Many species of sharks and rays, such as bull sarks and sawfish are believed to exhibit a behaviour called natal philopatry where females will always return to the same nurseries in which they themselves were born. Shark mothers possibly migrate over 1000s of kilometers every other year to pup in such nurseries, may they be rivers, estuaries or shallow bays. Juveniles will often remain in these nurseries for years at a time, safe from predators and with plenty of food.
Unfortunately, we have no clue about how prevalent this behaviour is along our coastlines and what implications this has for population connectivity and recovery potential of shark populations. If juvenile survival rates decline in particular nurseries because of habitat destruction, pollution or climate change but females always stick to the same nursery to pup instead of pupping somewhere with higher survival rates, will numbers eventually decline? Additionally, if we were to remove all females that pup in the same nursery, who will “refill” that nursery if every other female sticks to their own nurseries?
These are the pressing questions that need answering and to do that I collected as many genetic samples from juveniles and adult female bull sharks as possible to establish relatedness and population structure in bull shark nurseries. I also equipped all the females with acoustic trackers to monitor their movements. In that way, I can test if females always return to rivers where they are already had babies previously. This will reveal the degree of flexibility in natal philopatry and how this affects the overall movements of female bull sharks along the coast, which in turn will tell us about the resilience of shark and rays species with natal philopatry in coastal systems. Ultimately, bull sharks are a model organism that help us understand these patterns in other, rarer species with natal philopatry such as sawfish and hammerheads, which are critically endangered.
But how does one go about this practically? Well, I set out on a 3 month-long solo field trip in which I covered over 10.000 km of driving along the east coast of Australia, capturing and releasing over 250 bull sharks in total using rod and reel. From small, newborn babies of 60cm to large females up to 3m in length! I camped every night, fished for 12 hours each day, avoided crocodiles, got sunburnt (despite sunscreen), got soaking wet in torrential rains about every second day (thanks La Niña) and got bruised and cut (sharks have very rough skin!).
At the end of the day, all of this is more than worth it being able to spend so much quality time outdoors, along the river, out on the ocean and being around amazing animals in order to contribute to their conservation and our scientific as well as public understanding of their behaviours and importance in our coastal systems.