Segregation or aggregation? Sex‐specific patterns in the seasonal occurrence of white sharks Carcharodon carcharias at the Neptune Islands, South Australia

Abstract

The seasonal patterns of occurrence of male and female white sharks Carcharodon carcharias at the Neptune Islands in South Australia were reviewed. Analyses of a 14 year data series indicate that females seasonally aggregate in late autumn and winter coinciding with the maximum in-water availability of lactating female long-nose fur seals and seal pups. During this period, observed male:female sex ratios were similar; whereas during late spring and summer, males continued to visit, but females were rarely recorded. There was no evidence for segregation by sex or size at the Neptunes, but the highly focused seasonal pattern of occurrence of females compared with the year-round records of males suggests that there are likely to be differences between the sexes in overall distribution and movement patterns across southern Australia. It is suggested that foraging strategies and prey selection differ between sexes in C. carcharias across the life-history stages represented and that sex-specific foraging strategies may play an important role in structuring movement patterns and the sex ratios observed at such aggregation sites. Differences between sexes in distribution, movement patterns and foraging strategies are likely to have implications for modelling the consequences of fisheries by-catch between regions or jurisdictions and other spatially or temporally discrete anthropogenic effects on C. carcharias populations. Such differences urge for caution when estimating the size of C. carcharias populations based on observations at pinniped colonies due to the likelihood of sex-specific differences in movements and patterns of residency. These differences also suggest a need to account for sex-specific movement patterns and distribution in population and movement models as well as under conservation actions.

Spatiotemporal distribution patterns of immature Australasian white sharks (Carcharodon carcharias)

“…Sex-based differences in migratory patterns of juvenile and adult white sharks have been observed throughout their range, e.g. the northeast and central Pacific 13,44 , Australia 45 and South Africa 46 . Male-biased dispersal has previously been identified between Australasian and South African white sharks based on contrasting maternally and biparentally inherited genetic markers 47 .…”

Size-specific apparent survival rate estimates of white sharks using mark–recapture models

“…For many white shark populations around the world, researchers have reported male-dominant sex ratios (Chapple et al 2011;Domeier and Nasby-Lucas 2012;Kock et al 2013;Bruce and Bradford 2015). In this study, we estimated two males for every female shark in our sample over a period of 9 years after probabilistically assigning sex to individuals that remained unsexed throughout the study period.…”

“…Chapple et al (2016) reported that annual temporary emigration is more prevalent for females (0.57, 95% confidence interval (CI) = 0.40-0.72) than males (0.015, 95% CI = 0.00-0.31). This sex-specific unavailability for sampling occasions can inherently bias the observed sex ratio over a long study period and needs to be considered when conducting population dynamic studies (Bruce and Bradford 2015).…”

Communal and efficient movement routines can develop spontaneously through public information use

“…Animal aggregations can yield some of the most striking spatial patterns in nature. They are observed in almost all taxa (Ims 1988;Sumpter 2006;Bruce and Bradford 2015;Venkataraman et al 2017), and may comprise several species (Heymann and Buchanan-Smith 2000;Goodale et al 2010). In some cases, they result directly from the existence of large-scale resource heterogeneity, but in others, animals aggregate more than what would be expected on the basis of resource distribution alone (Brown and Orians 1970;Jeanson et al 2005).…”