What does removing sharks do to ecosystems?

Shark loss can have major effects on their ecosystems say researchers studying great white sharks in  South Africa and reefs in northern Australia.

“The removal of a top predator leads to cascading effects on the marine food web” says Dr Neil Hammerschlag of the Shark Research foundation.

Hammerchlag, then at the University of Miami, Florida, and colleagues from the University of Lancaster, U.K., documented great white shark disappearance, and its effects on the local marine food web in a paper published in Frontiers in Marine Science. 

Great whites used to feed on Cape fur seals and sevengill sharks in False Bay, near Cape Town, South Africa. 

Then they vanished.

Hammerchlag, then at the University of Miami, Florida, and colleagues from the University of Lancaster, U.K., documented their disappearance, and its effects on the local marine food web in a paper published in Frontiers in Marine Science. 

These apex predators disappeared from the surveys around 2016, Hammerschlag says, probably wiped out by beach nets, and a pod of orcas which developed a taste for shark liver.

Without the white sharks, their usual prey, seals and sevengill sharks increased, putting more pressure on their prey — anchovies, horse mackerel, smooth hound sharks and pyjama cat sharks — which progressively impacted more branches of the web. 

“The use of underwater video surveys conducted more than a decade apart provided us with a snapshot of the food web both before and after the disappearance of white sharks from False Bay,” says coauthor, Yakira Herskowitz.  

“Without these apex predators to regulate populations, we are seeing measurable changes that could have long-term effects on ocean health,” Hammerschlag adds.

Dr Mark Meekan of the University of Western Australia agrees that trophic cascades driven by the loss of top predators can have major effects on food webs.

Meekan told Cosmos that he and his colleagues hypothesise that shark removal could be responsible for crown-of-thorns-starfish outbreaks on the Great Barrier Reef.

The researchers reviewed dietary analysis using DNA, studies of feeding by lower-order predatory fish and experiments. Their paper is published in Nature Communications Biology

Coral reefs are one of the most threatened tropical ecosystems on earth and the coral-eating crown of thorns starfish (CoTS) are contributing to their decline, says Meekan

“Outbreaks of crown-of-thorns starfish are responsible for extensive loss of reef-building corals on the Great Barrier Reef and elsewhere,” says the Australian Institute of Marine Science (AIMS)

Meekan traces the decline in some northern Australian reef sharks, to harvesting by Indonesian fishermen.

Scott Reef, 270km off the coast of north-western Australia, is within the traditional fishing zone designated by the Australian and Indonesian governments in a Memorandum of Understanding (MOU).

The MoU requires that Indonesian fishers use traditional sail-powered fishing vessels and non-motorised equipment, and prohibits them from taking protected species such as turtles, dugongs and clams, but places no limitation on sea cucumbers (bêche-de-mer), trochus (topshell), reef fish and sharks. 

“It’s pursued in a very low tech way, says Meekan, by very poor fishermen from Indonesia, in dug-out canoes with a motor, puttering down to the reefs of Northern Australia.” Sharks are harvested for their fins. “You don’t need refrigeration or anything. You can dry it out on the boat and the shark fins are very high value, commodities, basically”

Sharks have low reproductive rates and are very curious, meaning they come to baits readily, says Meeken.

Meekan says top-predator removal means that their prey — fish such as snappers and emperors, can leave their usual invertebrate fare, things like marine snails and starfish, and move up into the water column to feed on small fish and squid, and the odd bait on a hook.

“They don’t have to eat the spiny, crunchy things on the bottom anymore, heading instead for the delicious squid and small fish.”

“It’s a predator release for the invertebrates on this reef and one of those is the crown of thorns starfish”

“We think that this predator release has a major effect on crown of thorns starfish (CoTS) outbreaks.

“Without emperors feeding on the very young CoTS, the mortality bottleneck is removed, and you get these big waves of starfish.”

There has been a lot of argument around the causes of CoTS outbreaks, whether its algal blooms providing more food for larvae or removal of predators.

Meeken says: “No one can deny that crown thorns outbreaks happen in many isolated coral reefs in the middle of nowhere, where they’re sitting in very nutrient-poor oceans — the Red Sea, the Seychelles, the Maldives.  There’s lots of places where CoTS are outbreaking without having huge terrestrial input of nutrients.

“One thing they all have in common is fishing, unless it’s a well-policed Marine Protected Area, and the first thing fishing targets are the top-order predators.”

Both the False Bay white shark disappearance and the potential links to CoTS on the GBR point to the same thing.

As Hammerschlag puts it: “The findings [of both studies] emphasize the importance of global shark conservation efforts, as their loss could have long term consequences on marine ecosystems. Given the global reliance on healthy oceans for food, recreation, and ecosystem services, protecting large sharks is essential to maintaining biodiversity.

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