Potentially lethal to fish and dangerous for humans, the summer’s toxic invader is caused by warming seas and strong winds

When Owen Francomb from Margate set out on a walk with his dog Gertie along Kent’s picturesque Thanet coast early this month, he didn’t imagine he’d need to be rescued from a tide of toxic sludge. But on the beach at Newgate Gap, French bulldog Gertie started sinking into a thick carpet of rotting seaweed and began to panic.

“She couldn’t move,” Francomb. says. “So I scrambled down the slipway and jumped down on to the beach, expecting the seaweed to be a foot deep, but it came up to my belt. I really struggled to wade through it.” Another dog walker had to help him and Gertie out of the stinking slime.