Non-native smallmouth bass are more aggressive toward endangered juvenile Atlantic salmon and more voraciously eat local food supplies at higher temperatures, according to a University of Maine study.
As a result of warming waters, more non-native warmwater fish species are invading watershed habitats, disrupting local freshwater food webs and competing with native species for limited resources. The combination of the two can be deadly for endangered fish in Maine, particularly the Atlantic salmon, which, in addition to their intrinsic value, is important to regional Indigenous cultures, recreation opportunities and the health of freshwater ecosystems in the state.
Nicole Ramberg-Pihl led a team of researchers during her Ph.D. in ecology and environmental sciences at the University of Maine in an experiment to observe the interaction between Atlantic salmon and smallmouth bass at different temperatures. The scientists collected fish from rivers and hatcheries across Maine and put them in tanks at the Aquaculture Research Center — some segregated by species, and others combined — at two different temperatures, 18 and 21 degrees Celsius, and filmed the fishes’ behavior.
In reviewing the footage, the researchers found that salmon ate less in the presence of smallmouth bass in general, but ate even less at the higher temperature, 21 degrees Celsius. They also observed that smallmouth bass were most aggressive at 21 degrees Celsius, particularly when there were salmon present.
“Our cameras provided a top-down view of the experimental tanks and this allowed us to observe aggressive interactions, such as chases (where a fish swims after another) and charges (where a fish quickly darts toward another) between individuals of both species at 18 and 21 degrees Celsius. We observed increased levels of aggression in smallmouth bass when salmon were present and when water temperature was higher,” says Ramberg-Pihl.
These findings illustrate the potential for invasive species like smallmouth bass to outcompete native salmonids for resources, especially under the warmer conditions predicted with climate change. The results could inform conservation efforts of the endangered Atlantic salmon in Maine.
“We found that bass are bullies when it is hot,” says Ramberg-Pihl. “Not only are factors such as temperature and food availability important for salmon performance, but smallmouth bass could outcompete Atlantic salmon in warmer temperature scenarios.”
The study was published in the journal Ecology and Freshwater Fish on March 22, 2023.