The surest sign that summer is here: The jellyfish have arrived. Swarms of them were sighted on the country’s southern beaches a week ago and have continued moving north − to the dismay of beach goers.

Jellyfish were seen over the weekend one kilometer off the beach of Hadera, west of Michmoret beach and near Netanya, according to reports that reached the Israel Oceanographic and Limnological Research Institute.

Just over a week ago, fishermen reported a “bloom” of jellyfish in an area 500 meters to two and half kilometers off the Ashkelon coast. That was the season’s first sighting. The jellyfish are estimated to be between 20-60 centimeters long.

Some Ashkelon beach goers complained of stinging. On Sunday it was Tel Aviv bathers’ turn to complain. The species of jellyfish arriving annually originally entered the Mediterranean through the Suez Canal several decades ago, and have since reproduced.

In addition to harming beach-goers, the jellyfish get stuck in fishermen’s nets and have been known to clog industrial marine cooling systems of power plants.The seasonal swarm of jellyfish which arrived on Israeli beaches over the last weekend made their way to the north of the country.