Saturday, 11 June 2016

Swarming of Asian Paste Shrimps (Acetes sp.)

Go into any Asian food shop and you can buy bags of dried shrimps or paste made from these shrimps, known as Jelly Prawns in Australia, so it is quite surprising that I cannot find much information on the net about them.  This week (10 June 2016), a swarm of shrimps formed in the swash zone of Holloways Beach and I was able to make a video of them.

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
The swarm was thickest around the mouth of Barr Creek (near Cairns), which was possibly less saline that usual due to unusual winter rains.  According to a paper published by Kemp in 1917 "The species of Acetes are found gregariously swimming in great numbers in mid-water or near the surface. They are apparently met with only in coastal waters; they occur near the shore in the open sea, and are frequently common in estuaries and backwaters. They are often found in water of low salinity, and occasionally in places where it is quite fresh, but penetrate little if at all beyond the reach of tidal influence. The species are fished commercially in India and Japan, the small size of the individuals being evidently compensated by the great abundance in which they are taken."

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
I have seen only three swarms in thirty years, however that is not to say that the shrimps are uncommon.  Rather they are probably observed only occasionally.  Conditions had been calms for several days in a row in Far North Queensland at a time when Tasmania was enduring some of its worst ever bad weather and floods.  Usually the shores are being pounded by wave after wave and it can even be difficult to launch a surf ski as being hit by large and continuous closely spaced breaking waves can stop even a strong young man.  In the unusual calm weather, the fish have the advantage over the shrimps and it appears that the shrimps crowd into the shallows for protection as fish are rightly very timid about entering shallow water.  The swarm at Barr Creek was present for at least three days and would have been 1 m wide and 10 m long, however the entire beach and adjoining Machans Beach had an abundance of shrimps which could be seen flicking from the surface to escape fish.

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
Under the water, there are silver flashes from fish nipping at the swarm.  The only fish to swim around in the swarm were three-banded toad fish and possibly striped grunter.  On the bottom were juvenile shovel nose sharks (rays).  On previous occasions I have seen 1.5 m long shovel noses cruising up and down 3 m off the beach when the shrimps are present.  At Ellis Beach I have even seen at least three fully grown manta rays feeding on a swarm.  In an attempt to make contact with a manta ray, I stood in the knee deep water but the rays know you are there and swim around you gracefully whilst taking in the goodness.

An article in Fishing World provides more information on Jelly Prawn biology.

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