When are there jellyfish in Portugal?

Portuguese man o' war are recorded most in January, April, May and June — and this is the one that matters, because the sting is severe.

Portuguese man o' war (severe sting)

Recorded well above the expected rate in January, April, May and June, peaking at 2.9× the expected rate. Based on 1,692 records.

Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Recorded vs expected	1.8	0.5	0.5	1.9	2.9	1.3	0.5	0.5	0.7	0.3	0.2	0.9

Mauve stinger (painful sting)

Above the expected rate in January, May, June, July and September — but those months sit in different parts of the year rather than forming one season, so the monthly figures below are worth more than any summary of them would be. Based on 136 records.

Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Recorded vs expected	2.1	0.5	0.1	0.6	1.8	1.5	1.3	1.1	1.4	0.4	1.2	0.0

True jellyfish (varies by species)

Above the expected rate in January, July, August and September — but those months sit in different parts of the year rather than forming one season, so the monthly figures below are worth more than any summary of them would be. Based on 1,663 records.

Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Recorded vs expected	1.9	0.8	0.2	0.2	0.3	1.2	1.5	1.4	2.1	1.2	0.8	0.8

What these numbers are, and what they are not

1.0 means "as often as expected". Nobody counts jellyfish; people report the ones they run into, and far more people are at the coast in summer. So a raw monthly count of sightings is mostly a picture of the holiday season.

To strip that out, each month's jellyfish records are divided by the records of a control group — molluscs, logged by the same people on the same coast in the same months. What is left is the seasonality that is not explained by there simply being more observers about. A 2.0 means jellyfish were reported twice as often as the level of recording that month would predict.

It is not abundance, and it is not your chance of being stung. It is also not comparable with another country: national totals reflect how much recording each country has digitised, not how many jellyfish are in the water.

Sea temperature in Portugal

Records from GBIF, the Global Biodiversity Information Facility. How the adjustment works