An unusually high concentration of jellyfish has caused a large amount of discomfort along Spain's Mediterranean coast this summer, according to officials.

The Red Cross says lifeguards have treated almost 11,000 people for stings on beaches so far this season in the northeastern region of Catalonia alone; twice the number from the same period last year, when the jellyfish count had already begun to rise.

Almunecar on the south coast had to cancel an annual swimming race across the bay last Sunday because of the number of jellyfish in the water, a town hall spokesman said.

Factors such as drought, heat and over-fishing all contribute to a rising jellyfish count, said Xavier Pastor, vice president of the international environmental group Oceana.