Fish Farming and Selective Breeding

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 5.9 — The methods used to farm large numbers of fish (bold B statement — Paper 2 only)
• 5.10 — How selective breeding develops plants with desired characteristics
• 5.11 — How selective breeding develops animals with desired characteristics

Specification section 5(a) Food production — also taught in Increasing Crop Yield, Yeast, Bread and Fermentation, Yoghurt and the Industrial Fermenter.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Fish farming is the energy argument from Topic 4 turned into a business. Only about 10 % of the energy in food is transferred at each trophic level, and most of the loss is respiration — so any way of reducing what a farmed animal spends on staying alive leaves more of the feed as food. Fish are unusually good at this before any farming begins: they do not maintain a body temperature, and being supported by water they spend far less energy holding themselves up. Everything a fish farm does is an attempt to push that advantage further.

How a fish farm increases yield — and the biological reason for each method

1. The fish are kept in TANKS OR CAGES, which prevents them from escaping and excludes predators — so none of the stock is lost to something else.
2. Restricting their movement also reduces the energy they spend swimming, so more of the food they eat goes into growth.
3. They are fed a HIGH-PROTEIN DIET in controlled amounts, supplying the amino acids for building tissue, with as little waste as possible.
4. Fish are SORTED BY SIZE and kept in separate tanks, which prevents larger fish eating smaller ones and stops the biggest monopolising the food.
5. The WATER QUALITY is monitored — oxygen concentration, temperature and waste products — because the fish are at far higher densities than in the wild.
6. SELECTIVE BREEDING produces fish that grow faster or larger, and eggs and young are protected in hatcheries, so far more survive than would naturally.
7. PESTS AND DISEASE are controlled, often with antibiotics or pesticides, because a dense population allows infection to spread rapidly.

Those last two points carry the real costs, and a good answer names them. High density means disease spreads easily, so antibiotics are used routinely — which selects for resistance in exactly the way Topic 3 described. Uneaten food and fish waste enrich the water beneath a cage, causing eutrophication. Escaped farmed fish interbreed with wild populations, and predators such as seals may be killed to protect the stock. Fish farming genuinely produces protein more efficiently than cattle; it is not free of consequences.

Now the second half of the page. SELECTIVE BREEDING — artificial selection — means choosing which individuals reproduce, so that a characteristic humans want becomes more common. The process is identical to natural selection except in one respect: the selecting is done by a person with a purpose rather than by the environment without one.

Selective breeding — the same four steps for any organism

1. From a population showing VARIATION in the characteristic, choose the individuals with the most desirable form of it — the highest-yielding wheat, the cow producing most milk, the fastest-growing salmon.
2. BREED those individuals together.
3. From their offspring, again select those showing the desired characteristic most strongly.
4. REPEAT over many generations. The alleles for the characteristic become more common in the population, and the characteristic becomes more pronounced.
5. That is the whole method, and it works on anything with heritable variation. Every domesticated plant and animal is its product.

Think of it like a sieve with a person holding it

Natural selection was described earlier as a sieve — it holds back whatever happens not to pass through, with no aim in mind. Selective breeding is the same sieve, but somebody has picked it up and chosen the mesh. The mechanism is unchanged: variation must already exist, only what is present can be selected, and change accumulates over generations. What has changed is that the criterion is now a decision — bigger fruit, more milk, a particular colour — rather than whatever happened to survive.

The examples are worth having ready. In PLANTS: wheat bred for higher yield, for shorter stems that do not fall over in wind and rain, and for resistance to fungal disease. In ANIMALS: cattle bred for higher milk yield or more meat, sheep for better wool, and chickens for faster growth. Domestic dogs are the extreme demonstration — every breed descends from wolves within a few thousand years, entirely by choosing which animals bred.

There is a serious cost, and it follows directly from what the method does. Breeding repeatedly from a small number of selected individuals REDUCES GENETIC VARIATION in the population, because the alleles of the animals not chosen are lost. Three consequences follow. The population becomes vulnerable to a new disease, since it is unlikely that any individual carries resistance. Harmful recessive alleles become more common, because closely related individuals are being crossed — which is why some pedigree dog breeds suffer characteristic inherited disorders. And the raw material for future breeding is reduced, since a variation that no longer exists cannot be selected.

That last point explains something that otherwise looks eccentric: seed banks and rare-breed farms, which preserve varieties nobody currently wants to grow or keep. They are storing variation. A wild wheat with a poor yield may carry an allele for resistance to a disease that has not appeared yet, and once that allele is gone no amount of selective breeding can recover it.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Selective breeding

Choosing which individuals reproduce, so that alleles for a desired characteristic become more common in the population over successive generations. Also called artificial selection.

Statements 5.10 and 5.11 — selective breeding, the four steps to write

1. From a population showing variation, select the individuals with the most desirable form of the characteristic.
2. Breed those selected individuals together.
3. From the offspring, select again those showing the desired characteristic most strongly.
4. Repeat over many generations.
5. The alleles for the characteristic become more common, so the characteristic becomes more pronounced in the population.

Statement 5.9 — how a fish farm increases yield, with the reason for each method

• TANKS OR CAGES — prevent escape and exclude predators, so stock is not lost; restricting movement also reduces the energy spent swimming, so more food goes into growth
• HIGH-PROTEIN DIET in controlled amounts — supplies amino acids for building tissue with minimal waste
• SORTING BY SIZE into separate tanks — larger individuals of the same species will eat smaller ones, and they also monopolise the food, so sorting prevents both losses
• MONITORING WATER QUALITY — oxygen, temperature and waste products, because the fish are at far higher densities than in the wild
• SELECTIVE BREEDING and protected hatcheries — produce faster-growing fish, and far more young survive than would naturally
• PEST AND DISEASE CONTROL — necessary because infection spreads rapidly at high density

Selective breeding against natural selection — identical mechanism, different selector

The examples worth having ready

Plants — yield

Wheat and other cereals bred for larger grains and higher yield per plant.

Plants — structure

Cereals bred for shorter, stronger stems so the crop does not fall over in wind and rain.

Plants — disease

Crops bred for resistance to fungal and viral diseases, reducing the need for chemical control.

Animals — yield

Cattle bred for higher milk yield or greater meat production; chickens for faster growth.

Animals — product quality

Sheep bred for finer or more plentiful wool.

The costs of selective breeding — all consequences of reduced variation

• Breeding repeatedly from few selected individuals REDUCES GENETIC VARIATION, because the alleles of those not chosen are lost
• The population becomes vulnerable to a NEW DISEASE, since it is unlikely that any individual carries resistance
• Harmful RECESSIVE ALLELES become more common, because closely related individuals are being crossed — hence inherited disorders in some pedigree breeds
• The raw material for FUTURE breeding is reduced: a variation that no longer exists cannot be selected
• Seed banks and rare-breed farms exist to preserve variation that is not currently wanted but may be needed

Fish farming and the energy argument

A. FISH convert feed into food efficiently because they do not maintain a body temperature and are supported by water, so less of their energy is spent on heat and on movement.

B. A FISH FARM pushes that further by restricting movement, excluding predators, controlling the diet and preventing losses — so more of the energy in the feed ends up as fish.

Not this: Selective breeding creates new characteristics that the population did not have before.

This: It can only work on variation that already exists. Selection makes existing alleles more common; new alleles arise only by mutation, which is random and rare.

Model answer [4 marks]

Describe how a farmer could use selective breeding to produce cattle with a higher milk yield. [4]

The farmer selects the cows in the herd that produce the most milk and breeds them with a bull from a high-yielding line. From the offspring, they again select the individuals producing the most milk and breed those together. This is repeated over many generations. The alleles for high milk yield become more common in the herd, so the mean milk yield of the cattle increases.

Model answer [3 marks]

Explain why selective breeding can leave a population vulnerable to disease. [3]

Selective breeding uses a small number of chosen individuals repeatedly, so the alleles carried by those not selected are lost and the genetic variation of the population is reduced. If a new disease appears, it is unlikely that any individual happens to carry an allele giving resistance to it. Since the individuals are all genetically very similar, a disease that affects one is likely to affect all of them, and the whole population may be lost.

Model answer [3 marks]

Explain why fish convert their food into body mass more efficiently than cattle do. [3]

Cattle are mammals and maintain a constant body temperature, so a large proportion of the energy released by respiration is used for that and transferred to the surroundings as heat. Fish do not maintain a body temperature, so they lose far less energy this way. Fish are also supported by the water around them and use less energy in movement, so a greater proportion of the energy in their food is available for growth.

Mark-losing trap. Selective breeding needs VARIATION to already exist. It selects; it does not create.

Mark-losing trap. Say 'over many generations'. A single cross is not selective breeding.

Mark-losing trap. The cost is REDUCED GENETIC VARIATION — then name a consequence: disease, or harmful recessive alleles.

Mark-losing trap. For fish farms, give the biological REASON for each method, not just the practice.

Mark-losing trap. Fish are efficient because they do not maintain a body temperature. That is the comparison examiners want against cattle.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — In selective breeding, what does the farmer choose the parents for?
2. Grade 7 · Explain [2 marks] — On a fish farm, fish are sorted by size and kept in separate tanks. Explain why.
3. Grade 8 · Describe [5 marks] — Describe how a plant breeder could produce a variety of wheat with shorter, stronger stems. Select every statement that belongs in a full-mark answer.
4. Grade 9 · Calculate [3 marks] — A fish farm feeds 1.4 kg of feed for every 1.0 kg of fish produced. A cattle farm requires 8.0 kg of feed per 1.0 kg of beef. Calculate how many times more feed is needed to produce one kilogram of beef than one kilogram of farmed fish.
5. 9+ · Analyse [5 marks] — A company has selectively bred a variety of salmon that grows to market size in half the usual time, and now farms it exclusively in every one of its sea cages. Select every statement that belongs in a full-mark evaluation of this decision.

The people behind this science

Two ways into the same idea — the one who used breeders' results to argue for nature's, and the one who explained why the method works. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Charles Darwin — the one who used breeders' results to argue for nature's

The Origin of Species opens not with finches but with domestic pigeons, and the choice was deliberate. Darwin bred them himself, joined pigeon-fanciers' clubs, and used what breeders had achieved in a few decades as his opening argument: if choosing which birds mate can produce forms as different as a fantail and a pouter, then something choosing over millions of years could produce far more. Selective breeding was his evidence that selection is powerful, offered to readers who already accepted it in their own lofts.

• “Why did you start your book with pigeons rather than wild animals?”
• “What did breeders' results prove that wild observations could not?”
• “How is a breeder's choosing different from what happens in nature?”
• “Did the pigeon fanciers understand what you were arguing?”
• “What surprised you most about what breeders had achieved?”

Gregor Mendel — the one who explained why the method works

Breeders had been selecting successfully for thousands of years without knowing what they were manipulating, and Mendel supplied the answer. Because inheritance comes in discrete units that separate and recombine, choosing parents changes which units the next generation receives — and because those units are not diluted by blending, a characteristic can be concentrated over generations rather than fading. He was also working in an agricultural monastery on precisely this problem, and his abbot had encouraged the work for its practical value to breeders.

• “What is a breeder actually selecting when they choose a parent?”
• “Why does a characteristic not fade away over generations?”
• “Could breeders have worked faster if they had known your results?”
• “What did your abbot hope your experiments would achieve?”

Carry on through the course

• Previous lesson: Yoghurt and the Industrial Fermenter
• Next lesson: Genetic Engineering
• Increasing Crop Yield — Everything a grower does is one of two things — remove whatever is limiting the crop, or remove whatever is competing with it.
• Yeast, Bread and Fermentation — One organism respiring without oxygen — and two industries built on the two different products it makes.
• Yoghurt and the Industrial Fermenter — Bacteria turned into a manufacturing process — and the vessel built to give them the best conditions they will ever have.
• Genetic Engineering — Cutting a gene out of one organism and making another organism read it — and the reason that works at all.
• GM Insulin and GM Crops — The same technique used twice — once to brew a human hormone in a tank, and once to change what a field can grow.
• All of Biology · Biology key terms · required practicals · Incandio Science · The 4BI1 specification