Stranding of Sea Hares in New South Wales

July 2, 2007
From: Brenda Harris

Dear Bill,
About three or four months ago after a few days of extremely high tides and rough water, I was walking the dog as I usually do along the waterfront at Davistown, a suburb of Gosford, 80 km north of Sydney, and noticed a few round things on the sand. I thought they were blown dead fish as sometimes happens but as I walked further along there were 30 or 40 of them all along a stretch of sand approx 200 metres long. There were more scattered further along up to probably a kilometre but the main concentration was in the one 200 metre area. I thought they were sea slugs but having searched the internet and found your site they are definitely sea hares.

I have been walking dogs down there for 13 years and have never seen them there before and have been talking to people who have lived there for 40 years and have never seen them there before. I walked along pushing them all back into the water. Since then the main concentration in the 200 metre stretch have been out every day I am down there at low tide - usually around 30 of them. I always throw them all back into the water. However I always walk the dog at the same time every day so it is not always low tide. This area is one of the more populous areas for people to walk along the waterfront and there is a ferry stop and jetty close by so the ferry makes fairly large waves when it comes in to stop.

The area of water is a wide channel, and across the channel from Davistown is Empire Bay. Would these sea hares have been washed in to the channel with the high tides? We have high tides every full moon but they haven't been in before. Are they choosing to come out on to the beach, or are they being washed up there and are unable to get back into the water? Am I injuring them by throwing them back out? They will only dry out if they are left there, - or will they find their own way back to the water if it becomes too hot? I am not down at the waterfront throwing them back in for 3 days out of 8. How do they survive on those days? How long can they live out of water assuming they are in the shade (which they're not) ? As they seem to be washed up every day, should I be gathering them up in a bucket and taking them to another area of the waterfront where the ferry is much further out and is not so populated with small boats and people? There is a 3 foot high honeycomb concrete block retaining wall separating the grassed area from the sand. Sometimes some of the sea hares have their heads buried in the tiny holes in the wall but their bodies are still very exposed to the sun. I try to prise these ones out so that I can throw them back in to the water. They come in all shapes and sizes, from approx 7 cm long to approx 25 cm long.

A few weeks ago there were approx 20 very small ones. They could bury themselves entirely in the holes in the besser block. Would they have come in originally to mate? Wouldn't they have done this in other years if that's the case? Since the wild storms we had 2 weeks ago I have not seen any of them - until a few days ago. I don't know whether they have been washed back out 'from whence they came', dashed against the besser block wall and killed, or washed over the wall and are buried, rotting, under the huge piles of seaweed that was washed over the wall. A few days ago I saw one of the regulars, a big fellow approx 25 cm long and approx 12 cm wide. It is always by itself 500 metres down from the main concentration, This is always the first one I come across on my walk and it is not always there - only every 3 or 4 days, but it is on a large area of exposed sand at low tide. I threw it back into the water as I always do. It was the first and only one I had seen since the storms. It was there again yesterday in a puddle of water about twice it's size. As it was an overcast day and it was in a puddle, my husband and I discussed it and decided it should be ok there and to leave it there rather than distress it by throwing it back. I went past today and it was dead in the same puddle we had left it in. What a waste - we had been right there! Also today there were 3 out on the sand a considerable way further along than they have been before. Maybe they have been washed further down. I am very much of the opinion that we as human beings have a brain and we have the dexterity and means - in many cases it is very easy for us to save stranded or injured wildlife. I'm walking down there most days - it doesn't take very much to throw 30 of them back in the water, although the dog gets somewhat impatient ! But I don't want to be doing the wrong thing. I don't want to be throwing them back in the water if they've just spent the last 2 hours slowly crawling out on to the sand, or if it is injuring them by doing so. I feel that if they are being washed up onto the sand in the same place every day, then surely the intelligent thing to do is to move them somewhere where they're not going to be washed up every day. But where is the right spot to move them? And are they being washed up or are they choosing to be there?

My apologies for such a long email. I hope you can find the time to answer my multitude of questions as I would really like to help the sea hares, but do not want to end up only hindering or injuring due to ignorance.

Thank you very much for your time.
Brenda Harris

Harris, B., 2007 (Jul 2) Stranding of Sea Hares in New South Wales. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Brenda,
I have a page on the Forum on mass mortalities which covers the phenomenon you have been experiencing. Essentially sea hares only last about a year and sometimes they spend their whole lifecycle unnoticed and other times some natural event gives them a nudge to a slightly early death - which often results in these mass strandings. Unfortunately there is not much you can do to help. Even putting them back in the water only delays the inevitable by a day or so as they get washed ashore again and stranded by another tide .. but I can understand you distress and concern.

Have a look at a recent message from northern NSW [#19452]. If you follow some of the correspondence back and look at the links to other species all around the world you will see that your thoughts and endeavours to save them are fairly universal. You are lucky however that you don't live in Western Australia. The large species, Aplysia gigantea, which washes ashore there often causes severe or fatal poisoning in dogs [see messages attached to that Fact Sheet].

Best wishes,

Rudman, W.B., 2007 (Jul 2). Comment on Stranding of Sea Hares in New South Wales by Brenda Harris. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from


Mass mortality

Related messages

  1. Mass Stranding of Sea Hares in nthn New South Wales
    From: Belinda Allman , February 14, 2007
  2. Re: Mass Stranding of Sea Hares in nthn New South Wales
    From: Inger Vandyke, January 16, 2007
  3. Mass Stranding of Sea Hares in nthn New South Wales
    From: Michael Hill, January 15, 2007
  4. Re: Mass mortality of Aplysia californica
    From: Ellen Hall, September 5, 2005
  5. Ebro Delta, Spain - Large Sea Hares
    From: Jorge Jimenez, August 20, 2005
  6. Migration inshore of Aplysia punctata
    From: Dr. Stephen Hoskins, June 11, 2003
  7. Bloom of Aplysia in Portugal
    From: Manuel Malaquias, September 17, 2002
  8. Swarming in Stylocheilus striata
    From: John Hoover, April 7, 2002
  9. Swarming behaviour in Aplysia
    From: Dr. Stephen Hoskins., April 3, 2002
  10. Re: Sea Hares - mass mortality
    From: Anne DuPont, September 28, 1999
  11. Re: Sea Hares - mass mortality
    From: Wayne Ellis, September 10, 1999
  12. Sea Hares - mass mortality
    From: Anne DuPont, September 9, 1999

Show factsheet and all related messages