Tiger, tiger

SHARKS: FEARED, ENDANGERED, THE ONLY SEAFOOD NOT WELCOME AT A BARBIE, and an essential part of the marine ecosystem. Now new research into one of the world’s largest populations of adult tiger sharks, found around Norfolk Island, is telling us about the extent of microplastics in our oceans - but also why these huge sea creatures just don’t seem to be interested in us.

Charlie Huveneers has had a thing for sharks nearly all his life. The Flinders University professor is the director of the Marine and Coastal Research Consortium and also research leader of the Southern Shark Ecology Group - but all that had humble beginnings.

“I’ve been interested in sharks since I was 12 years old,” he says. “Like most kids I was a bit afraid of them but wanted to learn more. So many of the books I read talked about how little we knew about sharks and that fascinated me - and that has grown into my life’s work.”

“Like most kids I was a bitafraid of them (sharks)but wanted to learn more.”

Most of his current work is related to shark bite mitigation and wildlife tourism around the world, but his recent focus on Norfolk Island has produced some quite surprising results. “Our shark programme on Norfolk Island started in 2020,” Charlie says, “and originated from the need for tiger shark samples, and stories emerging about Norfolk Island having a large number of tiger sharks. But, we never dreamt of being able to catch, sample, and tag as many sharks as we did. We took ten tags with us on our first field trip and would have been happy deploying five to seven of those tags, but ended up catching 30 large tiger sharks in only six days. And it has been the same story every year since. Over the last three years we caught, sampled and tagged 90 different tiger sharks, all from one small bay. These weren’t small tiger sharks either, and averaged 4m with many sharks reaching 4.5m.”

Which of course begs the question of just why you would swim with the sharks, and the ones in Norfolk in particular.

“We found that the main prey of Norfolk Island’s tiger sharks are mutton birds, which was unexpected”

“We first decided to go to Norfolk Island for the project of a post-doctoral fellow, Dr Lauren Meyer. Lauren is being supported by Georgia Aquarium (in Atlanta, USA) and is collaborating with Canadian scientists to understand microplastic loads in top predators. With tiger sharks occurring worldwide and having a broad and diverse diet, they were a logical, ideal species to investigate how much microplastic load varies between locations. Being so remote we expected Norfolk Island to have low microplastic levels, providing a good natural baseline or control location.

“As we were catching these sharks to collect samples, we also had the opportunity to tag them with acoustic and satellite tags thanks to the support of the Neiser Foundation. These tags showed us that tiger sharks are not permanently resident around Norfolk Island, and mostly use the island from November/December to May/ June. The rest of the year they travel far and wide to Fiji, Vanuatu, Australia, and New Zealand, and often spend extended time off New Caledonia. We already knew that tiger sharks in the Caribbean and Atlantic can travel long distances, but had no idea what the Norfolk Island tiger sharks did. Also, we found that the main prey of Norfolk Island’s tiger sharks are mutton birds, which was unexpected. As pelagic birds are notorious for consuming plastics, this could be a concerning source of plastic accumulation in tiger sharks.”

Should tiger sharks be a concern to us? “Tiger sharks have been responsible for many shark bites worldwide and are responsible for the most fatal shark bites in some regions like Hawaii. But tiger sharks have never been an issue around Norfolk Island,” Charlie says. “Even though the spot where we tagged these 90 tiger sharks is only a few kilometres away from popular surf spots, surfers have hardly ever seen a tiger shark while surfing, but I can nearly guarantee that tiger sharks have seen surfers! Whether this is linked to disposal practices is currently being assessed by our team. And while some shark species can be resilient to human impacts and overfishing, many species are at elevated risk of extinction. Tiger sharks are globally listed as Near Threatened on the IUCN Red List, but it appears that Norfolk Island has a healthy population of tiger sharks.”

The work of people like Charlie Huveneers and his team will ensure those marine ecosystems continue to be safe - and teach us all about what our addiction to plastics might mean for our top marine predators.