Fish Farming: Grade 9 Understanding for IGCSE Biology 5.9B

5.9B explain the methods which are used to farm large numbers of fish to provide a source of protein, including maintenance of water quality, control of intraspecific and interspecific predation, control of disease, removal of waste products, quality and frequency of feeding and the use of selective breeding.

fish-farm-l-eribollBefore we start it is worth me being completely clear – I hate the topic of fish farming.  I am not saying it isn’t important, just that when you read the specification point above, it hardly lifts your spirits does it?  Perhaps this is why I have waited until the evening before the 2016 paper 2 exam before writing about it on the blog.

Background understanding needed:

Humans have overfished all the world’s oceans over the past 20-30 years.  This means that fishermen have become so successful at collecting fish from the oceans due to advances in trawling technology and echolocation, that fish populations have crashed.  It is a truly sobering thought that almost every studied fish species in the ocean has only 10% of the numbers that we know were present 50 years ago.  90% of all fish have been removed by fishing.

Fish provide the  major source of dietary protein for a large proportion of the world’s population.  When you combine this with the fact that fish are becoming so scarce in the ocean, it is getting harder and harder to catch them, it is clear we have a potential problem on our hands.  Farming fish rather than catching wild fish might (stress might) provide a possible solution…..

Fish Farming

Farming fish is really as dull as it sounds.  Large floating cages are anchored often in estuary waters and small fish are added.  They are fed regularly (with fish food made from other less tasty fish that humans are extracting from the ocean), predators are kept out by the clever use of nets and fine mesh cage, antibiotics and other chemicals added to control lice and bacterial pathogens and after a few months, the fish can be harvested.


Maintenance of Water Quality

If the fish farm is in a tidal part of an estuary, then the tide will twice a day replace the water in the cage with fresh seawater containing high concentrations of oxygen for respiration in the fish.

Control of Intraspecific and Interspecific Predation

Intraspecific predation means predation within a species.  This means the fish will eat each other…. This is prevented by only having fish of the same age in any particular cage.  There won’t be any bigger fish to eat the smaller ones.  Interspecific predation means other species eating your precious fish.  This is prevented by keeping predators out of the cage using a mesh size that doesn’t let predatory fish and seals in plus covering your cage with netting to deter sea birds.

Control of Disease

The density of fish living in the cage will be much higher than would ever occur in the wild.  This means the conditions are perfect for fish parasites such as lice to spread.  This is controlled by adding toxic chemicals to the water.  Bacterial pathogens too can be a problem in which case antibiotics would be added to the fish food.

Removal of Waste Products

(See tidal solution above)

Quality and frequency of feeding

The farmed fish need to be fed a diet that is rich in protein for growth.  Obviously the frequency of feeding is important:  too often and food will not all be eaten and will leak out into surrounding waters, too rarely and the fish will grow too slowly.

Use of Selective Breeding

The characteristics a fish farmer wants in his fish are very different from those that might be selected for in a wild population.  For this reason, selective breeding might have to be used to produce fish that have the desired characteristics.  These might include an ability to grow rapidly when fed on a cheap diet of fish pellets and to produce tasty and attractive meat, a resistance to lice and other parasites, a willingness to grow even when swimming opportunities are rather limited, a friendly disposition so thousands can live happily crammed into a single cage etc.

I have written this post in a bit of a rush the night before the exam.  So there may be things I have left out, but at least I have given you something to read to help you sleep the night before the exam!  Good luck.




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