect of Environmental Variability in Wide-Ranging Antarctic Fur Seals

Strategies employed by wide-ranging foraging animals involve consideration of habitat quality and predictability and should maximise net energy gain. Fidelity to foraging sites is common in areas of high resource availability or where predictable changes in resource availability occur. However, if resource availability is heterogeneous or unpredictable, as it often is in marine environments, then habitat familiarity may also present ecological benefits to individuals. We examined the winter foraging distribution of female Antarctic fur seals, Arctocephalus gazelle, over four years to assess the degree of foraging site fidelity at two scales; within and between years. On average, between-year fidelity was strong, with most individuals utilising more than half of their annual foraging home range over multiple years. However, fidelity was a bimodal strategy among individuals, with five out of eight animals recording between-year overlap values of greater than 50%, while three animals recorded values of less than 5%. High long-term variance in sea surface temperature, a potential proxy for elevated long-term productivity and prey availability, typified areas of overlap. Within-year foraging site fidelity was weak, indicating that successive trips over the winter target different geographic areas. We suggest that over a season, changes in prey availability are predictable enough for individuals to shift foraging area in response, with limited associated energetic costs. Conversely, over multiple years, the availability of prey resources is less spatially and temporally predictable, increasing the potential costs of shifting foraging area and favouring long-term site fidelity. In a dynamic and patchy environment, multi-year foraging site fidelity may confer a long-term energetic advantage to the individual. Such behaviours that operate at the individual level have evolutionary and ecological implications and are potential drivers of niche specialization and modifiers of intra-specific competition.

The Marine Mammal Programme at the Prince Edward Islands: 38 years of research

The Marine Mammal Programme (MMP) conducts research on pinnipeds and killer whales Orcinus orca at Marion Island, Prince Edward Islands, under the auspices of the Mammal Research Institute, Department of Zoology and Entomology, University of Pretoria. The history of the MMP, which has benefited from collaboration with leading national and international researchers, is described from its start through to current research. The setting up of long-term studies such as the mark-resighting of southern elephant seals Mirounga leonina commenced in 1983. The elephant seal population declined by 87% between an initial census in 1951 and 2004. This was followed by a stabilisation period and a current increase. The recovery, and subsequent increase of sympatric populations of Subantarctic fur seals Arctocephalus tropicalis and Antarctic fur seals A. gazella (following cessation of commercial sealing), are documented. Insights into many aspects of elephant seal and fur seal biology, including life history, demography, diet, growth, foraging and ranging behaviour are described. Ancillary work on morphology, genetics, anthropogenic influences and rare events are mentioned, as well as the extent of current research that addresses population dynamics in an ecosystem context. Opportunistic photographic identification of killer whales and recent dedicated observations at Marion Island are used to determine population size, seasonal abundance and sociality of this population, and to further understanding of its potential impact on resident pinniped populations.

South for the winter? Within‐dive foraging effort reveals the trade‐offs between divergent foraging strategies in a free‐ranging predator

Summary Central to an animals fitness is its foraging strategy and understanding the choices made by foraging animals is a fundamental aim in animal ecology. For diving animals, quantifying foraging effort within dives provides a measure of foraging that can be integrated with location information to reveal how animals use their environment as well as the trade-offs associated with contrasting foraging strategies. We investigated the diving behaviour of 12 free-ranging Antarctic fur seals (Arctocephalus gazella) during their post-breeding winter migrations, quantifying within-dive foraging effort using a novel approach to identify divergent foraging strategies and determine the costs and benefits associated with foraging decisions. Significant differences identified in both diving behaviour and foraging effort of female Antarctic fur seals could be attributed to two main, contrasting foraging strategies. Habitat was a major determinant of diving and foraging behaviour, with clear differences occurring either side of the Polar Front, a prominent oceanographic feature in the Southern Ocean. Longer night duration and improved access to vertically migrating prey lead to increased foraging opportunities and a reduced foraging effort south of the Polar Front. Dives in this region were short and shallow. Conversely, seals remaining closer to the breeding colony north of the Polar Front had deep, long dives and an elevated foraging effort. The distinct foraging strategies of fur seals have associated trade-offs related to habitat availability, travel costs, prey accessibility and prey quality, which are likely driving their foraging decisions. This study highlights the trade-offs between contrasting foraging strategies that currently coexist within a population of a wide-ranging predator and raises questions about the viability of strategies with future change to population size or environmental conditions. Finally, understanding the trade-offs associated with foraging strategies is important for assessing the foraging decisions of animals across a variety of environments.

Multi-state mark-recapture models as a novel approach to estimate factors affecting attendance patterns of lactating subantarctic fur seals from Marion Island

Abstract Observer-based studies often underestimate key ecological parameters. Here a fresh approach was used to analyse six years (2006–11) of attendance cycles to estimate foraging trip lengths of a lactating flipper-tagged otariid: subantarctic fur seals at Marion Island. Multi-state mark-recapture models were used to calculate detection failures of females, correct estimates accordingly, and investigate the effects of year, season, pup sex and the presence of a telemetry device on attendance cycle parameters. There were no differences between corrected and uncorrected attendance data. This is attributed to the high capture probability across all seasons (range: 83–98%). This illustrates that observer-based studies are useful to augment telemetry studies. Only season and pup sex had a significant impact on female provisioning rates. In winter, foraging trip durations were longer (t-value=25.22, P<0.0001) and attendance durations shorter (t-value=-2.15, P=0.01) than during summer. Females with female pups spent a higher proportion of their time on land (χ2=6.6, P<0.05). Male pups have higher growth demands and are larger which suggests they can deplete female milk-stores faster.

Inshore Occurrence of Southern Right Whales (Eubalaena australis) at Subantarctic Marion Island

Opportunistic shore-based sightings of southern right whales Eubalaena australis for Marion Island (46°54′S, 37°45′E) were documented at five different times between 1974 and 2009. Whales were sighted between May and September and exclusively on the eastern lee side of the Island. Notwithstanding some observer biases over this time frame, the species appears to be an infrequent visitor to the inshore marine environment of Marion Island.

Opportunistic ship-based census of pack ice seals in eastern Weddell Sea, Antarctica

The distribution, density and percentage contribution of pack ice pinnipeds during ship-board censuses in the eastern Weddell Sea in summer 2015/2016 are presented. Of the four true pack ice seal species encountered, crabeater seals predominated. Despite the low survey effort, Ross seals continued to be relatively abundant in the pack ice off the Princess Martha Coast in mid-January 2016, similar to the situation here in the 1970s. Censusing of Ross seals is ideally carried out in late January/early February when the species’ haulout probability is at its maximum and the seals are gathered in the limited summer pack ice to moult.

Hitchhiking goose barnacles and their potential implications on the functioning of animal-borne instruments

Goose barnacles (Lepas australis) were found attached to satellite-tracking and time-depth recording (TDR) instruments carried by two lactating Subantarctic fur seals (Arctocephalus tropicalis) from Marion Island. We report on the movements of these seals, both of which crossed the Subantarctic Front. Barnacles surrounding the temperature probe of one TDR device did not appear to directly influence temperature recordings, although disturbances to recording accuracies by other high-precision devices carrying goose barnacles are considered likely.