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Part 4e. Methods of increasing food production and improving sustainability

4e. Overfishing, maintaining sustainable fish stocks in the seas and issues with fish farms

Doc Brown's Biology exam study revision notes

There are various sections to work through, after 1 they can be read and studied in any order.

Sub-index of notes on how to increase food production

Sub-index of notes on ALL aspects of food security


4(e) To avoid overfishing and keep fish stocks at sustainable levels

(Fish farms - advantages and disadvantages)

We must only take out numbers of fish that still allow breeding at a rate to keep a constant sustainable population.

A sort of 'rate of breeding = rate of fishing' = 'sustainable situation'!

You can do this in several ways:

(i) Placing limits on the numbers of fish extracted from the lake or fishing ground.

(ii) Fishing nets can have various mesh sizes depending on the species being fished.

This controls the size of fish caught and helps reduce the number of accidently 'unwanted' and resulting 'discarded' fish.

A bigger mesh will allow smaller fish to 'escape' and grow into breeding adults, hence helping to maintain fish stocks.

(iii) Fish can be 'factory farmed' in cages where they cannot escape (fish farms or aquafarms).

In fish farms, the cages are placed in more sheltered estuaries or bays to minimise the effect of rough water and their natural food is often supplemented with other fish-based protein products. There is a quite a lot of criticism of ALL factory farming methods (see section (f) below) and the following comments ....

Nearly half of all fish consumed worldwide each year are raised in aquafarms. Farmed fish spend their entire lives in cramped, sometimes dirty enclosures, and many suffer from parasitic infections, diseases, and debilitating injuries - which means chemicals and antibiotics are used to help the fish survive.

Food is added to the enclosed nets to feed the fish, who then produce large amounts of waste. The waste can leak out causing eutrophication and death of wild species - the waste contains pathogens. Fish farms are breeding grounds for parasites, again these can escape and threaten wild species.

Predators like seals and sea lions are attracted this food resource, but can get trapped in the nets and die.

The farmed fish can escape and compete with indigenous species of fish.

Fish tank farms only contain one species and are therefore low on biodiversity - they are kept free of any plants or predators, the parasites and microorganisms are usually killed.

For more see section on reduction of biodiversity

The Food and Agriculture Organization of the United Nations reports that the aquaculture industry is growing three times faster than land-based animal agriculture, and aquafarms will surely become even more prevalent as our natural fisheries become exhausted, is this good sustainability?


Summary of learning objectives and key words or phrases

  • Know and understand that fish stocks in the oceans are declining.

    • Know that it is important to maintain fish stocks at a level where breeding continues or certain species may disappear altogether in some areas.

    • Fish stocks of popular varieties are becoming alarmingly low in many regions of our seas and oceans due to over-fishing, which itself derives from the food demands of increasing population.

    • Its ironic that a lot of fish is used as 'unnatural' feed for the contained intensive farming of other species such as trout and salmon in fish farms.

    • Know and understand that net size and fishing quotas play an important role in conservation of fish stocks.

    • Fishing quotas are all about controlling the number of fish caught and the size of the fishes allowed to be caught e.g. fishing boat catches are carefully monitored and recorded and fishing nets inspected to check that the mesh spacing in the nets is not too small so that too many small immature fish (who can breed later) are not caught and hopefully unwanted species of fish or other marine creatures (eg crustaceans like crabs, lobsters, crayfish) can escape!

    • Understand that this is an example of sustainable food production, so that the numerical catch rate can match the numerical growth rate of the fish species we like to eat.

    • In fact in some areas, some fish species are so low in numbers, the catch rate is being highly restricted or banned all together to give the fish stock in the seas to get back to a sustainable level.

    • Such methods are needed because food chains are being disrupted and species may die out from particular oceanic regions from intensive fishing from huge trawlers and factory ships.


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