Â鶹ÒùÔº

The latest report on the state of the Gulf of St. Lawrence ecosystem could not be clearer: .

This warming is occurring both in its surface layer, which is directly exposed to global warming, and in its deepest layer, due to the recent increase in the inflow of warm water from the Gulf Stream into its deep channels through the Cabot Strait.

At depth, warming is combined with a significant decrease in oxygen levels, which amplifies habitat changes for marine species.

By affecting both near-surface and deep-sea organisms, the current warming is influencing the entire ecosystem and provoking a real upheaval in the balance of the species living there. This is having direct repercussions on the commercial fishing sector.

As a Canada Research Chair in fisheries ecology, I am interested in the causes and consequences of changes in the dynamics of commercially exploited species. In this article, I explain the changes underway in the balance of species that inhabit the bottom waters of the Gulf of St. Lawrence.

The decline of cold water species

Following the historic collapse of Atlantic cod stocks in the early 1990s, , species of Arctic origin, including northern shrimp, snow crab and Greenland halibut, took advantage of the cooling and a decrease in predation and competition in the system to settle comfortably in the Gulf of St. Lawrence.

The dominance of these species lasted for more than two decades, allowing for the development of lucrative fisheries, whose . However, as these species now face rapid warming of their habitat, their abundance is declining.

Northern shrimp in hot water

Shrimp can be considered a true barometer of the state of the demersal marine ecosystem, i.e. the layer of water located near the bottom, since its distribution fluctuates rapidly according to changes in the temperature of the environment. Preferring waters with temperatures between 1°C and 6°C, shrimp has seen a marked decrease in its habitat over the last decade.

Data from the monitoring survey conducted by Fisheries and Oceans Canada show that, due to the warming of the shrimp's habitat, the . This decrease in abundance has resulted in a 12 percent reduction in allowable catch in 2022. With an additional 18 percent reduction announced for 2023 and a high cost of diesel, the .

Snow crab and Greenland halibut: A fragile balance

Like the northern shrimp, the snow crab and Greenland halibut are two species of Arctic origin whose stocks in the St. Lawrence are at the southern limit of their distribution. Any warming of the waters of the Gulf, which are already warmer than the average for their habitat, can thus negatively affect the productivity of these stocks.

Snow crab are particularly vulnerable during their early life stages, and more specifically when juveniles are settling on the sea floor. . At present, snow crab abundance remains relatively high, especially in the southern Gulf of St. Lawrence, where the largest stock is located. However, , which will greatly reduce the potential for maintaining high abundance of the species and could lead to a collapse of the stocks in the medium term.

The abundance of Greenland halibut—the groundfish species that has been the most lucrative for fishers since the 1990s—. This decline has resulted in a decrease in allowable catch of almost 50 percent in five years, with the latter dropping from 4,500 to 2,400 tons from 2017 to 2022. While the exact causes of the decline of this stock in Gulf of St. Lawrence waters remain uncertain, warming and declining oxygen levels underway in deep channel waters are considered the most likely explanations.

Lobster and redfish: A massive red tide

In addition to the Atlantic halibut stock, which has recovered and is currently doing very well, two reddish-colored species are experiencing a remarkable population explosion in the Gulf of St. Lawrence. These are the American lobster, which lives on the bottom in coastal areas, and the redfish, a fish that is distributed on the bottom in deep waters.

Lobster is now by far the most lucrative resource in Atlantic Canada. For example, . In the southern Gulf of St. Lawrence, . In addition, with the gradual warming of the coastal areas of the northern Gulf of St. Lawrence, lobster is expanding there. , which represents a real windfall for fishers, who received .

Redfish, whose stocks had collapsed shortly after cod in the 1990s, took the fisheries sector by surprise by . .

These fish, which are now about 10 years old, are reaching the minimum size allowed for commercial capture. The return of large-scale fishing is therefore imminent, but the industry faces major challenges. Indeed, the anticipated value per unit weight of these small fish is about ten times less than that of lobster. Therefore, it will not be simple to make a sustainable harvest of this resource profitable.

Feedback loops generated by the relationships between species

The return of redfish to the waters of the St. Lawrence could well mean that, for cold-water species, there will be more bad news. Indeed, . However, . Although at this time we cannot accurately determine the amount of shrimp needed to sustain the , this predation pressure is expected to accelerate the decline of shrimp, whose productivity is already negatively impacted by warming waters.

Furthermore, the . Greenland halibut may therefore be competing for the resource at a time when the quality of its habitat is deteriorating. Such impacts caused by predator-prey relationships thus add to the physical changes in the environment to accelerate the rate of change in species abundance, which could cause the ecosystem to shift to a new steady state.

Research to guide the future

The return of redfish from very low abundance is a big surprise to scientists and a destabilizing event for the fisheries sector. It is, therefore, important to have a precise understanding of the causes and consequences of the demographic explosion of this stock in order to facilitate the sustainable development of the sector. To this end, university research teams from the group are teaming up with those from the of Fisheries and Oceans Canada as part of a program to better understand the ecology and dynamics of redfish.

Ultimately, the results of these multidisciplinary collaborations will provide valuable elements for developing an ecosystem approach to fisheries management, and thus promote the sustainable exploitation of our resources.

This article is republished from under a Creative Commons license. Read the .