Final answer:
El Niño is the weather system that might be occurring if the pressure gradient decreases and fish harvests in Peru are weaker than normal.
Step-by-step explanation:
If the pressure gradient decreases and fish harvests in Peru are weaker than normal, the weather system that might be occurring is El Niño. El Niño is a climate pattern that occurs when there is a weakening of the trade winds in the Pacific Ocean, resulting in warmer waters along the coast of South America and changes in global weather patterns. El Niño leads to reduced upwelling of cold, nutrient-rich waters that support fish populations and can result in weaker fish harvests.
Learn more about El Niño