Why is it beneficial that many predatory fish have larval and juvenile stages that feed at a low trophic level, while the adults feed at a tertiary or quaternary trophic level?

My answer was that the larval and juvenile stages would get more energy but my teacher said that this was wrong!!

It would have been useful for your teacher to have given an example fish species with the question. Sharks, for example, have few young and which eat small fish, crabs etc (depends on the species). The diet is therefore protein rich and enables fast growth. There is a lot of the food about so energy is not wasted looking for the food. Feeding at a higher trophic level usually means that there are less meals about and the individual needs to be larger to tackle it.

Does this help?

By having them feed at different trophic levels they are not competing for the same foods. The foods the adults eat are not the foods the juvenile eats so they are not intruding and competing against each other. That separation in trophic levels assists in the survival of the juveniles to the adult stage.

Understanding the reasons behind specific characteristics or adaptations in organisms often requires a deeper dive into ecological principles. In the case of predatory fish having larval and juvenile stages that feed at a low trophic level, while adults feed at a higher trophic level, there are several advantages to this phenomenon:

1. Energy Efficiency: By starting their lives at a lower trophic level, predatory fish can efficiently utilize the available energy resources. Lower trophic levels typically have higher productivity, meaning more food is available. This allows the larval and juvenile stages to acquire necessary energy for growth and development more easily.

2. Survival and Growth: Lower trophic levels often harbor abundant and easily accessible food sources, such as plankton or small organisms. In the early stages of life when vulnerability to predation is high, being able to access abundant food can enhance survival rates and promote faster growth.

3. Minimizing Competition: By occupying different trophic levels at different life stages, predatory fish can minimize competition for limited resources within their own population. By having distinct ecological niches at different life stages, they reduce the likelihood of resource depletion and promote their own reproductive success.

4. Ecological Interactions: By utilizing different trophic levels, predatory fish contribute to maintaining ecological balance and stability. This allows for the regulation of prey populations at lower trophic levels, preventing overpopulation or extinction of certain species and promoting overall ecosystem health.

To answer your specific question, stating that larval and juvenile stages "get more energy" is oversimplified and does not encompass all the ecological advantages mentioned above. It is important to consider the broader ecological context and understand the interconnectedness of organisms within their ecosystems.