Re: Sea Hares & sewage in Auckland, NZ

May 27, 2000
From: John Buckeridge

Dear Dr Rudman

Joel Cayford has asked me to look at your comments. It was my comment that sparked the discussion in Auckland. Things get a little garbled en route, so here are the main points of my argument:

•1. There are very large numbers of Bursatella glauca at Long Bay, on Auckland's North Shore.
•2. The numbers are higher than I have ever seen in 20 years. Residents who have been on eastern beaches for 65 years say that this phenomenon is something they have never experienced before.
•3. The greatest numbers of B. glauca are concentrated near a stream mouth at the southern end of the beach.
•4. The stream is polluted. It has high levels of coliform bacteria. (last week, a student group recorded levels at more than a magnitude above permissable "safe" levels).
•5. It is "unseasonally warm".
•6. There are blooms of cyanobacteria and green algae in the area, and these form thin layers on the silty-muddy sea bed.
•7. The breeding time of B. glauca (at least the end of the cycle), has coincided with this.
•8. The high levels of coliform bacteria are an indication of sewage in the stream. The blooms are an indication of high nutrient levels and warmer than normal temperatures. The abundance of B. glauca reflects the above. It does not eat sewage, rather, it eats cyanobacteria and green algae films. B. glauca is a symptom of environmental degradation e.g. sewage release and other trappings of urbanisation. The local council has for years denied that there is a sewage problem. This is most unfortunate, as the evidence has been overwhelming for years. (They are now in the midst of an expensive upgrade of the system, but it is far from complete, and there is still permission to release untreated sewage into the environment under certain circumstances).

It seems that "explosions" in the numbers of sea hares does at least focus the public interest and has galvanised the Council to take "remedial action".

I await developments with interest, but cannot escape the conclusion that our council will wait for this to calm down, and then get on with other agendae.

Kind regards

John

Professor and Director
Earth and Oceanic Sciences Research Centre
Auckland University of Technology

john.buckeridge@aut.ac.nz

Dear John,
I'm glad to hear from you and to get the story clarified. As I said to Joel, I suspected the Sea Hare may have been Bursatella, which by the way now goes under the name Bursatella leachii, B. glauca being an old unnecessary name. You'll see above your message some general information on the species, and below your message there is earlier correspondence on the species, including discussions on population explosions. Various subspecies are found around the world and all of them are characterised by 'population explosions'.

Although locals often say they have never seen anything like this before this does not mean that it has not occurred regularly in the past. They just missed it. There is a note about Bursatella leachii in Willan & Morton's (1984) book on the Opisthobranchs of the Cape Rodney Marine Reserve, just north of Auckland "becomes seasonally highly abundant..". I can also remember 40 years ago holidaying on nearby Waiheke Is in the Hauraki Gulf in January, walking along beaches so crowded with washed-up Bursatella that I had to stand on them, leaving a trail of purple foot steps behind.

The point that needs to be made is that a population explosion of Bursatella does not automatically signal a sewerage problem or an environmental disaster, it is a natural phenomenon. High nutrient levels in this case may have encouraged the growth of the blue-green algal mat that Bursatella feeds on, but as the blue-green algae flourishes naturally in shallow warm bays, which are not polluted, even this would need to be tested.

I sympathise with the North Shore's environmental problems, but I think the high levels of coliform bacteria in the water is more indicative of a problem, than the humble Sea Hare, even if sometimes they do seem to breed like rabbits..

Best wishes,
Bill Rudman.

Reference: Willan, R. & Morton, J. (1984) Cape Rodney to Okakari Point Marine Reserve. Marine Molluscs. Part 2. Opisthobranchia. 1-106. University of Auckland, Leigh Marine Laboratory: Auckland.

Rudman, W.B., 2000 (May 27). Comment on Re: Sea Hares & sewage in Auckland, NZ by John Buckeridge. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/2454

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