Invasive seaweed is taking over the Gulf of Maine, and new research says that’s bad news for fish

The reddish shrub-like seaweed known as Dasysiphonia japonica has taken over the seabed of the Gulf of Maine.JENNIFER DIJKSTRA/UNH

University of New Hampshire researchers have found that a species of shrub-like seaweed that has invaded the seafloor of the Gulf of Maine now dominates the seabed and may be reducing the number of fish in the areas it occupies.

Looking at decades of data and pictures of the ocean floor, researchers observed that warmer water temperatures caused by climate change are shortening the growing season of kelp, which thrives in colder conditions. This has opened the door for the invasive “turf seaweed,” a red, dense, and low-lying species known as Dasysiphonia japonica.

Jennifer Dijkstra, a research assistant professor at UNH’s Center for Coastal and Ocean Mapping, was the lead author of a paper about the findings published Nov. 8 in the Ecosphere journal. She had previously authored a study showing the invasion of the turf seaweed, which is originally from Asia and is spreading north along the Northeast coast, in 2017 in the Journal of Ecology.

The new research mapped a much larger area of the seafloor and showed that fish are looking for a habitat that provides continuous coverage over a large area, as opposed to the patchy coverage provided by the turf seaweed that is replacing other types of seaweed, Dijkstra said.

She said that with its long blades, kelp “provides cover or concealment for fish species that utilize that habitat,” much like the continuous canopy of a forest.

“When you lose a kelp forest, it’s very similar to losing a terrestrial forest,” she said. “It’s like going from a forested ecosystem to a shrubland . . . The canopy is important to species that visually assess their surroundings like fish.”

Fish use the coverage to hide from predators. The new study shows that there are fewer observed fish in the habitats with the turf seaweed, and that fish in these areas spend more time and energy seeking and defending their shelter.

The study is important because it is the first time that researchers have been able to make a connection between the spread of turf algae and fewer fish, said Karen Filbee-Dexter, a researcher at the Institute of Marine Research in Norway, who is studying the rise of turf algae around the world.

Filbee-Dexter said this phenomenon is becoming increasingly frequent and has only appeared in the past couple decades.

“This is a pervasive phenomenon,” she said. “It’s happening in at least four continents. . . . I think it points to a link between rising sea temperatures and a loss of kelp and increase of turf algae.”

Researchers are particularly concerned about cunner fish, a dominant species in the area. After dark and below temperatures of 41 degrees, cunner fish enter torpor, a state of mental and physical inactivity similar to hibernation.

“If exposed while in torpor, the fish is likely to be preyed upon by migratory species such as pollock, striped bass, sculpins, and harbor seals,” the study states. “Additionally, limited shelter availability may cause cunner to occupy less desirable shelters in which the risk of predation is greater.”