Trying to manage fisheries by imposing quotas may have its problems. But are there any viable alternatives? Here we look at five other approaches used in different parts of the world.

This is one of the options used by the European Union and other authorities responsible for fisheries management.

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The main reason in favour of limiting days at sea is that it is easy to track when boats are fishing and when they are in habour; and when they are in harbour, there is no chance of illegal or accidental catches.

The main downside is that fishermen will target the most valuable species during their allotted days at sea; and often the most valuable species are also the most scarce.

Also, it does not allow for the fact that different species may be able to stand very different catch levels sustainably.

It also makes no allowance for increasing efficiency of fishing gear.

Modern trawl nets can have mouths so big that they could encompass seven or eight jumbo jets.

Modern ships are also equipped with sonar systems capable of finding shoals of fish very efficiently, and of mapping the sea floor to show where prey could be hiding. Some types of fleets, for example tuna purse-seiners, use aeroplanes to spot shoals.

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Technological development means that the efficiency of fishing vessels continually increases.

Restricting the types of gear and size of vessels can be an effective way of managing fisheries, particularly combined with restrictions on days at sea.

An extreme example is the once great cod stocks of the Grand Banks off the eastern coast of Canada. Having been brought to commercial extinction by industrial fleets, the very limited fishing permitted now involves only small, traditional inshore vessels.

Globally, the most controversial technology, which some want banned entirely, is bottom-trawling, in which heavy rollers are dragged across the ocean floor, ripping up everything in their path.

The concept is simple; protect areas that are important to the production of young fish.

Usually these areas encompass spawning grounds and "nurseries" such as coral reefs.

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A number of conservation groups want Britain, and Europe, to protect a substantial percentage of its seas; the favoured scale is somewhere between 10% and 30%.

Fishermen have campaigned against the idea because they feel it would reduce their options.

Currently there is just one "no-take" zone in British waters, around the Isle of Lundy. Shellfish populations have increased since the zone was established.

The UN biodiversity convention suggests that about 10% of the oceans should be protected globally, but the current total is less than 0.5%.

As with conventional quotas, the starting point is a scientific stock assessment that leads to calculation of a Total Allowable Catch (TAC).

The difference is that the TAC is shared out between individual boats, or individual owners. Each boat or owner will be given their share for many years.

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This gives fishermen a stake in conserving the stock, because so long as the fish are there, they are guaranteed access and profit for years to come.

The quotas are "transferable" because owners can trade quota with each other.

Typically, transgressions such as illegal fishing are punished with heavy sanctions such as loss of licence, rather than in the UK where nominal fines are the normal penalty.

Transferable quotas are extensively used in Iceland, New Zealand and Australia, and have proved effective in many cases.

Aquaculture will become the dominant source of fish, shellfish and other seafood in the future whatever happens to wild fisheries.

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The UN Food and Agriculture Organization (FAO) calculates that aquaculture already accounts for about half of global consumption. China is by far the biggest producer.

Salmon, carp, shrimp, mussels, oysters and tilapia are among the species most heavily commercialised.

Aquaculture can affect wild fisheries through its environmental impact - for example, with salmon farms encouraging the growth of sea lice, which then transfer to wild salmon.

Also, some farmed fish are fed on food made from wild fish.

However, it also has the potential to take some pressure off wild stocks by supplying a cheaper and more predictable product.