Of all apex predators, the white shark Carchardon carcharias (commonly known as the great white) is perhaps the most fascinating. The potential danger from (very rare) human interaction has embedded the species in our national consciousness.
Debate as to the size and status of the white shark populations across the globe is both vigorous and often contested, and it is fair to say we have never had an accurate picture. Now, for the first time we estimate that the total number of adult sharks across the Australasian region is around 2,210. We’re lacking data on juvenile sharks in one region so it’s difficult to say what the total Australasian population is, but it’s likely to be in excess of 8,000-10,000 animals.
CSIRO researchers working with Australian and New Zealand scientists in the National Environmental Science Program have used world-first genetic analysis to investigate white shark populations. The results of this project, published on Thursday in the journal Scientific Reports, are the first estimates of white shark adult population size, trend and survival rates for the Australasian region.
One fish, two fish…
The widely used aphorism in marine and particularly fisheries modelling is that counting fish is like counting trees, but you can’t see them and they move around all the time.
Until now, researchers have had to rely on patchy sources, like historical catch data. The various shark control programs do not provide usable data on relative density over time. We do have information on white shark migration and population structure from electronic tagging and previous genetic studies, but these don’t tell us about shark numbers.
To address this key problem we worked with colleagues across Australia and New Zealand to use a highly novel method called close-kin mark-recapture, first developed by CSIRO in the late 2000s to monitor southern bluefin tuna.
Close-kin mark-recapture first involves taking a tissue sample from a shark, alive or dead, obtaining a genetic profile of the animal, and than comparing it to all the other sharks and asking: are these sharks related, and if so how are they related?
Due to a number of factors, it is easier to take genetic samples from juvenile white sharks (in the 3.5 to 4m or less range) than from adults.
In the first phase, we compared the genetic data from juvenile white sharks to look for half-sibling pairs – animals who shared either a mother or a father. The half-siblings are the close-kin side of the problem. The chances of finding these pairs in the samples is determined by (a) the size of the adult population, and (b) the survival rate of adult sharks.
Higher numbers of sharks, or sharks with low survival rates, make it less likely to find siblings in the samples.
This linkage between a specific type of relatedness (half brothers or sisters) and the size and survival rate of the adult population is the mark-recapture side of the equation. In traditional wildlife tagging studies, we “mark” an animal in some way (physically or in terms of visual or genetic ID) and try to “recapture” it again sometime in the future.
The mark-recapture principle is exactly the same with this method. The key difference is that a juvenile shark carries the “mark” of its parent within its DNA, which is “recaptured” when you find a half-sibling pair. Find enough of these half-siblings, and you can estimate both adult numbers and survival rates.
Currently, we believe there are two main populations of white shark in the Australasian region: the “Eastern” population, which is basically everything to the east of Bass Strait (including New Zealand), and the “Southern-Western” population, which appears to range from west of Bass Strait, around the South Australia and West Australia coasts as far north as Ningaloo Reef.