Scientists Track Bull Shark Population In And Around Mobile Bay

June 10, 2009

Release from: Ben Raines
Press-Register (Alabama)

The underwater ears are listening.

Based on what the ears hear, scientists hope to gain a better understanding of Mobile Bay's bull shark population, including how much time the famously aggressive creatures spend in the area's coastal rivers and the Mobile-Tensaw Delta.

Right now, it looks like the sharks spend a lot of time in those unsalty places, but that news sounds scarier than it actually is. Scientists say most of the sharks using the rivers are just little babies.

In an effort to track them, 30 so-called ears, which are more properly described as hydrophones, are being attached to pilings all around Mobile Bay, stretching up the western shore from Dauphin Island to the Causeway and back down the eastern shoreline to Fort Morgan.

The ears are actually high-tech receivers designed to pick up ultrasonic signals coming from $1,500 transmitter tags buried in the bellies of bull sharks or cownose rays.

"Our shark-tagging program has shown that the sharks are using these freshwater areas a lot. Bull sharks can handle freshwater while a lot of other sharks can't, and we think they are exploiting that niche around Mobile Bay," said Marcus Drymon, a graduate student working at the Dauphin Island Sea Lab.

"The hydrophones at the mouths of the rivers are essentially gatekeepers. We can tell what is going into or out of the rivers."

Drymon said Mobile Bay appears to be an important nursery for bull sharks. As evidence, he points to the large number of just-born sharks caught in the bay during recent surveys.

"We are catching a lot of sharks that still have the umbilical scar between their pectoral fins. That means those sharks are less than a week old. If they are not pupped in Mobile Bay, they are born very nearby," Drymon said.

Bull sharks give birth to live babies, with between one and 10 pups per litter. The sharks are around 2 feet long when they are born. In recent years, fishermen have caught young bulls near Gravine Island in the delta.

Working with a $100,000 grant from the Shelby Center for Ecosystem-based Fisheries Management, Dauphin Island Sea Lab scientist Sean Powers and a team of graduate students including Drymon are setting the hydrophones at the mouth of each river that feeds into the bay.

Transmitters attached to each shark or ray send out distinctive and barely audible chirps every five seconds. Anytime one of those transmitters passes within about a 1,500 feet of a hydrophone, a record is made. Using those records, the scientists can tell which shark entered which river and at what time.

"The hydrophones are basically underwater microphones," said Matt Ajemian, a Sea Lab grad student studying cownose rays. "Over the course of the next two years we plan to put out 105 transmitters, split between sharks and rays."

Scientists around the nation have become concerned about the impact large schools of cownose rays are having on shellfish populations. In North Carolina, the rays are blamed for decimating scallop populations, while in Mobile Bay the concern is for the blue crab. Ajemian said no one has really examined whether cownose rays exploit freshwater rivers. The rays are frequently seen in the bay, particularly around Point Clear.

Some believe the rays have become more numerous as shark populations have plunged. The decline in big sharks is believed to have profound — and often bad — implications for marine ecosystems.

"We are trying to figure out if Mobile Bay is essential fish habitat for the sharks, and if the freshwater areas are used as essential fish habitat," Drymon said.

Drymon said the Coastal Alabama Acoustic Monitoring Program could ultimately expand to include other creatures that swim in Mobile Bay, such as trout, redfish and even manatees.

"Once it is up and running, it is set up to be a long-term monitoring program for Mobile Bay. It's not just sharks and rays. As funding becomes available we can tag other species and follow them around the bay," Drymon said. "We'll be able to follow anything we tag."