Do you want to swim with sharks? What you should know about the biggest predators

Image copyright Scripps National Oceanographic Center Image caption Do you want to swim with sharks?

Humans are not dangerous to sharks, thank you very much.

Because not only are humans not dangerous to sharks, we’re actually a hazard to sharks in just a few places: northern California, Florida and Puget Sound, Washington State.

Sharks of this size typically live around the equator, at water temperatures just below 40 degrees Fahrenheit (4 degrees Celsius).

But in Pacific Ocean, we get these pretty big sharks, as big as 50 feet long. In San Francisco’s Alcatraz Island, visitors can swim with sharks.

Then there are Texas and New Jersey, which are inhabited by white sharks.

This is thanks to feeding between Cape Cod and their feeding grounds in New England.

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‘Do you want to fish with sharks?’

If you go beyond Florida and Puget Sound, sharks start disappearing. One reason might be that, when the ocean starts heating up, we start making it harder for them to turn on the spigot of fish food they can eat, such as cod.

If sharks’ food is becoming scarcer, their prey will be less likely to be available.

Sperm whales might be big enough to handle that extra weight, but less complex marine mammals, such as seals, don’t fare as well. The seals’ eating cycle is also disrupted as they move up the food chain to get more fish that sharks get.

In fact, over the past 30 years, researchers studying giant porpoises on Alaska’s Little Diomede Island have noticed fewer and fewer sperm whales.

Vast number of whale deaths?

Probably not, says Mark Setzer, a whale-entomologist at the National Oceanic and Atmospheric Administration. As of last year, there were 26,517 more whales than there were in 1982, when his team made their first detailed census.

In that case, the deaths were being swept into the North Pacific, Mr Setzer said.

But the human and shark numbers are not so different – 40,000 to 50,000 sharks and 10,000 to 20,000 sharks, though the exact opposite may be true.

Probably the more accurate comparison is to pelagic shark populations, which extend beyond the coast of the United States to the main migratory path for sharks, the North Atlantic Gyre.

Pelagic shark populations should be growing because they’re migrating north – that’s a normal, expected response to warmer waters.

Instead, the number of positive detection tags on sharks has dropped by 23% since 2004.

So maybe we’re bringing some shark populations down, but probably not counting all of them in our population counts, Mr Setzer said.

‘We may be taking sharks out of their natural habitat’

When sightings do happen, it’s too often not reported. (For the record, there were 1,018 big fish catches in the commercial fishing industry in 2018, according to the National Marine Fisheries Service.)

A typical story begins with a fishing trip, in which a fisherman gets a few grey or minnow-like fish caught on the hooks. They’re so small that they “stick out,” said Mr Setzer.

That’s how it starts. If he reaches deeper, he sees that there’s a huge shark – a great white, mako, greyhound or reef shark. He captures it with his boat or nets, then sometimes helps take it to the net spa, where a marine biologist relabels it a big-fish tag.

But still the shark is not reported to biologists. When Mr Setzer revisits the area, he’s often able to find it swimming by the dock, where it’s released into the environment.

In fact, he calls it “trash pile shark”.

Another risk is if you swim near a beach known for sea turtles, which often come ashore and breed.

The amount of sharks seen in Puget Sound have dropped since the 1950s.

Mr Setzer and his colleagues think the low marks have less to do with the fact that sharks are no longer there, and more to do with more people coming to the beach.

In 2017, near the age of our grandchildren, there were more rescues of unguarded sea turtles than for any other reason. Sea turtles are less strong swimmers than sharks, Mr Setzer points out.

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