- [Speaker] Sharks are turning up at U.S. beaches
farther north sooner and staying for longer,
ditching their usual hangouts for cooler waters.
- Now, when white sharks and other sharks like bulls
and tigers are moving further north,
it's less exploratory and more we're going here
to hang out for a while.
So it's an incredible change.
- [Speaker] Experts say that historically,
many sharks are pretty predictable
in their migration patterns.
- Most sharks, they have sort of like this set course
and this set pattern of like where they go,
when they go.
Once they're sort of adults,
they sort of go back and forth between these places.
- [Speaker] Tiger sharks typically spend the winter
in tropical waters around Florida and The Bahamas,
but travel north in the summer seeking cooler temperatures.
But these days, their summer vacations are starting sooner.
- About for every one degree Celsius
above the long term average, we see sharks,
our tiger sharks, extending their movements
about 250 miles further north.
And off like the northeast, like off of like New England,
they are starting to come there about two weeks earlier.
That has been a big change.
- [Speaker] The water off the U.S. northeastern coast
has historically been too cold for tiger sharks
to comfortably thrive there.
The average sea surface temperature
of the Earth's oceans has increased steadily,
about 0.14 degrees Fahrenheit per decade
from 1901 to 2020.
And the Gulf of Maine
is one of the fastest warming bodies of water in the world,
averaging a record high sea surface temp
of 59.9 degrees Fahrenheit in the fall of 2021,
more than four degrees more than the long term average.
This temperature shift might mean tiger sharks
that prefer tropical water temps could start to move in.
- I think tiger sharks to some degree
can act kind of like a canary in the coal mine
because their movements are so intertwined
with water temperatures.
- [Speaker] But it's not just tiger sharks
that are affected by changes in water temperature.
On the west coast, baby white sharks
are being spotted unusually far north
in areas like Northern Monterey Bay
as a result of a heat wave in 2014.
Warmer water temperatures
are motivating tiger and great white sharks
as well as bull sharks to move to different areas
for a couple of reasons.
One, is most sharks are considered cold blooded.
Their body temperature matches that of their environment.
If waters get too warm, they'll seek out cooler spots.
Another reason is that what sharks eat is moving.
As temperatures get too hot for prey to thrive,
they move out, and their predators follow.
When apex predators like sharks
who are at the top of their food chains move into an area,
it directly affects the prey already living there.
- If you're a striped bass and you're in an area
that suddenly sharks start coming through,
you're probably gonna leave that area
or you're gonna change what time of day
you go to those areas to try and avoid that predator,
and that can have a huge impact on an ecosystem.
So there's potential that sharks
are putting in a new predation pressure into these systems
that they haven't seen before
and it'll take time for them to adjust to it as well.
- [Speaker] Since 1970, the global population
of oceanic sharks and rays has decreased by more than 70%,
a dramatic decline
bringing some species to the brink of extinction.
One study found overfishing
to be the main driver of the decline.
And these new ventures also put sharks in danger,
drawing them away from parts of the ocean
that are protected from fishing
and interference like mining or drilling.
- They'll probably have to deal with human encounters more,
both in terms of humans being scared,
but more importantly of them getting fished
or killed advertently and inadvertently.
- [Speaker] And as ocean temperatures
continue to trend upwards...
- Sharks, fish, the plants, the birds,
everything that relies on the ocean
is sensitive to those temperature changes
and they all have thresholds.
- [Speaker] These changes in shark migration
tell us that even small changes in temp
can have a major impact on life in the ocean,
and that we're only just starting to see the effects
of those changes and fluctuations and patterns
that are likely just beginning.
(gentle music)