Something has happened to the kelp on the north end

Tonight I went for a ride on "Kelpie" to inspect the kelp on the north end of the island. I examined all plants between Lavender Bay and Entrance Island and was surprised to see that almost all bull kelp in the area have lost their blades. They are balder than I am - and that's not an easy thing to do!

The one plant that did have something remaining looked like this, and I am suspicious that something has been grazing on the surface, but have no idea what could do that. I did notice a large number of large Lion's Mane Jellyfish in the area.

This is somewhat worrisome given that when we mapped this area on August 3 2013 most of the plants had a full set of blades - and no sori were visible. I don't know how fast sori develop and drop spores. Does anyone know the answer to this question?