• White Sharks – Population Monitoring, Policy and Review

    • 20 October 2012
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    This post follows on from my review of Global Perspectives on the Biology and Life History of the White Shark. That book (a collection of scientific papers) is divided into three sections, and I’m going to highlight papers that I found particularly interesting in each of the sections.

    Here’s the first series of posts I did, on Biology, Behaviour and Physiology:

    Here’s the post I did on Migratory Patterns and Habitat Use.

    Application of Molecular Genetics for Conservation of the Great White Shark, Carcharodon carcharias, L. 1758- Gubili et al

    I didn’t follow a lot of the genetic discussion in this paper, but the gist of it is that white sharks are known – at least across part of their range – to exhibit philopatry, which means that females across generations return to the same sites for breeding. These regions are thus highly sensitive and critical to the survival of the species. Genetic testing indicates several distinct populations of white sharks which, although each of them conduct lengthy migrations, do not as a rule interbreed. The authors state that more information is required before management areas can be defined for white sharks. They conclude with the following:

    It is increasingly recognised that management policies are formulated by cooperation between scientists and decision makers, who must have up to date relevant information on which to base legislation.

    I found this a somewhat prescient reminder of the sentiments that had to be reiterated in South Africa to justify the work done with Ocearch in the first half of 2012.

    Use of Photo Identification to Describe a White Shark Aggregation at Guadalupe Island, Mexico – Nasby-Lucas, Domeier

    The authors used a variety of characteristic markers on white sharks’ bodies to identify individuals as they returned to Guadalupe Island off the Mexican coast. They matched sharks from year to year using their pigmentation patterns over the gill flaps, the shape of their pelvic, caudal (tail) and dorsal fin, and their gender. The study was done from 2001 to 2009. The authors took pictures, but they also received images from divers who visited the island to dive with the white sharks (in cages). Projects like these, that make use of data from laypeople are very exciting!

    The data led to the identification of 67 male and 46 female sharks (113 total). 83% of the sharks were sighted in more than one year. The longest gap between sightings of a shark was six years. The authors found that male sharks seasonally arrived at Guadalupe Island earlier than the females, by over a month. The female sharks were more often sighted in nonconsecutive years, reflecting that they have a nearly 2 year breeding cycle including an 18 month pregnancy.

    Implications of Increasing Pinniped Populations on the Diet and Abundance of White Sharks off the Coast of Massachusetts – Skomal, Chisholm, Correia

    White sharks are relatively rare in the north Atlantic, and no tagging studies have been done there. This paper describes an increase in white shark sightings and predations on pinnipeds (seals and sea lions) off the coast of Massachusetts in the north eastern United States. The authors contend that rather than implying an increase in the white shark population, the sightings should probably be attributed to the sharks expanding their diet beyond scavenging whale carcasses, as more seals become available as a result of protection accorded to them in legislation over the last few decades. The authors believe that this shift by the sharks to eating primarily seals is a reversion to a pre-existing scenario, and that the scavenging of whale carcasses was a response to low pinniped populations dating back several hundred years.

    The Use of a Nonlethal Technique to Assess the Reproductive Biology of the White Shark, Carcharodon carcharias - Sulikowski, Williams, Domeier

    This paper describes research conducted in the same manner to that recently conduced in Southern African waters. The white sharks were hooked, and brought on board a special cradle alongside the MV Ocean, the Ocearch research vessel. The research was done at Guadalupe Island off Mexico, and the Farallon Islands off California. The paper describes the kinds of tests that could be done on the blood extracted from the sharks, and the information that could be inferred from an examination of the claspers of the males.

    A Review of Research on the White Shark, Carcharodon carcharias (Linnaeus), in Southern Africa  – Dudley

    White shark research in South Africa has been going on for some 150 years. Research from the KwaZulu Natal shark nets has shed light on diet, parasites, age and growth patterns. No adult females and few mature males have been captured in KZN, leaving a gap in our knowledge. It is suspected that there is a pupping ground for white sharks in the Eastern Cape; this has yet to be confirmed. The movements and location of adult female white sharks in South African waters are mysterious.

    Research in the Western Cape has been around predator-prey interactions with Cape fur seals, possible conditioning as a result of eco-tourism, and movements around False Bay and some of the surrounding regions.

    This paper is an excellent survey of current, South African-based white shark research. I was heartened to see that hopefully many of the research priorities identified can be addressed with data collected on the recent Ocearch expedition in South African waters. Questions about parasites, sexual maturity and fecundity, and of course habitat use will all be addressed by the blood and tissue samples and data from the tags deployed on the project.

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