the environmental status of the orange river mouth as...
TRANSCRIPT
The Environmental Status of theOrange River Mouth as Reflected by
the Fish Community
MT Seaman • JGvanAs
Report to the Water Research Commissionby the
Department of Zoology and EntomologyUniversity of the Free State
WRC Report No 505/1/98
Disclaimer
This report emanates from a project financed by the Water Research Commission (WRC) and isapproved for publication. Approval does not signify that the contents necessarily reflect the viewsand policies of the WRC or the members of the project steering committee, nor does mention oftrade names or commercial products constitute endorsement or recommendation for use.
Vry waring
Hierdie verslag spruit voort uit 'n navorsingsprojek wat deur die Waternavorsingskommissie(WNK) gefinansier is en goedgekeur is vir publikasie. Goedkeuring beteken nie noodwendig datdie inhoud die siening en beleid van die WNK of die lede van die projek-loodskomitee weerspieelnie, of dat melding van handelsname of -ware deur die WNK vir gebruik goedgekeur of aanbeveelword nie.
by
MT Seaman and JG van As
Department of Zoology and Entomology,University of the Free State
WRC Report No 505/1/98ISBN 1 86845 388 X
THE ENVIRONMENTAL STATUS OF THE ORANGE RIVER MOUTH AS
REFLECTED BY THE FISH COMMUNITY
Final Report
PARTICIPANTS
Funding and material help by Cape Nature Conservation (CNC) and the Water Research
Commission (WRC).
Research by:
Dept. of Zoology and Entomology, University of the Free State.
Project leaders:
M.T. Seaman and J.G. van As.
Research teams six to eight persons during each field trip.
SUMMARY
Fish-wise the Orange River Estuary is neither particularly rich in estuarine species nor is it
important as a nursery area, which reflects the history of the estuary. The major concern
during an inevitable future regime of low flows in winter and less-low (certainly not large)
flows in summer is therefore the health of the riverine fish community. The extent to
which the large seasonal fluctuation in flows is necessary and can be simulated is
problematical. The low flows during the study period appeared to be adequate to sustain
the riverine fish community, but seasonality of flow (especially the summer increase
associated with rainfall runoff) is necessary to stimulate breeding in some of the species.
The long-term effect of flow-reduction on some marginal species is bound to be negative.
TABLE OF CONTENTS
1 INTRODUCTION 1
1. Normal estuaries 6
2. Hypersaline estuaries 7
3. Closed or blind estuaries 8
2. MATERIALS AND METHODS 9
3. RESULTS 14
4. DISCUSSION 33
5. CONCLUSIONS 39
6. ACKNOWLEDGEMENTS 39
7. REFERENCES 40
8. APPENDIX 42
- 1 -
1. INTRODUCTION
The Lesotho Highland Water Scheme has major environmental implications for the
Orange River. It has been predicted by consultants BKS (McKenzie and Roth, 1994) that
if the scheme is completed, then there will be a considerable shortfall of water at the
mouth, amounting to an average of more than 842 million m per year. This should be
compared with an original flow of 11500 million m per year which has already been
reduced to 6500 million m per year. This would result in a river mouth that is practically
dry for years on end apart from exceptional floods. In addition the flow pattern has
changed drastically from one of large flows in summer and negligible flows in winter to
one of reduced flows in summer and increased flow in winter. Consequently the mouth
stopped closing in winter. When it did close in the winter of 1994 this was the first
occasion in 16 years. Due to the unseasonal offtake of water by towns, agriculture
(notably in spring before the rivers waters would swell naturally), hydroelectrictty and
interbasin transfer (existing and potential), the natural flow pattern is lost. It is
questionable to what extent management of flow from reservoirs could ameliorate this
situation.
The massive floods which appear once every decade or more are probably important in
dictating the nature of the system.
There will be other related effects to the bird-rich salt-marsh, to the overall salinity regime
of the estuary and the drying-out of extended stretches in the middle and lower reaches of
the river. There are therefore potentially catastrophic effects on the Orange River
ecosystem unless adequate plans are made based on a suitable knowledge of the present
biota and of ecosystem function.
- 2 -
As the river mouth/estuary is the ultimate part of the ecosystem, where manipulative
effects will accumulate, the Orange River Environmental Task Group has requested that
baseline information on the plants, birds and fish of the mouth area be acquired as soon as
possible for incorporation in a model which will allocate sufficient water to maintain the
mouth area in a suitable condition.
Estimates resulting from the Instream Flow Requirements Workshop in Fishhoek during
April 1996 (Venter and Van Veelen, 1996) suggest that a minimum of 197 million m3
under drought conditions, and a preferred minimum flow of 294 million m of water per
year under "normal" conditions would be acceptable to maintain the river mouth3
ecosystem, at a potential selling value of at least R1.50 per m were it to be sold as
potable water. While a study of the fish cannot give monetary answers, we can certainly
provide an idea of the potential effects of freshwater loss and saltwater intrusion on the
fish populations in the estuary and to some extent upstream.
Rationale
The following extracts from Orange River Environmental Task Group (1989) explain the
rationale for this study:
"The following environmental management objectives were formulated:
Conservation of bird life, which includes conservation of the supporting
ecosystem.
- 3 -
Re-establishment of the saltmarsh on the southern bank of the ORM
(Orange River Mouth) -system, which is cut off from the system by the
access road to the beach.
Control of salinisation of the ORM-system, which is anticipated to become
a critical factor during low-flow or no-flow conditions. Under these
conditions the salinity of especially the groundwater may exceed the
human tolerance.
The identified remedial and control measures to manage the ORM-system can be
summarised as follows:
The control of the status of the mouth by means of regulated flow to the
system.
Periodic water flushing of the system to restore the freshwater regime.
Controlled inundation of the saltmarsh
Maintaining the present status of the oxidation pond system.
Installation of culverts and stormwater pipes under the access road to
provide unrestricted flow access to the saltmarsh.
- 4 -
In order to achieve the above objectives and measures, proposed management strategies
were formulated. These strategies distinguish between management under ideal
conditions, viz. years of average to above average streamflow, and the management of
the system under absolute minimum conditions, which represent extreme drought
conditions.
The management strategies formulate the guidelines to assess environmental water
requirements. Since only limited data are available these assessed water requirements
should be considered preliminary estimates and are subject to updating once more
substantial data can be obtained. The minimum annual environmental water requirement
for the two management conditions is as follows:
Under ideal conditions, 244 million m per year (this was updated
to 294 million m3 per year minimum under "normal" conditions - Venter
and Van Veelen, 1996).
Under absolute minimum conditions, 100 million m per year (this
was updated to 197 million m3 per year minimum under drought
conditions).
It is also important to stress that the above water requirements represent the minimum
water requirements under these two management conditions. Should the inflow to the
system exceed the defined minimum requirements, it is essential to note that there are
specific control measures built into the management strategies which should be complied
with, such as the annual inundation of the saltmarsh. These control measures are deemed
to be essential to ecosystem independent of the annual flow rate through the system.
A continuous monitoring programme was seen as a crucial part of the implementation of
the management strategies outlined in the report. Suggestions with regard to parameters
- 5 -
thai should be monitored are also included. It has been proposed that the responsibility
for the planning and implementation of this programme should be referred to the
ORETG or an appointed sub-committee"
Consequently the present study of the fish community in the mouth area is intended to
give an insight into the environmental water quality, past, present and future.
Aims
i) to provide a description of the present state of the river mouth environment
as indicated by the fish community composition and condition (with an
accent on the freshwater, but including estuarine and marine components),
bearing in mind the effects of both seasonal and tidal effects,
ii) to prepare a proposal on environmental management requirements on river
and mouth in order to maintain or improve the status of the mouth,
iii) to indicate future research needs.
Key questions
i) what is the fish community composition in terms of freshwater, estuarine
and marine components?
ii) what is the condition of river-dependent fish populations?
iii) to what extent do the fish populations appear to have suffered from river
regulation to date?
- 6 -
iv) what is the prognosis for the condition of the fish community of the river
mouth, given alternative scenarios?
v) what further research inputs are needed?
Definition of an estuary (Day, 1981)
"An estuary is a partially closed coastal body of water which is either permanently or
temporarily open to the sea and within which there is a measurable variation of salinity
due to the mixture of sea water with fresh water derived from land drainage."
This definition is very broad, but Day split estuaries into three further categories, namely:
"1. Normal estuaries
Most estuaries are normal or 'positive' in the sense that there is an increase in
salinity from the head where the river enters, towards the sea. Further, there is a net flow
seaward over a full tidal cycle. Normal estuaries may be subdivided according to the
degree of stratification or mixing of salt and fresh water.
la. Saltwedge estuaries. These are normal estuaries with a wedge of sea water
on the bottom and a layer of fresh water flowing out at the surface but no mixing between
the two. Such a condition is rare if not entirely theoretical, but possibly occurs in some
tranquil fjords.
lb. Highly stratified estuaries. These are normal estuaries with a layer of sea
water flowing in along the bottom, a layer of fresh water flowing out at the surface and
- 7 -
between the two is a layer of mixed water separated by marked haloclines. Most fjords
belong to this class.
1c. Partially mixed estuaries. These are normal estuaries in which the vertical
salinity gradient shows varying degrees of mixing or stratification between the outward-
flowing surface layer and the inward-flowing bottom layer. Many estuaries belong to this
class including the Thames and the Mersey in England, the Seine, Scheldt and Elbe in
Europe, and the Hudson and Chesapeake in the United States.
Id. Vertically homogeneous estuaries. These are normal estuaries with the
salinity decreasing from the mouth towards the head without a vertical gradient in
salinity at any point. There may, however, be differences in salinity across the width of
an estuary with the net current flowing landward along one side and seaward along the
other. The lack of a vertical salinity gradient is due to a turbulent mixing which often
occurs in the strong tidal currents near the mouth of a shallow estuary. Many South
African estuaries are homogeneous near the mouth but become partially mixed or even
highly stratified in the calm upper reaches. The Bashee, Swartkops, Knysna and Breede
estuaries show these features.
2, Hypersaline estuaries.
These have a reversed or 'negative' salinity gradient with the salinity increasing
from sea water values at the mouth to hypersaline values in the upper reaches where the
water level is below mean sea level, so that the net flow is landward Such conditions
occur during severe droughts. The Laguna Madre in Texas is a classical example. St.
Lucia in Zululand becomes hypersaline periodically and Milnerton Lagoon near Cape
Town becomes hypersaline every summer.
- 8 -
3. Closed or blind estuaries.
These are estuaries which are temporarily closed by a sandbar across the sea
mouth At such times there is no tidal range and thus no tidal currents. Fresh water
enters from the river and the circulation is dependent on the residual river current and
the stress of the wind on the water surface. According to the ration between evaporation
and seepage through the bar on the one hand, and fresh water inflow and precipitation
on the other, salinity will vary. The estuary may become hypersaline, it may retain its
normal value when the mouth is closed or it may become hyposaline"
Results will show that the category of the Orange River Mouth varies with season and
between years. Furthermore, its category has changed with time due to cultural influences.
Historic and current flow regime
Historically the river had scouring, flushing floodwaters reaching the mouth during
summer, with great variability of flow both within and between years. In winter the flow
often was so low that the mouth closed, making the estuary blind but apparently not
hypersaline. The flow volume at Vioolsdrif, the nearest monitoring station to the mouth,
was 350 million m3 per year in 1993 (data courtesy of the Dept of Water Affairs and3
Forestry), which means that the flow is already close to the lower level of 244 million m3
per year, but well above the absolute minimum level of 100 million m per year.
Furthermore there is a quasi-natural seasonality about the flow (Figure 4) which may be at
least partially lost as flow volume decreases as a result of upstream activity.
- 9 -
2. MATERIALS AND METHODS
Study area
The Orange River estuary was considered to be that part of the river in which tidal
influence occurred. For practical purposes this was taken to be the lower 35km of river
from Brandkaros to the sea (Figures 1 and 2). Although saline waters did not reach
Brandkaros during the study period, there was a slight (about 10 cm) but noticeable
change in water level due to tidal ebb and flow.
The natural position of the mouth during the study period was at the southernmost point,
eroding against the dyke built by Alexkor for their access road to the mouth area. The
artificial position of the mouth, i.e. where the berm was breached by Alexkor at intervals
from about June 1993 to January 1994 was always in the middle of the berm, whereafter it
migrated southward.
Choice of stations
Stations were chosen firstly to reflect a gradation from freshwater with mimmal estuarine
influences (Brandkaros 35 km upstream from the mouth) to estuarine and marine
influences and secondly according to accessibility. Stations (Figures 1 and 2) were more
abundant nearer the mouth. Stations are characterised as follows:
1. Brandkaros riverine sandy banks.
2. Bokdroldraai riverine sandy banks, used only once.
Brandkaros
N
t
t km.
Golf club
Blokhuis (5
Figure 1. The lower Orange River up to 35 km from the mouth, showing allquarterly sampling sites.
-11 -
1 km.
NAMIBIASir Ernest
Oppenheimer bridge
ATLANTIC OCEAN
Fipure 2. The Orange River estuary up to 12km from the mouth, showingquarterly sampling sites.
- 1 2 -
3. Dunvlei estuarine reeds, peripheral effects of salinity.
4. Dyk estuarine reeds, near total tidal exchange.
5. Blokhuis estuarine bare shore, total tidal exchange.
6. Golf Club estuarine bare shore, total tidal exchange.
In order to explore the extent of tidal influences (pattern of sea water penetration) within
the estuarine area a different set of stations was used (see Figure 3).
Routine Procedure:
i) Determine position of each station, consider accessibility and time
constraints - first visit only,
ii) Determine time schedule for each day - sampling at one station per day in
order to cover as much of the tidal cycle as possible,
iii) Setting of gill nets and supplementary netting by means of fine-meshed
seine net and throw-net.
iv) Taking of salinity (conductivity) and temperature measurements,
v) Recording of each fish according to protocol - date, time, sampling
method, species, mass, length, sex.
vi) Computer storage of all data at the UOFS.
vii) Data reduction and reporting.
500 m.
Artificial
Figure 3, The Orange River in the proximity of the mouth, showing sampling sitesused to investigate total infliience. The posilion of the mouth throughoutthe study varied from the point of artificial breaching lo the "natural"position at the southern end of the berm.
- 1 4 -
Sampting regime
Field surveys were carried out quarterly during approximately the following periods: 26/3
to 12/4/1993, 25/6 to 10/7/1993, 25/9 to 4/10/1993 and 10/1 to 21/1/1994.
3. RESULTS
Salinity patterns as related to river-flow and tide
Over the study period, monthly flow (taken at Vioolsdrif, the nearest measuring station to3 3
the mouth - DWAF data) varied between a low of 5 million m (ca. 2 m per sec) in
October 1993 and 160 million m (ca. 60 m per sec) in January 1994 (Figure 4). The
latter month's peak flows reached the mouth directly after final sampling. Flows during
sampling were therefore approximately 15, 11,4 and 60 m per sec respectively. Total flow3
for the year 1993 was approximately 350 million m . Seen in the light of untested
suggestion (Venter and Van Veelen, 1996) that 294 million m per year would suffice to
maintain the mouth's ecosystem, flow during the study period wasn't much higher. The
suggestion of 197 million m per year as minimum under drought conditions requires a
more critical evaluation in the field.
Depth changed up to 30 cm due to tidal influence at Brandkaros, 35 km from the sea. In
contrast, salinity changes due to marine inflow were found only within 5 km of the sea
(Table 1). Concentrations upstream of 5 km from the sea merely reflect river levels,
though these were as high as 0,94 mS per cm at Brandkaros and 1.00 mS per cm at
Dunvlei during January 1994 before seasonal flows increased. These can be compared
E
o
O N D J F M A M J J A S O N D F M A M
MONTHS 1992-1994
F'mure 4. Montiily flow-volume of the Orange River at Vioolsdrif(28° 45' 39" S, 17° 43' 49" E) which lies approximately 200 km from themouth for the study period..
-16-
TabJeJ Surface conductivity and tempeinture range at each station for all seasons
CONDUCTIVITY (niS/cm)
DATEMar./Apr. 93July 93Sep. 93Jan. 94
BRA0.49-0.530.50-0,60
0.94
BOK0.47
DUN0.49
1.00
DYK7.3-46.03.3-32.214.2-44.95.7-39.9
BLO11.3-46.96.3-45.121.7-46.35.3-42.1
GCL
16.0-44.32.6-36.5
SEA
43.9
TEMPERATURE (°C)
DATEMar/Apr. 93July 93Sep. 93Jan. 94
BRA21-24.816.4-18.4
26.0
BOK21.1
DUN18.5-19
19.1-19.8
DYK12.1-20.419.8-21.211.6-18.216.0-22.3
BLO16.6-20.815.6-21.210.5-15.015.8-21.6
GCL
12.2-15.016.6-23.2
SEA
1216.4
(BRA=I)r.iiidkai-05, BOK=Bokdroldi aai, DUN=Dimvlei, DYK=Dyke, BLO=nioklmis, GCL=GolfChib,SEA= Sea near mouth.)
- 1 7 -
with values of about 1.00 mS per cm for water flowing into Gariep Dam historically
(Stegmann, 1974).
Temperature dropped by as much as 10 C between Brandkaros and the sea (Table 1).
Tidal temperature influences were obviously similar to tidal salinity influences, due to the
colder seawater flowing in a wedge below the freshwater up to about 5 km from the sea.
For a more detailed insight into salinity and temperature changes with tide see the map of
the stations A to G (Figure 3) and Figures 5 to 8 for values at top and bottom, during low
and high tide, 30 September 1993. All stations were within 3 km of the sea.
It is evident that the seawater wedge pushed in below the freshwater beyond 3 km at both
high and low tides. At the surface, pure seawater pushed in at least 2.5km (Station F), as
reflected by both salinity and temperature profiles.
Fish community diversity
A checklist of the fish species found during the study (Table 2) shows only Cyprinus
carpio as a new freshwater record (compared with Bethune and Roberts, 1990) for the
Lower Orange River. We in turn did not find Barbus hospes, B. trimaculatus, Tilapia
sparrmanii or Austroglanis sclateri which were not expected near the mouth anyway.
The freshwater species were dominated by Labeo capensis and B. aeneus.
B. kimberleyensis, Clarias gariepinus and Oreochromis mossambicus were less common.
- 1 8 -
Top
Bottom
M\\\\\i\m\i\\i\\t\mm\MM\^^
• J - r F - J . " I I , . , ' .1 i
to 20 3 0 40 50
mS/cm
Station A EM Station B
EEH Station E CD Station FStation C
Statfon G
Station 0
Figure 5. Conductivity at Orange River Mouth on 30 September 1993 at low tide.
- 1 9 -
Top
Bottom
mS/cm
Station A
Station D
Station B
EZ1 Station F
[ H I Station C
WM Station Q
Figure 6. Conductivity at Orange River Mouth on 30 September 1993 at high tide.
- 2 0 -
Top
Bottom
degrees C
mm Station A SW Station B
EMI Station E • Station F
m Station C
Station Q
Station O
Figure 7. Temperature at Orange River Mouth on 30 September 1993 at !ow tide.
-21 -
Top
Bottom
'pmammmBBanMnEammmm
1
: • • . . • • • . . . |
iiiii
IDfl
10 15 20
Station
Station
A
D
mCD
degrees
Station B
Station F
c
iID Station
M Station
C
3
Figure 8. Temperature at Orange River Mouth on 30 September 1993 at high tide.
- 22 -
Table 2 Checklist of the fishes of the Orange River.
* present, R rare, E endemic, V vulnerable, A alien
FISHES
Cyprinidae
Barbus aeneus
Barbus anoplus
Barbus hospes
Barbus kimberleyensis
Barbus paludinosus
Barbus trimaculatus
Cyprinus carpio
Labeo capensis
Mesobola brevianalis
Austroglanidae
Austroglanis sclateri
Clariidae
Glorias gariepinus
LOWER ORANGE !
*
E R
*
*
*
*
R
THIS STUDY
*
*
*
*
- 2 3 -
Cichlidae
Oreochromis mossambicus
Pseudocrenilabrus philander
Tilapia sparrmanii
Poniatomidac
Pomatomus saltator
Sparidae
Diplodus trifasciatus
LUhognaihus tithognathus
Mugilidae
Mugil cephalus
Liza richardsoni
Sciaenidae
Argyrosomus inodorus
Carangidae
Lichia amia
Clupeidae
Gilchristella aestttaria
*
*
*
*
*
*
*
*
*
*
*
*
*
Bethune and Roberts (1990)
- 2 4 -
The marine specimens caught were only one or two individuals per species and are
therefore merely records, while the estuarine species were almost solely represented by
Mugil cephalus and Liza richardsoni, with minnows Gilchristella aestuaria and
Mesobola brevianalis less obvious but common nevertheless.
Distribution offish species
Although not all stations were sampled by gillnet in each season (Figure 9), there was
always good coverage of river (Station 1), reed beds (Station 3) and saline estuary
(Stations 4 to 6). In winter (July) the mouth was closed, raising water levels to the extent
that access to the area near the mouth was compromised. As a result samples from this
area could not be taken.
Distribution of the five most common species caught in gillnets, namely L. capensis, B,
aeneus, O. mossambicus, M.cephalus and L. richardsoni (Figures 10 to 14 and Appendix
Tables 1 to 4) indicates a lack of any obvious pattern of seasonally.
While B.aeneus was clearly an upper estuary species, and L. richardsoni preferred the
most saline waters near the mouth, M. cephalus and O. mossambicus were most common
in the mid-estuarine waters of intermediate salinity. This is borne out clearly in the Cluster
Analysis (Figure 15) which has four identifiable clusters, namely "strongly riverine" (L.
capensis)t "upper estuary" (B. aeneus, B. kimberleyensis and C. gariepinus ) , "mid-
estuary" (p. mossambicus and M. cephalus) and "mouth" (L. richardsoni).
Seasonally the gillnetted community at each station clustered according to salinity (Figure
16). The four clusters were, from top to bottom respectively, "fresh" (upper six cases),
I
in
250
200
150
100
50
Stations 1
Autumn •WinteiQSpringD
SummerQ
137
17
145
95
49
ND
ND
ND
99
38
109
109
6
130
51
180
28
ND
137
201
ND
ND
126
111
Figure 9. The number of individuals collected at each station for each season using gillnets.
60%
50%
40%
30%
20%
10%
0%Stations
AutumnO
WintertZI
Spring^
SummerC
... -
131%
43.7%
40.8%
57.7%
. .^. n2
10.7%
ND
ND
ND
31%
1.1%
0.9%
3.6%
- •
40%
0%
0%
0%
— • - • —
50%
ND
0%
0%
60%
ND
0%
0%
Figure 10. Relative abundance of L. capensis at each station for each season, expressed as a percentage of the total catch.
80%
i
60%
40%
20%
0%Stations 1
Autumn •WinterQSpringQ
Summer •
28.5%
46.7%
27.5%
23%
57.1%
ND
ND
ND
10.1%
41.3%
22.3%
71.4%
0%
0%
22%
35%
0%
ND0%
8.3%
ND
ND
0%
41.8%
Figure II. Relative abundance of B. aeneus at each station for each season, expressed as a percentage of the total catch.
I
oo
100%
80%
60%
40%
20%
0%StationsAutumnD
WinterfZI
SpringC
Summer •
117.3%
0%
2.3%
0.9%
216.1%
ND
ND
ND
~13
6.4%
5.7%
31%
4.5%
—
14
66.7%
2.33%
66%
11%
1 1
577.8%
ND
0%
0.6%
6ND
ND
0%
3.6%
Figure 12. Relative abundance of 0. mossambicits at each station for each season, expressed as a percentage of the total catch.
60%
50%
40%
30%
20%
10%
0%StationsAutumnQ
WinteiO
Spring C
SummerC
—- - —
10%
13.3%
14.4%
4.3%
20%
ND
ND
ND
318.3%
3.4%
31.3%
1.8%
—
...
40%
20.1%
0%
49.5%
-
•
53.7%
ND
0%
7.3%
—-t
6ND
ND
1.6%
8.2%
Fipure 13. Relative abundance of M cephalus at each station for each season, expressed as a percentage of the total catch.
o
120%
100%
80%
60%
40%
20%
0%StationsAutumnD
WinteiO
SpringD
SummeiO
l~I0%
6.7%
10.4%
2.6%
20%
ND
ND
ND
1— —
13
43.6%
47.4%
18.8%
7.3%
—
40%
81.4%
16%
0%
1 1
53.7%
ND
99.9%
16%
— • — - - - —
6ND
ND
96%
33.6%
Figure 14. Relative abundance of L. richardsonii at each station for each season, expressed as a percentage of the total catch.
-31-
Hierarchical Tree
Variable
LCAPENSIBAENEUS
BKIMBERLCGARIEPIOMOSSAMBMCEPHALULRICHARD
(Dlink/Dmax)*1001 10 2 0 30 40 50 60 70 80 9 0 100
Figure 15, The seven commonest gillnetted spectes from all samples clusteredaccording to co-abundance.Distance metric: 1-Pearson rJoining rule: Unweighted pair-group average.
- 3 2 -
Hierarchical Tree
Case No.
Autumn 1Winter 1Spring 1Summer 1Autumn 2Summer 3Autumn 3Winter 4Spring 5Spring 6Summer 5Autumn 5Winter 3Summer 6Spring SSummer 4Spring 4Autumn 4
l lo(Dlink/Dmax)*100
2 0 3 0 40 50 6 0 7 0 80 90—100
Figure 16, Stations clustered according to co-abundance of the seven commonestgillnetted species in each season.Distance metric: 1-Pearson r.Joining rule : Unweighted pair-group average.
- 3 3 -
11 mid-estuary to mouth" (next eight cases), "upper estuary" (next three cases) and "upper
estuary" (the bottom case) which was based on only six individuals and is therefore not
statistically reliable.
Condition oj fish populations
Condition is variously measurable. We have used length/mass ratio as a broad indicator
(Figures 1 to 28 in Appendix). There is no particular evidence of poor condition in any
species. The sampling regime was not sufficiently rigorous to allow the identification of
year classes, which is not important because the highly variable nature of river flow
between and within years would tend to produce a mish-mash of year class patterns.
Condition is therefore not as important as numbers within each species.
4. DISCUSSION
Salinity patterns, related to river-flow and tide
The Orange River estuary according to Day's (1981) categories would be classed as a
blend between a "normal partially mixed estuary" and a "closed or blind estuary". The
former refers to it having an inward-flowing bottom layer of water that is saline, an
outward-flowing upper layer that is fresh, with a degree of mixing. The latter refers to it
being temporarily closed by a sandbar across the sea mouth which prevents a tidal range
and tidal currents; according to the ratio between evaporation and seepage through the bar
on the one hand, and freshwater inflow and precipitation on the other, salinity will vary.
- 3 4 -
Pristinely, the estuary would suffer enormous, or devastating, flows in summer, which
would tend to flush out the estuary, taking much of its biota out to sea. In winter,
particularly in dry years, the flow would become a trickle or cease and the mouth would
close.
The next stage of estuarine function, from the start of modification of flow by humans to
the present, saw increasing attenuation of summer flows due to damming upstream, and
increased winter flows due to water-releases from dams for urban, agricultural and
hydroelectrical purposes. Only in exceptional flood years (the most recent being 1988)
have flows resumed something of their pristine pattern. These might be called the
"resetting" floods, which "reset" the ecosystem by restoring refuge pools ("kuile")
through scouring, clearing channels and making new channels. The value of these floods
to the ecosystem is probably great and vital to maintaining it.
In the latter part of this period, 1978 to 1994, the mouth did not close once. The estuary
therefore became a "normal partially mixed estuary". However in the winter of 1994 the
bar once more formed across the mouth due to low flows in the river. The bar was
breached artificially at least three times to prevent flooding of facilities important to the
human communities on either side of the mouth.
This last action stress the fact that the estuary has become a managed water body. In the
past, a dyke was built to cut off the estuary from the golf course to the north, while
another larger dyke was built to cut off the southern branch of the estuary. The area
gained to the south was put to farming, sewage works and mining. Managers of the
estuary will use the formation of the bar and related rise in water level to flood the
northern salt marsh.
- 3 5 -
That the estuary is now a registered transborder wetland of international importance
according to the Ramsar convention (Cowan, 1995) does not preclude the managing of
the estuary.
The future will be that more dams will be built on the Orange River system (according to
the Orange River Development Replanning Study, Department of Water Affairs and
Forestry), thus leading to greater potential management options, allowing:
a.) that more water be taken out of the system (for agriculture, urban use and
interbasin transfer) and
b.) that water will continue to be released for urban and agricultural needs
downstream as far as the mouth itself, including demands by Namibia.
It is important to note that "the natural environment" or "the ecosystem" of the river will
be different from what it once was in pristine times, but there will certainly always be
water at the mouth for the "natural environment".
The information gathered regarding the fish community should be seen in the light of the
management future.
Fish community diversity
The fish community has four components, namely mouth, mid-estuary, upper estuary
and strongly riverine. The mouth component, including the greatest variety of species, all
marine, is restricted to the proximity of the mouth as pure seawater is to be found only as
- 3 6 -
far as about two km from the mouth at high tide. Liza richardsoni dominated this
community. It is a species well-known for its ability to enter estuaries.
The mid-estuary component, two to five km from the sea was dominated by Mugil
cephalus and Oreochromis mosscanbicus, although the former has marine and the latter
freshwater origins.
Of the two freshwater components upper estuary and riverine, only Labeo capensis
(riverine) appeared to avoid even mildly saline water. The Barbus spp. and C. gariepims
in the upper estuary were less selective but were more common in freshwater.
The predominance of the introduced indigenous O. mossambicus in the mixed-saline area
suggests that, by its ability to Jive in saline water, it is able to utilise resources in that area
which other cichlids cannot. It is known to be present in the river upstream at least as far
as above the Vioolsdrif weir (Benade, pers.comm.).
Historically, apart from O. mosscanbicus, the community appears to have changed little in
the last three decades since major regulation started (Cambray, 1984; Benade, 1993).
O'Keeffe et al. (1994) expressed concern only about the future of Barbus hospes, a
minnow which is described as rare in the Red Data Book for Fishes (Skelton, 1987).
Prognostication depends on the modification to, and management of, the river upstream,
as well as of the management of the sandbar across the mouth. The following scenarios
should be considered. Scenario 3 is close to the way the system will probably be managed.
Given the information gained about the fish community of the mouth it is clear that no
species will be threatened. The ecosystem therefore appears to be sufficiently robust, as a
- 3 7 -
result of a long history of extreme conditions, to be able to tolerate the management
proposed.
Scenarios
Scenario 1
Pristine - unfortunately lost. Compare with those below.
Scenario 2
The river upstream is managed solely for its users excluding the environment and no
allowance is made for simulated seasonal patterns which broadly approximate the natural
pattern. Flow volumes will be much as they are at present, the Gariep/Vanderkloof Dams
providing water for hydroelectricity generation especially in winter, for urban needs
throughout the year, for agriculture all year but notably in spring, and for inter basin
transfer which will have its greatest effect by reducing overall flows.
Scenario 3
The above with an attempt to simulate a quasi-natural flow pattern and manipulate mouth-
closure and opening. This would be a highly managed system but the ecosystem would
have the best chance of functioning optimally. Flow volume and pattern will be something
like that suggested by the Orange River Instream Flow Requirements Workshop (Venter
and Van Veelen, 1996), as shown in Tables 3 and 4. The effect on the biota should be
monitored on a long-term basis.
- 3 8 -
Table 3 Management principles for the Orange River Mouth (from Venter and Van
Veelen, 1996)
MONTH OF YEAR
July to September
October to April
May to June
TYPES OF FLOW NEEDED
* Back-flooding for inundation of salt-marsh.
* Lowest flow during August and September to initiate mouth
closure.
* Need enough water to keep mouth open.
* Open mouth will create habitat for inter-tidal birds.
* Maintenance floods needed once every five years.
* Periodic maintenance floods for reseeding of plants, to reduce
salinity, clear sediments.
* During extended drought periods no maintenance floods are
necessary.
* The major maintenance floods should occur between January
and March.
* Low-flow months for proper germination of seedlings.
Table 4 Environmental requirements (Mm3) for the Orange River and Orange River
Mouth for normal (upper part) and drought conditions (lower part).
Oct
32.1
32.1
Nov
31.1
31.1
Dec
35.4
13.4
Jan
32.1
13.4
Feb
32.8
12.2
Mar
32.1
32.1
Apr
31.1
31.1
May
24.1
10.7
Jun
15.6
5.2
Jul
9.4
2.7
Aug
9.4
2.7
Sep
9.1
10.4
Total
294.3
397.1
- 3 9 -
5. CONCLUSIONS
In answer to the original five key questions:
1. The fish community is clearly divided into a small mouth component of marine fishes
dominated by Liza richardsoni, a mid-estuary component in mixed-saline waters
dominated by Mugil cephalus and the introduced Oreochromis mossambicus, an
upper estuary component ofClarias gariepinus and Barbus spp. and a purely
freshwater riverine component comprised of Labeo capensis,
2. The condition of the river-dependent fish population within 35 km of the sea are
healthy.
3. The fish populations appear, on the basis of evidence collected, to be uncompromised
by increased management of flow.
4. According to a highly managed scenario, the prognosis for the fish community is good,
but long-term monitoring will be needed to test this statement.
5. Further research inputs are necessary, firstly to monitor the effects of the managed
river in order to improve management guidelines and secondly to understand the
ecosystem better given its Ramsar status.
6. ACKNOWLEDGEMENTS
Mrs. Marie Watson and Messrs. Chris de Vries, Berndt van Rensburg and Niekie
Lambrechts for data collation and reduction.
- 4 0 -
7. REFERENCES
Benade, C. (1993) Studies in fish populations of the Orange River within the borders of
the Cape province. M.Sc. thesis, University of the Orange Free state, Bloemfontein.
Bethune, S. and Roberts, K.S. (1990) Checklist of the fishes of Namibia for each wetland
region. Madoqua 17 (2), 193-199.
Cambray, J. (1984) Fish populations in the Middle and Lower Orange River, with special
reference to the effect on stream regulation. J. Limnol. Soc. sth. Aft. 10 (2), 32-49.
Cowan, G.I. (1995) Wetlands of South Africa. S.A. Wetlands Conservation Programme
Series. Department of Environmental Affairs and Tourism, Pretoria.
Day, J. (1981) Estuarine Ecology with particular reference to southern Africa. Balkema,
Cape Town.
Me Kenzie, R.S. and Roth, C. (1994) The evaluation of river losses from the Grange
River downstream of the P.K. le Roux Dam. Report to the Water Research Commission
by BKS Inc. WRC Report No. 510/1/94
O'Keeffe, J, Avis, T, Palmer, R. and Skelton, P. (1994). Lower Orange River baseline
study. A survey of the fish, invertebrates, riparian plants, and water quality of the lower
Orange River from Sendelingsdrif to Arisdrif in the Sperrgebiet. A report by the Institute
for Water Research, Rhodes University for Namdeb Diamond Corporation (Pty) Ltd,
Oranjemund.
- 4 1 -
Orange River Environmental Task Group (1989) Orange River Ecology. Assessment of
environmental water requirement for the Orange River Mouth. Report on Orange River
Environmental Workshop Oranjemund 1989. compiled by Bruinette, Stofberg and Kruger
Inc. Consulting Engineers. Pretoria.
Skelton, P. (1987). South African Red Data Book - Fishes. South African National
Programmes Report No. 37.
Stegmann. P. (1974). Results of preliminary chemical studies of the Hendrik Verwoerd
Dam. In E.M. van Zinderen Bakker (Ed.), The Orange River, Progress Report. Institute
for Environmental Sciences, University of the Orange Free State. Bloemfontein.
Venter, A. and Van Veelen, M. (eds) (1996) Refinement of the Instream Flow
Requirements for the Orange River and Orange River Mouth. BKS Incorporated for Dept.
of Water Affairs and Forestry. Pretoria.
- 4 2 -
Ainmidix
Table 1 Fishes collected at end) station, lower Orange River, during March/April 199.3.
STATION
Labeo capensis
Barhus aeneus
OieochrnmismossambicttsMcsoboiabrevianalisPseudocreni-labms philanderCyprimts carpio
BarbuskimberleyensisMugil cephalus
Clarias gariepmus
DiplodustrifasciatusPomatomtis saltator
Liza richardsonii
RIVERBRAND-KAROS
56
16
31
36
6
0
0
0
0
0
0
0
. i
BOKDROL-DRAAI
6
18
9
7
0
t
1
0
0
0
0
0
REEDSDUNVLEI
1
10
0
0
3
0
4
22
9
0
0
43
DYK
0
0
3
0
0
0
2
0
0
0
0
0
ESTUARYI1LOKIIUIS
0
0
0
0
0
0
0
1
0
I
5
1
TOTAL
63
44
43
43
9
1
7
23
9
1
5
44
—I
- 4 3 -
Table 2 Fishes collected at each station, lower Orange River, during July 1993
RIVER REEDSWAMPSTATION
Labeo capensis
Bar bits aenetts
Liza richardsonii
Oreochromis mossambicus
Argyrosomus hololepidotus
Barbus kimberleyensis
Mugil cephalus
BRANDKAROS7
6
1
0
0
1
2
DUNVLEI
1
35
42
5
0
2
3
DYK
0
.0
100
3
1
0
26
TOTAL
8
41
143
8
1
3
31
- 4 4 -
Table 3 Fishes collected at each station, lower Orange River, during September 1993.
RIVER REEDS ESTUARYSTATION
Barbus aeneus
OreochromismossambicusMugil cephalus
Barbus kimberleyensis
Liza richardsonii
Barbus paludinostts
Labeo capensis
Clarias_gariepinus
Mesobola brevianalis
PseudocrenilabrusvhilanderLithognathuslithognalhus
BRAND-KAROS
103
35
27
4
18
0
135
1
42
6
0
DUNVLEI
69
42
142
7
22
1
3
9
0
4
0
DYK R.M.
11
39
126
2
8
0
0
0
0
0
0
BLOKHUIS
0
20
2
0
136
0
0
0
0
0
1
GOLF CLUB
0
0
2
0
121
0
0
0
0
0
0
TOTAL
183
136
299
13
285
1
138
10
42
10
4
- 4 5 -
Table 4 Fisiies collected at each station, lower Orange River, during January 1994
RIVER REEDS ESTUARYSTATION
Barbus aeneus
Bar bus kimberleyensis
Labeo capensis
Clarius gariepimis
OreochromismossambicusPomatomus saltator
LithogiiathuslithognathusMitgil cephalus
Liza richardsonii
ArgyrosomushololepidotusLichia amia
Gilchristella aestuaria
BRAND-KAROS27
4
68
3
1
6
0
4
3
0
0
1
DUNVLEI
81
10
4
1
5
0
0
2
8
0
0
0
DYK R.M.
74
0
0
1
22
6
0
98
0
0
0
0
BLOKHUIS
15
0
0
0
1
2
10
13.
137
0
1
0
GOLFCLUB47
1
0
0
4
11
0
S
38
1
0
0
TOTAL
244
15
72
5
33
25
10
125
186
1
1
1
- 4 6 -
1400
1200
1000
800
600
400
200
0
-200
Labeo <
n=63
Mar./Ap
. . . . i
:apensis
r. 1993
...
.,,, i
x l x j L — ^
y=o.ooo;
V=0.000,
' r i i :
1—,—,—. .
4.207!X (Mnlt
4.M7(X (rem
M 1•
. 1....1—i—i—
1 ' ' • •
kF
I *
Jk>
1 J.7
/:
, . . . i
i—•—•—•—•—i
Female/
/ : • /
/ /Malt
- .»---l *~.J-J
' / " • T"/ *
F
3
_ . i—t—i—•—
0 5 10 15 20 25 30 35 40 45
LENGTH (cm.)
Figure 1. The mass/length relationship of L. capensis individuals collected at allstations during March/April 1993.
- 4 7 -
1000
or>nOUU
oUU
in 4UU
•yon
0
-200C
Barbus
n=32
Mar.yV
. . . . j
i • • • •
i aeneus
ipr. 199!
X
1
i
i!
i
i
jI
105 1!
;
iy=0.D06
y=0.009
i
. . . . 1
„ 3.1508X (Me
3.037X (Fnr
-
k
iole)
u« V
....
u1 Male/''
• / /
V
1 • • • / •
1 / /
' //f
Temale
20 30 40 505 25 35 45
LENGTH (cm.)
Figure 2. The mass/length relationship of B. aeneus individuals collected at allstations during March/April 1993.
- 4 8 -
to
400
350
300
250
200
150
100
50
0
-50
Oreochrom
n-30
Mar/Apr.
|
Is mossamb
1993
+ . , « • • • •
X0
XS ;
I
. . ... .__.i
i • -
:
CUS
i ....„...,
:i
1
x...o
x
1.1.1X X o X
• IM. . . .
Iil*
•
•
*
!
•
!
FJ a.,..
t
- • • + •
]
1i1
«... .-L i ^ *
10 15 20
LENGTH (cm.)
25 35
Figure 3. The mass/length relationship of O. mossambicus individuals collected atall stations during March/April 1993.
- 4 9 -
10
9
,1600
1400
1200
1000
800
600
400
200
0
-200
! 1j
Borbus kimberlI1
..JEr.7 j:!|
Mar./Apr. 1993!jr
iI
X
j
syensts
, . . . . , . . . . , . . ( . , . . . .
1 1 1! ... .1 1 1 !. .' •
!y=O.O26
y=0.003
F
1
1 J.7HD< (Mai*
: 3.391X (Feme
........ j/..^,
j
i •
F•
i
i !. . , , i . . . . i
\ 1 /Femolej
/
. . . .
/
"Male
-
-
•
0 10 2015 25
LENGTH (cm.)
3550
Figure 4. The mass/length relationship of B. kimberleyemis individuals collected atall stations during March/April 1993.
- 5 0 -
3000
2700
2400
2100
i nnn» S-l V W
1i 1500to"
^ 1200
goo
600
300
0
-300C
Mugil
n=23
Mar./
r ™ • ' j • ' • • j • • • •
i I1
cepholus
frpr. 1993
!•
i
I_ . ~~*-F ———'
. . . .
y=0.0
\ijI
1
Vnalt)
r . . l , , .
J!
//..rX
tk/...\
r/
TJ^;:
. . . . i . . . .
!
i |. . . . i . . . . i . . . . . . . .
VF /
/Fern lie
i 10 20 30 40 50 605 15 25 35 45 55
LENGTH(cm-)
Figure 5. The mass/length relationship of M. cephalus individuals colleclec! at allstations during March/April 1993.
- 5 1 -
3500
3000
2500
2000
i/i 1500
1000
500
-500
Clarias c
n=9
Mar./Apr
ariepinus
. 1993
"•"" j
!
, , , ,
. , . .
i • -•—•—•—i
I y=
i y=
• • • •
3.0086x"
3.0068xW
^ ^Ij
I
\ ""—'—'—'—I
2 1 (Malo)
G1(Female)
i
. . . . i
i/>
u
Male/// / '
/Female
/ /
i
-
10 20 30 . 40 50 60 70 80
LENGTH (cm.)
Figure 6. The mass/length relationship of C. gariepinus individuals collected at allstations during March/April 1993.
-52-
700
600
500
i/i 300
200
100
0
-100
Pomatomi
n=5
Mar./Apr.
s saltator
I
1993 |
i—-—"—""
^ ^
! • • • •
P^
j
S53
\
/
/ :
"emaleT \ •
-
0 10 15 20
LENGTH (cm.)
25 30 35
Figure 7. The mass/length relationship of P. saltator individuals collected at allstations during March/April 1993.
-53-
600
500
400
300
t/f
15 20
LENGTH(cm.)
25 30 35
Figure 8. The mass/length relationship of L. richardsomi individuals collected at allstations during March/April 1993,
-54-
1400
1200
tooo
800
400
200
0
-200
..Labeo...
n=8
Juki?.
:opensis
)3
^ ~ - — "
I
y=O.QQl
\ !
•
JJ90.
\[•
\
Mert.+r«rwh)l.
1•
Ii
1
y
i . . , .
1
/1! /V/7
/ •
i
i w -i *
1
•
10 20 3015 25
LENGTH(cm.)
3540
4550
Figure 9. The rnass/length relationship of L capensis individuals collected at allstations during July 1993,
-55-
1600
1400
1200
1000
^ 800
10
600
400
200
-200
Barb i
n=41
Jul.
, . . .
,ts aene
993
. i i r i
US
X-.—-—f-5
l._ I • t
i
. . . .
i 1 ' • - • —
•
. . . .
, . , ,
• i i i
' ' ' '
y=0.0C
y-O.OC
• • *
3.07
. , . •
>
i, (|Fenwls)
I
Ferr
. . . . i
ale//n
/Male
V
0 10 20 30 40 50 605 15 25 35 45 55
LENGTH (cm.)
Figure 10. The mass/length relationship of/?, aeneus individuals collected at allstations during July 1993.
- 5 6 -
500
450
400
350
300
250
200
150
100
50
0
-50
. Orcochromis
n-8
Jul.t993
j
1
mossambicus
!
_
r • • •]
! yJo.OII4xJW4(Mj
1I
:
i
~7^1^ / !
...... . . ,....
fJ
7\
/V.
y
' ':
!
-* 1
1i
•
10 15 20
LENGTH(crn.)
25 30 35
Figure 11. The mass/length relationship of O. mossambicus individuals collected atall stations during July 1993.
-57-
400
350
300
250
200
150
100
50
0
-50
:..B.or.bU3...M.:
nhedeyens
...ns.S J|
Jol. 1993!•
: IiI
: I" I
Si\
I• 1 '
. . . . i . . . . I
:[ - ••
s
:. . , . i . . . .
i—.—•—i • , • •—«•• •
! i
1 I
X <
, , , ,
t
. . • -
: X: *
_
-
-
-
i
i0 10 15 20
LENGTH(cm.)
25 30 35 40
Figure 12. The mass/length relationship of B. kimberleyensis individuals collected atall stations during July 1993.
- 5 8 -
25
LENGTH(cm.)
35 45
Figure 13. The mass/length relationship ofM cephalus individuals collected at allstations during July 1993.
- 5 9 -
i
200
150
100
50
0
-50
Lizo richardsonii
Jut... 1.99.3.
.=y=p..pO71.x.;
10 15 20
LENGTH (cm.)
25 30
Figure 14. The mass/length relationship of L nchardsonii individuals collected at allstations during July 1993.
- 6 0 -
1000
800
600
en
l/l 400
200
-2000 10 15 20 25 30 35 40 45
LENGTH (cm.)
Figure 15. The mass/length relationship of L. capemis individuals collected at allstations during September 1993.
- 6 1 -
400
350
300
250
S 2001/1
150
100
50
0
-50
• Barbus ae
n=60
Sep. 199J
ieug
, . . ,
- 0 05 2'54!
_ 0 0 4 1 X 2 . 6 ;
M
a-\-4-a \&*
(remalo)
Fe
M -••_-)
. . . . I
/
• /y
'//Male
V
A /
•
0 10 15 20
LENGTH (cm.)
25 30 35
Figure 16. The mass/length relationsliip oTB. aeneus individuals collected at allstations during September 1993.
-62 -
350
300
250
200
; IOreochromisi mossambicus!
n=65
150
-5010 15 20
LENGTH (cm.)
25 30
Figure 17. The mass/length relationship of O. mossambicus individuals collected atall stations during September 1993.
-63-
550
500
450
400
350
300
250
200
150
100
50
- 50
: Barbug k
: n=4
: Sep. 199
i . . , , —
imberleyen
!i-—*—I i 1 i < r.,. ,i
T ' ' ' •—
Ms
_ ^
. . . .
i—.—.—.—,—
. . . . i
y=0.0442x
M y
y
....
2.530(Mol.)
... /
A
. . . .
! ' • ' '
/
T/Male
1
" • /
/ '
7
•
-
-
0 10 15 20 25 30
LENGTH (cm.)
Figure 18. The mass/length relationship oF5. kimberleyemis individuals collected atalt stations during September 1993.
-64-
600
500
400
^ 300
200
100
-1000 10
f i
15 20
LENGTH (cm.)
25 30 35
Figure 19. The mass/length relationship of M cephalus individuals collected at allstations during September 1993.
- 6 5 -
350
300
250
200
150
100
50
0
-500 10 15 20 25 30
LENGTH (cm.)
35
Figure 20, The mass/length relationship of L. richardsomi individuals collected at allstations during September 1993.
- 6 6 -
180Q
1500
120Q
900
600
300
0
- 3 0 0
Lobeo
n=71
Jan. IS
—I—•—J—t—
— , - * . i-
:
i
:opensis
94
U4-J 1—
. . . . r , , , ,
y=0.00~i
y=0.0K
. . . ,
o 3-1 BoOX
1 x 3.052
IS \^ri
i:
i iI i
! 1
. I..I_I_.I—1 1 k • i 1— -J—• L_ i
Femolfl)
. . . .
Female JF //* \A/y
0
. , . .
' /
Mole .
-
• • i •
0 1Q 2015 25
LENGTH (cm.)
35 4550
Figure 2l . The mass/length relationship oFL. capemis individuals collected at allstations during January 1994.
-67-
1400
1200
1000
800
600
400
200
-2000 10 20 30 40
LENGTH (cm.)
50 60
Figure 22. The mass/length relationship of B. aeneus individuals collected at allstations during January 1994.
- 6 8 -
200
160
120
80
40
0
-40
Oreochromis mobsambicus
..n=..3.1,
Jan. 1994
0 10 15
LENGTH (cm.)
20 25
Figure 23. The mass/length relationship of 0. mossambicus individuals collected atall stations during January 1994.
- 6 9 -
V)
1600
1400
1200
tooo
800
600
400
200
0
-200
Barbu
T • • ' '
j
s kimbe
1n=17
Jan.
!
994
rleyensi
_-——
i
i
F
:
Ii:
j - i -I -r ,
i
1 1y=0.02
y=0.01
>
u r %s?r°.
1
)1x 4
rex1"1
! ' • '
ir
1I
/[*!!•>.
!!(Female)
/
ii
| 1 "/
- ^/
•
•
0 10 20 30 40 505 15 25 35 45 55
LENGTH (cm.)
Figure 24. The mass/length relationship of B. kimberleyensis individuals collected atall stations during January 1994.
- 7 0 -
1400
1200
1000
15 20 25 30 35 40 45-200
Figure 25. The mass/length relationship oFM cephalus individuals collected at allstations during January 1994.
- 7 1 -
1000 r
800
GOO
CD
to 400
200
-200
Pomaton
n=19
Jan. 19£
IUS saltal
4
iiiIj
i . . . .
or
. . . . . , : .
• • • - •
y-0.005/
^ ^
'x
"y
x yX K/
y(
.... i ....
. , , ,
. . . .
. . . .
/
. ¥"
/
. . . .
?d
10 15 20 25 30 35 40 45
LENGTH (cm.)
Figure 26. The mass/length relationship of P. saliator individuals collected at allstations during January 1994.
- 7 2 -
450
400
350
300
250
200
150
100
50
- 5 00 10 15 20
LENGTH (cm.)
25 30 35
Figure 27. The mass/length relationship of L. richardsonii individuals collected at allstations during January 1994.
- 7 3 -
2000
1500
i / i1000
500
0
I ! :
:
Lithogriothus lilhognatl
~ 1
Jon. 1&94
— — — . H i
IUS
X J
I
•::
!
y=0,04
!
:iI
i
:.
!5x
/
/
.. ../
/
|
j 1 . . .
7/
Unsex^
/
• /r
d
•
|
i
t : :
: i ii I iI t i
, j ,,, j j i
.. , . i . . . . 1 . . . . ! . . . .
i
•
•
10 20 3015 25 35
LENGTH (cm.)
40 5045 55
Figure 28. The hrass/Iength relationship of L. lithognalhus individuals collected at allstations during January 1994.