Counting fish to prevent overfishing

Is the sea impossible to measure as a resource?

How many herrings, mackerels, skates or sea bass are there? Are there more or less than in previous years or decades? Are the fish getting bigger or smaller? How many will there still be next year? These are the kinds of questions that challenge the minds of scientists at IFREMER, the French national institute for ocean science and technology. Why? In an effort to guide fisheries management. 

Every year, nearly 90 million tonnes of fish and shellfish are caught worldwide, including 4 million in the European Union (EU). Over 13 million tonnes are consumed across the 27 countries of the EU. Humans have long considered the sea to be an inexhaustible source of food - as fertile as it is unfathomable. And yet there have been warnings, followed by disasters which have laid bare the limits of this invisible resource, and the risks of unchecked exploitation. The most infamous case is the collapse of Newfoundland’s cod population in the late 1980s due to overfishing—a stock that, even after forty years of industrial fishing restrictions, has yet to recover.

To avoid reaching these tipping points, it is vital to know what the status of fish populations is – i.e. their ‘ stocks ’ –, their composition and their trends. Scientists must not only quantify but also determine the age and sex of individuals... These are challenging tasks to accomplish at a distance. So for sampling purposes, they have to head out to sea. Welcome aboard the Thalassa, for a carefully planned trip in the English Channel and the North Sea.

From sea-based collection to fishing quotas

Sea campaigns the likes of IBTS are one of the links in a long chain leading to fisheries management at European Union level. This scientific work is crucial for establishing total allowable catches, which enable fish stocks to rebuild. 

To properly assess the quantities of fish and shellfish, IFREMER collates different types of information. Surveys are conducted among fishing vessels, once on land, to calculate their activity figures on a monthly basis. Auction measurements are also performed. Other observers go aboard fishing vessels to estimate total catches, i.e. not only what is landed and sold, but also what is thrown back into the sea (undersized fish or species that are not of commercial interest, etc.). All this data is necessary to gain an idea of the structure of the population of each species, in terms of size and age, and to estimate the quantity of juveniles as well as the natural mortality rate – from disease or predation – and fishing. 

Once the lab-based analyses have been performed, the data is entered into an international database and used to run stock assessment models, track population trends and deliver short-term projections (1 to 3 years). The goal is to define the quantities that could be fished the following year without jeopardising a population’s ability to rebuild. 

This scientific expertise is submitted in the form of an opinion to European policymakers who, at the end of long negotiations, divide the total allowable catches between Member States in the form of fishing quotas.

Marine biodiversity under threat

Fishing is not the only human activity to have an impact on the ocean: myriad other factors also disrupt the marine environment. For instance, the building of ports or use of resources such as oil or sand destroy precious habitats. Toxic chemical contaminants – mercury or persistent organic pollutants – accumulate all the way up the food chain to the final consumer: humans. Nitrates from agricultural fertilisers cause ‘ blooms ’, outbreaks of microalgae which use up the oxygen in the water to the point of creating ‘dead zones’. 

Commercial maritime traffic is inadvertently introducing invasive exotic species into the sea, not least via the dumping of ballast water from ships, which is leading to biodiversity loss. The rising carbon dioxide emissions in the atmosphere are also disrupting the balance of the oceans, which absorb this surplus CO₂ and become more acidic. Finally, the oceans are suffering from rising water temperatures caused by global warming, and this is taking a toll on habitats. How will marine species adapt? Will some fish have to migrate further north, or live at greater depths? How will ocean life respond to these cumulative stress factors? 

Plenty of research projects are endeavouring to address these questions. At IFREMER’s base in Boulogne-sur-Mer (on the North coast of France), scientists are analysing the effects of climate change on herring or sardine larvae and embryos, for the development of these fish and the future of their populations depend on these first stages of life.

What does the future of fishing look like?

‘Conserve and sustainably use the ocean’: this is the goal of the third United Nations Ocean Conference organised in Nice in 2025 by France and Costa Rica. We have our work cut out: in the twentieth century, total catches fell tenfold, with rates worsening since the 1950s. Still today, nearly 38% of global fish stocks are overexploited, according to the Food and Agriculture Organization of the United Nations (FAO). 

Various measures could be adopted and expanded, however, including limiting vessel capacities, prohibiting the most destructive practices and banning fishing in nursery grounds where juveniles are growing, as well as marine protected areas. The quota policies adopted since the 1980s have also proven to be effective. In Europe, fishing pressure mounted until the 1990s. But thanks to the introduction of restrictive quotas in the early 2000s, fish stocks have been able to rebuild, especially in the Atlantic Ocean. That said, these regulations do not apply to all seas worldwide, and the situation in Africa and Asia is still of immense concern. 

Today, the principle of ‘ maximum sustainable yield ’ is given precedence in fisheries management: i.e. the maximum catch that can be taken without impairing a stock’s ability to replenish itself. But is this standard enough to define sustainable fishing at a time when climate change is further undermining the delicate balance of ecosystems? Experts are already advocating for an ‘under-exploitation’ of marine resources to safeguard their resilience.

Thank you to the Thalassa’s crew and the IFREMER team leading the IBTS 2025 campaign, particularly Pierre Cresson for his role as scientific adviser