Counting Adults by Studying Offspring

One of the most difficult tasks in shark conservation is estimating population numbers, particularly for the large pelagic sharks that spend much of their time out of sight.  For animals such as white sharks, where protection is controversial because of the public fear associated with shark bites, population assessments are even more critical.  Given my background in genetics, I’ve been interested in (and a bit skeptical of) a new technique to estimate total shark population numbers by determining the number of juvenile half-siblings, individuals which share one parent, within the population.  Using this method, the genetic information of the pups serves as a sort of “proxy” for the genetic profiles of the parents, and therefore the number of adult animals.  A recent paper by Hillary et al used this approach to determine white shark populations along the eastern coast of Australia and New Zealand, work that was also presented at the recent 5th International Whale Shark Conference in Exmouth, Western Australia.

Typical population studies are done by counting actual animal numbers, or through mark-recapture studies where animals are tagged or photo-identified and the number of new versus returning animals is examined at a later time.  These are time- and labor-intensive methods, and are impacted by biological variables such as age and breeding activities, and by ecological variables such as changes in food supply that can alter return rates to the study site.  Fisheries data can also be used to estimate population size, but fishing intensities vary over time, and poor record-keeping often renders such data inconsistent.  Traditional genetic analysis can also be used to estimate population size, determining levels of genetic variability among animals sampled from one or more populations, but is most effective when large numbers of individuals can be analyzed.

A variation on standard population genetics is the technique called Close-Kin Mark-Recapture (CKMR).  CKMR uses genetic profiles obtained from juvenile animals to determine the ratio of half siblings to unrelated individuals in the sample set.  If the ages of the juvenile animals is known (this can be determined from vertebral band counts for dead animals, or estimated from body length for live ones) the number of adult animals can be estimated and adult survival times can be calculated.

It is known that white sharks off Eastern Australia and New Zealand form a single mobile population, with juveniles remaining coastal, while adults range farther offshore.  The open sea habitat of adult white sharks makes it difficult to obtain tissue samples in large numbers, but coastal juveniles are more accessible.  Hillary et al sampled live individuals transiently captured, as well as dead sharks from fisheries, and genotyped them at more than 2500 positions within the genome.  Using the genetic profiles of 100 juvenile sharks sampled over a 15-year period, the authors identified 20 half sibling pairs (those that share 50% of the markers analyzed), and sequenced the full genomes of these 20 animals.  This data was used to compute the number of adult sharks that would be expected to produce such a proportion of half siblings.  For this particular white shark population, CKMR determined that there are 280-650 adult sharks, and a total population size of 2500 to 6750 animals.  This large range is typical of most population estimates that result from indirect methods of estimating animal numbers.  Future studies should incorporate more animals sampled over a longer interval to produce a more precise population estimate, and to follow population changes over time.

As adult animals have more offspring the longer they live, CKMR can also estimate adult survival from juvenile genetic data.  In this study adult survival was found to be high, greater than 90% over the 15-year time period used for modeling.  Many of the juveniles that were restrained for biopsy sampling also had acoustic transmitter tags attached, allowing them to be tracked along the Australia and New Zealand coasts where acoustic listening stations have been placed.  This technique can directly measure survival during the juvenile period, and the authors found that within the study population juvenile survival was greater than 70%.  While overall white shark numbers in the area are low, the apparent high survival rates may somewhat offset the population small size.

It is difficult from one relatively small dataset alone to predict the conservation status of East Australian white sharks, but CKMR studies suggest a population that is likely stable, but is not increasing.  High survival rates of juveniles are a positive sign for future of the population.

The paper is: Hillary RM, Bravington MV, Patterson TA, Grewe P, Bradford R, Feutry P, Gunasekera R, Peddemors V, Werry J, Francis MP, Duffy CAJ, Bruce BD. (2018) Genetic relatedness reveals total population size of white sharks in eastern Australia and New Zealand. Scientific Reports, 8:2661. DOI: 10.1038/s41598-018-20593-w