The three species we are interested in occur in very low numbers as
result of the introduction of exotic fish species and damming -
the Clanwilliam yellowfish, Barbus capensis, the Clanwilliam
sandfish and the sawfine. However, management strategies to conserve
these fish urgently need to be developed. Extremely low numbers and
threatened status of these species makes this a difficult job. I am
interested to know how fisheries biologists in the US have gone about
acquiring information on aspects of life history of fish which are
rare. Failing this, I would appreciate it someone could refer me
to any other people or publications which may be able to help in this
regard
First of all acquiring information on age-growth, age at
maturity, fecundity, survival at all life stages and recruitment,
require killing fish. I would need to sacrifice a minimum of 100 fish
per species in several age classes. Firstly I am not sure the present
populations could stand this kind of sampling pressure, and secondly,
but more realistically, I don't think we could catch that many fish
over the 3 year project.
Secondy, the fish are so rare and mobile that mark-recapture has
proved almost impossible - in the last year and a half I have had no
recaptures. Estimates of absolute abundance would not therefore be
possible using this technique. To give you an inidication of the
densities - in 18 gillnet hours of sampling on the mainstem of the
Olifants River we caught two fish. Smaller fish are more abundant in
the tributaries, but large adult fish in the mainstem are almost
impossible to find. We don't even know where they are spawning, or if
they have spawned at all in the last ten years.
Combined with this is that there are is no historical data on these
species apart from anecdotal information which suggests that they were
originally present in the river in their thousands and would
regularly migrate up the river to spawn in riffles of the middle and
upper reaches. The presence of the Clanwilliam dam now prevents this
and recruitment by the remaining fish is being severely impacted by
introduced largemouth and smallmouth bass populations (which are now
very abundant in the river).
Unless I am missing something, I am almost inclined to believe that
studies should be laid aside in favour of a programme to eradicate
bass and restore habitat.
Regards
Bruce Paxton
Freshwater Research Unit
University of Cape Town
John Day Zoology Building
Rondebosch
7701
Cape Town
8000
Tel: 021 650 4634
Cel: 072 2162904
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