Gray reef sharks and blacktip reef sharks near Tahiti, French Polynesia. Alexis Rosenfeld/Getty Images/The Conversation

Oceans without sharks would be far less healthy – new research

WORLDWIDE
Saturday, August 03, 2024, 17:30 (GMT + 9)

The following is an excerpt from an article published by The Conversation:

There are more than 500 species of sharks in the world’s oceans, from the 7-inch dwarf lantern shark to whale sharks that can grow to over 35 feet long. They’re found from polar waters to the equator, at the water’s surface and miles deep, in the open ocean, along coasts and even in some coastal rivers.

With such diversity, it’s no surprise that sharks serve many ecological functions. For example, the largest individuals of some big predatory species, such as tiger and white sharks, can have an oversized role in maintaining balances among species. They do this by feeding on prey and sometimes by just being present and scary enough that prey species change their habits and locations.

In a newly published study, colleagues and I surveyed decades of research on sharks’ ecological roles and considered their future in oceans dominated by people. We found that because sharks play such diverse and sometimes important functions in maintaining healthy oceans, their current decline is an urgent problem. Since 1970, global populations of sharks and rays have decreased by more than 70%.

People are killing many types of sharks at unsustainable rates, mainly through overfishing. We see a need for nations to rethink where and how to conserve sharks for healthy oceans.

How sharks foster seagrasses

Along the remote coast of Western Australia, more than two decades of work shows that the mere presence of tiger sharks shapes the entire seagrass ecosystem by changing where and how big grazers, such as sea turtles and sea cows, feed.

A healthy seagrass bed in Shark Bay, Western Australia, protected from overgrazing by the presence of tiger sharks. Michael Heithaus, CC BY-ND

Having tiger sharks nearby protects wide swaths of seagrass from being overgrazed, allowing it to grow into thick underwater meadows that provide habitat for juvenile fish and shellfish. These species are important food for other animals and for humans.

In places where tiger sharks have declined and turtle populations have expanded, seagrasses are being overgrazed. In Bermuda, for example, the exploding turtle population has led to an almost total collapse of seagrasses.

This seagrass bed in Australia’s Shark Bay is in an area with few sharks. It has been heavily grazed and offers little cover for fish or other species. Michael Heithaus, CC BY-ND

White sharks produce some of the same effects. Along the California coast, where white shark numbers are increasing, otters are spending more time in the safety of protected inland waters and less time in the open waters of Monterey Bay. The otters prey on crabs, which in turn feed on grazing invertebrates such as sea slugs that clean algae from seagrasses. More otters means fewer crabs, more grazers and healthier seagrasses.[continues....]

Author:  Michael Heithaus | Read the full article by clicking the link here

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