Is regional variability in environmental conditions driving differences in the early body condition of endemic Australian fur seal pups?

Abstract

Context. Good body condition in juvenile marine mammals is crucial for survival and, therefore, population demography. Australian fur seals, endemic to Australia, recently established a breeding colony at the southern edge of their range, at The Needles, a small group of islands in southwest Tasmania (43.6614°S) and a significant distance from their core breeding range in Bass Strait. Aims. We aimed to compare pup body condition at two breeding colonies, distinct in time since establishment and latitude. Specific aims were to: (1) establish the timing of peak pupping, to compare condition of known-age pups, and determine a baseline at The Needles; (2) investigate pup body condition over time at an established colony; and (3) gain insight into the effects of environmental conditions on pup body condition. Methods. We conducted a colony comparison of pup body condition using condition indices at The Needles and an established breeding colony in Bass Strait, Tenth Island, for the 2019/20 and 2020/21 breeding seasons. Pup body condition was quantified at Tenth Island over 18 years (2003-2020) using a long-term morphometric dataset. To establish breeding phenology at these two colonies, we determined peak pupping date for the 2019/20 breeding season using daily pup counts. We assessed the effect of environmental parameters on body condition for the long-term dataset. Key results. Pups from The Needles displayed significantly higher body condition than those from Tenth Island, despite similar peak pupping date. Breeding phenology was consistent with published timing for Australian fur seals. Pup body condition at Tenth Island over the 2-year colony comparison was comparable to the historical average. Environmental drivers that affect maternal foraging efficiency are linked to pup body condition. Conclusions. Higher pup body condition at The Needles is likely underpinned by better foraging conditions resulting in increased pup provisioning levels. Our results indicate that south-west Tasmania is a region of foraging and emerging breeding importance for Australian fur seals. Implications. Future research to monitor pup body condition, maternal foraging behaviour and ecosystem productivity at The Needles will help to provide greater understanding of likely population trajectories at this southernmost breeding site for Australian fur seals.

Long-term decline in body condition of female Australian fur seals: potential causes and implications

“…Alternatively, in response to reductions in nutritional resources and adult female body condition, the population could have experienced decreases in offspring birth mass and pre-weaning growth rate (Boyd and McCann, 1989;Arnbom et al, 1997;Boltnev and York, 2001). Indeed, recent studies have highlighted regional interannual variability in AUFS pup growth and condition in relation to environmental variables (Wall et al, 2023). Hence, further monitoring of these parameters is needed to assess whether they have changed at Kanowna Island since the start of the study period when they were last measured (Arnould and Hindell, 2002).…”

“…While environmental influences on Australian fur seal foraging behaviour, habitat use and diet have previously been documented (Speakman et al, 2020;Speakman et al, 2021;Geeson et al, 2022;Kliska et al, 2022;McIntosh et al, 2022), little is known of factors impacting body condition over the long term in the species (Arnould and Warneke, 2002;Wall et al, 2023). Such information is crucial for predicting how the species, and the ecosystem they dominate, may respond to future environmental change.…”