Leopard seal with a shark spine lodged in its face. Photo / Kirsty Moffatt
The groundbreaking discovery this week that New Zealand leopard seals feed on sharks has come as a surprise to Northland marine biologist Dr Ingrid Visser.
The discovery is a world first after years of research conducted by a New Zealand group that highlights sharks are not always top of the food chain.
Visser, renowned for her role as a scientist at Orca Research Trust, is co-founder of LeopardSeals.org and a co-author of the study, and said she had no idea leopard seals were eating sharks, until they started this research.
"As this is a world first, we didn't expect it based on what other researchers have found.
"I've been researching one of New Zealand's top predators, the orca, for 30 years and I've seen them eat sharks on a regular basis. To know that there is another marine mammal also munching on sharks, well, that has implications for the whole food web and our understanding of how it all is interlinked."
She described it as groundbreaking and interesting to see how complex the ecosystem is, "when you have top predators predating on top predators and it really helps us understand how complex the marine environment is".
The study was carried out over several years by LeopardSeals.org, which sorted through over 100 seal poops (technically referred to as scat). Senior author of the study Krista van der Linde said new findings show this was the first record of leopard seals, anywhere in the world, feeding on sharks.
"Leopard seals [research] in New Zealand is still in its infancy and we have been building on our knowledge in recent years. When we founded LeopardSeals.org I knew we were going to find some interesting things, but this is the next level of incredible."
Van der Linde added, "The relatively high number of shark remains also indicates that sharks could be an important part of the seals' diet around New Zealand."
While leopard seals are known as apex predators who feed on a variety of prey such as penguins and other seals, sharks have now been discovered to be an important menu item.
Co-author of the study, Te Papa Museum's Dr Lara Shepherd said, "We looked at the remains left in the scat and combined the morphological examination with DNA sequencing, just like a real-world CSI show in order to identify the shark remains."
This study continued to produce surprises when the researchers found two other unique cartilaginous (non-bony) fish, both close relatives of sharks, were part of the seals' diet.
Van der Linde added, "We were blown away to find that sharks were on the menu, but then we also found that elephant fish and ghost sharks were being hunted by the leopard seals. These fish have large spines which help protect them from predators and sure enough, there were wounds on the leopard seals, sometimes even big spines embedded in their faces – one leopard seal had at least 14 such wounds."
It was not known if leopard seals' presence was becoming more common in New Zealand in recent years or if "people were becoming better scientists with the use of phone cameras". Van der Linde said it was also unknown at this point which countries they were arriving from.
At least three leopard seals had been commonly sighted around Northland in recent years, named Hātea, of the Town Basin, Owha, of Marsden Cove and Novy, of Urquharts Bay. However, there had been no reports of leopard seals in Northland this year.
''The last one was Owha, in Marsden Cove, so we are super-keen to hear if anyone has seen any, and we are always interested in photos so we can try and match the individuals to our identification catalogue."
Said Visser: "The key to doing this type of research is the involvement of citizen scientists – and that means anyone can help out."
For further information, go to leopardseals.org, the Leopard Seals NZ Facebook page or call 0800LEOPARD.