Fisheries in the whirlpool of climate change: fish migrate, the economy suffers

The following text is a translation and reproduction of the article written in Greek by Chrysanthi Archontidou at Parallaxi.

How climate change is affecting fish and fisheries – shifts in their size and habitat – the impact on fishers, and the role of overfishing.

As climate change impacts every aspect of daily life, sea temperature is rising dangerously, marine ecosystems are transforming, many fish species are under threat, and fisheries are left in the vortex of the grim consequences of the climate crisis.

According to WWF, since the 19th century, the ocean has absorbed about 28% of the carbon dioxide (CO₂) produced by human activity and more than 90% of the excess heat it generates. By the end of this century, ocean acidity is expected to increase at a rate ten times faster than any previous acidification event in the past 55 million years. As a result, lower oxygen levels are affecting the survival, reproduction, and growth of marine species.

Due to ocean warming, many fish species are moving away from the coasts, migrating to deeper waters or even towards the poles to find suitable temperatures. Climate change is driving aquatic animals to migrate at a rate ten times faster than terrestrial animals.

A recent report by the United Nations Food and Agriculture Organization (FAO) cites research findings that indicate declines in fish populations if global temperature rises by 3–4°C above pre-industrial levels by the end of the century.

In such a scenario, nearly 50 countries would face a 30% reduction in fish abundance in their waters. This forecast raises serious concerns among professional fishers, who will be confronted with the severe consequences of the climate crisis. Fewer fish mean less food and, consequently, lower income.

The director of the Laboratory of Ichthyology, head of the MarinOmics research group, and professor in the Biology Department of the Aristotle University of Thessaloniki (AUTH), Athanassios Tsikliras, along with Dr. Donna Dimarchopoulou, speak to Parallaxi about the impacts of climate change on fish and fisheries.

Fish are affected by climate change in many ways. One of these has to do with their physiological growth — specifically, their body size.

“When fish lose the ideal living conditions that exist for each species in the sea, they follow those conditions until they find them again somewhere else. If the water temperature increases or changes, they will move to waters that ‘suit’ them. Temperature affects not only the distribution of fish, but also their biology. Some species change their reproductive habits, and almost all species reduce their physical growth” explains Prof. Tsikliras.

“Warmer water due to rising sea temperature holds less oxygen, while at the same time it increases the metabolic rate of fish. This means that fish require more oxygen in an environment that contains less of it. This way, fish — as well as all marine organisms with gills — face pressure from both sides, which makes it harder for them to breathe and forces them to move”

Dr. Dimarchopoulou adds.

Dr. Dimarchopoulou also explains the theory of Daniel Pauly, a Killam professor at the University of British Columbia in Canada and honorary doctorate recipient of the AUTH Biology Department, which states that: “The gill surface area of fish is two-dimensional and does not grow at the same rate as the rest of their body. This means that small fish have a relatively greater gill surface area in proportion to their body volume, while as the fish grows in size, the gill surface area becomes proportionally smaller. This leads fish to become smaller in size in order to survive”.

At the same time, there is an ecosystem-level impact of climate change on fish, which forces their movement from one sea to another.

“Each fish species prefers to live in specific water temperature. Some prefer colder waters, others prefer warmer waters. When sea temperature rises in a geographic region, organisms either move to deeper, cooler waters, or migrate northward to maintain their preferred thermal conditions. In the Mediterranean Sea, where a geographic barrier exists to the north, they either move to deeper waters—if their biology allows them to—or they are forced to reduce their body size” explains Dr. Dimarchopoulou.

According to Dr. Dimarchopoulou, tropical regions face the most severe consequences, as the water heats up dangerously and fish avoid it. “No new species arrive; only existing ones leave. That’s where the risk of species extinctions is greatest” she emphasizes.

The Mediterranean Sea is “boiling”

With sea temperature steadily rising and successive records being broken one after another, the Mediterranean is turning into a “boiling pot”. It is one of the regions most severely affected by the climate and biodiversity crises. It is the most overfished sea globally, experiencing a rapid loss of biodiversity, and is warming 20% faster than the global average.

“In the eastern Mediterranean, we have the Suez Canal, a human-made intervention. The Mediterranean would never have been connected to the Indian Ocean without this opening. As a result, the eastern Mediterranean and Greek seas are more affected by climate change and fish migrations. Likewise, areas with large dams experience altered hydrology and are affected by the climate crisis, as freshwater inflow to the sea decreases. In addition to the northward shift of some species, others are entering the Mediterranean basin through the Canal and have reached the Aegean Sea” notes Prof. Tsikliras.

“The most characteristic fish that has entered the Aegean Sea is the round herring of the genus Etrumeus (Etrumeus golanii), which is now abundant in the Cyclades”

Prof. Tsikliras adds.

The economic impacts on fisheries

Fish migrations mean fewer fish in certain seas. Fewer fish translate into reduced catch and lower income for fishers.

Dr. Dimarchopoulou explains that under these conditions, fishers are forced to adapt, affecting the overall economy of the fishing sector:

“If the traditional fishing grounds they have been visiting for years no longer hold the fish they used to target, they must follow the species to wherever it has moved. In some cases, when the species has shifted to deeper waters, they’ll need to spend more on fishing trips, equipment, and fuel.

On the other hand, they may adapt by targeting a different species. But that too requires changing fishing gear, fishing zones, and possibly even complying with new regulations, as they may not hold the necessary permits for the new species” she notes.

On the other hand, Prof. Tsikliras presents a different dimension of how fish migration affects fisheries—one that is directly linked to the arrival of new fish species:

“Fishers are now catching different species than those they were used to for years. In many areas of the southern Aegean, there is a high number and large biomass of these new species. Some of them, like the lionfish (Pterois miles), are very tasty, commercially valuable, and expensive. This means that such catches could potentially boost fishers’ income. However, we are currently in a transitional phase: we as consumers are not yet accustomed to eating these species, and neither are fishers used to catching them” Prof. Tsikliras concludes.

The challenging factor of overfishing

According to experts, the same outcome as that caused by climate change—reduced fish size—is also driven by overfishing.

“In addition to climate change impacting fisheries, it’s important to recognize that fishing itself significantly impacts fish populations. Fishers typically target the larger species or the largest individuals of a species. Over time, this reduces their average body size. So, we’re seeing the same result from two different pressures”

stresses Dr. Dimarchopoulou

“For many decades, we have been fishing beyond the rebuilding capacity of fish stocks. Overfishing has created chronic problems. Now, climate change is acting on top of that, leaving its own footprint and further reducing fisheries catches” adds Prof. Tsikliras.

The sustainable fisheries of tomorrow

To protect our seas and fisheries, there is a need for better and more effective regulation of fishing activity, investments in improved equipment, and stronger scientific data—especially since climate change is here to stay. This can be achieved through sustainable fishing.

“Unfortunately, climate change is here to stay and is a global phenomenon. It is something we will continue to face in the coming decades. We must certainly aim to reduce its future impacts and conduct research that provides modern and flexible results. What is more immediately addressable, however, is overfishing and the limits we set on fishing. We need to move towards a model of sustainable fishing” concludes Dr. Dimarchopoulou.

Through sustainable fishing, fishers can adopt practices that protect the populations of their target species. In this way, stocks will not be depleted beyond recovery, and their environment will not be damaged. Fish populations can remain at healthy levels without negatively affecting other species in the ecosystem—including humans.

At the same time, according to FAO data, fish stock losses can be stabilized at less than 10% if the global average temperature increase is kept within 1.5 to 2°C. Provided that all countries meet their global climate targets and achieve net-zero emissions by 2050, the consequences of climate change will be significantly milder, making it possible to effectively address them.