Overfishing making fish smaller

Shrinking cod


From Nature:

Ocean conservation: A big fight over little fish

Size limits have been a part of fisheries management for decades, but some fear that they are doing more harm than good.

30 January 2013

SHRINKING FISH: For Northeast Arctic cod, the age, size and weight of first-time spawners have fallen dramatically.

One April day, a fisherman named Johan Norman reeled in a female cod near the Norwegian village of Moskenes, where snow-capped mountains rise straight from the sea. He measured the fish: 82 centimetres from the tip of its snout to the tip of its tail. Then he pulled out his knife and sliced off several scales, placing them in a small envelope to deposit at the Institute of Marine Research in Bergen, Norway. The year was 1913.

Over the next century, as those scales sat in a repository, radical changes took place in the world’s oceans. The small sailing vessels of Norway and other fishing nations were replaced with industrial bottom trawlers. In 1968, the North Atlantic cod harvest started a precipitous decline, as did other stocks, including salmon, sole and lobster. Then, in the early 1980s, biologists began to report another worrying phenomenon. Fish in some areas were growing more slowly, maturing earlier and laying fewer eggs than before1. Not only was this an ominous sign for the sustainability of these fisheries, but smaller fish are less valuable than larger ones because they yield smaller fillets.

Explanations for the shrinking fish have ranged from changes in seawater temperatures to a decline in food resources2. But the real culprit could be the practices devised to protect the fisheries. As mandated by various laws and treaties, most trawlers’ nets sport a large mesh that allows small, young fish to wriggle free. The reasoning is simple: harvest only the oldest, fattest members of the population and let young fish live to spawn and contribute to the next generation. Fisheries scientists and conservationists support size restrictions because they are thought to protect populations, and fishermen are happy to concentrate on large, high-value fish.

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But what if the underlying theory is wrong? Over the past five decades, scientists have come up with little evidence that reducing the catch of juveniles or small fish has improved the annual harvest. Instead, a small chorus of researchers is now arguing, fish are adapting to size restrictions by investing their energy into reaching sexual maturity earlier instead of growing large (see ‘Shrinking fish’). And as a result of their small size, they produce fewer eggs. Although these scientists do not deny that overfishing is the greatest threat to fisheries, they say that this evolutionary pressure will have a pernicious impact that will be hard to reverse. “You can safely ignore it for a couple of years, but it’s accumulative, so the problem keeps growing,” says Mikko Heino, a biologist at the University of Bergen.

The theory is controversial, and many scientists are unconvinced. So last year, Heino turned to Norman’s 100-year-old preserved cod scales for help. He extracted DNA from them and is piecing together the whole genome sequence of this fish and others in a hunt for changes in growth and development genes that might explain the species’ shrinking size.

But even if the evolution idea is true, there is some disagreement over what to do about it. Only “a shrinking minority of fools” think that increasing fishing pressure on juveniles is smart or sustainable, says Carl Walters of the University of British Columbia in Vancouver, Canada.

The theory of fisheries-induced evolution can be traced back to 1981, when the Canadian fisheries scientist William Ricker suggested that coho salmon (Oncorhynchus kisutch) and pink salmon (Oncorhynchus gorbuscha) were maturing at a smaller size because Japanese gill-net fishermen were targeting only the largest fish on the high seas1. By the 1990s, researchers had begun to notice the phenomenon in other species too. But for many years, the consensus was that environmental factors such as climate change and pollution were at play, not genetics.

Then, in 2002, David Conover and Stephan Munch at the State University of New York in Stony Brook published a contentious experiment3. They caught Atlantic silverside (Menidia menidia) off the coast of Long Island and established six captive populations of around 1,000 individuals each. After 190 days, they removed 90% of the fish from each population. In the first two populations, they took only the largest fish; in the second two they took only the smallest fish; and in the final two they took individuals of random size. They then stimulated the remaining 10% to breed. After four generations, the fish in the large-harvested populations were about one-third the average weight of those in the random-catch group.

But critics called the experiment unrealistic. The stimulated breeding essentially created a population with a fixed age at sexual maturity, so it was no surprise that removing larger fish favoured those that matured at a smaller size. By contrast, in a natural population, the size at maturity is relatively stable, but age at maturity varies. Slower-growing fish mature later, and faster-growing fish mature earlier. Thus, size limits could select for faster growth, a possibility that Conover and Munch’s experiment did not allow. “I was outraged,” recalls Walters. “They did an experiment that could only give one result.”

Precocious cod

The dispute intrigued Heino, a theoretical biologist, who had begun working on his own approach to studying the life history of fish. In the past, researchers would chart a population’s maturation reaction norm — the size and age at which fish typically become sexually mature. But Heino realized that comparisons of maturation reaction norms between populations could be misleading if they didn’t take into account the variation in growth rates caused by food availability, climate or other environmental factors. So Heino developed a probabilistic approach that considers growth-rate variations.

Using this technique, he showed in a 2004 paper in Nature4 that northern cod (Gadus morhua) born in 1987 were maturing at a younger age and a smaller size than those born in 1980, and these changes preceded a dramatic collapse of the species off the coast of Canada in the late 1980s and early 1990s (see ‘A shift in maturity’).

Shift in maturity


“It’s the most famous fisheries collapse in recent times,” says Heino, “You would expect the potential for rapid evolution.” Heavy fishing was the main cause of these changes, Heino says, but size-selective fishing compounded the problem. Critics point out that the trend coincided with colder water, heavy sea-ice cover and other factors2.

Nevertheless, Heino’s technique opened up a new field, called Darwinian fisheries management, and evolutionary biologists were soon trying to measure the impacts of size restrictions on other wild populations. A 2009 study5 used Heino’s method to conclude that, of 37 commercial fish stocks, the majority were maturing earlier and at a smaller size than in the past, and that these effects were strongest in heavily fished populations.

Jeff Hard, a geneticist with the US National Oceanographic and Atmospheric Administration Fisheries Service in Seattle, Washington, says that in 1976 the largest class of female salmon — those greater than 100 centimetres in length — accounted for more than 20% of the fish spawning in one Alaskan river. Today, that number is less than 4%, and the number of eggs that females are producing has declined by 16%. But without genetic data from this and other populations, the findings can always be attributed to environmental changes. “It’s almost impossible to prove these things,” says Andrew Hendry, an evolutionary ecologist at McGill University in Montreal, Canada.

That is why Heino and others are looking to the DNA from historical samples of cod and other species for help. Filip Volckaert of the Dutch-language Catholic University Leuven in Belgium, for example, is sequencing DNA from otoliths, or ear bones, of yellowfin sole (Limanda aspera) from every decade back to the 1950s to identify genetic changes that might be linked to growth.

And Heino is complementing the genetic work with his own brand of lab experiment. Inside a special room at his university, he now has nine populations of guppies, and harvests between one-quarter and one-half of the population on the basis of size. To make the experiment more natural than that of Conover and Munch, he allows the guppies to reproduce freely at any age. And, as in nature, the breeding populations contain a wider range of ages and sizes. He expects the experiment, which he started in 2009, to run until 2014.

But it will take a lot to convince the sceptics. “Fisheries-induced evolution is an interesting side issue, but it’s been greatly overblown,” says Ray Hilborn, a fisheries scientist at the University of Washington in Seattle. There is no question that fished populations are evolving, he says, but some traits, such as earlier age of maturation, may make some fish populations more productive, not less so. The data suggesting that growth rates are slowing are also not yet convincing, he says. The best way to preserve fish populations is simply to fish less, he says.

Heino agrees, but wants to see other changes in marine policy. For example, he does not think that marine reserves should protect only spawning grounds — a common conservation strategy — because that gives another advantage to early-maturing fish, which return to the spawning grounds to breed sooner than late-maturing fish. Second, he says that it is time to abandon most size limits.

Support is growing for these views. Last year, an international group of fisheries experts published a policy paper in Science6 rejecting size limits for a wide range of reasons, including evolutionary issues. Jeppe Kolding of the University of Bergen studies small-scale fishing in Africa, and has found that areas where fishermen use illegal nets that catch large and small fish alike tend to have food webs that are diverse, intact and resemble unharvested areas, only with lower biomass. When fishing pressure is spread across species and sizes, he argues, fishermen can net more fish, yet the risk of wiping out individual populations is lower. “How can you tell me this is a bad fishing method?” he asks.

Heino knows that overturning entrenched fishing practices could take decades, and for now he is focusing just on the data. “It requires patience,” he says. “The practical implications are something that will keep developing for a long time.”

Nature 493, 597–598 (31 January 2013), doi:10.1038/493597a


  1. Ricker, W. E. Can. J. Fish. Aquat. Sci. 38, 1636–1656 (1981).
  2. Kuparinen, A. & Merilä, J. Trends Ecol. Evol. 22, 652–659 (2007).
  3. Conover, D. O. & Munch, S. B. Science 297, 94–96 (2002).
  4. Olsen, E. M. et al. Nature 428, 932–935 (2004).
  5. Sharpe, D. M. T. & Hendry, A. P. Evol. App. 2, 260–275 (2009).
  6. Garcia, S. M. et al. Science 335, 1045–1046 (2012).

Author information


  1. Brendan Borrell is a fellow with the Alicia Patterson Foundation in New York.

End overfishing – support the Invest in Fish Campaign: here.

What are distant-water fishing fleets, and how do they affect #overfishing? Here.

Cod in the North Sea: here.

Two recent scientific studies point to the effects of global warming on Atlantic cod populations, which were already reduced to historically low sizes by decades of overfishing during the 20th century: here.

As the global population booms, equitable access to healthy food sources is more important than ever. But a new study shows that wealthy countries’ industrial fishing fleets dominate the global oceans. This skew in power and control has important implications for how our planet shares food and wealth: here.

Film about overfishing on the Internet

This video says about itself:

Overfishing and more: SEA THE TRUTH — In 2048 The Oceans Will Be Empty! (16:9, full-length, HiQ)


(NOTE for the YouTube-Team: we have uploaded this movie with the permission of the Nicolaas G. Pierson Foundation)


Sea the Truth is based on numerous scientific publications that examine the problems of seas and oceans. Below follows an overview of the themes addressed in the film and a brief explanation.


According to a report of the New Zealand news channel 3News sea mammals, among which whales, are dying of malnutrition. The makers claim that this is caused by overfishing. Watch the report here: http://www.3news.co.nz/Deep-Trouble-/tabid/371/articleID/169002/Default.aspx


Fishing policy around the world is destructive. Recommendations from scientists on quotas are ignored by policy makers, wealthy countries plunder the fishing territories of poor countries and bottom trawlers sow destruction all over the seafloor with their dragnets. In Europe, 88% of fish stocks have been overharvested, such as the blue fin tuna which sadly is threatened with extinction.


In addition to the effect on the fish stocks, fishing also affects all other organisms in the same habitat or ecosystem. Whether the fish being harvested are predatory or prey, the balance of the ecosystem is disrupted and this can have serious consequences. The degree of disruption strongly depends on the fishing method employed.


The term bycatch has come to be used to refer to fish caught unintentionally when fishermen fish for commercial fish. These kinds of fish are not interesting to sell and as a consequence they are thrown back into the ocean either death or mutilated. The average bycatch worldwide is about 40.4% of the total amount of fish being caught. This means that 3 kilos of consumed fish brings about 2 kilos of bycatch. In total, 37 billion kilos of fish per year is wasted bycatch.


People once thought that fish could not feel anything when they are caught. This idea was probably motivated because fish are cold blooded; this is in contrast with humans who are warm blooded. However, the ability to feel pain does not have anything to do with body temperature. From research studying the behavior of fish, as well as the study of anatomy and physiology, it turns out that fish have feelings and are in fact able to feel pain. This means that the current methods to catch and kill fish are in truth a torture for fish, moreover captured fish die of suffocation: a process that can take up to several minutes or hours.


Between Hawaii and San Francisco floats an enormous amount of rubbish — a plastic soup with a surface area of 8.6 million square kilometres. To compare: This is 33 times greater than the surface area of the Netherlands (41,528 km2). This plastic soup was ‘discovered’ by Charles Moore when he sailed through this area with his boat and found himself surrounded day in day out by plastic waste. He later returned with scientific equipment to determine the soup’s total size. The plastic soup is a huge threat to a number of marine animals and mammals.


We’re told we should eat fish twice a week as it is packed with nutrition. These healthy nutrients are however easily obtained from other food sources, whereas fish may also contain large amounts of toxins. Mercury and dioxins ‘enjoy’ the status of most researched toxins in fish.

Without knowing it, most of you daily contribute to the Plastic Soup by scrubbing your body, brushing your teeth, cleaning your face or washing your hair, because the plastic micro beads inside these products will be washed down the drain: here.

September 2012. Greenpeace activists on an inflatable boat intercepted the world’s second largest factory fishing trawler, the FV Margiris, and blocked the monster ship’s attempt to sneak into Port Lincoln in South Australia. One activist boarded the vessel and blocked the entrance to the Margiris to stop the harbour pilot from bringing the vessel into port. Greenpeace is calling on the Australian government to refuse to grant a fishing license to the FV Margiris and introduce a policy to ban all super trawlers from Australian waters: here.

Film Sea the Truth, world premiere

Not just at the Cannes film festival there are world premieres of movies.

Today, there was one in Tuschinski theatre in Amsterdam, the Netherlands.

It was the film Sea the Truth; about the beauty of marine life and the many threats to it.

Sea the Truth is in English (with Dutch subtitles).

A central character in this documentary is diver, underwater photographer and conservationist Dos Winkel.

This video was filmed for the movie Sea the Truth: Barbara van Genne interviewing Dos Winkel about overfishing.

Other prominent roles are for two young Dutch female marine biologists: Marianne van Mierlo and Barbara van Genne. Ms Van Genne is the chair of PINK!; the youth branch of the Dutch Party for the Animals.

The film was made for the think tank of this party, and Party for the Animals MP Marianne Thieme and Senator Niko Koffeman contributed to it.

Before the film started, beautiful photos of fish and other marine animals by Dos Winkel were projected on the screen.

Two islands are important in the film: Newfoundland and Bonaire.

On Newfoundland, Ms Van Genne interviews an ex-fisherman from Portugal Cove coastal village. Pius Coombs started being a fisherman at 12 years of age. Then, there was small-scale, comparatively sustainable, fishing. However, since the 1960s, big trawlers came from Europe and North America. In 1992, almost all the cod for which Newfoundland is historically famous was gone, and over 40,000 fishermen lost their jobs.

Some people blame this on seals. However, the ex-fishermen, and Canadian fisheries biologist Daniel Pauly interviewed blamed it on the big trawler businesses. They overfished; they destroyed much of the ocean bottom, damaging the food chain. Many ex-cod fishermen of Newfoundland are now working in crab fishing. According to a former colleague of them, there is crab overfishing now, and the same disaster as with cod fishing may happen.

Bycatch is a terrible waste in fishing, especially big industrial trawl fishing. The film also mentioned pollution, eg, by plastic in the sea, eaten by marine animals who mistake it for jellyfish.

The film debunks human health claims about fish. Swordfish from the Indian Ocean (considered to be a sea with comparatively few pollution problems) was tested on unhealthy mercury. It turned out that the swordfish contained far more mercury than what is medically acceptable.

Fish oil businesses have extravagant claims about how healthy their product supposedly is. These are wild exaggerations. Also less than 5% of a fish consists of oil. So, over twenty fish have to die for a tiny bit of fish oil.

Bonaire island in the Caribbean, according to the film, has the best protected marine environment in the world. The movie shows beautiful footage of Bonaire coral reefs and mangroves, and fish, turtles, and other animals living there. Yet, even here, marine life is in danger. Every day, eg, thrash from elsewhere in the world beaches on Bonaire’s coast.

After the film, Marianne Thieme told about the scornful reactions after she had proposed to Parliament that the parliamentary restaurant should stop serving endangered eel. However, two years later, the restaurant did stop it.

Then, Dos Winkel called for applause for some of the people present who had worked on the film.

There was a video message by comedian Paul van Vliet, reading a song by himself about pollution of the sea.

Finally, there were snacks. Not fish snacks, but vegetarian soybean snacks tasting like fish.

Finally, a few critical remarks about this good film. The premiere is at a time when BP oil is polluting much of the Caribbean. A documentary film is not a TV newsreel. And a film of about one hour cannot show all sides of a big complex issue. So, I understand that the BP Louisiana oil scandal is not in the film. Still, the film does not mention any pollution by the oil industry, or, more widely: capitalism. Neither does it mention war and militarism, military dangers to marine life.

However, let these points not deter anyone from seeing this film, and from becoming active about the important points which it raises.

ScienceDaily (May 13, 2010) — A new, less selective approach to commercial fishing is needed to ensure the ongoing productivity of marine ecosystems and to maintain biodiversity, according to a paper in the Proceedings of the National Academy of Sciences: here.

Britons want to buy sustainable fish but labels leave us baffled: here.

Young coral ‘threatened by noise pollution’: here.

The most endangered marine turtle in the world is threatened by trawl fisheries operating on turtle feeding grounds: here.

When did humans first start eating fish? Here.

The Globosa Mangrove, Heritiera globosa, is listed as ‘Endangered’ on the IUCN Red List of Threatened Species™. This species of mangrove is one of Indonesia’s many endemic species and is only found in the West Kalimantan province, on the island of Borneo. It is a very rare species of mangrove that has a very patchy distribution within a specific salinity range and prefers the more freshwater-dominated areas of upstream riverine habitats. It is a slow-growing species, but can reach large sizes of up to 25 metres: here.

Alternative fish feeds use less fishmeal and fish oils: here.

A BLAZING row has broken out between fisheries scientists over whether fishing fleets are depleting the world’s oceans of their large species: here.

Around 40 percent of hake is mislabeled: here.

Halifax, Nova Scotia: After decades of little hope in what was once one of the world’s major fisheries, Atlantic Cod is showing signs of recovery on the Grand Banks off the coast of Canada. But WWF is warning that fisheries managers must not rush to reopen the cod fishery that has been under moratorium since 1994: here.

New US report finds more than 20,000 marine mammals, sea turtles, and birds caught as bycatch in 2005: here.

A new investigation put in evidence the key role of cod as regulator of the whole Baltic Sea ecosystem. The study shows that when the cod population in the central Baltic increases, it spreads into larger areas and spills over into adjacent marginal systems where it usually does not occur, as for example the Gulf of Riga: here.

‘Sea the Truth’ shown on Bonaire: here.