In late spring, estuaries along the U.S. Gulf Coast come alive with newborn fish and other sea life. While some species have struggled to adjust to the region’s rising water temperatures in recent years, one is thriving: juvenile bull sharks.

We study this iconic shark species, named for its stout body and matching disposition, along the Gulf of Mexico. Over the past two decades, we have documented a fivefold increase in baby bull sharks in Mobile Bay, Alabama, and a similar rise in several Texas estuaries, as our new study shows.

Despite the bull shark’s fearsome reputation, baby bull sharks are not cause for concern for humans in these waters.

While adult bull sharks are responsible for an occasional unprovoked attack, baby bull sharks haven’t fully developed the skills needed to hunt larger prey. And you’re still far more likely to be killed by bees, wasps, or snakes than sharks.

Waters are warming

As the planet warms, coastal ocean temperatures are rising. In the Gulf of Mexico, water temperatures have risen more than 3 degrees Fahrenheit (more than 1.5 degrees Celsius) due to climate change.

On a global scale, warming waters are harming more fish species than they are helping. Higher temperatures increase food requirements and stress levels, while making fish more susceptible to disease and reducing the survival of their young. A variety of fish populations in the Gulf of Mexico, including mullet and flounder, have declined as warmer conditions affected their spawning.

At the same time, the waters used by baby bull sharks have expanded in part due to this warming, creating a dynamic habitat.

An easy way to understand how sharks use dynamic habitat is to capture them with nets and measure the characteristics of the surrounding environment. In our sampling data, we could see that the mean annual water temperatures on the Alabama and Texas coasts increased at the same time the bull shark populations rose.

 

In coastal Alabama, we found that the relative abundance of baby bull sharks has increased fivefold over the past 20 years. Slight increases in temperature over that time provided the best explanation for this population increase.

Of all the temperatures recorded in that study, there was no maximum temperature threshold detected for baby bull sharks. So far, at least since 2003, it’s been “the warmer the better” for a baby bull shark.

We observed a similar trend in coastal Texas from Sabine Lake to Matagorda Bay, where warming estuaries supported increased abundances of baby bull sharks up to eightfold over the past 40 years. Warmer waters allowed baby bull sharks to remain in their natal estuaries longer during their first year before overwintering in the Gulf of Mexico, increasing their survival to the next life stage.

 

Collectively, our recent studies indicate that warming waters are currently beneficial for young bull sharks. But just like your favorite dessert, too much of a good thing can be detrimental.

All animals, including bull sharks, have maximum and minimum temperatures at which they can function. If temperatures get too hot or too cold, this can lead to problems, whether through direct stress on the shark’s bodily functions or on its ecosystem at large.

Some of our previous work from Florida shows that baby bull sharks will leave coastal nurseries in response to episodic cold snaps to avoid cold-stress. Sharks that didn’t leave died. The same may be true for hot temperatures, although conditions have not yet reached that point in the Gulf of Mexico based on our research.

A changing world

It’s clear that climate change is altering coastal ecosystems. Our work shows the direct benefit to young bull sharks, but how the observed population growth is affecting other species in the coastal estuaries remains to be seen.

The rise in bull sharks may affect other fish species, including bull shark prey like mullets, drums, herrings, and catfish. More bull sharks could eventually mean fewer of the fish that humans rely on. In warmer water, sharks burn more energy.

Ultimately, tracking how the distributions of species like bull sharks change over time remains a critical priority for understanding future shifts in fish populations and the health of our coastal ecosystems.

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James Marcus Drymon is an associate extension professor in marine fisheries ecology at Mississippi State University.

Lindsay Mullins is a Ph.D. student in marine science at Mississippi State University.

Philip Matich is an instructional assistant professor of marine biology at Texas A&M University.

This article is republished from The Conversation under a Creative Commons license. Read the original article.