Explain why the efficiency of energy transfer between trophic levels is important when considering the sustainability of different types of farming.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Salmon farms raise fish in large enclosures and feed them on smaller fish caught from the sea. Some farmers are switching to plant-based feeds for their salmon instead.
Model answer (5 marks)
Only about 10 % of the energy in a food chain is transferred from one trophic level to the next; the rest is lost as heat, respiration, movement and waste.
When salmon are fed on smaller fish, the energy must pass through an extra trophic level. Each level loses roughly 90 % of the energy, so the salmon receive far less of the original energy available in the sea.
If the salmon are fed plant‑based feed, the feed is produced directly from primary producers, removing that intermediate fish level. More of the energy from the plants reaches the salmon, and less energy is wasted as fish biomass that would otherwise have to be harvested.
Because plant‑based feeds require fewer wild fish, less land and fewer resources to produce the same amount of salmon biomass, they are more sustainable.
When salmon are fed on smaller fish, the energy must pass through an extra trophic level. Each level loses roughly 90 % of the energy, so the salmon receive far less of the original energy available in the sea.
If the salmon are fed plant‑based feed, the feed is produced directly from primary producers, removing that intermediate fish level. More of the energy from the plants reaches the salmon, and less energy is wasted as fish biomass that would otherwise have to be harvested.
Because plant‑based feeds require fewer wild fish, less land and fewer resources to produce the same amount of salmon biomass, they are more sustainable.
Examiner tips
- Mention the 10 % transfer efficiency and the extra trophic level for fish‑fed salmon.
- Explain the energy losses (heat, respiration, waste).
- Show that plant feed removes a level, so more energy reaches the salmon.
- Link higher energy efficiency to lower resource use and sustainability.
Common mistakes
- Saying that all energy is transferred between levels; ignoring the 10 % rule.
- Not recognising the extra trophic level when salmon eat fish.
- Failing to connect energy efficiency with resource use and sustainability.
Mark scheme (5 marks)
- Only a small percentage (approximately 10%) of energy is transferred from one trophic level to the next
- Energy is lost through processes such as respiration / movement / heat loss / waste products (excretion/faeces)
- Feeding salmon on smaller fish means there is an extra trophic level, so more energy is lost overall
- Using plant-based feed removes a trophic level, so more energy reaches the salmon / less energy is wasted
- Plant-based feeds are therefore more sustainable because fewer resources / less land / fewer wild fish are needed to produce the same amount of salmon biomass
Key terms in this question
trophic level · energy transfer · sustainability
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Energy and Ecosystems questions
- Explain why farmers who rear livestock often use indoor housing and controlled f…
- A farmer grows wheat and uses it to feed cattle. The farmer is considering wheth…
- Describe how energy is lost from a food chain at each trophic level, and explain…
- Explain why only a small percentage of the energy available at one trophic level…