IGCSE Biology / Human influences on the environment

Feeding people without emptying the sea

Edexcel International GCSE Biology (4BI1), chapter 15: Human influences on the environment, specification point 5.9B. Free lesson.

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Fish stocks collapse when fish are caught faster than they can replace themselves by breeding. Conservation methods protect breeding populations, while fish farms increase fish production by controlling conditions, but they can also harm the environment.

Why fish stocks collapse

A fish stock is stable when the number caught is matched by the number replaced through breeding. If fishing effort increases, catches can exceed the breeding rate. The stock then falls, and the collapse can speed up because too few adult fish remain to replace those being caught. Catches may fall to almost nothing even if fishing effort stays high. Recovery is much slower than the collapse, and may not happen if the ecosystem has changed.

How fishing methods protect stocks

Each conservation method protects fish at a different stage or in a different place. They all aim to keep the catch below the number of fish replaced by breeding, so the stock does not keep falling. Quotas limit the mass of each species that a boat may land, keeping the total catch below the breeding rate. Net mesh size sets a legal minimum hole size. Young fish can escape and live long enough to breed at least once. Closed seasons ban fishing during the breeding season, allowing fish to spawn undisturbed. Exclusion zones ban fishing in an area, so a breeding population…

How fish farms increase production

Fish farming produces fish without catching them from wild stocks. Farms control conditions that would otherwise use up energy or reduce the number of fish that survive. This gives a higher yield per unit of feed than in the wild. Cages or tanks keep fish together so they can be fed and harvested. A controlled, high-protein diet supports fast growth with little waste. Cages exclude predators, so more young fish survive. Sorting fish by size and age into separate cages controls intraspecific predation, where fish eat each other. Restricted movement means less energy is spent swimming and…

Costs of fish farming

Fish farms can produce more fish, but they can also create environmental and health problems. Their benefits need to be weighed against these drawbacks. Eutrophication can result from waste entering the water. Disease can spread, and antibiotics used to treat it are a concern. Farmed fish may be fed wild fish, so farming can still put pressure on wild fish stocks.

More lessons in Human influences on the environment

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