The unusual discovery in New Zealand waters is based on the remains of scat and scars on seal’s bodies
Leopard seals are apex predators but evidence that they attack sharks puts them in a tiny club of marine species. Photograph: Minden Pictures/Alamy
The Guardian-Eva Corlett in Wellington
In a world first, New Zealand leopard seals have been found to feed on sharks, making them part of a tiny and exclusive club of marine predators that do so.
The study, led by Krista van der Linde of leopardseals.org, found shark remains in the scat of leopard seals, and visible signs of struggle with sharks on seals’ bodies, indicating the marine mammals predate on sharks, rather than scavenge their remains.
“We were blown away to find that sharks were on the menu,” Van der Linde said.
“But then we also found elephant fish and ghost sharks were also being hunted by the leopard seals. These fish have large spines to 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.”
Van der Linde is uncertain why the seals are targeting sharks, especially given how risky it is to hunt them. “There could be something nutritionally about sharks that makes them desirable, it could be sort of a treat for [the seals].”
Leopardseals.org was set up by Van der Linde and Ingrid Visser, an orca expert, to better protect, educate and research the seals. It has the largest collection of leopard seal scat in the world, gathered by researchers and volunteers who comb the coastline collecting it for the project, in a bid to learn more about the rare animals. In 2019, a still-working USB stick was found in a one-year-old frozen scat sample.
“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 said.
Leopard seals are already known as apex predators, who feed on penguins and other seals, but this is the first time evidence has been produced of them hunting sharks.
The leopard seals that arrive on New Zealand’s shores are originally from Antarctica. Their presence in the country’s waters are becoming more common, and research is trying to ascertain whether this is new, or whether more frequently reported sightings give the illusion there are more. The new diet research raises questions for Van der Linde over whether the seals are coming to New Zealand because there is more variation in the diet, and whether climate change could be affecting food sources farther south.
The researchers are still in the process of trying to determine whether leopard seals have always feasted on sharks, or if this is also new. “It kind of indicates to us so far that leopard seals are opportunistic predators that are coming to New Zealand and feasting on whatever they can get their jaws on.”
Van der Linde said it is very rare to see predators feeding on other predators, and that could have implications for the food chain.
“A top predator feeding on another top predator is quite interesting in itself. If the leopard seals do keep increasing in the numbers and then that affects the shark populations, we really don’t know how that will affect things.”