Sustained reduction in numbers of Australian fur seal pups: Implications for future population monitoring

Abstract

Fur seal populations in the Southern Hemisphere were plundered in the late 1700s and early 1800s to provide fur for a clothing industry. Millions of seals were killed resulting in potentially major ecosystem changes across the Southern Hemisphere, the consequences of which are unknown today. Following more than a century of population suppression, partly through on-going harvesting, many of the fur seal populations started to recover in the late 1900s. Australian fur seals (Arctocephalus pusillus doriferus), one of the most geographically constrained fur seal species, followed this trend. From the 1940s to 1986, pup production remained at approximately 10,000 per year, then significant growth commenced. By 2007, live pup abundance had recovered to approximately 21,400 per year and recovery was expected to continue However, a species-wide survey in 2013 recorded a 20% decline, to approximately 16,500 live pups. It was not known if this decline was due to 2013 being a poor breeding year or a true population reduction. Here we report the results of a population-wide survey conducted in 2017 and annual monitoring at the most productive colony, Seal Rocks, Victoria that recorded a large decline in live pup abundance (-28%). Sustained lower pup numbers at Seal Rocks from annual counts between 2012–2017 (mean = 2908 ± 372 SD), as well as the population-wide estimate of 16,903 live pups in 2017, suggest that the pup numbers for the total population have remained at the lower level observed in 2013 and that the 5-yearly census results are not anomalies or representative of poor breeding seasons. Potential reasons for the decline, which did not occur range-wide but predominantly in the most populated and long-standing breeding sites, are discussed. To enhance adaptive management of this species, methods for future monitoring of the population are also presented. Australian fur seals occupy several distinct regions influenced by different currents and upwellings: range-wide pup abundance monitoring enables comparisons of ecosystem status across these regions. Forces driving change in Australian fur seal pup numbers are likely to play across other marine ecosystems, particularly in the Southern Hemisphere where most fur seals live.

Pacific Gulls (Larus pacificus) as Potential Vectors of Coxiella burnetii in an Australian Fur Seal Breeding Colony

“…The high prevalence of oral and cloacal swabs of PGs sampled on KI in the present study supports the hypothesis that most gulls feeding on placentas at this location are exposed to C. burnetii. Kanowna Island is the third largest breeding colony for AuFS, having approximately 15,000 animals present on the island [ 6 ] and more than 2200 pups born annually [ 16 ]. A prevalence of up to 56.7% for C. burnetii in AusFS placentas has been reported from KI during the peak of pupping [ 7 ].…”

“…It was first described in marine mammals in the southern hemisphere, in 2022, in Australian fur seals (AusFS, Arctocephalus pusillus doriferus ) breeding on Kanowna Island (KI) in the Bass Strait, southeastern Australia [ 5 ]. Australian fur seals have recently had decreased pup production, more obviously in their largest breeding colony at Seal Rocks [ 6 ]. A post-pupping environmental DNA prevalence of over 90% for C. burnetii has been detected in this breeding colony [ 7 ].…”

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

“…Similar to their conspecific, the Cape fur seal (Arctocephalus pusillus pusillus; CFS; Warneke and Shaughnessy 1985), Australian fur seal populations have undergone a recovery. However, it is estimated that current population levels of AUFS represent less than half pre-exploitation levels, and have now either stabilised or are in decline, following an increase until 2007 (McIntosh et al 2018;McIntosh et al 2022). Despite this overall trend, a number of new colonies were detected in 2013 and 2017 surveys (McIntosh et al 2018(McIntosh et al , 2022, one of which was The Needles, south-west Tasmania.…”

“…However, it is estimated that current population levels of AUFS represent less than half pre-exploitation levels, and have now either stabilised or are in decline, following an increase until 2007 (McIntosh et al 2018;McIntosh et al 2022). Despite this overall trend, a number of new colonies were detected in 2013 and 2017 surveys (McIntosh et al 2018(McIntosh et al , 2022, one of which was The Needles, south-west Tasmania. This site is over 300 km from Tenth Island, which is at the southern edge of their core breeding range, therefore representing a southward range shift.…”

“…This new colony represents a shift in the recent breeding distribution, with associated differences in environmental conditions, and so requires investigation. Tenth Island is subject to variability in pup survival due to its susceptibility to wave inundation during summer (December to February) storms, although there is a long-term morphometric dataset for pups there making it a useful site for monitoring (McIntosh et al 2022). Understanding the mechanistic processes through which fur seals respond to their environment at each location may help identify differences in trends or give insight into the rapid expansion of breeding at the southern edge of their range.…”