The Portuguese man-of-war: Gone with the wind

- More than 3,500 Portuguese man-of-war arrived at the Basque coast in August 2010.
- The wind drag velocity was used to estimate their region of origin and routes.
- The region of origin was probably located in the North Atlantic Subtropical Gyre.

The Portuguese man-of-war (Physalia physalis) is a siphonophore that lives at the air-water interface of the sea. The wind is the main mechanism controlling its drift. In August 2010, a significant number of individuals of this species arrived at the Basque coast (southeastern Bay of Biscay), causing a great socio-economic impact. Here we investigate the most likely region of origin and routes of these individuals using the Sediment, Oil spill and Fish Tracking model (SOFT). This model was run backwards in time using only the wind drag velocity (i.e., the wind velocity multiplied by a wind drag coefficient) to estimate the drift of these Portuguese man-of-war for one year and taking into account that the final destination was the Basque coast. The wind data were obtained with the Weather Research and Forecasting model (WRF). Six different simulations were carried out with SOFT using the following wind drag coefficients: 0.02, 0.025, 0.03, 0.035, 0.04 and 0.045. The simulation period covered from the end of August 2010 to the beginning of August 2009. After the first eight months of simulation (i.e., at the beginning of January 2010), the virtual Portuguese man-of-war used in SOFT were located near or on the northwest and southwest coasts of France and England, respectively, and in the English Channel, the southern Celtic Sea and the northwestern Bay of Biscay. However, at the end of the simulation period (i.e., at the beginning of August 2009), most of these Portuguese man-of-war were located between the central part of the Bay of Biscay (∼5° W) and the open North Atlantic Ocean (∼35° W), depending on the wind drag coefficient. From these results, we conclude that the region of origin of the Portuguese man-of-war arriving at the Basque coast in August 2010 was probably located in the northern part of the North Atlantic Subtropical Gyre. This conclusion is in agreement with the general wind-driven circulation in the North Atlantic Ocean.