Newswise — The specter of ghost crab pots looms large beneath the waters of the Delaware Inland Bays. In addition to littering the seafloor, these derelict crab pots — pots that have been lost or abandoned — can cause injury to swimmers, damage to boat propellers and have the potential to ‘ghost fish,’ luring sea creatures into their midst and trapping them.

The University of Delaware’s Art Trembanis and Kate Fleming of Delaware Sea Grant (DESG) conducted a preliminary assessment of the state of these ghost crab pots in three areas of Rehoboth Bay — Love Creek, Herring Creek/Guinnea Creek and Bay Cove.

Surveying 100 acres, they found 160 ghost crab pots, with Bay Cove, a section of Rehoboth Bay near Dewey Beach, accounting for almost half the findings, or 3.5 pots per acre.

Fleming, a coastal ecology specialist for DESG, said that the initial findings suggest that there’s a “derelict crab pot problem in the Delaware Inland Bays.”

Cannibal crabs

Blue crabs are cannibalistic and so when these baited crab pots get lost beneath the water, they continue to attract additional crabs which can, in turn, attract other organisms.

On their survey, the researchers got approval to pull some of the pots they located and in some of the pots, they found dead blue crabs, live blue crabs, a dead sunfish, oyster toadfish and a largemouth bass.

“This gives us a little bit of a window into what species we might expect to find if we get back out there to pull at a larger scale,” said Fleming.

Of the six pots they hauled, only one was outfitted with a bycatch reduction device, which is a tool utilized on pots to minimize the unintended capture of turtles. Finding these devices on only one of the pots suggests that there could be a problem with recreational crabbers failing to use the required devices.

“In the future, when we haul more derelict pots we can more comprehensively document the presence of bycatch reduction devices, and use that information to help educate crabbers about why it’s important to use them,” said Fleming.

Side-scan sonar

On two cold days this past February, the researchers set out aboard the R/V Dogfish with a side-scan sonar to conduct exploratory surveys of three sections within Rehoboth Bay, the northernmost of the Inland Bays, which consists of Rehoboth Bay, Little Assawoman Bay and Indian River Bay.

Trembanis said they wanted to use a low-cost, consumer-friendly sonar system — one that could be found at outdoors stores and are usually utilized by recreational fisherman — to develop a method of tracking ghost pots that could be recommended to agencies, private citizens or civic groups to adopt if they wanted to help aid in the mapping efforts in the future.

They have also tested the sonar with smaller autonomous surface vessels (ASVs) and an autonomous kayak to provide a novel set of robotic platforms particularly well suited for mapping in the very shallow inland bays.

“There are a lot of areas to cover,” said Trembanis, associate professor in UD’s College of Earth, Ocean and Environment. “My interest is to try and help explore as much as we can, but even more than that, we want to help develop the methods and protocols that we can share with other groups.”

Trembanis said it was tricky to use the sonar in such a confined, shallow environment. In the open ocean, it can be easier to operate the sonar, which covers a larger area the further it gets from its source before hitting something, usually the bottom of the waterbody.

“It’s like mowing the lawn,” Trembanis said. “The shallower you get, the narrower the width of your lawn mower is. When we’re in deeper waters, it’s like a whole armada of lawn mowers. Now when we get into shallow waters, it’s a smaller, single lawn mower.”

Public Participation

To help inform the public on the hazards of derelict crab pots, Fleming appeared in a series of Delaware Sea Grant’s 15-second science videos focused on the topic.

In addition, Fleming and Trembanis both appeared at the Center for Inland Bays’ (CIB) Scientific and Technical Advisory Committee meeting on Friday, May 10.

Presenting at the CIB meeting allowed them to share the new findings with many stakeholder groups. They were also able to elicit questions and ideas for further partnering to expand the understanding of the problem and address how best to reduce the number of ghost pots and combine forces towards an effective sustained removal program.

This summer, the researchers will host four interns from the Naval Academy who will work with CEOE and DESG to test the efficacy of drones to help map and look for crab pot buoys, which can serve as a proxy for the amount of crabbing going on in an area.

The more help they can get, the better, as Trembanis said it was disheartening to find such a large number of ghost pots.

“We did 21 acres in Bay Cove and saw more than 75 ghost pots, so that’s 3.5 per acre,” said Trembanis. “Once we ground truthed the sonar data and knew that we were seeing pots, it was a crestfallen sort of shock.”