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Overfishing: a threat to marine biodiversity
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DescriptionOverfishing: a threat to marine biodiversity
As catches have gradually become smaller, so the mesh sizes used in fishing nets have decreased, allowing smaller and smaller fish to be caught. Many of these are too small to be used as food, so they are crushed to be made into either animal food or fertiliser.
Fishing using nets is indiscriminate. Any fish which get in the way of the net will be caught in it if they are too big to get through the mesh. For every one tonne of prawns caught, three tonnes of other fish are killed and thrown away. 20,000 porpoises die each year in the nets of salmon fishermen in the Atlantic and Pacific Oceans and tens of thousands of dolphins are killed each year by tuna fishermen.
How Commercial Fishing Works
Some sea fish live in the upper parts of the water. They are called ‘pelagic’ fish, and are caught by drift netting. This is where a net suspended from floats is stretched between two boats so that fish swim into it. Fish are unable to swim backwards, so once they are caught in the net, there is no escape unless they are small enough to fit through the net’s mesh.
Fish which live lower down – mid-water and bottom-feeders – are caught by trawling, which involves dragging a large net through the water, catching whatever happens to be in the way. The size of the net holes is again very important, and it is vital for the conservation of fish stocks that nets with a very small mesh are banned, as these catch young fish before they have even had a chance to breed.
The ‘Cod War’ and Other Overfishing Incidents
A serious dispute broke out between British and Icelandic fishermen over the Icelandic cod fisheries. British trawlers continued to fish for cod despite a ban on fishing put into place by the Icelandic government, and there were confrontations between British and Icelandic trawlers, which became known as the “Cod War’.
There is now a 100-mile exclusion zone around Iceland, in which foreign boats are not allowed to fish, so Icelandic cod stocks are starting to improve, though it is unlikely that they will ever recover fully.
It used to be a matter of luck whether fishermen would catch any fish at all, as fish are difficult to find, but now modern radar technology has allowed them to be located much more precisely than was possible before.
In Peru, a small fish called the anchoveta was caught in huge numbers to be made into fish meal for animals. In 1970, more than 69,000 tonnes were caught, making it the biggest fishery in the world. One thousand five hundred boats were catching 100,000 tonnes of anchoveta every day. By 1972, the daily yield had risen to 180,000 tonnes! The fishermen ignored warnings from the United Nations Food and Agriculture Organisation that their catches were too big, and when a natural upwelling of warm water entered the normally cool, nutrient-rich waters the anchoveta inhabited, this, combined with a lack of breeding stock in the population was enough to cause the total collapse of the anchoveta fishing industry.
On the Dagger Bank, off the east coast of England near Great Yarmouth, overfishing caused the annual catch of herring to fall 30-fold in just 15 years. By 1966, only 10,000 tonnes of the fish were caught in the whole year. Further north, a ban was placed on herring fishing, and in 1977, a total ban was placed on herring fishing, throughout the North Sea. The ban lasted for six years.
These are just a few examples of how overfishing can seriously affect not only the fish stocks, but also the livelihoods of many people who depend on fishing as a job. There is a delicate balance to be struck between catching large numbers of fish so as to make more money and ensuring that there are enough fish left alive to be able to replenish stocks for future years. It is human nature to try to make as much money as possible, but this has to be weighed against the economic hardship that whole communities have suffered as a result of overexploiting their own fisheries, not to mention the grave consequences of overfishing for fish populations.
The Effects of Overfishing on Other Wildlife.
The overfishing of a particular species does not just damage the population of that fish alone. It can have serious effects further up the food chain. Herring is a vital prey species for the cod. Therefore, when herring are overfished the cod population suffers as well. The sandeel is the main food for seabirds such as the puffin. Sandeels have been fished around the Shetland Islands since the mid-1970s, though catches were declining throughout the 1980s. At the same time, the colonies of seabirds nesting around Shetland declined, with some even falling to breed for several years.
In the Antarctic, fishing for krill is threatening to disrupt the delicate balance of nature in these waters. Krill are small, red shrimps, about 6cm long, found in huge numbers in areas of plant plankton, and they make up a significant part of the animal plankton. Krill occur in huge swarms many kilometres across, and it has estimated that there could be up to 650 million tonnes of them in the Antarctic Ocean.
Since the early 1980s, six countries, including Japan and the former USSR have been harvesting krill, which is the main food for the great whales, and which also supplements the diets of seals, penguins, squid and fish. We have no idea what effects this will have on the populations of animals which feed on krill. The natural balance in the Antarctic has already been upset by the overexploitation of the great whales, and heavy fishing of krill may well worsen the situation.
What can be done?
Properly maintained fisheries could and should be a renewable and possibly even endless supply of protein. At present, short-term economic pressures are preventing sensible long-term planning for a sustainable yield (only taking out as many fish as can be replaced by reproduction the following year.)
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