ANSWER:
The weather system that might be occurring if the "pressure gradient" decreases and "fish harvests in Peru" are weaker than normal is El Nino.
Step-by-step explanation:
- El Nino is the phase that has the warmer weather due to its association with the warm ocean water.
- During El Nino phase the winds has the weakened effect in the central region and pacific region.
- So, the fish harvest in this weather system in Peru is weaker than normal weather conditions.