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IB DP Biology- C4.1 Populations and communities - IB Style Questions For SL Paper 2 - FA 2025

IBDP Biology Paper 2 - All Chapters

Question

The feed conversion ratio (FCR) of grouper fish, Epinephelus coioides, was compared at two different fish densities in two types of tanks over a period of 10 weeks. The recirculation tank reused the same water, which was constantly purified. The flow-water tank had a continuous flow of clean water.

FCR comparison graph
(a) State the effect of increasing fish density on feed conversion ratios.
(b) Evaluate the rearing of grouper fish as food for humans at high density using recirculation tanks rather than water-flow tanks.
▶️ Answer/Explanation

(a)

When the number of fish increases (higher density), the feed conversion ratio (FCR) goes down, meaning the fish are using food more efficiently.

(b)

Using recirculation tanks at high fish density has some good and bad points:

  • On the positive side, the water is reused after being cleaned, so it saves water and reduces pollution going into the environment.
  • But on the negative side, fish in recirculation tanks need more food to grow compared to those in flow-water tanks, which can make it more expensive to raise them.

Markscheme: 

(a) Feed conversion ratios (FCRs) decrease with increasing fish density.

(b) Evaluation of recirculation tanks vs water-flow tanks:

Advantages:

  • Less water is wasted (as it is purified and re-used)
  • Less impure water is released to the environment

Disadvantages:

  • Recirculation tanks have higher feed conversion ratio
  • More food/energy needs to be supplied to the fish
  • OR recirculation systems may cost more to maintain
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