April 22, 1998
From: David & Leanne Atkinson
I have been on holidays and diving a lot at Port Stephens over the last few weeks. About 3 weeks ago the nudibranch population more than suddenly halved and has remained static since. The water temp has remained at 20 deg for the last 6 weeks, so I can't blame that. All the "exotic" species have gone and we are left with the species that are here most of the year. Polycera capensis seems to still around in reasonable numbers. Hopkinsia sp. is still around, but numbers have declined.
Any suggestions as to the cause of this phenomenon?
David & Leanne Atkinson.
firstname.lastname@example.orgAtkinson, D. & L., 1998 (Apr 22) Population decline - Port Stephens. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/153
Without longterm studies of each individual species it is difficult to come up with "general rules" for phenomena such as you have observed. The causes of population fluctuations, both explosions and declines, are often difficult to determine. The most common and most spectacular examples, probably because they are intertidal and relatively large, concern Sea Hares. Often they are found on beaches packed so thickly that one is forced to walk on the animals to get past. These are often interpreted as breeding swarms, but although some may be copulating, as adults usually do when they find themselves in close proximity, the usual cause of the great numbers is a recent storm which has washed them off the grass or algal beds on which they were living, up on to the beach.
In the case of your observations at Port Stephens it may just be the end of the life span of some of the species involved. I suspect that many of the tropical species at Port Stephens arrive as larvae from further north when the southerly flowing current or eddies move in close to shore. If the larvae that settled at Port Stephens in the spring arrived at the same time it is possible that their life span or the life span of the colonial animals they feed on has ended somewhat synchronously as well. The populations of non-tropical species are also declining around Sydney as winter approaches. To better understand these fluctuations we need to study the lifecycles of many more species and their food..
The other thing to realise is that unlike many land animals, most sea slugs have free swimming larvae which hatch out of the egg ribbon. These larvae can spend many weeks in the plankton and can be swept far from their "place of birth". The makeup of a population at say Port Stephens is therefore very dependent on things that have happened far from Port Stephens. No matter how ideal conditions may be at Port Stephens, if conditions did not favour the production of eggs and larvae larva further up the coast, or if the currents were not close enough to shore to bring the larvae to Port Stephens, then populations at Port Stephens the next year may be dismal... Bill RudmanRudman, W.B., 1998 (Apr 22). Comment on Population decline - Port Stephens by David & Leanne Atkinson. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/153