Hot and Bothered: How heatwaves amplify the ecological impacts of nearshore clam fishing

On hot summer days, you might find yourself feeling tired and sluggish. It makes sense that some animals would feel the same way. According to a recent study conducted by Fisheries and Oceans Canada (DFO) Research Scientist Dr. Jeff Clements, some species of clams are also negatively affected by heatwaves.

Clam fishing, or clam digging, has been practiced for thousands of years. In Atlantic Canada, Indigenous peoples harvest clams for food, and social and ceremonial purposes. Recreational clam digging and commercial clam fisheries are also very popular. However, in the southern Gulf of St. Lawrence, commercial clam landings have declined drastically.

Clements conducts research on soft-shell clams and other mollusc species, aimed towards supporting the sustainable management of Canada’s nearshore fisheries. In a recent study on the east coast of New Brunswick, Clements and his team found that human activities like fishing can alter interactions between clams and other co-occurring species. And that’s even more true during heatwaves, which can amplify the ecological impacts of fishing.

During the study, one of the team’s experimental trials happened to take place during one of the worst heatwaves ever recorded in Atlantic Canada, which allowed them to test how sweltering temperatures affect clams when they are fished. “Field studies documenting the effects of climate events such as abnormal heatwaves are rare, so we were excited at the opportunity to leverage such an event in our experiments,” Clements says.

His field experiments at the Kouchibouguac National Park (located on the east coast of New Brunswick along the Acadian Coast) focused on undersize clams (below 50 mm in length) that clam diggers are not allowed to keep. Under DFO fishing regulations, undersize clams that are caught during clam fishing must be tossed back to allow them to grow to adult size and reproduce.

Clements and his team dug up undersize clams at low tide five times throughout the summer of 2024. They glued metal washers and fishing line to these clams to serve as markers, then placed them back onto the sand in designated areas. Half of the clams were protected from crabs — main predators of the clams — by bottomless plastic buckets covered with mesh placed over them, which allowed the clams to dig back into the sand. The others were left unprotected.

“Clams live underneath the sediment and need to dig their way back in when removed. This might sound simple in practice but it can take a long time and a lot of energy for clams to do this. This leaves them vulnerable to being picked off by hungry predators,” Clements explains.

During the experiment, the marked clams were tracked to see how many burrowed, how many died, and how many predators were found in the designated tidal areas within one to two days. Under normal conditions - during the trials with regular temperatures - the team found that most of the clams were able to dig back into the sand within one to two hours, and the vast majority (85 per cent) were back in the sediment by the next day, regardless of whether they were protected or exposed to predators.

However, during the heatwave trial, only 5 per cent of the clams had managed to burrow back into the sand after two days. Most of the clams had been eaten by predators. Clements and his team found that the number of predatory crabs and scavenging snails in the designated plots was about four times higher during the heatwave trial than during their other trials conducted on days with typical temperatures.

“This follows a typical ecological pattern when temperatures increase: predators’ metabolism increases and they need to eat more to compensate for that, making them hyperactive,” Clements clarifies. “On the other hand, prey like clams can become sluggish in high temperatures as they decrease their activity to conserve energy and hide from predators. But when they are removed from their hiding spot, this sluggishness works against them and makes them easier for predators to catch.”

While the results of Clements’ study focused on nearshore clams and their predators, they underscore a knowledge gap for other nearshore species as well.

“Nearshore and intertidal fisheries exist all over the world. While the specific temperature thresholds we uncovered in our experiments won’t apply to all species, it is possible that human engagement with species in different habitats during climate extremes like heatwaves could have lasting ecological ramifications,” Clements says. “Unfortunately, heatwaves are becoming more frequent and extreme as the climate continues to change, and that is something we need to grapple with in relation to our coastal fisheries.”

Clements encourages people to consider the impacts when harvesting clams during hot weather.

“The purpose of throwing back small clams when you dig them up is to help them survive long enough to reproduce and replenish the stock, but our work shows that the vast majority of those clams won’t survive being thrown back during heatwaves,” he says.

“My hope is that our research helps people understand these impacts and consider them before digging up vulnerable clams during hot weather.”

Citation: Clements, J.C., Harrison, S., Roussel, M., Hunt, J., Power, B.-L., Sonier, R. (2025) Fishing during extreme heatwaves alters ecological interactions and increases indirect fishing mortality in a ubiquitous nearshore system. Communications Biology 8: 735. https://doi.org/10.1038/s42003-025-08158-w