'Important role to play': Expert talks about the need to protect sharks and their necessary roles on ocean health

Halifax scientist on sharks and ocean health Author and Dalhousie University professor Aaron MacNeil discusses the role sharks play in healthy oceans.

The number of sharks has continued to decrease over the last five decades, hindering the important role they play in keeping oceans healthy.

The abundance of sharks has plummeted by 71 per cent, according to a new study led by Dalhousie University and Florida International University. As well, the populations of the top five reef shark species have decreased by 63 per cent.

Sharks help shape and maintain balance in an ecosystem from the bottom-up, meaning a variety of sharks in a variety of sizes are needed.

“Sharks are important in many ways – from helping to protect seagrass beds from overgrazing to keeping seal populations in check, sharks are key players in oceans globally," said Aaron MacNeil, co-author and professor of biology at Dalhousie and Canada Research chair in Fisheries Ecology.

"This study lays out the various roles that sharks play in marine ecosystems and makes the case that healthy shark populations correspond to healthy oceans. Understanding how sharks contribute to ecosystem health is critical for deciding what steps to take in restoring them to full function."

Besides helping to maintain balance within the food web, reef sharks feed in offshore waters and bring nutrients back to the reef. Others move nutrients around that are used at the base of the food chain.

“So particularly on, say coral reefs, a lot of fish will live on something the size of this table,” said MacNeil, as he points to a table about three-feet by three-feet, during an interview with CTV Atlantic’s Todd Battis.

“But the sharks that are in that ecosystem will move around a lot and so what you get is an evening out of nutrients rather than a concentration in very specific places and that just contributes to the overall health of the ecosystem.”

Sharks can also serve as food for other species and even as scratching posts for fish to remove parasites.

Many factors play a part in the species’ numbers declining, including climate change, habitat loss, energy mining, shipping activities, and particularly overfishing.

“Overfishing is the thing, that’s what’s led to their decline. We did a study a couple years ago that showed that in about 20 per cent of the coral reefs in the world, they’re (sharks) functionally absent. That role that they play that we are talking about today, it just doesn’t exist, and that has all come from overfishing and it’s the same here in the north Atlantic,” said MacNeil.

MacNeil, and others who took part in the study, says shark conservation becomes more critical as temperatures increase, leading some sharks to head to new areas to find temperatures they can thrive in.

“Specifically white sharks, which is what people are often interested in, that’s a population that’s been protected in its major habitat for a long time. They increase very slowly, as well there’s warmer waters. So they tend to reside here (the Atlantic region) for a longer period of time in the summer. Those two things combined and the fact that everyone has a camera in their pocket is leading to the interest and the sightings,” he said.

MacNeil hopes the study helps people understand the importance of sharks in our oceans.

“I think what this does is it puts them into a context where we understand that they’re necessary,” said MacNeil.

“Understanding that a healthy shark population leads to a healthy ocean is a big deal and hopefully people will take this type of study and take that to heart and understand that they’re not a threat. They’re actually something that has an important role to play.”