Every few minutes the swarm erupts from the surface, with shrimps flicking in all directions. These eruptions seem to be mainly chain reactions where shrimps falling back in make other shrimps jump. A single small toadfish swimming close to the swarm can trigger the eruption which then spreads across the swarm.
|The swam appears as a reddish band in the water|
|Usually the prawns are completely clear but swarms are dull red|
|The prawns leap into air on the slightest hint of danger|
|As the swarm hugs the breaking waves it is a dense swirling mass of shrimps, sand and bubbles|
|A fishes eye view of the edge of the swarm|
As the swarm builds, up it becomes very dense and I suspect that oxygen depletion occurs. Small fish continually nip at the edges of the swarm and keep it compact. Whilst the shrimp can survive the oxygen depletion, they appear to become sluggish and when a slightly larger wave breaks in deeper water and within the swarm, it can swash large numbers of the sluggish shrimps up the beach where they are stranded. The seagulls think it is Christmas but as there are not so many seagulls in the north, they are quickly sated and just stand around and try to digest as quickly as possible so that they can some eat more.
|Jelly Prawns washed up by waves for bands along the beach|
An article in Fishing World provides more information on Jelly Prawn biology.