In April this year white shark researchers Michael Domeier and Nicole Nasby-Lucas published a paper detailing some of their research into white shark migration patterns using data from satellite tags. The scientists show that white sharks in the northern Pacific Ocean are generally segregated by gender except when they are at Guadalupe Island, where it is hypothesised that they mate. The females then head offshore while they are pregnant (for about 15-16 months), returning to pupping sites along the coast of Mexico. The complete cycle for a female takes about two years. The evidence from the tracks generated by the tags is very compelling. Furthermore, working backwards using the gestation period for white sharks from when young of the year white sharks start to appear off the Mexican coast implies that mating takes place during the months when white sharks are known to aggregate at Guadalupe Island, which provides further support to the theory.
Domeier’s tagging methods have until recently involved the fairly invasive satellite tags that are bolted onto the sharks’ dorsal fins (see the Shark Men series for how this works, and some photos of a tagged shark here). The tags that provided the results of this study were actually affixed during an Ocearch expedition. The data obtained from such tags is remarkably detailed and will be invaluable in defining conservation objectives for this species.
Barbara Block’s team at Stanford, whose work I profiled yesterday, is responsible for a paper detailing a competing hypothesis, that white sharks in fact mate during their time offshore. This is science at work: competing ideas are not a bad thing at all, as they stimulate original thought and drive the process forward. The theory of the white shark’s life history that is best supported by the observations and experimental data is likely the most accurate one.
Read Domeier and Nasby-Lucas’s paper here. It’s quite short and very easy to understand. These are the kinds of results we can expect to see out of the tagging work done off the South African coastline in early 2012. Can’t wait!
