The latest report on the state of the Gulf of St. Lawrence ecosystem could not be clearer: It shows undeniably that the system is warming rapidly.

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.

This article is part of our series, The St. Lawrence River: In depth. Don’t miss new articles on this mythical river of remarkable beauty. Our experts look at its fauna, flora and history, and the issues it faces. This series is brought to you by La Conversation.

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, caused by a combination of overfishing and very cold conditions, 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 revenues not only replaced, but greatly exceeded the value of revenues from the cod fishery prior to its collapse. 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 area it occupies has decreased by half since 2008, and the abundance estimate in 2021 is among the lowest values in the history of monitoring this resource. This decrease in abundance has resulted in a 12 per cent reduction in allowable catch in 2022. With an additional 18 per cent reduction announced for 2023 and a high cost of diesel, the profitability of businesses that rely on this resource is threatened in the near term.

graph showing the decline in northern shrimp abundance

The combined biomass of the four northern shrimp stocks in the Gulf of St. Lawrence has been declining since 2004. The data is taken from the latest Fisheries and Oceans Canada stock assessment report. (Dominique Robert), Author provided

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. Their survival then depends on the availability of very cold water, with temperatures between 0°C and 2°C. At present, snow crab abundance remains relatively high, especially in the southern Gulf of St. Lawrence, where the largest stock is located. However, ocean models indicate that by 2050, bottom water temperatures in this area of the Gulf will exceed 3°C, 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 — has been declining for 15 years. This decline has resulted in a decrease in allowable catch of almost 50 per cent 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.

graph presenting the evolution of snow crab and halibut abundance

While the biomass of the southern Gulf of St. Lawrence snow crab stock remains stable, the abundance index of Greenland halibut shows a decrease in this stock since 2004. The data are from the latest Fisheries and Oceans Canada stock assessment reports. (Dominique Robert), Author provided

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-coloured 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 in its natural habitat

Lobster is now by far the most lucrative resource in Atlantic Canada. (Jean-Daniel Tourangeau Larivière), Author provided

Lobster is now by far the most lucrative resource in Atlantic Canada. For example, for Québec fishers, it alone represented 42 per cent of total revenues in 2020. In the southern Gulf of St. Lawrence, Magdalen Islands fishers landed record catches in 2022. In addition, with the gradual warming of the coastal areas of the northern Gulf of St. Lawrence, lobster is expanding there. Off the North Shore, the increase in catches between 2015 and 2021 varied from 388 per cent to 850 per cent depending on the sector, which represents a real windfall for fishers, who received approximately $8 per pound of lobster landed in 2022.

Redfish, whose stocks had collapsed shortly after cod in the 1990s, took the fisheries sector by surprise by making a strong comeback in the 2010s. This dramatic return occurred as a result of strong survival of juvenile redfish born in the 2011-2013 period, likely due to favourable environmental conditions for the larvae.

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.

graph showing the increase in abundance of lobster and redfish

Redfish and lobster have experienced a population explosion in the Gulf of St. Lawrence over the past 15 years. The redfish biomass data are taken from the latest Fisheries and Oceans Canada stock assessment report, while the lobster landings data are the sum of catches from Québec, New Brunswick and Prince Edward Island. (Dominique Robert), Author provided

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, recent research on the diet of redfish shows that once they reach a size of 25 to 30 cm, northern shrimp become one of their main prey. However, redfish born in the 2010s were about 24 cm in 2021. Although at this time we cannot accurately determine the amount of shrimp needed to sustain the 2.8 million tons of redfish currently inhabiting the St. Lawrence, this predation pressure is expected to accelerate the decline of shrimp, whose productivity is already negatively impacted by warming waters.

Goldfish on a conveyor belt

University research teams from the Ressources Aquatiques Québec group are teaming up with those from Fisheries and Oceans Canada’s Maurice Lamontagne Institute as part of a program to better understand the ecology and dynamics of redfish. (Joëlle Guitard), Author provided

Furthermore, the redfish diet shows a significant level of overlap with that of Greenland halibut. 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 Ressources Aquatiques Québec group are teaming up with those from the Maurice Lamontagne Institute 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.

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