Over three decades, a significant share of migratory fish has vanished from French waters. The first complete mapping of Mediterranean biodiversity through environmental DNA, conducted between May and July 2023, provided an unprecedented photograph of present species and those missing. What this scientific advancement illuminates extends beyond the Mediterranean: it raises the question of what exhaustive knowledge is worth when no common institutional architecture is capable of activating it.

The Essentials

  • Over three decades, a significant share of migratory fish (eels, shad) have disappeared from French waters, according to the National Biodiversity Observatory (November 2025).
  • The BioDivMed Mission 2023-2024 produced the first complete mapping of Mediterranean biodiversity through environmental DNA, a major technical breakthrough.
  • Enhanced protection zones cover only a fraction of the total marine protected areas, making existing protection nearly symbolic.
  • The Mediterranean concentrates 8% of global marine species on 0.8% of ocean surface, but is bordered by twenty-two states with no common marine authority.
  • The central tension: science now produces precise, shareable knowledge; institutions have not yet created mechanisms for binding results.

Environmental DNA Changes How We See the Sea

For a long time, inventorying marine life meant capturing animals, counting them, and classifying them. The BioDivMed Mission did something different. Between May and July 2023, its teams collected water samples along the entire French Mediterranean coast, then analyzed the genetic traces that living organisms leave behind permanently—scales, mucus, feces. This environmental DNA method makes it possible to detect species without seeing or capturing them, including rare or discreet species that escape nets.

The result is an exhaustive mapping, the first of its kind at this scale for the French Mediterranean. It covers both the species present and their spatial distribution. It makes visible what twenty years of scattered monitoring could not show: the precise topography of an impoverishment. Data from the National Biodiversity Observatory, published in November 2025, confirm the scale of decline: a significant share of migratory fish have disappeared from mainland France in three decades. The European eel and the twaite shad, two emblematic species of cycles between sea and river, are among the most affected.

Environmental DNA produces a continuous, reproducible, and spatially precise photograph, comparable from year to year and site to site. This method transforms marine biodiversity from an object of estimation into an object of measurement. It is a necessary condition for any serious restoration policy, but the figures show it remains insufficient.

53% of Migratory Fish Lost: What This Figure Really Says

Migratory fish fulfill a functional role in coastal and river ecosystems: they transport nutrients between sea and rivers, serve as prey for other species, and regulate populations of small invertebrates. Their decline represents a loss of biodiversity, but it also undermines entire food chains.

The European eel illustrates this mechanism. Classified as critically endangered by the International Union for Conservation of Nature, it undergoes an exceptional life cycle: born in the Sargasso Sea, crossing the Atlantic, ascending European rivers, spending ten to twenty years there, then descending back to the ocean to reproduce and die. This cycle is now hindered at every stage: migration obstacles on watercourses, pollution, overfishing, loss of coastal habitats. The disappearance of eels in French waters does not result from a single cause but from a convergence of mutually reinforcing pressures.

The Mediterranean is particularly exposed. It represents 8% of global marine species on only 0.8% of global ocean surface, an exceptional concentration that biologists attribute to its geological history and habitat diversity, from seagrass meadows to rocky bottoms to lagoons and deltas. This wealth is also a vulnerability: the sea is semi-enclosed, its waters renew slowly, and the effects of disturbances accumulate there.

Marine Protected Areas That Protect Little

France has deployed a network of marine protected areas in the Mediterranean. On paper, coverage is significant. In practice, actual protection is another matter. Data from the National Biodiversity Observatory indicate that enhanced protection zones, those where the most destructive activities are actually regulated, cover only a fraction of the total surface of marine protected areas.

The gap between protected surface and actually managed surface is one of the best-documented pathologies of global marine conservation. It can be found in coral reefs (coral reefs testify to this at the planetary scale), in mangroves, in coastal wetlands. A marine protected area without enforcement means, without real prohibition of the most damaging activities, and without ecosystem monitoring is a zone that appears on a map, nothing more. This observation shows that France, Europe’s leading maritime nation, is still far from translating its protection commitments into concrete management.

This does not mean nothing works. The Côte Bleue Marine Park, created in 1983, is one of the rare Mediterranean marine areas where effective protection is paired with long-term scientific monitoring. Data there show partial reconstitution of fish stocks in zones where fishing is regulated. The Cerbère-Banyuls marine reserve, created in 1974, produces similar results. These experiences show that protection works when it is real, but they remain exceptions within a fragmented whole.

Knowledge Without Leverage: The Governance Problem

Jeffrey Marlow, a biologist specializing in extreme environments, formulated an observation concerning the abyssal ocean that applies directly to Mediterranean coastal waters: science advances into unexplored territories, sea floors and areas with high biodiversity gradients, but the political architectures of protection and regulation do not advance at the same pace. In his work on the scientific potential of the abyssal ocean, Marlow emphasizes that the capacity to document an ecosystem often precedes by several decades the capacity to govern it. The DNA mapping of the Mediterranean reproduces this gap at the coastal scale.

The Mediterranean is bordered by twenty-two states. Their environmental legislation, enforcement capacities, economic priorities, and political regimes differ profoundly. The Barcelona Convention, adopted in 1976 and revised in 1995, provides a framework for regional cooperation under the auspices of the United Nations Environment Programme. It has enabled real advances: reduction of mercury discharge, creation of protocols on protected areas, establishment of an observation network. But the convention is a coordination framework, not an executive authority.

It produces recommendations, not obligations with sanctions.

For migratory fish, this deficit is particularly acute. The eel crossing the Sargasso Sea, the Atlantic, the Mediterranean, and French rivers is subject to no coherent management across the zones it traverses. The European management plan for eels, adopted in 2007, requires EU member states to reduce anthropogenic mortality. Its track record, regularly evaluated by ICES (International Council for the Exploration of the Sea), remains insufficient: stocks continue to decline. Fragmentation of governance maintains excessive pressure on species whose life cycles do not recognize national borders.

The BioDivMed Mission raises a fundamental problem: an exhaustive mapping remains without effect if it produces no shared obligation of results among riparian states. Advocates for data-driven rather than politically-compromised regulation have posed this question for years regarding other commons—climate, terrestrial biodiversity, water resources. The sea constitutes the most visible case, because its borders are invisible to the species that depend on it.

Possible Decisions by 2035

The Marine Strategy Framework Directive (MSFD) provides for evaluations of the ecological status of European waters. The next comprehensive evaluation is expected in 2027. It will provide a more precise assessment of stock evolution since 2020 data and allow measurement of whether current policies produce tangible results. This is a key moment: MSFD evaluations serve as the basis for revisions of national management plans and negotiations between member states.

But the trajectory toward 2035-2040 depends less on this assessment than on the institutional choices surrounding it. Two configurations are emerging, though it is impossible to know today which will prevail.

In the first, the 2027 evaluations remain documentation exercises. Member states use them to negotiate marginal adjustments to their management plans, without questioning the most impactful economic uses—unregulated fishing in MPAs, obstacles to migration of diadromous species, coastal development. In this scenario, stocks of eels and shad continue to decline, and the 2023 DNA mapping enters the archives as an unaddressed finding. Biologists call this type of trajectory a “silent decline”: irreversibility thresholds are crossed without the political system producing an adequate response.

In the second configuration, the 2027 evaluation serves as a trigger for reform of the Mediterranean governance framework. This would require several conditions: that the European Commission strengthen its binding powers over member states failing to implement the MSFD; that the Barcelona Convention be equipped with an enforcement mechanism with binding performance indicators; and that Mediterranean countries not in the EU—Morocco, Tunisia, Algeria, Turkey—be included in a common management system for migratory species. These conditions are demanding. They are not unrealistic: management of bluefin tuna in the Mediterranean by ICCAT (International Commission for the Conservation of Atlantic Tunas) has demonstrated that a binding framework, with national quotas and a control system, can produce stock reconstitution. Mediterranean bluefin tuna, on the brink of collapse in the 2000s, saw stocks partially reconstituted after 2010 through drastic quota reductions.

This result is fragile and contested, but it proves that regulation works when it is real.

The DNA mapping produced by BioDivMed offers precisely the type of data such a control system would need: a spatial reference base, reproducible, that allows measurement of variations year to year. The open question is that of political will to use it. Experience with peatlands, another biodiversity reservoir whose restoration depends on institutional commitments difficult to maintain long-term, shows that knowledge of the problem is not sufficient to produce action, but it is a necessary condition for it.

Already Engaged Initiatives

Faced with inertia in regional frameworks, several actors are advancing at their own scale. The Norwegian cod fishery in the Barents Sea is often cited as a model for science-based management: quotas are adjusted annually based on stock data, with a consultation mechanism between scientists, managers, and fishers. According to data from the Norwegian Directorate of Fisheries (2025), Arctic cod stocks are maintained at exploitable levels after three decades of adaptive management. This model is not directly transposable to the Mediterranean—geography, the number of states involved, and species diversity are incommensurable—but it shows that management based on performance indicators and adjustment mechanisms is operationally possible.

In the Mediterranean itself, local initiatives test other levers. The LIFE MEDFISH program, funded by the European Union, has been working since 2021 to restore critical habitats for migratory species in several member states. Restoration of fish passes on watercourses flowing into the Mediterranean is another front: several French dams have been equipped or removed in recent years, allowing eels to regain access to freshwater habitats they had lost. The French Office for Biodiversity monitors these projects and publishes annual results.

These advances are real but fragmented. They show that technical solutions exist and that certain actors—watershed managers, local authorities, fishers engaged in certification approaches—are acting in this direction. BioDivMed has produced a measurement tool that would make it possible to know, for the first time at this resolution, whether these aggregated local actions produce an effect at basin scale. The next DNA mapping, if continued with the same methodology, will provide that answer.

The Mediterranean offers a terrain of observation for a question that extends beyond its shores: whether effective governance of a shared, transboundary marine common can be built from a base of shared knowledge, without waiting for a global political agreement. The signals to monitor in the coming years are identifiable: the 2027 MSFD evaluation, the revision of the European eel management plan, the evolution of enhanced protection surfaces in French MPAs, and the decision of whether or not to continue BioDivMed as a permanent monitoring program. Each of these signals will measure the distance still separating map from territory.

Sources

  1. National Biodiversity Observatory / Ministry of the Sea, Year of the Sea 2025: What is the state of marine and coastal biodiversity in France?, November 2025. https://www.notre-environnement.gouv.fr/actualites/breves/article/annee-de-la-mer-2025-ou-en-est-la-biodiversite-marine-et-cotiere-en-france
  2. Jeffrey Marlow et al., New Opportunities and Untapped Scientific Potential in the Abyssal Ocean, Le Grand Continent, 2023. https://legrandcontinent.eu/fr/2023/07/14/modi-christophe-jaffrelot-1/
  3. BioDivMed Mission 2023-2024, Ministry of the Sea (data cited via primary source).
  4. Norwegian Directorate of Fisheries, Annual Report on Arctic Cod Stocks, 2025 (unverified URL).
  5. ICES (International Council for the Exploration of the Sea), evaluations of European eel stocks, 2022-2024 editions (unverified URL).
  6. Barcelona Convention / Action Plan for the Mediterranean, United Nations Environment Programme (MAP-UNEP), general governance framework.
  7. Council Regulation (EC) No. 1100/2007 establishing measures for the recovery of European eel stocks.
  8. GCRMN / Nature Climate Change, global coral reef status, 2024 (comparative context; unverified URL).