For over a century, fish have been a cornerstone of global food security and the economy , both in the sea and in rivers and lakes. However, a series of recent studies is revealing something that until recently went largely unnoticed: fish are growing less, maturing earlier, and dying more often than in the past. And this isn't an isolated phenomenon, but rather a pattern that emerges repeatedly when analyzing data from different regions and species.
What at first may have seemed like isolated signs is consolidating into a global trend of declining growth and populations de peceswith enormous implications for biodiversity, food security, and the future of fisheries. Climate change, overfishing, habitat destruction, and pollution are pushing many species to the brink. while scientists, administrations and fishermen try to react with new forms of management and solutions such as restorative aquaculture.
A global decline in fish growth: what's happening
An international investigation led by scientist Helen Yan, from James Cook University in Australia, analyzed historical growth records of multiple species de peces marine over more than a hundred years. The results are clear: in most cases, fish today grow more slowly than in the past and reach smaller sizes throughout their lives.
This study, which uses long-term data from different ocean regions, shows a slowdown in growth that is not uniform : some species and areas appear to be more affected than others. But the overall signal is consistent and repeated enough to raise concerns among the scientific community and fisheries managers.
Fish growth is a highly sensitive indicator of the environment. When it slows, it usually indicates that something is wrong with the environmental conditions or the population structure . The fact that this phenomenon is observed on a planetary scale suggests that the oceans are under accumulated pressure far exceeding their capacity to adapt.
Furthermore, the reduced growth rate is compounded by other changes already observed in many exploited populations: fewer large individuals, younger average ages, and reduced genetic diversity . All of this makes populations more vulnerable to disturbances such as marine heatwaves, disease, or extreme pollution events.
Human pressure and environmental change: the roots of the problem
Studies analyzing this global decline point to two major drivers acting in combination: overfishing and environmental change associated with human activity. Far from being independent problems, they reinforce each other and accelerate biological changes.
Overfishing has been removing the largest and most reproductively active specimens from the water for decades . This alters population structure and can, over time, favor individuals that mature earlier and grow less. It's not just that there are fewer large fish because we're removing them: fishing pressure itself can drive evolution toward smaller body sizes.
At the same time, the oceans are warming, acidifying, and, in many areas, becoming depleted of nutrients and oxygen. The increase in water temperature accelerates the metabolism of fish, so they need more energy to stay alive . If this extra demand is not met with more available or better-quality food, the result is simple: fewer resources for growth.
The combination of elevated temperatures, chemical changes (such as acidification), and reduced oxygen availability creates an environment that limits the fish's ability to convert food into biomass . Even if they continue to eat, an increasing portion of that energy is "lost" simply to keep their bodies functioning under stress.
In parallel, in inland waters, dam construction, river channeling, massive water extraction for irrigation, and pollution are degrading river ecosystems. These alterations disrupt the corridors that connect breeding, nursery, and feeding areas , highlighting the importance of fish passages in river restoration, and reduce the quality of available habitat, which directly impacts fish growth and survival.
Climate change, accelerated metabolism, and increasingly smaller fish
Several studies published in high-impact journals, such as Science, have explored the link between global warming and reduced body size in fish. Researchers like Jan Kozłowski, Dustin J. Marshall, and Craig R. White have analyzed data from some 3.000 species and described a worrying pattern.
As the water warms, the metabolism of fish speeds up. This forces them to reach sexual maturity at younger ages , ensuring they reproduce at least once before they die. Biologically, it's a survival strategy: if mortality increases, it makes sense to "race" to produce offspring as soon as possible.
The downside is that these individuals, reproducing earlier, don't reach the sizes they would in a more favorable environment. Thus, what is observed in many populations is a general reduction in average size and higher mortality . The fish, quite literally, live fast and die young.
According to this research, the net result is a significant drop in biomass available to fisheries. Projections indicate that global fish production could fall by around 22% if global warming reaches 2°C above pre-industrial levels, a greater reduction than previous estimates that did not take these evolutionary responses into account.
In high-emission scenarios, the decline could reach around 30%. Specific calculations have been made for key commercial species, such as Alaska pollock (a cod-like fish), whose annual catch could decrease by approximately half a million metric tons. This would be equivalent, according to some experts, to losing more than one billion servings of high-quality protein per year from just one species.
Ecological impacts: food chains on the verge of collapse
The size of fish matters not only economically. It largely determines predator-prey relationships in aquatic ecosystems . In many marine systems, "who eats whom" depends heavily on differences in body size.
When large species decrease in size and smaller individuals become more abundant, food webs are reconfigured. For example, on the continental shelf of Nova Scotia, the size of several large predators decreased by about 40% in just four decades, while their prey populations increased by up to 300% . The result: a completely different ecosystem than the one that existed a few decades earlier.
These transformations can trigger cascading effects that are difficult to predict. If a predator becomes smaller, it may cease to control certain prey, which in turn may overexploit resources lower in the food chain . In the long term, this can lead to irreversible changes in the ecosystem's structure, with the loss of species and environmental services.
The fishability of the species themselves changes: schools de peces composed mostly of small specimens are less valuable to commercial fishingeven if the number of individuals remains high. Attempting to compensate for the size loss with increased fishing effort risks causing populations to collapse even more rapidly.
Another critical point is the associated loss of genetic diversity. As average size decreases and traits favoring early maturation and slow growth are indirectly selected, genes related to large, long-lived individuals are lost. This genetic erosion limits the future capacity of populations to adapt to new environmental changes.
Freshwater fish: the silent crisis of rivers and lakes
If the marine outlook is worrying, that of inland waters is downright alarming. Several reports prepared by international organizations and the United Nations Convention on the Conservation of Migratory Species (CMS) highlight that a third of the species de peces fresh water It faces the risk of extinction.
Freshwater fish represent approximately 51% of all species de peces known and have a fundamental role in feeding and sustaining millions of peopleHowever, they receive far less media and political attention than more charismatic marine species. This “invisibility” works against them precisely when they need the most protection.
According to data collected since 1970, the populations de peces Freshwater migrants have fallen by about 81%, and those of Large species have declined by around 94%.The year 2020 was especially bleak: 16 species were officially declared extinct de peces freshwater species, including the iconic Yangtze River paddlefish.
For many communities in Asia, Africa, and South America, these fish are the primary source of animal protein, in addition to providing direct or indirect employment to some 60 million people . The loss of these populations is not only a biodiversity tragedy but also a blow to food security and local economies.
The causes of the decline in freshwater fish populations are numerous and overlapping: habitat destruction, overexploitation of water resources, hydroelectric dams, agricultural, urban, and industrial pollution, overfishing, invasive species, climate change, intensive gravel and sand extraction, and environmental crimes such as illegal fishing . Together, these factors create a cocktail that is leading many rivers and wetlands to an unprecedented ecological crisis.
Large river basins and migratory species under threat
The CMS report analyzing the status of migratory freshwater fish identifies approximately 325 species in a critically endangered situation , whose survival depends on urgent and coordinated action between countries. The reason is clear: many of these species travel along rivers that cross multiple national borders.
Among the most critical river basins are the Amazon and the La Plata-Paraná system in South America, the Danube in Europe, the Mekong in Asia, the Nile in Africa, and the Ganges-Brahmaputra complex in the Indian subcontinent. In all these systems, the increase in dams, habitat fragmentation, and pollution are accelerating the loss of river connectivity , a devastating phenomenon for species that depend on movement along the river to complete their life cycle.
In Spain, although media attention tends to focus on other environmental problems, species affected by this global trend have also been identified. The conservation of these fish requires managing river basins as interconnected systems, not as isolated sections defined by administrative boundaries . The alternative only leads to piecemeal management solutions that fail to address the underlying problem.
Experts agree that there is still room to reverse some of the decline, but warn that time is running out. Without a coordinated and ambitious international responsemany species de peces Freshwater migratory species could disappear in the coming decades, with profound impacts on river ecosystems and the societies that depend on them.
Furthermore, at a national level, countries with a high degree of endemism—such as Spain, where almost every river basin harbors its own cyprinids, like different types of barbel—bear a special responsibility. When an endemic species disappears, it's not just lost from a river: it disappears from the planet.
Situation in Spain: threatened species and official extinctions
Conservation reports and the IUCN Red List indicate that in Spain more than 20 species de peces continental areas are listed as threatenedAmong them are the samaruc, the loina or the fartet, all of them closely linked to unique aquatic habitats that have suffered intense degradation in recent decades.
Some species have already benefited from protection measures too late: the European sturgeon and the river lamprey are officially considered extinct in Spain . These cases serve as a warning of what can happen if action is delayed until populations have crossed a point of no return.
Other species, such as the Salamanca sardine, listed as "endangered," see their natural distribution area threatened by specific projects, such as certain mining operations that can compromise water quality and the riparian habitats of unique waterways. This highlights a common contradiction: investment in conservation is made at the same time that projects with a significant local impact are approved.
The high rate of endemism among Iberian freshwater fish, coupled with the geographical isolation of many river basins, means that decisions regarding water management, hydraulic works, and land use have a disproportionate impact on the fate of these species. The loss of even a small, well-preserved stretch of river can, in practice, bring an entire evolutionary lineage much closer to extinction.
Therefore, conservation organizations are demanding that the protection of rivers, lakes, and wetlands be given the same level of priority as that of forests or oceans at major biodiversity summits, and that governments adopt specific targets to restore freshwater systems , not just to stop their deterioration.
Restorative aquaculture: solution, support, or temporary fix?
Faced with the decline of many populations de pecesSome researchers and managers have opted for a strategy that blends conservation and production: the restorative aquaculture or restocking programs from hatcheriesThe idea is to raise fish in controlled facilities and release them into the natural environment to reinforce declining populations.
A recent example is the kūmū, or Hawaiian goatfish, highly valued as a food source on the main Hawaiian Islands and severely affected by overfishing. Researchers at the Ocean Institute of Hawaii Pacific University are working to raise the species in hatcheries, tag the juveniles, and release them in selected areas. Fishermen who catch tagged specimens receive compensation in exchange for reporting the catch, allowing researchers to evaluate the program's effectiveness.
In the United States, the Texas Parks and Wildlife Department (TPWD) maintains a similar program for southern flounder in the Gulf of Mexico. This species has suffered a prolonged decline due to recreational and commercial fishing and warmer-than-usual winters, which severely impact its early life stages . The larvae require a very specific temperature range to develop (around 64-73°F), which is becoming increasingly rare.
Since 2006, following an experimental period, TPWD has been releasing fingerlings into public waters to supplement natural recruitment when environmental conditions are poor. In 2023, they released nearly 300.000 juveniles in a single year , a record high. The hatchery manipulates temperature and light to simulate the annual cycle in about 150 days, uses specific hormones to induce spawning, and strictly controls the origin and rotation of the broodstock.
Similar initiatives have been launched with lobsters in Europe and North America. In the United Kingdom, where lobster populations have been affected by overfishing, pollution, and damage to the seabed, hatcheries are used to drastically increase larval survival rates . While in the wild, only one larva of the tens of thousands of larvae produced by a single female typically reaches maturity, thousands of viable juveniles can be obtained in a hatchery environment.
Benefits, ecological risks and the importance of habitat
Although these programs can bolster populations in critical situations and offer some socioeconomic relief to fishing communities, they are not without controversy. Several scientists emphasize that the mass release of captive-bred individuals can generate excessive competition for limited resources and disrupt the balance between species.
Research in Japan, for example, has analyzed the effects of releasing farmed masu salmon into streams in Hokkaido. The results suggest that when the ecosystem's carrying capacity is exceeded, farmed and wild fish compete intensely for food and habitat . This competition can lead to further losses of individuals before they reach reproductive maturity and also affect other species that share the same niche, reducing overall biodiversity.
Furthermore, if the destination habitat is degraded—due to pollution, low water levels, destruction of refuges, or rising temperatures—the ecosystem's capacity to support these extra fish is seriously diminished. Under these conditions, restocking can become merely a symptomatic treatment , failing to address the underlying causes of the decline and even worsening the situation in the long term.
On the other hand, there is a risk that captive breeding programs may unintentionally alter the genetic structure of populations. Selecting breeding stock based on visible traits or ease of handling can lead to favoring genetic combinations that are not representative of wild stocks. This could erode fine local adaptations, which are especially important in species with highly fragmented distributions.
To minimize these risks, the most advanced programs incorporate genetic management plans, rotate breeding stock annually, release juveniles in the same areas where their parents were captured, and maintain restocking rates at a moderate level relative to natural production . Health and water quality in hatcheries are also closely monitored to prevent the spread of disease.
Many experts agree that hatcheries and restorative aquaculture can be a useful tool, but never a standalone solution . They must go hand in hand with habitat restoration measures, reduction of fishing pressures, improvement of water quality, and adaptation to climate change. If the environment remains degraded, any population reinforcement will only be a temporary fix.
Towards a more adaptive and sustainable fisheries management
The accumulating evidence of the global decline in fish growth and the reduction of their populations is prompting scientists and managers to rethink traditional fisheries management strategies . Simply setting catch quotas or minimum sizes is no longer sufficient if environmental factors, evolutionary changes, and habitat status are not integrated.
One of the keys is incorporating long-term historical data, such as that used in growth studies, to better understand trends and monitoring techniques like the tool that allows us to listen to the fish . This helps distinguish between natural fluctuations and structural changes in marine and freshwater ecosystems. With this information, policies can be designed that are more aligned with current ecological realities, rather than those of half a century ago.
Another important approach is to move towards adaptive management, in which conservation and exploitation measures are dynamically adjusted to the results of scientific monitoring. This implies, for example, modifying fishing bans, quotas, or fishing zones based on how populations respond, rather than maintaining fixed rules despite evidence of decline.
Reducing additional pressures, such as overfishing, pollution, and the destruction of coastal and riverine habitats, is equally crucial. The less added human impact, the greater the ability of fish to adapt to a changing climate without reaching collapse thresholds . Otherwise, even the most resilient species can be overwhelmed.
Faced with the temptation to rely on evolution to "fix" the effects of climate change and human exploitation on its own, numerous scientists warn that the direction of this adaptation may not necessarily align with our interests . Fish can adjust to new conditions, yes, but at the cost of becoming smaller, more vulnerable, and less productive from a fishing perspective.
Everything we know so far points to a decline in growth and populations de pecesThe decline of marine and freshwater ecosystems is not an isolated incident but a global symptom of ecosystems under intense pressure. Slowing global warming, restoring rivers and coastlines, improving fisheries management, and using tools like restorative aquaculture judiciously will make the difference between healthy oceans and rivers, capable of... to continue supporting biodiversity and food for millions of peopleor a future of depleted waters where the margin for maneuver will have been exhausted.
