USA Biology Olympiad (USABO) Practice Exam

What does evidence suggest about anadromous migration in fish such as Salmo salmon?

1. It is primarily for reproduction in saltwater

2. It is linked to feeding adaptations

3. It has no evolutionary benefit

4. It suggests a preference for warmer waters

The correct answer is: It is linked to feeding adaptations

Anadromous migration in fish like Salmo salmon is primarily associated with their life cycle, where these fish are born in freshwater, migrate to the ocean, and then return to freshwater to spawn. The evidence surrounding this migration highlights several significant factors, particularly relating to feeding adaptations. As salmon migrate to the ocean, they encounter a different ecological environment rich in diverse food sources, which promotes growth and development. This transition to saltwater allows them to take advantage of the abundant prey available, such as small fish and invertebrates, which are crucial for their energy needs and overall health before they return to their natal freshwater streams for spawning. This strategy is functionally advantageous because the growth achieved in the ocean can improve their reproductive success, as larger and healthier individuals tend to have better chances of producing viable offspring. Thus, the link between anadromous migration and feeding adaptations supports the concept that this behavior has significant evolutionary benefits related to foraging in nutrient-rich environments and optimizing fitness for reproduction. The other options do not capture the essence of the evidence supporting the ecological and evolutionary significance of salmon migration. It's this feeding adaptation associated with the anadromous life cycle that highlights why such migration is a critical adaptation for these salmon species.