As ocean water gets warmer, it can cause fish to change their habitat range. Some species venture into new habitats with cooler waters, others lose some of their range and are no longer found in areas where they used to be.
Marine biologist Dr. Donna Dimarchopoulou recently collaborated with Dr. Heike Lotze and Dr. Boris Worm at Dalhousie University, along with Dr. Jesús Pineda and Dr. Rubao Ji at Woods Hole Oceanographic Institution, to assess the effects of ocean warming on fisheries catches in the Northwest Atlantic Ocean. This research was supported by OFI’s International Postdoctoral Fellowship program.
“Climate-induced habitat shifts impact the entire ecosystem structure,” says Dr. Dimarchopoulou.
“This can lead to significant changes in fisheries. The aim of this project is to improve our understanding of these impacts so that we can help inform adaptive management strategies.”
Warming waters means changing fisheries
The first stage of the project focused on fisheries catches. This data provided researchers with details on which species have been fished where and when in the Northwest Atlantic ocean. Scientific survey data, collected from fisheries management agencies in the United States and Canada, was also used to provide a more accurate estimate of species abundance.
The next step was for researchers to determine the mean temperature of the catch (MTC), which is an index of the average temperature preference of fish and invertebrates weighted by their annual catch.
“Like humans, most fish have a preferred temperature range where they do well,” explains Dr. Dimarchopoulou.
“Multiplying the temperature preference of each species by their catch amount in kilograms and dividing by the total catches gives us a time series of the MTC index. If the index changes over time, this reflects a change in the average preferred temperature of the species caught in a specific region.”
An increase in MTC means fishers are catching more warm-water species. This means that the warmer it will get in the coming years, the more changes fishers will see in their nets and consumers on their dinner table.
The data shows that Canadian and U.S. fisheries have become tropicalized over the past few decades, with more and more warm-water species being caught over time. The range of these species is expanding further north, while cold-water species have in many cases been declining.
These changes point towards a clear impact from climate change, that is visible despite the confounding effects of historical overfishing or local management regulations, which could be masking some of these signals.
Climate-adapted fisheries management
The tropicalization of fisheries has consequences for ecosystems, seafood supply, and fisheries revenue. In general, it alters species composition, which can disrupt existing food webs, and force fishers to travel further away into new fishing grounds and target new species.
For example, the warm-water longfin squid used to occur more in the mid-Atlantic ocean. During periods of time when the temperature was very warm, fishers have encountered them north of their historical range, in an area where northern shrimp are typically found.
“Higher longfin squid predation has been suggested as a significant factor in the collapse of the northern shrimp population in the Gulf of Maine that used to support a locally important fishery,” says Dr. Dimarchopoulou.
“The expectation is that tropicalization will cause some disruptions of established fisheries in the Northwest Atlantic and could complicate transboundary management as stocks move across national borders.”
Current and future management plans, therefore, need to be climate adaptable. The MTC can add value as an indicator in risk-based management frameworks by measuring exposure of fisheries to various hazards, including increasing ocean temperatures.
Dr. Dimarchopoulou presented the project’s findings at several research conferences, including the 2024 World Fisheries Congress in Seattle, the 2024 Ocean Sciences Meeting in New Orleans, and the 5th International Symposium on the Effects of Climate Change on the World’s Ocean, held in 2023 in Bergen. The lead researchers are also preparing several publications on their data that will be submitted to scientific journals.
About Dr. Donna Dimarchopoulou
Dr. Donna Dimarchopoulou is a marine biologist holding PhD, MSc, and BSc degrees from the Aristotle University of Thessaloniki in Greece.
For the past four years, she has been conducting postdoctoral research at the University of Rhode Island, Dalhousie University, and Woods Hole Oceanographic Institution. Dr. Dimarchopoulou’s research focuses on marine fisheries ecology.
She uses fisheries-dependent and -independent data, ecosystem modeling, and fish stock assessments to shed light into the multifaceted impacts of anthropogenic pressures, such as fishing and climate change, on marine ecosystems, fish, and fisheries.
