The Port Jackson shark
One of the most common sight in Sydney's waters is this shark who gets the name from the Port Jackson Harbour (more commonly known as Sydney Harbour). Endemic to East and South Australia, they show a very distinct migration pattern. We know that PJs aggregate in shallow areas for breeding from July to October and then they are virtually absent for the rest of the year, this is what our community of divers at VIZ have been observing for two seasons at the main dive sites such as Cabbage Tree Bay:
What scientists have discovered, by tagging and tracing 50 individuals, is where they go.
Every year they leave our waters to swim South, sometimes as far as to Tasmania, 600 km away, and then somehow they manage to come back to the very same reef one year later!
In the references below the link to the research paper with all the details.
But before leaving, females need to lay the eggs they are incubating hence they stay a bit longer compared to males (see graph lower left). As the temperature of the water is rising above what they find comfortable, they move to areas where the water is still a bit colder, and one of those "labour rooms" is what I came across in late November, one month after the PJs have disappeared from the usual sites. 25 PJs, (most of them females) were resting 400m South of Bare Island in the space of 10 meters, in 18 degrees of water temperature. Such an unexpected find!
Marco Bordieri for the Sydney Underwater Gazette, 11/12/2020
Extracts from the research paper:
"It has previously been suggested that female Port Jackson sharks show preference for rocky gutters and crevices on reefs to avoid male harassment, thus allowing them to co-occur at the same sites. Smaller males may avoid rocky gutters and crevices because of the presence of spotted wobbegongs in these habitats, which have previously been found to predate upon Port Jackson sharks"
"Given the lower abundance of males on the reefs because of their earlier departure, females likely delay egg laying to decrease the likelihood of egg mortality by predation by males, thus maximising the survival rates of the eggs laid late in the season."
References: "Long-term migration patterns and bisexual philopatry in a benthic shark species" January 2016, Nathan Charles Bass, Johann Mourier, Nathan A. Knott, Joanna Day, Culum Brown and Tristan Guttridge, CSIRO PUBLISHING
Thanks to Dr Julianna Kadar, Port Jackson shark specialist, for the help in putting together this article and for hosting the excellent presentation on Port Jackson sharks, recording available on Youtube, below
Below, an excellent documentary on the Port Jackson sharks