Fish shrinkage probed in lab

Scientists are starting a novel project to investigate whether overfishing alters fish behaviour and changes their pattern of development.

Overexploitation of stocks has already been shown to select for smaller fish.

A team reporting at the meeting of the European Society for Evolutionary Biology in Germany will deliberately remove the largest individuals from populations of lab-bred guppies.

The experiment is designed to uncover what is happening in our oceans.

“There are clear indications that almost all… commercial fish are shrinking,” said marine biologist Carl Lundin, who directs the International Union for the Conservation of Nature’s Global Marine and Polar Program.

For mass spawning fish such as cod, there is a great advantage to maintaining older, larger females because they are very efficient at restocking the population.

And if industrial fishing selectively removes the largest individuals, explained Dr Lundin, the industry suffers as populations are reduced to the smallest fish.

However, smaller seafood is unlikely to be the only consequence of industrial fishing; research has also shown that fish in the oceans are reproducing earlier.

Experimental evolution

Now evolutionary biologist Beatriz Diaz Pauli and her colleagues from the University of Bergen, Norway have begun an experiment that they hope will help uncover what other changes we can expect to see in the oceans’ fishes.

The team established nine populations of guppies, each comprising 500 to 900 individuals. Over the next few years, Ms Diaz plans to remove all the fish that measure over 16mm from three of her tanks.

In the remaining tanks, Ms Diaz will purge fish under 16mm, or take fish independent of their size – regimens that will act as a control for the effects of changing the density of fish in the tanks.

The team will then painstakingly record the changes that they see in the fish’s growth rate, age and size of maturation, reproductive effort, and mating and feeding behaviours.

The team hopes to unpick whether the shifts they see are a result of fish moulding themselves to a new environment – a so-called plastic response – or are a consequence of genetic changes.

Plastic responses are not inherited. For example, an organism might reach a smaller body size if it gets little food as a juvenile, but its young would not inherit this propensity to be small.

Genetic responses, by contrast, are inherited, and even if a future generation is returned to an environment where food is plentiful, it would remain small.

Determining the nature of the changes in the fish will help scientists understand how stocks might recover if overexploitation stopped or breeding grounds were protected.

“If we set aside 20-30% of the habitat where reproduction… of key commercial fish stocks [occurs], we are much more likely to avoid these types of problems,” said Dr Lundin.

He added that carrying out experiments of this type allows researchers to control other factors that could affect the fishes’ survival and concentrate on just the consequences of overexploitation.

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Over-fished tuna in ‘hot water’, study finds

Two more species of tuna have been added to the Red List of Threatened Species.

They join the Southern bluefin tuna – listed as critically endangered.

The report, published in this week’s Science, is the first global assessment of this highly prized family of fish, which are at risk of being over-fished.

The World Conservation Union (IUCN) says there is a lack of resolve to protect against overexploitation driven by high prices.

Until this latest study, attempts to assess the health of scombrid and billfish populations, families of fish that include tuna and swordfish, have been carried out at a regional scale.

This study, which relies on the IUCN Red List criteria to judge the stocks’ health, took a more global approach.

Of the 61 species of fish assessed, seven were earmarked as either vulnerable, endangered or critically endangered. All suffer from over-fishing, habitat loss and pollution.

Along with the two species of tuna, two mackerel and two marlin joined the Red List.

The ‘sapphires of seafood’

Per kilo, bluefins are among the most expensive seafood in the world.

“All three bluefin tuna species are susceptible to collapse under continued excessive fishing pressure. The Southern bluefin has already essentially crashed, with little hope of recovery,” said one the the study’s authors Kent Carpenter the IUCN’s Marine Biodiversity manager.

Southern bluefin numbers have reached levels that are one twentieth of those recorded before industrial fishing began.

Atlantic bluefins have probably gone the same way, add the authors, while bigeye tuna is labelled “vulnerable”.

“Tunas are highly migratory fish, swimming across ocean basins and between the waters of various countries during their lifetimes. Conserving them requires regional and global co-operation,” commented Susan Lieberman, director of international policy with the Pew Environment Group in a statement.

What is more, tuna’s restricted spawning grounds make them exceptionally susceptible to collapse if over-fishing continues, reports the international team of scientists.

And tuna’s long lifespan means it would take their population several years to recover if fishing stopped altogether.

Pew’s Dr Lieberman adds: “The IUCN Red List assessment reinforces that it is time for governments to live up to their responsibilities.”

The report comes days before the tuna regional fisheries management organizations (RFMOs) assemble in in La Jolla, California for the Kobe III meeting.

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Marine Mud Is High in Fish Poop

Will you still enjoy feeling the beach between your toes this summer knowing it’s partly fish feces? In a paper published online this week in the Proceedings of the National Academy of Sciences, researchers report that 14% of the calcium carbonate that makes up the muddy floors of shallow tropical seas is fish poop. Fecal samples from 11 common tropical fish, including barracudas and snappers, reveal that calcium carbonate forms a key component of the excrement. The team estimates that every year, tropical fish excrete 6.1 million kilograms of calcium carbonate, equivalent to the weight of 1000 adult elephants, over an area of 111,577 square kilometers. Each fish may even have its own unique “fecalprint”, with specific sizes and shapes of calcium carbonate crystals (as seen in the black and white image), which could allow future oceanographers to analyze an ocean’s mud to track changes in the numbers and diversity of fish species.

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Sharks Chew Off More Than They Can Bite

Teenage great white sharks might look all grown up, but they don’t have the awesome bite of a fully grown adult, researchers report online today in the Journal of Biomechanics. The team built a computer model based on measurements taken of the head of a young great white shark and its cousin, the sand tiger shark (seen here chomping down on a bait fish). Based on the structure of the sharks’ cartilage and muscle, the model indicates that the jaws of young great whites haven’t fully stiffened. This means that, although their gape is wide enough, the sharks can’t feast on the flesh of seals and other large mammals without damaging their jaws. The researchers hope the work helps conservation biologists better understand the sharks’ feeding behavior and protect their prey species.

See the movie of the biting shark.

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Fish Sleep Soundly in Mucous Cocoons

Even the ocean has bedbugs. Tiny blood-sucking crustaceans (inset) roam the seas, nipping at the scales of passing fish. But the parrotfish (Chlorurus sordidus) has evolved an unusual defense. According to a study published online today in the Proceedings of the Royal Society B, the fish spend up to an hour spinning cocoons from their own mucous before they settle down to slumber for the night. These transparent, gelatinous balls of spit are large enough to envelope the fish from head to tail. By gently pushing fish from their cocoons without waking them, researchers showed that those sleeping without protection were 80% more likely to be bitten by the crustaceans than those they left untouched. Producing these mucous membranes costs just over 2% of the fish’s daily energy budget; apparently a worthwhile investment against things that go bite in the night.

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Ancient shark had colossal bite


The great white shark may have awesome jaws but they are nothing compared with those of megalodon, its gigantic, whale-eating ancestor.

A new study of the extinct creature’s skull shows it had an almighty bite, making the prehistoric fish one of the most fearsome predators of all time.

All the more remarkable, scientists say, because the crushing force came from jaws made of cartilage, not bone.

The researchers report their skull work in the Journal of Zoology.

The Carcharodon megalodon super-shark swam in the oceans more than a million-and-a-half years ago.

It grew up to 16m (52ft) in length and weighed in at 100 tonnes – 30 times heavier than the largest great white – and must have been one of the most formidable carnivores to have existed.

“Pound for pound, your common house cat can bite down harder, ” explained Dr Stephen Wroe of the University of New South Wales, Australia. “But the sheer size of the animal means that in absolute terms, it tops the scales.”

Measuring up

Dr Wroe’s team used a technique known as finite element analysis to compare the skulls of the great white with that of the prehistoric megalodon.

The approach is a common one in advanced design and manufacturing, and allows engineers to test the performance of load-bearing materials, such as the metal in the body and wings of an aeroplane.

CT (X-ray) scans were taken of megalodon remains to construct a high-resolution digital model.

A model of a modern 2.4m-long male great white shark (Carcharodon carcharias) was developed for comparison.

Artist's impression: Megalodon (BBC)

A recent BBC series imagined a face-to-face encounter

The model of Megalodon’s muscles was based on those of the great white, and the simulations were then loaded with forces to see how the two skulls, jaws, teeth and muscles would have coped with the mechanical stresses and strains experienced during feeding.

By looking at the distribution of stress and strain on the sharks’ jaws, researchers found that the largest great whites have a bite force of up to 1.8 tonnes, three times the biting force of an African lion and 20 times harder than a human bites.

Megalodon, though, is more impressive. It is estimated to bite down with a force of between 10.8 to 18.2 tonnes.

The team said biting with such force was quite a feat given that the jaws of these ancient creatures were made of flexible cartilage.

In contrast to most other fish, sharks’ skeletons are made up entirely of cartilage. Scientists think that cartilage, being a much lighter material than bone, is one adaptation that allows sharks to swim without the aid of a swim bladder.

With finesse

The Australian research team was interested in how a cartilaginous jaw performs compared with a bone jaw.

The scientists’ study shows that the cartilaginous jaw is almost as strong as a bony jaw of the same size – losing only a few percent – in measures of bite force. What is more, the elasticity of the cartilage jaw increases the gape of the sharks to devastating ends.

“The shark’s upper jaws can be dislocated: the whole upper and lower jaw pull out and forward as the shark twists and shakes its head from side to side to bite a chunk out of its prey,” explains Dr Wroe.

These sharks feed on very large prey: the great white shark eats sea lions and the megalodon is thought to have eaten whales.

“These sharks ambush their prey and immobilise them with a bite, then wait for them to die,” Dr Wroe told BBC News. “They are actually delicate feeders and take care not to damage their teeth by biting down too hard on the large bones of their prey.”

To keep their teeth sharp, sharks have a battery of them that is continually replaced.

It is the combination of their size, their razor-sharp teeth and the element of surprise that makes these sharks such deadly predators.


Shark graphic
Megalodon Great white shark
Type Cartilaginous fish Cartilaginous fish
Size 16m (52ft) 6m (20ft)
Diet Whales, including the now extinct Odobenocetops, seals Fish, turtles, seals, sea lions, squid and crustaceans
Predators None known Occasionally caught by fishing industry as bycatch

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