Port Jackson sharks are quite common in NSW waters. During the austral winter–spring season, the adults move onto shallow rocky reefs along the coast to mate. From late October to early July, they migrate to unkown locations. If you dive or snorkel, surely you’ve seen them swimming around or stacked up in crevices and caves. Yet, how much do you know about these sharks?
Understanding the movements, habitat preferences and the behaviour of species like the Port Jackson shark is very important to understand their role in our ecosystems and assess potential effects of human impacts on their populations.
For this reason, the BEEF Lab has been tagging and monitoring the movements of Port Jackson sharks in the Jervis Bay Marine Park since 2012. We use acoustic tags, a small device that sends acoustic pulses or “pings” through the water. These “pings” are then detected by hydrophone receivers deployed in the bottom of the ocean. The Jervis Bay Marine Park has some of these receivers around and at the entrance of the bay, so we know when our sharks get in or out of the bay and have a general idea of where they like to hang out within the bay.
So far, we’ve found that PJs show very high levels of site fidelity at their breeding aggregation sites. This means that throughout the winter mating season and from year to year, these sharks have been coming back to specific reefs within the JB Marine Park. Interestingly, we also found that males show even higher fidelity than the females – on average, 98% of all male detections occurred at their preferred location in Jervis bay!
When we looked at their movements and activity during the course of the day, we found that it depended a lot on the sharks. Some of them were more active around dawn and night time, while others were more diurnal.
These results are pretty impressive, and for now lead to a few more questions. Why do they come back to these specific reefs? And how do they know which way to go when they return every year? Moreover, the fact that they have some degree of individual variation in their behaviour means we should be careful when generalizing some observed behaviours to the whole species or population.