Kyle fish mortality.qxd
A selective fish mortality and some notes on Rhabdosargus sarba at
Kosi Bay, South Africa
R. KYLE
ISSN 0075-6458 117 Koedoe 45/2 (2002)
On 6 July 2000, while crossing Lake
Nhlange, the largest of the Kosi Bay lakes,
several dead fish were seen, lying bloated on
the surface. On investigation they were
found to be large Natal stumpnose Rhab-
dosargus sarba. Occasionally, during twenty
years of residence at the Kosi lakes, the
author has seen individual Natal stumpnoses
dead or dying in this lake, particularly in
winter, but never so many at one time.
On investigation, by questioning other local
residents, it was found that the mortality had
been taking place for several days and it sub-
sequently extended for at least another 14
days. Efforts were made to identify and mea-
sure as many dead and dying fish as possible
(Table 1) but those recovered were a small
proportion of the total number of fish which
died. The total mortality was probably sever-
al thousand fish. The vast majority of dead
fish were mature R. sarba (Tables 1 &2) and
dead fish were not reported in any of the
other Kosi Bay lakes. Nearly all the fish were
affected by a white fungal growth over much
of their bodies, particularly behind the dorsal
and anal fins. Initially there was deep con-
cern among locally resident anglers that a
sickness or poison was killing the fish, as
many rely on the fish resource for their liveli-
hood. Soon, however, they began collecting
dead and dying fish on the lee shore each
morning and eating or selling them. No ill-
ness or deaths were reported from people eat-
ing the fish and so the mortality was later
regarded by local residents as fortuitous.
Results of several days’ collecting and mea-
suring of dead fish show that 85 % of the fish
by number, and almost 94 % by mass, were
R. sarba (Table 1). The next most common
species were R. holubi (Cape stumpnose), a
much smaller member of the genus, then
Pomadasys commersonni (spotted grunter)
followed by a few very small fish and a sin-
gle Acanthopagrus berda (riverbream).
Many other R. sarba were collected by locals
but they reported few other species and
Natural History Note
Table 1
Species composition and total mass estimates of dead fish found in lake Nhlange between 6 and 28 July
2000
Species Number % Total mass estimate (Kg) %
Rhabdosargus sarba 215 85 322.5 93.9
Rhabdosargus holubi 17 6.7 2.5 0.8
Pomadasys commersonni 6 2.4 12 3.5
Gilchristella aestuaria 6 2.4 0.1 0.0
Caranx spp. 4 1.6 6 1.7
Hyporhamphus capensis 3 1.2 0.1 0.0
Ambassis spp. 2 0.8 0.1 0.0
Acanthopargus berda 1 0.4 0.3 0.1
Total 254 100 343.6 100.1
Kyle fish mortality.qxd 2005/12/09 10:06 Page 117
would probably not have noticed the very
small fish.
Mass fish mortalities have been recorded in
several estuaries including Kosi Bay (Blaber
& Whitfield 1976). Although many estuarine
fish are resistant to extremes of temperature
(Kyle 1984) this is often the reason suggest-
ed for mortalities. These have been ascribed
to particularly high temperatures (Kyle
1998) or particularly low temperatures
(Blaber & Whitfield 1976, Kyle 1989). They
have also been ascribed to chemical pollu-
tants (Ramm 1992) as well as a combination
of low temperatures and low salinity (Blaber
1973a, 1973b) Sometimes they appear to
have affected most fish present (Blaber &
Whitfield 1976; Kyle 1989) while often they
only affect certain species or size classes
(Blaber 1973a, 1973b; Kyle 1998).
A large amount of information is now avail-
able on the fish and the environment of the
Kosi Bay lakes, and it is possible to use this
to examine the selectivity of this mortality.
The mortality took place in winter and there
were no records of, or information on any
unusual chemicals in the lake at the time.
The catches of the legal gillnet fishery, oper-
ating in these margins, should give an
Koedoe 45/2 (2002) 118 ISSN 0075-6458
Table 2
Species composition of gillnet catches in Lake
Nhlange for May and June 2000 and for larger fish
found dead during July 2000
Species % Gillnet % Mortali-
ty
Gerres methueni 56.9 0
Mugil cephalus 20.1 0
Rhabdosargus sarba 9.1 88.9
Pomadasys commersonni 6.6 2.4
Myxus capensis 2.9 0
Acanthopagrus berda 2.3 0.4
Oreochromis mossambicus 1.7 0
Rhabdosargus holubi 0 6.7
Caranx spp. 0 1.6
Other species 0.4 0
Number of fish examined 145 253
insight into the fish species and size present
at the time. Although gillnetting is a selec-
tive way of catching fish, gillnet catch
species composition (Table 2) shows that
there were large numbers of species other
than R. sarba present, which were not
recorded in the mortality. The gillnet catch
species composition in the two months prior
to the mortality (Table 2) show that Gerres
methueni, Mugil cephalus, Myxus capensis
and Oreochromis mossambicus, together
comprised over 80 % of the catch but were
not recorded in the mortality. Pomadasys
commersonni and Acanthopagrus berda
appeared to die in numbers lower than
expected. Only Rhabdosargus sarba, R. hol-
ubi and Caranx species were found in num-
bers greater than their gillnet catch abun-
dance would have suggested, had all fish
been equally affected. The mean total length
of gillnetted R. sarba (395 mm) was similar
to that of the fish found dead (389 mm).
Although local residents occasionally report
dead and dying fish, when questioned they
said that they had never seen so many. Tem-
peratures are recorded daily at a jetty in the
NE of Nhlange lake and these showed levels
no lower than usual for this time of year.
Salinity is also recorded in the lake and over
the last few years it had been slowly rising,
from about 0.5 parts per thousand to almost
three parts per thousand. Extremely heavy
rainfall in late summer, more than double the
average, flushed the system and reduced
salinities markedly and rapidly back to
about 0.5 parts per thousand in Nhlange
Lake. The salinity in Lakes Mpugwini and
Makawulani, which also contained large R.
sarba, remained above 3 parts per thousand.
Data from Kosi Bay National Marine Line-
fish Survey voluntary catchcards (Prader-
vand pers. comm.) show an increase in mean
R. sarba mass, from 0.7 kg in 1994 to 1.4 kg.
in 1999, which was the period of rising
salinity. Most of the R. sarba found dead
were well above the size reported to be
found in estuaries by Whitfield (1998) who
stated that a proportion of the spent individ-
uals return to estuaries but these are con-
fined to fish less than 35 cm (standard
Kyle fish mortality.qxd 2005/12/09 10:06 Page 118
length). Whitfield also reported in an earlier
publication (Blaber & Whitfield 1976) that a
combination of salinities below 3 parts per
thousand and temperatures below 12 ºC were
lethal to R. sarba. The lowest daily water
temperature recorded in Nhlange in July
2000 was 15 ºC. Most of Whitfield’s work
was carried out south of, and thus in cooler
areas, than in Kosi Bay. Data reported here
suggest that salinities of around 3 parts per
thousand and warmer temperatures allowed
much larger R. sarba than previously report-
ed to enter or grow up in the Kosi Bay lakes.
When temperatures and salinities fell, the R.
sarba probably suffered severe osmoregula-
tory stress which led to often fatal fungal
infections (Blaber & Whitfield 1976). It
appears that, in the preceding years a combi-
nation of salinities above 3 parts per thou-
sand and temperatures above 15 ºC allowed
R. sarba well above the predicted maximum
size, to live in the Kosi Bay lakes.
It seems likely that the larger R. sarba will
now disappear from Lake Nhlange until
salinities rise again. Other larger marine-ori-
gin fish species found in estuaries seem to be
much more resistant to falling salinities and
temperatures.
Acknowledgements
I thank Ezemvelo KwaZulu Natal Wildlife for per-
mission to publish this paper.
References
Blaber, S.J.M. 1973a. Temperature and salinity toler-
ance of juvenile Rhabdosargus holubi (Stein-
dachner) (Teleostei). Journal of Fish Biology. 5 :
593 - 599.
Blaber, S.J.M. 1973b.Population size and mortality
of juveniles of the marine teleost Rhabdosargus
holubi (Pisces : Sparidae) in a closed estuary.
International Journal on Life in oceans and
Coastal Waters. 21 (3) : 219 - 225.
Blaber, S.J.M. & Whitfield, A.K. 1976. Large scale
mortality of fish at St. Lucia. South African
Journal of Science. 72: 218.
Kyle, R. 1984. Tolerance of high temperature by the
Cichlid Oreochromis mossambicus in the Kosi
System. Lammergeyer. 32:50.
Kyle, R. 1989. A mass mortality of fish at Kosi Bay.
Lammergeyer. 40:39.
Kyle, R. 1998. An interesting, possibly man
enhanced, fish mortality at Kosi Bay. Lam-
mergeyer 45:37-38.
Ramm, A.E. L. 1992. Aspects of the biochemistry of
sulphur in Lake Mpugwini, Southern Africa.
Estuarine, Coastal and Shelf Science 34: 253 -
261.
R. KYLE,
P.O. Box 43, KwaNgwanase, 3973, South Africa.
(Ezemvelo KwaZulu Natal Wildlife;
rkyle@iafrica.com).
ISSN 0075-6458 119 Koedoe 45/2 (2002)
Kyle fish mortality.qxd 2005/12/09 10:06 Page 119
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/SVE <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>
>>
>> setdistillerparams
<<
/HWResolution [2400 2400]
/PageSize [612.000 792.000]
>> setpagedevice