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Off the coast of Norway,
scientists onboard the 'Star Atlantic'
are investigating a strange phenomenon.
The once huge cod are getting smaller and smaller...
The case of the shrinking cod
Scientists want to find out why this is happening.
The Case of the Shrinking Cod
For centuries, Bergen in Norway has been the centre of the fishing trade in northern Europe.
Many generations of fishermen have been wrestling cod from the sea.
Cod has become a staple of the european diet
and a way of life for people living on the coast of Norway.
but in the 1950s, something happened.
The cod, which the fishermen found in their nets, became smaller.
The large cod had disappeared.
The scientists on the 'Star Atlantic' haul in a fishing net full of cod for analysis.
Since the 1930s scientists have been recording the size and age of the cod.
To determine the age of the fish, they study the cod's ear bones, better known as otoliths.
Ear bones or otoliths, they grow a bit like trees.
So, as the fish grows, it lays rings.
And you can age a fish on the basis of this growth rings in Otoliths.
After 1940s you see an increase in the fishing intensity,
and a continuous decline in age and size of maturation.
And that is the big discussion, like one hypothesis is that when you have less fish in the Ocean,
it is more resources for the fish that is left.
So, they can have more food and etc., so they can grow faster and reach maturation faster.
This ecological hypothesis says that the cod is shrinking because its surroundings have changed...
That more and more fish are being caught than before.
To let the cod stock recover, cod fishing was strongly regulated through minimum mesh sizes and quotas,
limiting the amount of fish the fishermen were allowed to catch.
But the fish stayed small. The norwegian cod stock was in trouble.
However, evolutionary biologists have an ingenious theory:
That the cod had evolved to escape the fishermens nets!
They expected it to recover quite soon,
and it would if it had been only an ecological effect, but it didn't.
Then people started considering indeed it might have some evolutionary effect that was happening here, evolutionary changes.
With this special camera, which has been attached to a fishing net,
we can see that only the large fish are being caught.
The small fish can easily escape through the mesh, but the large fish are trapped inside.
So small fish, they can generally escape, and thats even the idea with these large meshes.
They want to be sure that the small fish escape and can grow larger.
In a natural situation, usually it is an advantage to be big, because then you are safe from more predators.
Small fish are eaten by almost anybody, but big fish can escape.
Fishing reverses this situation.
Because we preferentially target large fish, because they are more valuable as food and resource.
That makes it dangerous to be large, because those are targeted.
So, it pays to stay a bit smaller, because then you are less exposed to fishing.
If these scientists are right, then fishing induces evolution, evolution towards smaller cod.
And the more we fish this way, the smaller the cod will become.
When we are fishing, we are doing the opposite of what is done in animal breeding.
An interesting way of understanding how fishing is influencing fish stock, is to think of animal breeding.
And in animal breeding, breeders select the best animals to breed.
So, their genes are passed on and over time you get better and better cattle or whatever.
Whereas in fishing, it is pretty much the opposite.
So, we fish the best individuals, the large fish, and prevent them from spawning.
So, instead those fish that are not so attractive, they get the chance to spawn and spread their genes.
For a nation that relies heavily on the fishing industry to drive its economy, the shrinking cod is worrying news.
After all, the biggest cod bring the biggest profits at market.
Since the cod is economically important, then we want to find out, what is the economic impact of the fishery.
They are maturing earlier, they are smaller, does this have an economic impact?
And then, since everyone wants to have a large yield as possible, and the fish is getting smaller, is this effecting the yield?
And for the economic impact, evolutionary changes are especially bad, because this could mean that the cod will stay small.
Is this only driven by ecology or are there also evolutionary effects?
Because if there are evolutionary effects, these are more difficult to reverse.
The evolutionary scientists are close to proving their theory,
but their critics say that without genetic proof they won't believe the theory.
Evolutionary change is always a genetic change, but in the case of the cod, not much is known about the genetics.
We are now sequencing the cod genome, and that is interesting in itself.
But it might help us understand what the genetic basis for this is.
So, we do not have a genetic understanding of this at the present, but we are working towards that,
we are working toward that, other groups in the world are also working towards that.
So my expectation will be that in a few years we will have a much better understanding of the genetic basis for this.
It remains to be seen if norway's cod will ever return to the size it was at the beginning of the 20th century.
However, studying the evolutionary theory will help scientists better understand what is happening in the northern european seas,
and go some way to ensure that future stocks of cod are protected.