Jellyfish season — November to February In general, jellyfish can be anywhere that the water is calm since they don't like wind or waves. They are present year-round but are more prevalent in the winter months, particularly in January. If you are swimming, keep an eye out.

Excellent question! In Puerto Rico, there isn’t a single, sharply defined “jellyfish season” like in some temperate regions. Instead, jellyfish presence is year-round but highly variable, influenced by several key factors.

Here’s a detailed breakdown of what to know:

Key Factors Influencing Jellyfish Presence:

1. Wind and Currents (The Biggest Factor): The most reliable predictor is the wind. When strong, persistent easterly trade winds blow, they push surface water (and any floating jellyfish) toward the island’s northern, eastern, and western shores. Calm periods with little wind usually mean fewer jellyfish near shore.
2. Rain and Runoff: After heavy rainfall, freshwater runoff can lower salinity near river mouths and urban coasts (like parts of San Juan). Some jellyfish species, like the Sea Nettle, can tolerate this and may even be found in higher concentrations in these brackish areas.
3. Lunar Cycles: Spawning events for some marine creatures, which jellyfish may feed on, are tied to the lunar cycle. Increases in plankton after a full moon can sometimes attract jellyfish.
4. Seasonal Blooms (Mild): While not a strict season, a general increase is often noted during the warmer, wetter months of late spring through fall (roughly May to October). This coincides with calmer seas in the summer, which can allow jellyfish to drift closer to beaches.

Most Common Jellyfish in Puerto Rico:

• Sea Nettle: Very common, with long, trailing tentacles. Sting is painful but typically mild-to-moderate.