Juvenile genes reveal shark numbers

Genetic sampling of juveniles has produced the best population estimate yet for Great White Shark numbers off the coast of Australia.

The Great White (Carcharodon carcharias) inspires both awe and fear among people in many parts of the world. It is a species that enjoys a global distribution, lives for more than half a century and can grow up to 6.5 metres long.

However, it reproduces comparatively slowly – meaning it is vulnerable to human predation, whether deliberate or accidental.

As a peak predator, its numbers are naturally low and its range vast, so compiling reliable population estimates is a challenging exercise.

Previous attempts have used photography to identify individual animals, or a tag-and-release strategy. Both produce unreliable results, with low numbers (and frequent tag loss) making results uncertain.

To try to rectify this, a team led by Rich Hillary, principal research scientist at Australia’s peak state-owned research organisation, the CSIRO, decided to adopt a different approach.

Instead of trying to count the adult sharks, they opted to take tissue samples from juveniles in a bid to determine their genetic relatedness. They did this by using catch-and-release strategies at known Great White hotspots, augmented by material recovered from sharks killed as fishing bycatch.

Genetic material was then used to identify half-sibling pairs – sharks that share one parent – and unrelated pairs. This information, combined with additional acoustic tag data, and allowed Hillary and his colleagues to estimate juvenile and adult abundance, population trend, and survival rate.

There are two distinct Great White populations off the coast of Australia – one that extends along the east coast and down to New Zealand, and another that extends along the southern and western coasts.

The research concentrated on the eastern population, and information was obtained from 183 individual sharks.

Based on extrapolations from the data, the team estimate that the eastern population comprises between just 280 and 650 adults. Adding in the juvenile sharks takes the total population estimate to between 2500 and 6750.

Despite the small numbers, the implication for Great White shark numbers is good. Adult survival rate – that is, the proportion of adults expected to still be alive in one year – is 90%, with the juvenile rate at around 75%.

Hillary and colleagues acknowledge that their estimates are quite wide – a consequence of small sample sizes. They suggest their sibling genetic approach, however, can be usefully applied to other species, with estimates automatically narrowing in line with increases in the available information secured.

The research is published in the journal Scientific Reports.