Re: [RML] P. cyanodorsalis revisited

David Wilson (rwilson at taunet.net.au)
Wed, 27 Jan 1999 00:34:22 +1030

Mach
I lived in Broome 1986 - 1988 and fished regularly in Crab Creek. I saw
cyanodorsalis but never collected any specimens for the aquarium. I can
comment about the environment they come from. Crab creek is a good place
to catch threadfin salmon which I think is a fish you have in Hawaii. You
call them something else, a name which Bruce mentioned in one of his posts.
(Bruce, what is threadfin salmon in Hawaii)

P. cyanodorsalis live in salt water and that changes to fresh for several
months in Darwin and hardly at all in Broome WA where Crab Creek runs into
Roebuck Bay. The annual rainfall at Broome is very low and there is no
permanent freshwater for about 165 km (Fitzroy River at Wilare) to the east
and probably 600 km to the west. Crab Creek has tides up to 9 meters and
very sloppy light grey mud banks with a shell grit bottom. Once you crawl
and slide into the creek and walk towards the sea the mud banks are above
eye level. A friend and I walked for about half an hour toward the sea at
low tide and ended up being swept back to the mangroves by the incoming
tide. It was not a nice experience. Last time I drove to Broome from
Darwin it took 22 hours on the road and there are only about 4 small towns
in between. Mack I think you need to come and have a look where these
little fishes live. I can draw you a few mud maps and then you can get
plenty of mud on you. Probably 99% of the places where these fish spawn
would be silty mud. Question - what happens to the eggs in this low oxygen
environment. In your calculations about cyanodorsalis spawnings you need
to factor in fast rushing water one way for 6 hours and the other way for
six hours and what happens to any eggs left in the mud which does not dry
between the tides. The spring tides in Darwin are up to 7 meters (about
25 ft) and the spring tides in Broome are up to 9 meters (about 30 ft)
which diminish down to a couple of meters during neap tides. I have
explored many places where there is fresh water creeks running into the sea
and have not seen any cyanodorsalis inland past the mangroves. You would
think they would migrate inland if they could, perhaps they need the salt
dose after a period in fresh water. I don't know what happens to P.
cyanodorsalis in Queensland mangrove habitat cause I have not seen it.
When the wet season drops great loads of water there is a decrease in water
temperatrure. The pearl farmers at Broome and Darwin keep year round sea
temperature statistics. As far as salinity goes the salinity near the
opening of Darwin Harbour at the moment is 30 ppt, just a little below
normal sea water.

I don't think cyanodorsalis would interbreed with inconspicuus because they
inhabit different parts of the mangrove creek.

Palintest may make an iodine test kit. I will check my catalogue tomorrow.

Dave Wilson