Much of the lives of the most iconic shark species – including the tiger shark – remains a secret, even to shark researchers. This is because these large sharks are capable of migrating thousands of miles across oceans in a single year. One of the most unsolved mystery of sharks’ lives is where adult females go during their pregnancy. Discovering the habitats that are important during this life stage will be critical for creating conservation protections for mother sharks and their developing pups.
Tiger sharks are wide-ranging marine predators that can carry around 10-80 (yes, up to 80!) pups within their womb while pregnant. Finally, after about 15 months in the womb, the mother tiger shark will give birth to the live pups that are around 75 cm long. While we know this basic information about tiger shark reproduction, we have yet to uncover many of the breeding grounds, gestation grounds, and pupping grounds for this migratory species. Discovering this information will require the use of novel technologies, and that’s where our research comes into play.
My name is Brooke Anderson and I am a PhD student at Arizona State University studying sharks, their movements, and their movements relate to reproduction. I am lucky to be a part of this team of researchers trying to figure out if and how pregnant tiger sharks are using the Yarari Sanctuary and the wider Caribbean.
To help solve this mystery, we must first set out to the Saba Bank and do some fishing. Once we catch a large female tiger shark, we will secure her along the research vessel and take several size measurements to confirm that she is healthy and mature. Female tiger sharks mature at a whopping 3 meters in length! We can also examine her for fresh bite marks on her fins or body, which indicates that she had recently mated and could be pregnant.
After we collect this information on her maturity, we will rotate her upside down in the water to initiate tonic immobility. Tonic immobility is a natural reflex in sharks that induces a trance-like state of inactivity. This trance-like state help keeps the shark calm and still for the next part of the workup where my expertise comes into play. I will be able to use a portable ultrasound (from E.I. Medical Imaging) – just like we could use on a human – to get a look inside the shark’s womb for hidden pups. If she is pregnant, we will see on the ultrasound many miniature tiger sharks inside their mother’s womb! We can even use the ultrasound to take measurements and determine the size of the pups – this helps us to estimate how far along in her pregnancy that the mother tiger shark is.
Image of a tiger shark embryo as seen on the portable ultrasound.
Next, we can attach a satellite tag to the mother tiger shark to track where she goes throughout her pregnancy in near real time. This will allow us to determine the extent that the Yarari Sanctuary, Saba Bank, and the surrounding eastern Caribbean are used as important habitats for pregnant tiger sharks for the very first time. With this information, we can help assess the effectiveness of current conservation and management strategies for this near-threatened and ecologically important species. Stay tuned to see if we were able to find pregnant tiger sharks and where they might be headed on their journey to motherhood.
Article included in the Special Edition BioNews: Tiger Shark Expedition