The influence of turbidity on juvenile marine fishes in estuaries. part 1. field studies at Lake St. Lucia on the southeastern coast of Africa

3. Exp. Mar. Biol. Ecol., 1987, Vol. 109, pp. 53-70 Elsevier

53

JEM 00888

The influence of turbidity on juvenile marine fishes in estuaries. Part 1. Field studies at Lake St. Lucia on the southeastern coast of Africa D. P. Cyrus’ and S. J. M. Blaber2 ‘Department of Zoology. University of Zululand, Kwa-Dlangezwa, Natal, South Africa; zDepartment of Zoology, University of Natal, Pietermaritzburg, Natal, South Africa

(Received 23 June 1986; revision received 2 March 1987; accepted 4 March 1987) Abstract: Lake St. Lucia, the largest estuarine system in Africa (325 km*), was chosen as the field study

area for a 3.5yr (1980-83) investigation into relationships between water turbidity and estuarine fish distribution. The variety of habitats, from clear water, open sandy shores to shallow muddy substrata and turbid waters, together with high species diversity (108 species) rendered the area suitable for this study. The relationships between fish distribution and environmental factors were monitored by monthly seine netting of fishes at seven sites representative of the range of conditions in St. Lucia. Simultaneously, water turbidity, salinity, and temperature were recorded. The possible influences of substratum type and food availability were also investigated by using recently published data on invertebrate benthos and zooplankton distributions. Published data were also used to determine the diet of the common fish species. The results showed that the distribution ofjuveniles of the 20 commonest fish species were statistically correlated only with water turbidity, water temperature, and food availability. The correlation with temperature was related to seasonal not spatial temperature patterns. Turbidity and food type influences were difftcult to separate but exceptions were the anchovy Thryssa vitrirostris(Gilchrist & Thompson) and the sole Solea b[eekeriBoulenger which occurred only in turbid water despite the widespread occurrence of their prey, and Gerres acinaces Bleeker, G. rappi (Barnard), and G.$lczmentosus Cuvier, all of which occurred only in clear water although the greatest densities of their bivalve prey were in turbid waters. Similarly, the sparids Rhobdosargus holubi (Steindachner) and R. sarba (Forsskal) were distributed according to turbidity and not their preferred foods. Principal component analysis with a minimum spanning tree plot and a canonical correlation test showed that the fish fauna could be divided into five groups according to their occurrence in various turbidities. These were: clear water species (e.g. Gerreidae) in < 10 NTU, clear to partially turbid species (e.g. Liza dumerilii (Steindachner) and L. macrolepis (Smith)) in < 50 NTU, intermediate turbidity species (e.g. Valamugi? cunnesius (Valenciennes) and Leiognathus equula (Forsskal)) in lo-80 NTU, turbid-water species (e.g. Elops mnchnata (Forsskal) and Thryssa vitrirostris)in > 50 NTU, and species indifferent to turbidity (e.g. Acanthopagrus berda (Forsskal) and Terapon jarbua (Forsskal). It is, therefore, suggested that turbidity plays a significant role, either singly, or in combination with other variables in determining the distribution of juvenile marine fishes in estuaries. Key words: Estuary; Juvenile marine fish; Turbidity; Distribution; Field study

Correspondence address: D.P. Cyrus, Department of Zoology, University of Zululand, Private Bag X1001, Kwa-Dlangezwa, Natal 3886, South Africa. 0022-0981/87/$03.50 0 1987 Elsevier Science Publishers B.V. (Biomedical Division)

54

D.P. CYRUSAND S.J. M. BLABER

Turbidity has long been recognized as affecting aquatic life (Wallen, 195 1) and fishes in particular (Ellis, 1931, 1936). Much early work concerned freshwater species in the U.S.A., where suspended erosion silt may play a limiting role in the environment of aquatic animals that are dependent on light. It was concluded that erosion turbidity caused changes in the species composition of fish populations (Langlois, 1941). Further work by Doan (1941, 1942) found that turbidity was inversely correlated with commercial catches of sauger Stizostedion canadense. These results were not accepted by Van Oosten (1945) who proposed that overtishing was the major cause of the depletion of fishes in the Great Lakes. Subsequent research attempted to establish the direct effects of turbidity on fishes (Wallen, 195 1; Herbert & Merkens, 1961; Neumann et al., 1975 ; O’Connor et al., 1976, 1977). Experimental studies related to fishes and turbidity are reviewed by Cordone & Kelly (196 l), Hollis et al. (1964) and Sorenson et al. (1977). These reveal little besides the results of direct effect laboratory experiments, and do not extend findings to field situations. Moore (1973) demonstrated that the invertebrate fauna in kelp beds could be divided into clear-water, turbid-water, and turbidity-indifferent species. Swenson et al. (1977) however, found no relationship between turbidity and fish biomass in the Great Lakes, but did detect a relationship between turbidity and species composition. Blaber & Blaber (1980) found in Moreton Bay, eastern Australia, that juvenile marine fishes inhabiting estuaries could be divided, according to distribution, into the categories proposed by Moore (1973). Swenson (1978) found that Stizostedion vitreum showed a preference for turbid waters when in a gradient while Salvelinus namaycush avoided them. Subsequently Gradall & Swenson (1982) showed that Semotilus atromaculatus prefer turbid waters and that Salvelinus fontinalis appear to be indifferent to turbidity. These studies have shown that fishes could also be divided into three turbidity categories. The estuaries of southeastern Africa are important nursery grounds for juvenile marine fishes of B 100 species (Wallace et al., 1984). It has been suggested (Blaber, 1981) that the occurrence of juveniles of many marine species in subtropical southeastern African estuaries may be related more to water turbidity than to an absolute need for any other factors present within the estuaries. The aim of the present study was to establish the role of turbidity in determining the distribution of juvenile marine fishes in Natal estuarine systems. This paper forms the first of two on this subject and presents results of the field surveys on the distribution and occurrence of estuarine fishes in relation to turbidity and other variables at Lake St. Lucia on the southeastern coast of Africa. The second paper in this series (Cyrus & Blaber, 1987) reports on laboratory experiments on the effects of turbidity on fish and the correlations between experimental and field results. The initial part of this study was to determine which environmental variables could be affecting fish distribution. A total of eight variables were measured, these were

INFLUENCEOF TURBIDITYON FISHES - FIELD STUDIES

55

turbidity, salinity, water temperature, wave action, wind speed, lake level, substratum particle size, and the composition of the benthic fauna. These data were analysed and the correlations between them and fish distribution considered by Cyrus (1984). Salinity, always considered important in determining the distribution of fishes, was of no importance in the St. Lucia system as all species investigated are known to frequent the range of 32-45.5x, recorded during this study (Whitfield ef al., 1981). Cyrus (1984) determined that only turbidity, temperature, and the amount of food available have any direct influence on fish distribution and these are reported on here. MATERIALSAND METHODS STUDY SITE The Lake St. Lucia system (325 km2) on the southeastern coast of Africa (Fig. 1) comprises two major parts: (1) the estuary and adjoining channel, known as the “Narrows”, extends from the sea to the furthest point of tidal influence at the southern end of the South Lake, and (2) the estuarine lake component. The average depth below MSL is 1.5 m, the maximum depth is 2.04 m (Johnson, 1977) and the mouth of the system is kept open during periods of low river flow by dredging. During the study period freshwater input was low and salinities in South Lake fluctuated between 32 and 45.5x,. Biological data on the system have been reviewed by Whitfield (1977), Begg (1978), and Taylor (1982), and its present environmental condition is classed as “good” (Begg, 1978). Some 108 fish species have been recorded, with juveniles of many marine species being common (Whitfield, 1977). South Lake (Fig. lc) was chosen as the study site because it provided a wide range of turbidities. The eastern shores are “predominantly clear” while the western shores are “predominantly turbid”. FIELD SAMPLING The study was carried out from January 1980 to June 1983, with the system being visited on 20 occasions, including monthly visits during 1981 when the major part of the fish sampling programme was undertaken. Fishes

Fishes were caught with a 70 m x 2 m x 12-mm bar mesh seine. The system has relatively uniform substrata with no rocky outcrops, hence the choice of sampling sites (Fig. lc) was determined by the presence of suitable beaches on which to haul the nets, and by water depth. The fact that a number of species were only caught in “clear” waters and that some “turbid’‘-water species were netted in “clear” areas in small numbers when they were migrating into the system (Cyrus, 1984) led to the conclusion that the

D.P. CYRUS AND S.J.M. BLABER

anies Island

harter’s Creek outh Lake f

Fig. 1. Study site; (a) in relation to southern Africa, (b) the St. Lucia system, (c) South Lake; 0 = seine netting sites.

seining technique used was effective. The standard length (SL) of each captured fish was measured, to the nearest centimetre, before they were returned to the water.

The definition given by the U.S. Geological Survey (1964) was adhered to in this study: “Turbidity is the optical property of a suspension with reference to the extent to

INFLUENCE OF TURBIDITY ON FISHES - FIELD STUDIES

51

which the penetration of light is inhibited by the presence of insoluble material. Turbidity is a function of both concentration and particle size of the suspended material”. Prior to seine-netting, water samples for turbidity determination in nephelometric turbidity units (NTU) were collected at NN 3 and 10 m from the shore and then measured on a Hatch Model 16800 portable turbidity meter. The reading at 10 m gave an indication of the general turbidity of the area which was to be sampled by netting, while the 3-m sample provided data on turbidity increases which might have occurred as a result of onshore wave action. Water temperature

Temperatures were measured 30 cm below the water surface prior to all seine-netting, simultaneously with turbidity sampling. Benthic fauna of South Lake, St. Lucia

Benthic samples were collected monthly for a year, from water depths of x 0.7 m, at four sites in South Lake. The sites were Ukwakwa and Nkazana Stream (sandy substrata), and Gillies Point and Indicus Indent (muddy substrata) (Fig. lc). A Zabalocki-type Ekman grab which samples a surface area of 0.0236 m* to a depth of 4.5 cm was used. Four grab samples were taken each month at each site. After collection, larger organisms were separated by washing the sample through a I .O-mm sieve. A solution of 4% formalin was added to the remainder, the sample stirred into suspension and decanted through a OS-mm sieve. This procedure was repeated five times to ensure the collection of all animals in the sample. The animals were preserved in 10% formalin and the vital dye Phloxin added to aid sorting and counting in the laboratory. ANALYTICAL PROCEDURES

Benthic analysis

Samples were sorted to species where possible, counted, and dried to constant weight at 60 ’ C and weighed to 0.01 mg on a Cahn 29 automatic electrobalance. Molluscs were weighed shell-free after dissolving the shells in a nitric acid solution. The results of the benthic analysis have been published separately (Blaber et al., 1983). Food preferences The diet of juveniles of the 20 most common marine species in Lake St. Lucia was determined from published data and personal observations. The following were consulted in this regard: Blaber (1974,1976,1979,1984), Blaber & Cyrus (1983), Cyrus & Blaber (1983), Day et al. (1981), Kyle (1986), WhitfIeld (1980), and Whittield & Blaber (1978a,b,c).

58

D.P. CYRUSAND S.J.M. BLABER

Statistical analyses of data Comparison of turbidity, temperature, and benthic data withjsh

distributions in South

Lake.

In order to determine which of the factors was having the greater influence on fish distribution three methods were employed. (1) Correlation analysis: this was carried out in order to determine whether significant correlations existed between the three factors and the 20 commonest fish species. The analysis took into account summer, winter and site variations. (2) Principal component analysis: this multivariate technique for examining relationships among several quantitative variables is described by Ray (1982). (3) Canonical correlation analysis: this technique, used for analysing the relationship between two sets of variables each containing several variables, is described by Ray (1982). Comparison of turbiditypreferences of common species. To determine whether juveniles of the common estuarine species could be grouped according to their turbidity preferences, a principal co-ordinate analysis and minimum spanning tree plot were performed. This analysis of the catch per unit effort data (CPUE) for each species, in four turbidity groups, produced a two-dimensional plot showing the relationship between the different species in terms of their preferences for different turbidities. The minimum spanning tree data assisted interpretation by indicating the way in which the different species were linked in terms of their overall turbidity preferences. The values calculated allowed the linking together of the plotted results of the principal co-ordinate analysis in the form of a dendrogram. This facilitated interpretation when a number of species were positioned close to each other in the two-dimensional plot.

RESULTS TURBIDITY In South Lake (Fig. lc) during 1981, when monthly sampling was undertaken, the mean turbidity range recorded was 5.6-238.1 NTU while the ranges of monthly minimum and maximum turbidities were 2.5-9.0 and 99.0-544.0 NTU, respectively. The mean turbidity and range for each of the sampling sites are given in Table I. WATERTEMPERATURE This varied seasonally but not spatially, the seasonal range from 16 to 31.5 “C corresponded with the records of Hutchison & Pitman (1973). Measurements taken during sampling runs showed that temperatures at any one time were uniform across South Lake, the only variation occurring in very shallow areas along the shore. Differences in temperatures, throughout South Lake, recorded during each of six such runs made over a 12-month period were between 0 and 1.5 “C, with a mean of 0.66 “C.

INFLUENCE OF TURBIDITY ON FISHES - FIELD STUDIES

59

TABLE I

Mean turbidity and ranges recorded at seven sampling sites in South Lake, St. Lucia (Fig. lc) between January and December 1981.

Western shores Gillies Point Charters Jetty Public Jetty Indicus Indent Eastern shores Nkazana Stream Old Jetty Ukwakwa

x 10 m from shore

YZ3 m from shore

Site ?I

Range

SE

j?

Range

SE

141.7 79.5 63.2 91.0

18-544 10-300 5-172 3-324

42.4 22.9 18.0 29.9

98.9 76.9 60.6 85.2

6.5-324 lo-306 3.5-198 2.5-324

28.9 23.0 17.4 29.7

3.2 3.3 2.6

11.3 9.7 10.0

2.5-37 3.5-29 4.0-24

2.9 2.0 1.5

14.0 14.8 14.1

5.5-40 2.0-41 2.7-35

No significant differences were found between temperatures recorded on the eastern and western shores of South Lake. BENTHOS OF SOUTH LAKE The

mean monthly biomass values for the eastern and western shores are shown in Fig. 2, no definite seasonal variations were evident. The overall mean biomass for the year (dry mass) was 2.63 g - me2 with mean values for the substrata of the “muddy” west and “sandy” east being 4.19 and 1.07 g * m- 2, respectively. F and t tests on the data (Cyrus, 1984) showed that there was a significant difference between the values

I,,,,,,, JFMAMJ

I

I

I4

JASOND

Month Fig. 2. Mean monthly standing stock of benthos (g . m -a dry wt) in South Lake, St. Lucia: 0 = eastern shores; A = western shores.

60

D. P. CYRUS

AND

S. J. M. BLABER

of animals present in the benthos on the two shores. The taxa present, their densities, distribution, and seasonality have been discussed by Blaber et al. (1983). The taxonomic authorities for all invertebrate species are given in Day (1974). Thirty-seven taxa were recorded with the bivalve Solen cylindraceus being the greatest contributor to the benthic biomass, particularly in the “muddy” areas on the western shores. Other bivalve species present were widely distributed but were also most abundant in the west. The polychaete Marphysa macintoshi was the second most important contributor to the overall biomass and, although abundant at all sites, had its largest standing crop on the “muddy” west at Indicus Indent (Fig. lc). There were considerable differences between numerical densities of the dominant taxa in different substrata. Cumacea were the only dominant group on the “clear” eastern shores. Bivalvia, Polychaeta, Nemertia, Amphipoda, Tanaidacea, and Mysidacea were all present in their greatest numbers on the “muddy” western shores. RELATIONSHIPS

FROM

FIELD

AND

FOOD

DATA

The preliminary analysis showed that 17 of the 20 species were affected by turbidity (Table II). Ten avoided turbid waters, five avoided clear waters and two occurred in intermediate turbidities. A further three species were apparently indifferent to turbidity.

TABLE II The relationship between fish species and three variables in South Lake, St. Lucia, as shown by the field data and food preferences: turbidity preference + = turbid, - = clear, + = intermediate, I = indifferent; temperature preference + = warmer, - = cooler, k = none; food preference YES = with available food, NO = not related to food, ? = unknown; [] = not a benthic-feeder. Species Gerres acinaces Gerres rappi Monodactylus argenteus Rhabdosargus holubi Caranx sexfasciatus Liza dumerilii Liza macrolepis Rhabdosargus sarba Gerres filamentosus Valamugil buchanani Leiognathus equula Mugil cephalus Valamugil cunnesius Acanthopagrus berda Pomadasys commersonni Terapon jarbua Elops machnata Solea bleekeri Johnius belengerii Thryssa vitrirostris

Turbidity

Temperature

Food

NO NO YES [YES1 [YES1

_

+ + + +

Wsl [?I YES NO

[NOI

* + +

+ + +

YES

I I I + + + +

+

YES NO

i+ + + + +

[?I 1YW [?I [YEsI YES

[YEsI [NOI

INFLUENCE OF TURBIDITY ON FISHES - FIELD STUDIES

61

As far as food is concerned, 11 of the species were at their greatest densities in areas where their favoured food was also at its greatest density. Six species showed the opposite trend, and for three species it was not possible to determine any relationship. The latter group consisted mostly of species which are not benthic invertebrate feeders. In terms of temperature 16 species increased in numbers as the temperature increased, two showed the opposite trend, and a further two showed no trend. DISTRIBUTION OF FISH SPECIES IN SOUTH LAKE During field sampling it was noticed that the CPUE for a number of species declined above or below certain turbidities; consequently when catch data were analysed four natural groupings were apparent. The turbidity groupings were designated as follows: Type A < 10, Type B 10-50, Type C 50-80, and Type D > 80 NTU. The distribution of the four turbidity categories among the 6 1 seine hauls carried out during 198 1 are given in Table III. Catch data from these seine hauls were used to assess the abundance of juveniles of the 20 common marine species in the different turbidity ranges. Table III also shows the distribution of 61 seine hauls according to season. During 198 1,33 seine hauls were carried out on the eastern shores and 28 on the western shores.

TABLE III The seasonal distribution of 61 seine hauls and number of hauls in each turbidity range in South Lake, St. Lucia, during 1981: X = mean. Spring Season 2 temperature (“C) Number of hauls Turbidity range (NTU) Number of hauls

Summer

Autumn

24.3

27.6

25.9

16.6

15

16

15

15

10-50 15

51-80 8

< 10 28

Winter

>80 10

The distribution of each of 20 species in terms of their percentage catch in the four turbidity ranges is shown in Table IV. These are ranked with “apparent” clear-water species at the beginning, intermediate, and indifferent species in the centre, and “apparent” turbid-water species at the end. Taxonomic authorities for all fish species are given in Smith & Heemstra (1986). SPECIES GROUPINGS ACCORDING TO TURBIDITIES OCCUPIED The results of a principal co-ordinate

analysis to determine whether specific turbidity

groupings were indeed present among the species are shown in Fig. 3. The “goodness of fit” criterion (T) for the plot of the field data was reasonably however,

not possible to determine probability

good (54.2%);

it is,

values for this test. The point locations

62

D.P. CYRUS AND S.J.M. BLABER TABLE IV

The percentage (catch per unit effort) distribution of the catch of each of 20 species in four turbidity ranges from South Lake, St. Lucia: n = number of fish. Species

Gerres acinaces Gerres rappi Monodactylus argenteus Rhabdosargus holubi Caranx sexjasciatus Liza dumerilii Liza macrolepis Rhabdosargus sarba Gerres filamentosus Valamugil buchanani Leiognathus equula Mugil cephalus Valamugil cunnesius Acanthopagrus berda Pomadasys commersonni Terapon jarbua Elops machnata Solea bleekeri Johnius belengerii Thryssa vitrirostris

Turbidity range (NTU)

n


10-50

51-80

>80

89.5 60.0 64.0 65.0 54.5 51.0 49.0 42.0 23.0 20.0 0 5.5 1.0 7.0 17.0 36.0 1.0 1.0 0 0.2

10.5 35.0 8.0 22.0 36.5 42.0 28.0 32.0 77.0 70.0 36.0 22.0 4.0 35.0 23.0 33.0 18.0 12.0 0 0.4

0

0

5.0 13.0 6.0 9.0 7.0 14.0 23.0 0 10.0 56.0 61.5 86.0 48.0 33.0 10.0 66.0 32.5 32.0 54.0

0

15.0 7.0 0 0 9.0 3.0 0 0 8.0 11.0 9.0 10.0 27.0 21.0 15.0 54.5 68.0 45.4

430 177 134 237 24 231 767 1143 40 16 145 20 28 174 222 180 39 552 47 148

were linked using the data from a minimum spanning tree plot, thus indicating the relationships between each point by highlighting the “distortions” produced by the two-dimensional ordination plot. The linking of the ordination plot points with the minimum spanning tree data clearly reveals the relationship between the species in terms of their turbidity preferences. By comparing the CPUE data for each species (Table IV) in relation to its position on the ordination plot with those of its nearest neighbours, it was found that the species could be divided into live major groups (Fig. 3), despite the fact that four natural turbidity ranges had been indicated by the catch data. The live groups shown in Fig. 3 are: (1) Clear-water species (common in < 10 NTU): Gerres acinaces (G.a), Gerres rappi (G.r), Monodactylus argenteus (M.a), and Rhabdosargus holubi (R.h). (2) “Clear to partially turbid” species (common in < 50 NTU): Liza dumerilii (L.d), Liza macrolepis (L.m), Rhabdosargus sarba (R.s), Valamugil buchanani (V.b), Gerres filamentosus (G.f), and Caranx sexfasciatus (Cs). (3) Species occurring predominantly in waters of intermediate turbidity (lo-80 NTU): Valamugil cunnesius (V.c), Mugil cephalus (Mc), and Leiognathus equula (L.e). (4) Species occurring predominantly in turbid waters (> 50 NTU): Elops machnata (E.m), Solea bleekeri (S.b), Johnius belengerii (J.b), and Thryssa vitrirostris(T.v). (5) Species indifferent to turbidity

INFLUENCE OF TURBIDITY ON FISHES - FIELD STUDIES

63

Fig. 3. Principal co-ordinate and minimum spanning tree plot of catch data of 20 commonly occurring estuarine species divided into five major turbidity groupings: letters = abbreviations of species names given in turbidity preference groupings.

(found throughout): Pomadusys commersonni (P.c), Acanthopugrus berdu (A.b), and Terupon jurbuu (T.j). STATISTICAL ANALYSIS OF DATA

Correlation analysis

As the data set comprised a number of 0 values for species catches, and the fact that these could affect the significance values of certain correlations, it was decided to deviate from the normal procedure of significance acceptance only at the 5 % level and to include data significant at < 10% (D. J. Lubmsky, pers. comm.). This enabled a better interpretation of the relationship between the three factors and the different tish species, it revealed that correlations existed with 10 species for turbidity, 14 with temperature, and eight for the benthos. Princ@ul component analysis The principal component analysis of the data showed that only nine of the 23 factor patterns generated, accounting for 76 y0 of the common variance, had eigenvalues > 1.

64

D.P. CYRUS AND S. J.M. BLABER

Of the nine factors only the first two, those with the highest eigenvalues, contained factor loadings (correlations) for turbidity, temperature, or benthos which were > + 0.40 or < - 0.40. The high values are considered as contributing significantly to the determination of the factor grouping. Factor loadings reaching + 1 or - 1 have the highest direct or inverse relation with the factor while those tending to 0 have no relation. The composition of the remaining seven factor groups was determined within the species themselves. Turbidity was the most important of the three factors, and the only one showing a significant correlation value within Factor 1 (Table V). It showed signiticant correlations with nine of the 20 fish species (two negative and seven positive). Factor 1 had the highest eigenvalue and accounted for 16y0 of the total variance within the correlation matrix. Turbidity and seven of the fish species attained their highest correlation values within Factor 1. Factor 2, which also had a high eigenvalue, accounted for 13 y0 of the total variance. In this factor, however, turbidity, temperature, and benthos all had high correlation

TABLE V

Variables with high correlation values (> + 0.40 or i - 0.40) associated with the first two factors of the principal components analysis of major physical and fish data from South Lake, St. Lucia: * = highest correlation value for the variable within the whole factor pattern. Variable Physical factors Turbidity Temperature Benthos Fish species Gerres acinaces Gerres rappi Monodaciylus argenteus Rhabdosargus holubi Carnax sexfasciatus Liza dumerilii Liza macrolepis Rhabdosargus sarba Gerres filamentosus Valamugil buchanani Leiognathus equula Mugil cephalus Valamugil cunnesius Acanthopagrus berda Pomadasys commersonni Terapon jarbua Elops machnata Solea bleeheri Johnius belengen’i Thryssa vim’rosms

Factor 1

Factor 2

- 0.47*

0.46 0.43* 0.52*

0.57* 0.41 0.60* 0.62* 0.47 0.80*

0.43 0.54 0.60* 0.65*

- 0.46* - 0.41*

0.60* 0.42 0.40 0.60*

INFLUENCE OF TURBIDITY ON FISHES - FIELD STUDIES

65

values (Table V), as did seven fish species, two ofthe latter having also shown significant values in Factor 1. The correlation values within this factor indicate that all three factors (variables) influenced those fish species within the group which also showed high correlation values. Canonical cowelation analysis The results of the canonical correlation analysis of the relationship between turbidity,

temperature and benthic food, and fish distribution are given in Table VI based on the 59 data sets collected during 198 1. Only the first canonical correlation was significant, with the probability of getting a greater F statistic than observed if the hypothesis is true being 0.0054. The standardized correlation coefficients for the environmental measurements and the correlations between the physical measurements and the canonical variables of the species CPUE are given in Table VII. TABLE VI Results of the canonical correlation analysis comparing turbidity, temperature, and benthic food with the distribution of 20 fish species in South Lake, St. Lucia: df = degrees of freedom, Vl = greater df, V2 = lesser df. F

No.

Canonical correlation

Variance ratio

statistic

Vl

v2

Prob. > F

1 2 3

0.82634 0.70962 0.49364

2.1531 1.0144 0.3222

1.7738 1.1642 0.6444

60 38 18

102 70 36

0.0054 0.2839 0.8393

TABLE VII Standardized canonical coefficients for environmental measurements ( = ENVI 1) and correlations between the environmental measurements and the canonical variables of the species CPUE ( = CATCH 1) for the first canonical correlation (see Table VI). Factor

ENVI 1

CATCH 1

Turbidity Temperature Benthos

- 0.9155 0.5252 0.0021

- 0.7049 0.3453 - 0.1699

DISCUSSION INFLUENCE

OF IMPORTANT FACTORS

To determine to what extent turbidity, temperature, and benthos influence species distribution, a number of statistical tests of the data were undertaken.

66

D. P. CYRUS AND S.J.M. BLABER TABLE VIII

Summary of correlations, from three tests, between three physical factors and 20 fish species from data collected in South Lake, St. Lucia, during 1981: [ - ] = negative, [ + ] = positive, * = agreement with preliminary analysis of field data, Fig. 2. Species _...._ ._-.__

Turbidity

Temperature --

Food

Gerres a&aces Gerres rappi ~onoda~ty~us argenteus Rhabdosargtls hoiubi Carnar se.xfasciatus Liza d~le~li~ Liza macro~ep~ Rhabdosargus sarba Gerres filamentosus Valamugil buchanani Leio~tathus equula Mug8 cephalus Valamugil cunnesius Acanthopagrus berda Pomadasys commersonni Terapon jarbua Hops rna~bn~fa Solea bteekeri Johnius belengerii Thryssa vitrirostris

t-1*

[+I*

I-l”

[+I*

i-1* [+I*

Total per factor : agreement

14: 11

I -1”

-1 +I +I

+I*

17: 16

12:8

Table VIII summarises the correlations obtained from the three statistical tests as well as comparing them with those from the preliminary analysis of the data. It shows that there is good agreement between the two. The main discrepancies or lack of correlation occurred with species found in intermediate turbidities or those that are indifferent to turbidity, while under benthos, those which are not benthic ~ve~ebrate feeders showed least agreement. The results of the canonical correlation analysis showed that, of the three factors, turbidity exerts the greatest overall influence on fish dis~bution. This is shown by the high canonical coefficient value, and the fact that turbidity has the highest value, by SO%, of the three factors in determining the species catch of the first and only significant canonical variable within the data set (Tables VI, VII). The importance of turbidity over the other two factors is also shown by the results of the principal component analysis (Table V), with its significant correlation within Factor 1 which accounted for the greatest amount of variance within the data set. The use of a principal co-ordinate analysis provided information indicating that species in South Lake could be divided into five turbidity groupings as shown in Fig. 3. These groupings provided a good basis for the study of the distribution of fishes and turbidity in other Natal estuaries (Cyrus, 1984). In the international context, however,

INFLUENCE OF TURBIDITY

ON FISHES - FIELD STUDIES

the det~a~on of what constitutes turbid water is not st~d~~~ discussed by Cyrus (1984) and Cyrus & Blaber (1987).

COMPARISON

67

and this has been

WITH OTHER STUDIES

The present study has shown that the fish fauna of South Lake may be divided into similar groupings to those found by Moore (1973). Due to the turbidity range occupied by some species, as well as the wide range of t~bidities present, two additions categories were include. These were for species occupying “clear to partially turbid” waters (< 50 NTU).and those in an intermediate range of turbidities (lo-80 NTU). Studies on turbidity and the distribution of marine and estuarine fishes are limited. A number of broader studies on specific groups have discussed distribution and turbidity, these include: Sphyraenidae (barracudas) (Blaber, 1982), Carangidae (kingfish) (Blaber & Cyrus, 1983), Gerreidae (pursemouths) (Cyrus & Blaber, 1982), and ~o~ro~~ f~~~e~~ (red shiner) (Mathews & Hill, 1979). Those species in common with this work have been found to show similar patterns to those obtained in the studies just mentioned, The work of Blaber 8z Blaber (1980) is one of the few estuarine studies in which turbidity and species distribution formed an important part. This work, carried out in Moreton Bay and its associated estuaries on the eastern coast of Australia, provides useful comparison with the results of this study, due to similar species and genera being present at both study sites. BIaber & Blaber (19805 found that the species present could be divided into four main groups according to their ~s~bution aad that the ~s~bu~on patterns found corresponded closely with the turbidities present in the system. Blaber & Blaber (1980) also found that fish movement commenced with a drop in turbidity during winter and that juveniles of turbid water species which were not approaching maturity, moved to the upper reaches of the estuaries where turbidities remained relatively high during winter, In Lake St. Lucia, there was also a distinct drop in mean monthly turbidity during winter when mean t~bi~ties of the turbid areas dropped to 20-40 NTU. A complete range of turbidity from < 10 to > 80 NTU was, however, nevertheless present during each month. That fish movement related to turbidity occurred within St. Lucia on an almost daily basis has been shown by the fact that large numbers of clear-water species have on occasions been caught on the “turbid” western shores when the weather was calm and turbidities on the west were low, ~20 NTU (Cyrus, 1984). Mathews & Hill (1979) found that juvenile Notropis lutrensis occupied a different turbidity range to that of the ad&s. They postulated that this led to a r~uction in competition between the diierent age groups. BIaber & Blaber (1980) found a simiIar situation in Moreton Bay, where two of their turbidity groups consisted of species where adults occupy areas of low turbidity while juveniles occur predominantly in h&b turbidities. Both adults and juveniles of another group were found in turbid waters and those of a fourth occurred only in clear waters. Although this study concentrated on the

68

D.P. CYRUS AND S.J.M.BLABER

distribution of juvenile marine fishes in estuaries, Blaber & Cyrus (1983) have shown that in the genus Curanx the adults and juveniles of some species occupy different turbidities within the St. Lucia system. The main results from the field work indicate that although turbidity, temperature, and food each exert some influence on the occurrence and distribution of juveniles of the common marine species in an estuarine system such as Lake St. Lucia, turbidity is, overall, the single most important factor. This has been verified by statistical correlations generated from the data collected. The juveniles of different species show a preference for a particular range of turbidity, and this has led to the formulation of five groups with distinct species compositions. The results obtained show good agreement with the limited work on the influence of turbidity, carried out in estuaries in other parts of the world.

ACKNOWLEDGEMENTS

We are grateful to the Natal Parks Board for permission to work in areas under their control. The financial assistance of the South African National Committee for Oceanographic Research is gratefully acknowledged. We would also like to thank D.J. Lubinsky of the National Research Institute for Mathematical Sciences for assistance with the statistical analysis of the data, D. Hay, N. Kure, T. Martin, and R. Cyrus for assistance in the field, and the Universities of Natal and Zululand for use of facilities. The figures were produced by the Language and Audio Visual Centre of the University of Zululand.

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