January 15, 2007
From: Michael Hill
Just returned from a short holiday at Crescent Head in New South Wales, Australia. On the main beach on two separate days mass strandings of sea hares (Aplysia extraordinaria?...but darker?...and then maybe one in fifty of a paler species). Literally thousands of animals when we arrived had been washed up and stranded - as the days progressed they made a truly awful smell.
After they cleared away with the tide (nothing else wanted to eat them - except a few flies) the beach was pristine again. Then with a north-easterly another crop came in yesterday. You couldn't enter the water to swim without having them brush against you. All sorts of sizes from 4cm to about 12 cm - so it didn't seem like a single generation of individuals - very active swimmers too. Literally thousands in the water and then as we left in the afternoon on the low tide they were getting stranded again.
Both strandings occured at the same time as the north-easterly, and were accompanied by a very thick deposit of red weed. Locals thought the red weed sucked the oxygen out of the water. (A couple of days earlier in Port Macquarie we saw red weed covered rocks and dozens of dead sea stars on the rocks.) Not sure if the weed and the strandings are related, but anyway, it looked connected.
Locality: Crescent Head, Shoreline, New South Wales, Pacific Ocean, 12 January 2007, Intertidal. Length: Various
email@example.comHill, M., 2007 (Jan 15) Mass Stranding of Sea Hares in nthn New South Wales. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/19225
These mass strandings of Sea Hares are a fairly common occurrence around the world and often raise thoughts of ecological disaster, pollution etc. For this reason I have a Fact Sheet on mass mortality where I discuss the topic and give links to further observations and messages on the Forum. In your example the stranding of all sizes would suggest its not a post-mating death of the old and exhausted, so I would suspect the north-westerlies have caused rough waters off-shore which have stirred up the bottom and washed many animals ashore. Although some species of Sea Hare can swim, it's not a normal method of locomotion, so I guess they became exhausted and battered in the surf. I suspect the red weed has suffered the same fate. Red algae does not live as free-floating plants in the ocean, so rough water must have ripped the weed off the bottom, where it would have been attached, and washed it ashore. I would say the washed-up weed is a sign there has been a storm severe enough to wash both weed and sea hares ashore.
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