Nearshore Fish Community of the Port River ... - CSIRO Publishing

Mac Freshwater Res., 1996,47,785-99

Nearshore Fish Community of the Port River-Barker Inlet Estuary, South Australia. I. Effect of Thermal Effluent on the Fish Community Structure, and Distribution and Growth of Economically Important Fish Species G. K. Jones, J. L. Baker, K. Edyvane and G. J. Wright South Australian Research & Development Institute, Aquatic Sciences, PO Box 120, Henley Beach, SA 5022, Australia.

Abstract.

The nearshore fish community of the Port River-Barker Inlet Estuary was sampled between January 1986 and May 1987 using a beach seine to determine the effect of thermal effluent on the community structure and nursery function of the estuary. A total of 41 species was found in the estuary, with decreasing numbers of species with decreasing distance from the thermal outfall. Cluster analyses and multi-dimensional scaling ordination separated the thermally polluted sites from the non-affected sites. During the summer/auturnn period, thermal effluent only affected water temperature and the species compositions in the inner estuary, and the estuary-opportunistic species Aldrichetta forsteri, Arripis georgiana, A. truttacea and Hyporhamphus melanochir avoided the area at this time. During winterlspring months, thermal effluent acted in the opposite way, with A. forsteri attracted to the warmer waters of the inner estuary. The extended growth season for this species and significantly higher growth rates promoting premature movement out of the inner estuary for S. punctata were additional direct effects. These latter effects may alter the population structures of these species by increasing their vulnerability to heavy localized fishing intensity, aggregation of natural predators and point-source pollution. The species composition of the fish fauna of the estuary may also be indirectly affected by the thermal pollution-mediated seagrass loss in the inner estuary and a method is described to test this hypothesis.

Introduction The Torrens Island Power Station is an important seawater-cooled power-generating station for South Australia, situated in the largest estuary (Port River-Barker Inlet) in Gulf St Vincent, and it is adjacent to the City of Adelaide (Fig. 1). The estuary is of considerable regional importance as a nursery area for commercial and recreationally important fish stocks in Gulf St Vincent and beyond (Jones 1984; McGlennon 1992). Several studies have been undertaken on the temperature tolerances of some of the mobile fauna in the area (Neverauskas 1977; Neverauskas and Butler 1982), and Thomas et al. (1986) studied the effect of changes in power production on the benthic communities. However, until the present study, there have been no analyses of the mobile faunal communities and the effect of thermal effluent on them. Although there have been several studies on thermal effects on fish communities in estuarine ecosystems elsewhere (e.g. Can and Giesel 1975; Shapot 1978), investigations of effects on the mobile fish communities have been few (see review: Langford 1990). The present study aimed to investigate the nearshore fish community in this thermally altered estuary through a traditional pollution-gradient approach. In addition to community analyses, the possible effects on the nursery function of the estuary were assessed by examining both the

seasonal fluctuations in size frequencies of economically important species and the relative abundances of the age groups. Together, these approaches can assist in indicating the possible effects of thermal effluent on estuarydependent and -opportunistic species. Estuary-dependent species are those found almost exclusively in the estuary for some part of their life cycle, whereas estuary-opportunistic species benefit from using the estuary through achieving faster growth rates or from low exposure to predation, but are not dependent on estuaries for their survival (Lenanton and Potter 1987). The economically important fish species examined in the present study included Sillaginodes punctata (King George whiting), Aldrichetta forsteri (yelloweye mullet), Arripis truttacea (Australian salmon), Arripis georgiana (tommy ruff), Hyporharnphus rnelanochir (sea garfish) and H. regularis (river garfish). Although gradient analysis may make it difficult to detect factors that may be confounded with the effects of thermal effluent and is limited in its extrapolation to other estuaries, the present study is a preliminary examination and description of the nearshore fish community in a regionally important, but thermally altered, estuary. Finally, the implication of changes in nursery function is discussed in terms of increased vulnerability of economically important species to fishing, pollution and toxic algal blooms.

G. K. Jones et al.

Materials and Methods Study Area The Port River-Barker Inlet estuary is a sheltered water complex of mud flats, mangroves and samphire marshes dissected by narrow channels with only small and intermittent inflow of fresh water. The intertidal sediments range from coarse to fine sand (Thomas et al. 1986). The estuary is flushed diurnally, with a maximum tidal amplitude of 2 m. Water from Gulf St Vincent enters and leaves the estuary from two regions, the Port River and the channel just north of the Section Bank (Fig. 1). It then flows down both sides of Torrens Island, eventually meeting in the area just north of the Eastern Passage (P. Petmsevics, personal communication). Water from the Torrens Island Power Station usually flows through Angas Inlet to Eastern Passage, the North Arm and into the Port River (Thomas et al. 1986). At times, however, some of the heated effluent enters the southern part of Barker Inlet. The power station discharges 3.9 million m3 of heated effluent per day (Thomas et al. 1986). Seagrasses (Zostera muelleri and Heterozostera tasmanica) are found along the intertidal and subtidal edges of the northern channels of Barker Inlet and the Port River adjacent to Torrens Island, and the eastern side of the 'mainland' coast. The rest of the channels are devoid of seagrasses or attached algae, but extensive areas of the floating macroalga Ulva australis are found at times in Barker Inlet. Stands of the mangrove Avicennia marina surround most of Barker and Angas Inlets and the eastern end of the North Arm (Butler et al. 1977). Sampling Strategy The nearshore fish community was sampled over a large part of the estuary by beach-seine. As part of a monthly beach-seine-netting survey that has been running since 1981, during which the relative abundance of

juveniles of a number of economically important fish species at three stations in the Barker Inlet is monitored, two additional sites (Site I, Section Bank; Site 3, Quarantine Station) were sampled between January 1986 and May 1987 (Fig.1). The sampling sites were chosen to cover the whole estuary, and to ensure that the same sampling gear could be used in all areas at similar tidal levels. Two of the sampling sites (Site 4, Angas Inlet; Site 5, North Arm) were within the influence of the thermal effluent described in Thomas et al. (1986) and are referred to throughout this paper as the inner-estuary sites. Sites 2 (King's Beach) and 3 are referred to as the mid-estuary sites and Site 1 as the outermost site. Netting at the five sites was undertaken over two consecutive days every month during the daytime low tide, on days when the greatest tidal amplitude occurred. Sampling Gear The net used was the same as the one described by Connolly and Jones (1996) to sample other nearshore fish communities in South Australia. At each site, the beach- seine net was set in a semi-circle with the aid of a 6m planing-hull net boat, and then hauled onto the mud or sand bank. All fish that accumulated in the central fine-mesh area of the net were sorted and transferred to a holding net of 10-mm mesh. All fish were identified and counted. Total lengths were recorded for all economically important species. For the two garfish species, H. regularis and H. melanochir, the total length was the length from the tip of the upper jaw to the end of the caudal fin. All lengths were measured to the 1 cm below. When large numbers (>loo) of fish were caught, subsamples were scooped from the net and measured but the rest of the fish were identified and counted without being measured. Species counts and length-frequency distributions for the main species were generated. At each site the surface water temperature was measured and a water sample was collected for salinity measurement in the laboratory with an 'Autolab' (Model 601 Mk 3) salinometer. In this paper, values of salinity are expressed as practical salinity units. Analysis of Data

138' 30' E

Mangroves

Fig. 1. Map of the Port River-Barker Inlet estuary showing locations of the sampling sites in relation to the Torrens Island Power Station.

Temperature data. To determine the influence of thermal discharge at the sampling sites, the water temperature at the sampling site nearest to the discharge (Site 4) was regressed against the temperature at each other respective site, and slopes of the regression relationships were compared. Cluster analyses of fish assemblages. To compare species compositions between sampling sites from seasonal and annual data, cluster analyses and multi-dimensional scaling (MDS) ordination were carried out on log-transformed (logl$ + 0.1) species-abundance data (Wilkinson 1990). The severe transformation was chosen so that the few highly abundant species in this suite would not be over-emphasized in the determination of clusters. Normalized Euclidean distance was used as a dissimilarity measure, and the Euclidean matrices of the inter-site dissimilarities for the annual, summer-autumn and winter-spring periods were clustered using the unweighted pair-group method with arithmetic means (UPGMA) algorithm (Field et al. 1982). Non-metric multidimensional scaling was chosen as the ordination procedure, using the normalized Euclidean metric to compute distances between points in the MDS configuration, and Kruskal's F-stress formula 1 (Kruskal and Wish 1978) as the objective function. The cluster analysis was used as a verification measure for the MDS-derived site groupings. To determine which species consistently contributed to the variation between clustered groups, tables similar to that described in table 1 of Clarke (1993) were generated for both seasonal and annual sets of data, using a spreadsheet to compute the 'SIMPER' routine (i.e. the average Bray-Curtis dissimilarity between all pairs of sites making up each cluster group). Association between water temperature and fish species composition was determined by overlaying the water temperature gradients on the MDS species groupings.

787

Nearshore Fish Community. I.

Fish densities. For comparison of the abundances of fish between this study and others using the same sampling gear, fish densities at each site were calculated by converting the average numbers of fish per haul of the net to the number of fish per 100 m2, based on the average area covered by each beach-seine haul at each site (2290 m2). Studies on the efficiency of beach seines (Charles-Dominique 1989) have shown that species- and sizedependent factors should be considered when estimating fish abundances from this gear; however, recent studies (Jones, unpublished) on the efficiency of the beach-seine used in the present study have shown little difference in the efficiency of the gear among sites. During the beach-seining operations, some species were found to be extremely fragile (e.g. H. melanochir exhibited very high rates of mortality), and it is not possible to estimate the relative efficiency of the gear for such a species. Therefore, the densities at each site have not been adjusted upwards and hence should be considered as minima. Size-frequency distributions and the assignment of year classes for the economically importantfish species. Because of the monthly time series and wide separation of the lengths of different age groups, it was possible to calculate the mean lengths of each year class for these species. Different age groups were assigned respective year classes, with a specific year class being that calendar year when the youngest age group sampled was spawned. Information on spawning dates for most of these species is summarized in Jones et al. (1990) and for A. forsteri in Hanis (1968). For S. punctata, the mean lengths of the same age group were compared among sites by Student's t-test analysis.

Results Temperature and Salinity For all sampling sites, the fluctuations in temperature and salinity were similar (Fig. 2), although in warmer months differences of up to 3 in salinity occurred between sites. There was no trend in salinities with distance from the source of thermal effluent. In contrast, both temperature maxima and seasonal variations were most pronounced at the site closest to the outfall (Fig. 2a) and progressively decreased with distance from the outfall (Figs 3 and 4). The linear relationship between temperature at the site closest to the outfall (Site 4) and at other sites was significant in all cases (Table 1, Fig. 3), with higher values of the y intercept with increasing distances from the source of the thermal effluent. The slope of the temperature relationship between the outfall site (Site 4) and a mid-estuary site (Site 3) was much lower than for the other sites, indicating that the thermal effluent affected Site 3 during the cooler months (Fig. 3).

Jan

Mar

May

Jul 1986

Sep

Nov

Jan

1

Mar 1987

May

Month

.

Fig. 2. Fluctuations in (a) water temperatures ('C) and (b) salinity (psu) within the Port River-Barker Inlet estuary, January 1986May 1987.0 Site 1, Site 2, A Site 3, V Site 4, x Site 5.

Fish Communities Data from 41 species (1 elasmobranch, 36 teleosts, 3 decapod crustaceans and 1 mollusc species) were included in the cluster analyses, and the relative abundances of each species and the combined species at each site are shown in Table 2. Total fish densities at Sites 1, 3 and 5 were similar (12.7-14.9 per 100 m2) but relatively low compared with Site 4 (58.0 per 100 m2) and Site 2 (32.0 per 100 m2).At Site 4, the high density was made up almost solely by the high numbers of A, forsteri. The number of species at Sites 1, 2, 3 , 4 and 5 were 31,29, 27, 18 and 22, respectively.

r

Angas lnlet - St I

+ Angas lnlet - St 2 + Angas lnlet - St 3 18

20

22 24 26 28 30 Water Temperature at Angas lnlet PC)

32

34

Fig. 3. Patterns of water temperature within the Port River-Barker Inlet estuary. Regression analyses at inner, mid and outermost sites of the estuary.

788

G. K. Jones et al.

Table 1. Regression equations for relationship between water temperatures within the thermally affected Port River-Barker Inlet estuary (January 1986-May 1987) Y, water temperature ("C) at Sites 1, 2, 3 and 5 respectwely, X, water temperature ("C) at the site closest to the thermal outfall (Site 4) All h e a r regressions sign~ficantat P < 0 001 level

2

Site comparison

Regression equation

Site &Site 5 (inner estuary)

Y = -0.109

+ 0.88X

0,634

Site 4-Site 3 (mid estuary)

Y = -4.109

+ 0.62X

0,617

Site 4-Site 2 (mid estuary)

Y = -6.11

+ 1.014X

0,886

Site 4-Site 1 (outermost site)

Y = -10.70

+ 1.145X

0.778

Multi-dimensional scaling ordination and cluster analysis. Fish species composition and water temperature were closely associated in the thermally affected estuary. Fig. 4 shows the ordination plots of fish distribution and abundance for the annual, summer-autumn and winterspring periods respectively, with species cluster groups superimposed on the left, and water temperature relationships on the right. For the annual and summer-autumn periods, exactly the same clustering and levels of association between sampling sites occurred. Relatively low dissimilarity levels occurred between the thermally affected inner-estuary sites (Sites 4 and 5 ) and the mid-estuary sites (Sites 2 and 3). However, the site-temperature relationship differed between these two periods because the temperature at the mid-estuary sites was more similar to that at the outermost site (Site 1) over the summer-autumn period. For the winter-spring period, the species composition and abundance at the inner-estuary sites (Sites 4 and 5) and Site 3 (mid-estuary) were similar enough for them to cluster together as one group. The distribution of temperatures during this period was similar to that for the annual temperature data. Species contribution tables. Between 7 and 10 species contributed to about 50% of the total calculated variation between cluster groups (Table 3). Appendix 1 explains the meaning and derivation of the columns in the species contribution tables. For the annual data (Tables 3a-3c), the main source of variation between the inner-estuary (Sites 4 and 5 ) cluster and the mid-estuary (Sites 3 and 2) cluster stemmed from the relatively high abundance of Arenogobius bifrenatus, H. regularis and Liza argentea at one or the other of the inner-estuary sites, and the relatively high abundance of A. georgiana and S. punctata at both the mid- and the outerestuary sites (Table 3a). Note the high consistency of A, georgiana (high ratio, low standard deviation), owing to its low abundance at both of the inner-estuary sites, and high

Species list and average densities (No. per 100 mZ)at all sites in the Port River-Barker Inlet Estuary Those species taken comnerc:ally or recreat~onallyare mdicated C or R, respecuvely Table 2.

Species

Fishery

Teleosts Sillaginodes punctata Sillago schomburgkii Sillago bassensis Spratelloides robustus Hyperolophus virtatus Kestratherina esox Atherinosoma microstoma Nesogobius sp. Arenogobius bifrenafus Arripis truttacea Arripis georgiana Enoplosus armatus Pelates octolineatus Aldrichetta forsteri Liza argentea Pseudaphriris cirvillii Acanthopagrus butcher; Rhombosolea taparina Anunotretis rostratus Hyporhamphus melanochir Hyporhamphus regularis Kaupus costatus Stigmatophora argus Meuschenia freycineti Brachyluteres jacksonianus Cristiceps australis Haletta semifasciata Platycephalus speculator Platycephalus bassensis Cynoglossus broadhursti Gynmopistes marmoratus Vincentla conspersa Cynoglanis macrocephalus Tetractenos glaber Diodon nichthemerus Syphognathus argyrophanes

1

2

Site 3

4

0.01 0.01 0.01 0 0 0.01

0.01 0 0 0.23 0.01 0

0.02 0 0 0.55 0 0

0.01 0 0 0.02 0 0

0.01 0.01 0 0.02 0 0

0

0.01

0

0

0

C, R C, R

0.12 0.06 0.02

0 0.15 0.03

0.02 0.02 0.01

0 0.02 0

0 0.19 0

C, R

0.02

0

0.02

0

0

C, R

Elasmobranch Tvgonorrhina guanerius Crustaceans Ovalipes australiensis Portunus pelagicus Nectocarcinus integrijrons Molluscs Sepioteuthis australis Total density (No. per 100 mZ)

14.89

32.01

12.67

58.03

5

14.75

abundance at both of the mid-estuary sites. The main source of variation between the inner, thermally polluted sites and the outermost site is attributable to A. forsteri (Table 3b), although this species was recorded in high numbers at only one of the inner-estuary sites (Site 4) and hence is a poor


Annual species composition II I I

I

I

Summer/autumn species composition

Winterlswrina swecies composition

Dimension 1 Euclidean Dissimilarity Level

18

17

18

19

20

21

22

23

Seasonal average water temperature (OC)

Fig. 4. Non-metric MDS plots and cluster analysis overlays of annual and seasonal species compositions and associated water temperature distribution within the Barker Inlet-Port River estuary: (a) annual, (b) summer-autumn and (c) winterspring.

790

Table 3.

G . K. Jones et al.

Rank order of importance of species contributing to the average variation between cluster groups for annual (a-c), summer-autumn (d-ft and winter-spring (g-i) fish community data (see Appendix 1 for meaning and derivation of the columns)

(a) Annual data: Cluster Group 1 (inner-estuary Sites 4 and 5) compared with Group 2 (mid-estuary Sites 2 and 3) Average dissimilarity between Groups 1 and 2 = 3 1 . 8 4 ~

(4 Summer and autumn data: Cluster Group 1 (inner-estuary Sites 4 and 5) compared with Group 2 (mid-estuary Sites 2 and 3) Average dissimilarity between Groups 1 and 2 = 3 4 . 2 0 ~

Species

Species

Abundance Dbar(i) SDd Ratio Cumulative Group 1 Group 2 bar(i) contribution

Dbar(i) SDd Ratio Cumulative Abundance contribution Group 1 Group 2 bar(i)

(%I --

Arenogobius bifrenatus Hyporhamphus regularis Liza argentea Arripis georgiana Aldrichettaforsteri Sillaginodespunctata Tetractenos glaber

27.7 21.3 16.4 1.0 466.4 21.8 1.0

1.0 0.0 0.1 12.0 90.9 124.8 8.1

3.03 2.60 2.30 2.28 2.07 2.05 1.94

1.37 1.86 2.23 0.52 1.28 0.70 0.85

--

2.21 1.40 1.03 4.36 1.62 2.92 2.28

9 17 24 31 38 44 50

Arenogobius bifrenatus 41.4 Tetractenos glaber 0.0 Hyporhamphus regularis 45.8 Arripis georgiana 0.2 Spratelloides robustus 47.7 Haletta sentifasciafa 0.5 Brachyluteres jacksonianus 0.0

1.3 14.3 0.0 17.2 14.4 18.7 1.2

3.48 3.41 3.28 2.47 240 2.25 0.94

-

-

*37 species contribute to the total variation between Groups 1 and 2.

A29specles contr~buteto the total vanahon between Groups 1 and 2.

(b) Annual data: Cluster Group 1 (inner-estuary Sites 4 and 5) compared with Group 3 (Site 1) Average dissimilarity between Groups 1 and 3 = 5 ~ . 9 6 ~

(e) Summer and autumn data: Cluster Group 1 (inner-estuary Sites 5 and 4)

compared with Group 3 (Site 1) Average dissimilarity between Groups 1 and 3 = 5 9 . 1 3 ~

Species

Species



(%I Aldrichetta forsteri Pelates octolineatus Arenogobius bifrenatus Platycephalus bassensis Nesogobius sp. Spratelloides robustus Hjporhamphus regularis Sillago bassensis Atherinosoma microstoma

(%)

Pelates octolineatus Arenogobius bifrenarus Spratelloidesrobustus Platycephalus bassensis Aldrichetta forsteri Sillago bassensis Hyporhamphus regularis Nesogobius sp. Arripis georgiana Atlierinosoma microstoma

60.8 41.4 47.8 0.0 126.8 0.0 45.8 33.9 0.2 124.8

0.0 0.0 0.0 16.5 4.8 12.7 0.0 4.2 6.7 29.3

5.72 5.11 4.88 4.34 4.28 3.97 3.90 2.89 2.89 2.14

0.89 1.84 1.53 0.26 0.79 0.24 2.53 0.42 0.39 0.46

6.39 2.77 3.19 16.46 5.42 16.46 1.54 6.88 7.39 4.65

10 18 27 34 41 48 54 59 64 81

A37 species contribute to the total variation between Groups 1 and 3 A30 species contribute to the total variation between Groups 1 and 3. (c) Annual data: Cluster Group 2 (mid-estuary Sites 2 and 3) compared with

Group 3 (Site 1) Average dlssimlanty between Groups 2 and 3 = 37 4A Spec~es

V) Summer and autumn data: Cluster Group 2 (mid-estuary Sites 2 and 3)

compared with Group 3 (Site 1) Average dissimilarity between Groups 2 and 3 = 4 0 . 1 5 ~

Abundance Dbar(~) SDd Ratlo Cumulatwe Group 2 Group 3 bar(!) contr~but~on

Spec~es

Abundance Dbar(i) SDd Ratio Cumulative contribution Group 2 Group 3 bar(i)

(%)

(%)

Pelates octolineatus Aldrichetta forsteri Sillaginodespunctata Platycephalus bassensis Spratelloides robustus Tetractenos glaber Sillago bassensis

34.6 91.0 1244 0.1 10.8 8.1 0.0

0.0 2.2 4.5 10.5 0.2 0.0 5.9

3.73 3.67 3.54 2.68 2.37 2.32 2.19

2.11 1.77 1.75 2.10 1.35 2.62 0.63 4.23 0.13 18.92 0.13 17.15 0.52 4.21

10 20 29 36 43 49 55

-

-

Pelates octolineatus Sillaginodes punctata Platycephalus bassensis Sillago bassensis Tetractenos glaber Aldrichetta forsteri Nesogobius sp.

A41 species contribute to the total variation between Groups 2 and 3. A33species contribute to the total variation between Groups 2 and 3.

discriminator between groups. Pelates octolineatus also contributed to the variation, being absent from the outermost site and present in relatively high numbers at the

outfall site. Other consistent contributors to variation included Nesogobius s p , and Taeniomembras microstomus (both species being recorded in high numbers at both

Nearshore Fish Community. I

Table 3.

continued

(g)Winter and spring data: Cluster Group 1 (inner- and mid-estuary Sites 5,4 and 3) compared with Group 2 (mid-estuary Site 2) Average dissimilarity between Groups 1 and 2 = 37.9gA

Species

Abundance Dbar(i) SDd Ratio Cumulative Group 1 Group 2 bar(i) contribution (%)

Arripis georgiana 1.6 Hyporhamphus melanochir 11.0 Aldrichetta forsteri 419.1 Spratelloides robustus 1.7 Sillaginodespunctata 22.4 Haletta semifasciata 0.0 Pelates octolineatus 4.3 Arenogobius bifrenatus 11.3 A

18.6 0.6 266.4 14.0 33.6 3.6 13.6 0.8

2.97 2.75 2.71 2.64 2.31 2.21 2.02 1.98

0.46 6.40 0.62 4.40 1.20 2.25 0.66 4.00 1.24 1.86 0.10 22.49 1.49 1.35 1.59 1.25

8 15 22 29 35 41 46 51

34 species contribute to the total variation between Groups 1 and 2

(h) Winter and spring data: Cluster Group 1 (Sites 5, 4 and 3) compared with Group 3 (Site 1) Average dissimilarity between Groups 1 and 3 = 53.67A Species


(%I Aldrichetta forsteri Nesogobius sp. Arenogobius bifrenatus Platycephalus bassensis Haletta semifasciata Liza argentea Pelares octolineatus A

419.1 24.6 11.3 0.1 0.0 16.0 4.3

0.0 0.0 0.0 6.3 3.7 0.0 0.0

8.04 5.24 3.53 3.16 2.49 2.45 2.44

3.84 1.38 2.27 0.71 0.49 3.47 1.73

2.09 3.80 1.56 4.42 5.11 0.71 1.41

15 25 31 37 42 46 51


(i) Winter and spring data: Cluster Group 2 (mid-estuary Site 2) compared with Group 3 (Site 1) Average dissimilarity between Groups 2 and 3 = 59,32A Species

Abundance Group 2 Group 3

Dbarfi) Cumulative contribution

(%I Aldrichetta forsteri Nesogobius sp. Arripis georgiana Hyporhamphus melanochir Pelates octolineatus Spratelloides robustus Sillaginodes punctata Atherinosoma microstoma A


thermally polluted sites, and in low abundance at the outermost site). Conversely, Sillago bassensis and Platycephalus bassensis were consistent contributors to the

variation between the two groups, owing to their abundance at the outermost site and their absence at the inner-estuary sites during the sampling period. The main species responsible for the separation of the mid-estuary sites from the outermost site (Table 3c) were I? octolineatus, A. forsteri and S. punctata (all recorded in very low abundance at the outer site and in relatively high abundance at each of the two mid-estuary sites). Most of the species responsible for the annual patterns of species groupings were also prevalent in the same areas during the summer-autumn period (Tables 3d-38. The variation between the inner-estuary sites and the midestuary sites was attributable mainly to A. bifrenatus, Tetractenos glaber and H. regularis, but only 2: glaber was a consistent contributor (being found in high numbers at both inner-estuary sites and absent from the mid-estuary sites). The same species were also consistent indicators in the annual patterns of variation between the inner-estuary sites and the outermost site (Table 3e). The main species contributing to the variation between mid-estuary Sites 3 and 2 and the outermost site (Site 1) were I? octolineatus, S. punctata, I? bassensis and S. bassensis. During the winter to spring period (Tables 3g-3i), Site 2 and Site 1 formed two separate groups according to the abundance data, and the remaining mid-estuary and innerestuary sites (3, 4 and 5) clustered together. The main species responsible for the variation between Sites 3, 4 and 5 and Site 2 were A, georgiana, H. melanochir and A. forsteri. For example, A. georgiana was found in consistently low abundance at Sites 5, 4 and 3, and was relatively abundant at Site 2. The most consistent contributor to the variation between this group was Haletta semifasciata, owing to its absence at all of the former sites. For the cluster groups with most variation during the winter-spring period (Sites 4, 5 and 3 v. Site 1; Table 3h), much of the variation is accounted for by the abundances of A. forsteri (absent from Site 1 and generally with high abundance at the other sites), Nesogobius sp. (relatively abundant at Sites 3, 4 and 5), and I? bassensis and H. semifasciata (each virtually absent from the inner estuary and more abundant at Site 1). Species partitioning also occurred between Site 1 and the mid-estuary Site 2 during winter-spring, mainly owing to the relative abundance of A. forsteri and Nesogobius sp. at Site 2 and their absence from Site 1 (Table 3 9 . H. melanochir was relatively abundant only at Site 1. Between-region Comparisons in Seasonal Size Composition and Abundance of Economically Important Species Two general effects were detected for a number of the species.

G. K. Jones et aL

Outermost site (Site 1)

Mid estuary (Sites 2 8 3)

Inner estuary (Sites 4 8 5)

SiNaginodes punctata

Aldrichetfa forsteri 83 yc

.t . .+

84 yc

'I 85 yc

Arripis georgiana

Sillaginodespunctata. The smallest fish (2-3 cm, 1986 year class) were first caught in October and November in both the inner and mid regions of the estuary. Thereafter, growth rates in these two regions differed, and by May 1987 these fish had grown to an average length of 15.1 (+ 1.7) and 13.2 (+ 0.4) cm respectively. The difference is significant (P < 0.001). The length-frequency data of the 1985 year class show that relatively smaller fish remained in the mid estuary for a longer time than the faster-growing fish in the inner estuary; this suggests size-dependent movement out of the inner estuary. By the time that the new year class of fish were first caught, however, relatively few fish of the 1985 year class remained in all these areas (Table 4). In the outer estuary, the opposite occurred with small numbers of the 1986 year class and mainly fish from the previous year class when water temperatures were at their highest. Fish from both year classes remained in this area until April 1987. Small numbers of the 1984 year class (length 26-30 cm) were caught during April-July in the mid estuary; however, it is likely that most of these fish would be found in the deeper channels of the estuary and marine waters, as shown from mark-recapture data from Gulf St Vincent (D. Hall and K. Evans, personal communication).

Arripis truttacea

Hyporhamphus melan~ rir 85(f)Yc 65(2) yc

Hyporhamphus regula

Jan 86

Jut 86

Jan 87 Jan 86

(1)With the exception of H. melanochir and H. regularis, all economically important species studied were first caught in the beach seines during the winter-spring period. The mesh size of 10 mm precludes the capture of postlarval fish. Some species, such as S. punctata, enter the estuary as larvae from April onwards and overwinter as postlarvae (A. Fowler and D. Short, personal communication), whereas others, such as A. truttacea and A, georgiana, enter as small juveniles and are vulnerable to the beach-seine as soon as they enter the estuary (Lenanton et al. 1991). ( 2 ) All the species studied, with the exception of H. regularis, avoided the most thermally affected region of the inner estuary (Sites 4 and 5) during the warmest months (January and February 1987). The monthly length-frequency data for S. punctata, A. forsteri, A. georgiana, A. truttacea, H. melanochir and H. regularis are shown for the three regions in Fig. 5 and Table 4.

Jul 86

Jan 87 Jan 86

Jul 86

Jan 87

Fig. 5. Changes in the size of year classes of Sillaginodes punctata, Aldrichetta forsteri, Arripis georgiana, Arripis truttacea, Hyporhumphus melanochir and H. regularis in outer, mid and inner estuary.

Aldrichetta forsteri. This species was found mainly in the mid and inner estuary, with five and three age groups, present respectively. Youngest fish were caught in the estuary in July 1986 and in May 1987. In the mid estuary (Sites 2 and 3), little growth of fish from the 1986 year class occurred until late December (average length 5.9 cm); however, average size increased thereafter. In the inner estuary, the average size began to increase one month earlier, with a mean length of 10.7 cm in December 1986.


During the winter-spring period, the 1985 year class was absent from the mid estuary, but this and the 1984 year class were found in high abundances in the inner estuary, and growth continued during this period. Numbers of these fish again decreased during January-March 1987 but increased once more in April.

Arripis georgiana. Recruitment of the 1986 year class occurred at all sites in October-November when they were 6-7 cm length, and growth rates were similar for all areas. No year classes were caught in the inner estuary when the water temperature was higher than 26°C; however, fish from both year classes were found in relatively large numbers at the outermost site. Fish from the 1985 year class were found during the cooler months in the inner estuary and during warmer months at all other sites. Arripis truttacea. Abundance fluctuates markedly from year to year (Cappo 1987), and the total numbers caught during the study period were relatively low in comparison with other years (Lenanton et al. 1991). The smallest fish (6 cm) were caught in the inner estuary in August 1986 and fish of the 1986 year class remained in this region until October 1986. Small numbers were also caught in the mid estuary in October and November. Fish of the 1985 year class were found during the cooler months (May-September 1986) in both the inner and mid estuary. Little can be interpreted from the small number of fish caught at the outermost site. Hyporhamphus melanochir. Abundances were similar at all sites. Two cohorts of the same year class recruited; fish from the first cohort were caught in May-July 1986 and from the second in February-March 1987. At all sites the relative abundances of fish from the second cohort were higher. Relatively low numbers of both year classes were caught in the inner estuary during January and February 1986 and in February 1987. Hyporhamphus regularis. This estuary-dependent species was found only in the inner estuary, and highest numbers occurred at inner-estuary Site 5 when water temperatures were at their highest in February 1986. The 1985 year class remained there until June 1986. The 1986 year class was caught between February and May 1987, but in lower abundances than the previous year class. Discussion Three broad factors determine the distribution and abundance of fish in the Barker Inlet-Port River Estuary: life-history patterns, direct environmental effects (water salinity, temperature) and indirect environmental effects such as habitat differences. These factors can be influenced by the thermal effluent from the Torrens Island Power Station.

Life History Patterns According to the definition of estuary-dependent, estuary-opportunistic species and marine stragglers (Lenanton and Potter 1987), the following species that were confined to the inner and mid estuary were considered to be estuary dependent: H. regularis, A. bifrenatus, Acanthopagrus butcheri and Liza argentea (Table 2). Marine stragglers included S, bassensis, P: bassensis, Hyperlophus vittatus and Kestatherina esox, found in areas where thermal effluent either was not detected (Site 1) or was relatively minor (Site 2). The estuary-opportunistic species included the juveniles of S. punctata, A. georgiana, A. truttacea and H. melanochir, and both juveniles and adults of Sillago schomburgkii and A. forsteri (Jones 1984). Because these estuary-opportunistic species were found at times throughout the estuary, they were considered to be the best ones to indicate any effect of thermal effluent. Direct Environmental Effects (Salinity and Temperature) Many studies have indicated that salinity is a prime factor in determining the distribution and abundance of fish in estuaries (sensu strictu) with significant freshwater influx (e.g. Loneragan and Potter 1990). The present study found that salinity varied seasonally between 35 and 41, but with no significant geographic variation during winter months, and with only slightly higher salinities in the inner and mid parts of the estuary during the warmer months. Salinity was not an important determinant of the distribution or abundance of the fish fauna in the Port River-Barker Inlet estuary: although the greatest differences in salinity occurred between the mid-estuary sites (Sites 2 and 3) during the summer-autumn period, the fish fauna of the two sites was similar at this time; during the winter-spring period, there was little difference in salinity among all sites yet the fish fauna at Site 2 clustered as a separate group from all other sites. On the other hand, water temperature (as a function of the distance from the source of the thermal effluent) influenced the fish fauna of the estuary during both the summerautumn and winter-spring periods, albeit in opposite ways. For the warmer summer-autumn period, the inner-estuary sites (Sites 4 and 5) were the only sites affected by the thermal effluent, with average temperatures as high as 29°C. The fish species compositions were most similar at these sites (Fig. 4), with A, bifrenatus, Nesogobius sp. and I! octolineatus being the most consistent species to provide variation with the other species clusters. Most of the economically important species (A. forsteri, A. georgiana, A. truttacea and H. melanochir) markedly decreased in abundance of all year classes during the warmer months in the thermally affected area. During the cooler winter-spring period, the water temperatures at all four sites inside the estuary were, on

Sites 2 and 3 Sillaginodes punctata 84 Yc 85 Yc 86 Yc Aldrichetta forsteri 82 Yc 83 Yc 84 Yc 85 Yc 86 Yc 87 Yc

Jan.86

Feb.

Mar.

Apr.

May

June

July

Aug.

Sept.

Oct.

Nov.

Dec.

Jan.87

Feb.

Mar.

Apr.

May

Relative abundance (average number per net haul) of the six economically important species taken outside Barker Inlet (Site I), the mid estuary (Sites 2 and 3) and inner estuary (Sites 4 and 5) from January 1986 to May 1987 (This table accompanies Fig. 5)

Site 1 Sillaginodes punctata 84 Yc 85 Yc 86 Yc Aldrichetta forsteri 82 Yc 83 Yc 84 Yc 85 Yc 86 Yc 87 Yc Arripis georgiana 84 Yc 85 Yc 86 Yc Arripis truttacea 85 Yc 86 Yc Hyporhamphus melanochir 85-1 YC 85-2 YC 86-1 YC 86-2 YC

Table 4.


G. K. Jones et al.

average, higher than at Site 1 and were affected to varying degrees by the thermal effluent. The temperature range at Site 1 during this period was indicative of the temperature range in other nursery areas within Gulf St Vincent at the same time (Jones, unpublished). The greatest degree of similarity amongst species compositions occurred at the inner- and mid-estuary Sites 4, 5 and 3, and these were similar to the other site of the mid estuary (Site 2). In contrast to the summer-autumn period, thermally affected waters attracted A, forsteri, with highest densities in winter-spring occurring in the most thermally affected inner estuary (Sites 4 and 5). This species was also the most abundant species at Site 2, whereas very low numbers were recorded at Site 1. The direct effects of temperature on the distribution of fish and other fauna in the thermally affected area of the estuary can also be demonstrated by observing the changes in the distributions with the increase in power production, and hence, flow of thermal effluent into the estuary. Between 1967 and 1979, the rate of discharge of thermal effluent from the power station was 1.9 million m3 per day, at 6-7°C above the intake temperature; however, additional power production occurred in 1979, increasing the flow of cooling water to 3.9 million m3 per day and at temperatures up to 10°C above intake temperature (Thomas et al. 1986). During the initial period, several studies were undertaken on the thermal tolerances of fish and crustaceans. R. Ainslie (personal communication) carried out preliminary surveys on fish with set gill-nets of various mesh sizes at varying distances from the source of thermal effluent; he found that in waters closest to the source, L. argentea was attracted and A. forsteri avoided the area. However, A. forsteri was found in relatively large numbers further away from the source. Neverauskas (1977) also found that the shrimp (Leander

Table 5.

serenus) demonstrated avoidance and attraction behaviour during summer and winter months, respectively; however, Neverauskas and Butler (1982) found that during the period prior to increase in power production, the distribution of blue swimmer crabs (Portunus pelagicus) was not determined by the thermal effluent, and ambient water temperatures did not exceed their lethal temperatures (39.6"C). Thomas et al. (1986) studied the changes in intertidal benthic fauna over the time that the power production increased; they found decreased species diversity, increased dominance of a 'tropical' polychaete worm and increased area affected by the thermal effluent. The direct effects of temperature on the distributions of some fish species were observed in Angas Inlet prior to the increase in power production in 1979 (Ainslie, personal communication). However, these data are difficult to compare with those from the present investigation because of the different sampling methods used. Between March 1974 and February 1975, set gill-nets of mesh sizes 3 and 5 cm were deployed in the channels of the estuary and, owing to mesh selectivity, smaller fish were not sampled. Locality 3 of that study was the same distance from the source as was Site 4 of the present study, and a comparison of the species composition, temperature and salinity between the two periods is seen in Table 5. The minimum and maximum water temperatures were lower in 1974-75 (16-30°C) than in 1986-87, and the major species caught were S. schomburgkii, R octolineatus and A. forsten, whereas the species composition in 1986-87 was dominated by A. forsteri. At stations closer to the effluent source, Ainslie found that abundance of L. argentea was positively correlated and A. forsteri negatively correlated with water temperature. Water temperature in February 1975 reached 34°C nearest the source.

Comparison of environmental and species composition data at similar sites between 1974 and 1975 (Ainslie, personal communication) and the present study (Site 4)

Method of sampling

5-cm-mesh gill-net

3- and 1-cm beach-seine

Salinity range

34440.0

36.7-40.5

Species composition (for species >1% of total abundance)

S. schomburgkii P: octolineatus A. butcheri H. melanochir

51.5% 39.4% 4.5% 2.8%

A. forsteri A. microstoma P: octolineatus H. regularis S. neopilchurdus S.punctata C. mucosus S. schomburgkii H. melanochir

Total no. of species

18

Temperature range

19.8-33.2"C

71.1% 8.2% 4.8% 3.4% 3.4% 2.4% 2.2% 2.1% 1.2%


In 1986-87, with the exception of the site closest to the outfall, the overall fish densities from all sites within the estuary (4-32 fish per 100 m2) were in the same range as those estimated for a semi-tropical mangrove and seagrass estuary by Morton (1990) using a beach-seine of similar dimensions. For a fish community of similar species composition over seagrass beds in Western Port, Victoria, Robertson (1980) (using beach-seines up to half the length of those in the present study but similar in mesh size), obtained higher annual average densities (100 fish per 100 m2; adjusted for efficiency of net). In the present study, the relatively high density of 58.03 fish per 100 m2 closest to the outfall was mainly due to the aggregation of A. forsteri during winter-spring (41.23 per 100 m2). The highest density for this species at other sites within the estuary was 7.27 fish per 100 m2 at Site 2, which was near to the extent of the influence of the thermal effluent, and thus the aggregating power of the thermal effluent may be as much as six times. This method could be used in future assessments of the effects of thermal discharge on fish populations. There are no other published studies with which to compare these results. Indirect Environmental Effects on the Distribution and Abundance of Fish Thermal discharge may indirectly affect fish distribution by means of changes in sediment structure that result from increased flow rates (e.g. Saenger et al. 1982); however, we discount this possibility because Thomas et al. (1986) found no association between sediment type and distance from the source of thermal discharge in 1982, which was after the flow rate was increased with the further development of the power station. Another possible indirect effect may result from the possible disappearance of seagrass beds in the thermally affected area. The annual data show increased importance of A. forsteri and other estuarine-dependent species (L. argentea, A. bfrenatus and H. regularis) and diminished densities of S. punctata in the inner estuary. Some species, such as S. punctata, are commonly found over seagrass beds during the newly settled stages of their life history (Robertson 1977; Connolly 1994). The relatively lower densities of this species in the inner estuary (Sites 4 and 5) may be due to the absence of seagrass beds (Zostera and Heterozostera) there. Seagrasses are adversely affected by thermal effluent (Van Tine 1981; Thorhaug et al. 1978; Ainslie et al. 1994). Thermal tolerance experiments on the seagrass species found in Barker Inlet as well as studies on their distribution within other mangrove estuaries in Gulf St Vincent would provide useful corroborative evidence for any indirect effects of thermal discharge. Unfortunately, there is no information on the distribution of seagrasses within Barker Inlet prior to the installation of the Torrens Island power plant.

Consequences of Thermal Efluent on the Fish Nursery Function of the Estuary Species taken in the commercial and recreational fisheries within and adjacent to the estuary, as listed by McGlennon (1992), accounted for 67.6% of the fish density for the combined sites. (The respective percentages for Sites 1-5 are 53.4%, 58.2%, 62.7%, 81.9% and 40.8%.) Moreover, for the estimated landings in 1990-91 by both recreational and commercial sectors (McGlennon 1992), 85% of the total catch was of species that used the estuary for part of their life history. The importance of the area is probably even greater when it is considered that the remaining species may be important in the food chain as prey species (e.g. Atherinosoma microstoma). Seasonal length comparisons of seven economically important species indicated direct effects of thermal discharge, including significantly larger fish of the youngest age group of S. punctata in the thermally affected areas of the estuary, longer growth season and marked aggregation of A. forsteri during the cooler, winter months, and the avoidance of thermally affected regions by several species (H. melanochir, Arripis truttacea and A. georgiana) when water temperatures exceeded 26°C. There are several consequences of aggregation and avoidance by these species on their population dynamics. The aggregation of estuary-opportunistic and estuarydependent species to smaller areas may make the populations more vulnerable to a number of environmental and human perturbations such as point-source pollution (chemical spills), toxic algal blooms, or seasonally heavy fishing intensity. All these perturbations occur in the Barker Inlet-Port River estuary. The aggregation of fish during winter months, promoting higher fishing pressure, is a well known phenomenon (e.g. Marcy and Galvin 1973; Shuter et a1 1985). In the Barker Inlet, although the increased fishing effort as a result of the thermal effluent has not been quantified, there is circumstantial evidence to suggest that it does occur for some species. For example, tagging experiments on S. schomburgkii in 1978-79 found that the majority of tagged fish recaptured during the autumn-winter months came from the area of thermal effluent, although some fishing effort still occurred outside the area of thermal influence during the same period. During warmer months, the fish migrated out of the estuary to the southern inshore waters of Gulf St Vincent for spawning (Jones 1981). Another consequence is a decrease in the potential nursery area for estuary-opportunistic species in the inner estuary during the warmer months, so in years of high recruitment to the estuary survival rate of new recruits could be adversely affected. Also, the premature movement of S. punctata out of the nursery area through size-dependent movement may result in vulnerability for a longer time in the main fishing areas

798

G. K. Jones et at.

outside the estuary. McGlennon (1992) found recreational effort outside the estuary on King George whiting to be significantly higher than inside the estuary. The relative importance of the migration of S. punctata from the thermally affected area to the main fishery needs to be evaluated through a mark-recapture experiment. It is also possible that a change in the predator-prey relationship may occur in the thermally affected area. Gray (1990) found that juvenile salmonids were susceptible to heavier predation in the thermally affected areas, resulting in the decreased strength of that year class. One of the main reasons for the seasonally high fishing pressure in the thermally affected area of the Barker Inlet is the attraction of larger predatory fish, such as Argyrosomus hololepidotus, to the warm water area. Further studies are required on the predator-prey relationship between this species and some of the estuary-dependent and estuary-opportunistic species using the area as a nursery. Acknowledgments We are grateful to Mike Retallick and Ken Burrell who provided invaluable support in 1986-87 during field operations. We thank Bob Ainslie and Peter Petrusevics for their comments during the writing of the manuscript, and Alan Butler, Patrick Hone, Gary Jackson and Scoresby Shepherd for their constructive comments on the manuscript. References Ainslie, R. C., Johnson, D. A., and Offler, E. W. (1994). Growth of the seagrass Posidonia sinuosa Cambridge et Kuo at locations near to, and remote from, a power station thermal outfall in northern Spencer Gulf, South Australia. Transactions of the Royal Society of South Australia 118, 197-206. Butler, A. J., Depers, A. M., McKillup, S. C., and Thomas, D. P. (1977). A survey of mangrove forests in South Australia. South Australian Naturalist 51, 34-49. Cappo, M. (1987). The biology and exploitation of Australian salmon in South Australia. Sajish 12(1), 4-14. Carr, W. E. S., and Giesel, J. T. (1975). Impact of thermal effluent from a steam-electric station on a marshland nursery area during the hot season. Fisheries Bulletin 73(1), 67-80. Charles-Dominique,E. (1989). Catch efficiencies of purse and beach seines in Ivory Coast Lagoons. Fisheries Bulletin 87,911-21. Clarke, K. R. (1993). Non-parametric multivariate analyses of changes in community structure. Australian Journal of Ecology 18, 117-43. Connolly, R M. (1994). A comparison of fish assemblages from seagrass and unvegetated areas of a southern Australian estuary. Australian Journal of Marine and Freshwater Research 45, 1033-44. Connolly, R. M., and Jones, G. K. (1996). Determining effects of an oil spill on fish communities in a mangrove-seagrass ecosystem in southern Australia. Australasian Journal of Ecotoxicology (in press). Field, J. G., Clarke, K. R., and Warwick, R. M. (1982). Apractical strategy for analysing multispecies distribution patterns. Marine Ecology Progress Series 8, 37-52. Gray, R. H. (1990). Fish behaviour and environmental assessment. Environmental Toxicological Chemistry 9, 53-67.

Harris, J. A. (1968). The yellow-eye mullet. Age structure, growth rate and spawning cycle of a population of yelloweye mullet Aldrichetta forsteri (Cuv. and Val.) from the Coorong Lagoon, South Australia. Transactions of the Royal Society of South Australia 92, 37-50. Jones, G. K. (1981). Yellowfin whiting (Sillago schomburgkii) studies in South Australian waters. Sajic 5(4), 20-3. Jones, G. K. (1984). The importance of Barker Inlet as an aquatic reserve; with special reference to fish species. Sajic 8(6), 8-13. Jones, G. K., Hall, D. A., Hill, K. L., and Stmiford, A. J. (1990). The South Australian Marine Scalefish Fishery-Stock assessment. Economics. Management. SA Dept Fisheries Report (Green Paper). 186 PP. Langford, T. E. L. (1990). 'Ecological Effects of Thermal Discharges.' (Elsevier Appl. Sci.: London and New York.) 468 pp. Lenanton, R. C. J., and Potter, I. C. (1987). Contribution of estuaries to commercial fisheries in temperate Western Australia and the concept of estuarine dependence. Estuaries 10(1), 28-35. Lenanton, R. C., Joll, L., Pem, J., and Jones, K. (1991). The influence of the Leeuwin Current on coastal fisheries of Western Australia. Journal of the Royal Society of WesternAustralia 74, 101-14. Loneragan, N. R., and Potter, I. C. (1990). Factors influencing community structure and distribution of different life-cycle categories of fishes in shallow waters in a large Australian estuary. Marine Biology (Berlin)106, 25-37. Marcy, B. C., and Galvin, R. C. (1973). Winter-spring sport fishery in the heated discharge of a nuclear power plant. Journal of Fish Biology 5, 541-7. McGlennon, D. (1992). Recreational boatfishing. The 199C-91 metropolitan survey. Sajish 16(3), 4-10. Morton, R. (1990).Community structure, density and standing crop of fishes in a sub-tropical Australian mangrove area. Marine Biology (Berlin) 105, 385-94. Neverauskas, V. P. (1977). Some biological effects from warm water effluent discharged from the Torrens Island Power Station. M.Sc. Thesis, University of Adelaide. 198 pp. Neverauskas, V. P., and Butler, A. J. (1982). Tolerance of blue crab, Portunus pelagicus (L), to high temperature. Transactions of the Royal Society of South Australia 106,215-16. Robertson, A. I. (1977). Ecology of juvenile King George whiting Sillaginodes punctata (Cuvier & Valenciennes) (Pisces :Perciformes) in Western Port, Victoria. Australian Journal of Marine and Freshwater Research 28, 35-45. Robertson, A. I. (1980). The structure and organisation of an eelgrass fish fauna. Oecologia (Berlin)47, 76-82. Saenger, P., Stephenson, W., and Moverley, J. (1982). Macrobenthos of the cooling water discharge canal of the Gladstone Power Station, Queensland. Australian Journal of Marine and Freshwater Research 33, 1083-95. Shapot, R. M. (1978). Initial power plant effects on fish distribution in a small Florida estuary. Proceedings of the Annual Conference of the Southeastern Association of Fish and Wildlife Agencies 32, 529-46. Shuter, B. J., Wimer, D. A., and Matuszek, J. E. (1985).An application of ecological modelling: Impact of thermal effluent on a small-mouth bass population. Transactions of the American Fisheries Society 114,631-51. Thomas, I. M., Ainslie, R. C., Johnson, D. A,, Ofller, E. W., and Zed, P. A. (1986). The effects of cooling water discharge on the intertidal fauna in the Port River Estuary, South Australia. Transactions of the Royal Society of South Australia 110, 159-72. Thorhaug, A., Blake, N., and Schroeder, P. B. (1978). The effect of heated effluents from power plants on seagrass (Thalassia) communities quantitatively comparing estuaries in the subtropics to the tropics. Marine Pollution Bulletin 9, 181-7. Van Tine, R. F. (1981). Ecology of benthic seaweeds and seagrasses in a thermally impacted estuary of the eastern Gulf of Mexico. Proceedings of the International Seaweed Symposium 18,499-506.


Appendix 1.

Derivation and Meaning of the Columns in Table 3

The contents of the species contribution and consistency tables (Table 3) were calculated by using equations and procedures as outlined by Clarke (1993). The f i t column of each table lists in rank order of importance the species that most contribute to the average variation between groups x and y. That variation is calculated as the average Bray-Curtis dissimilarity between all pairs of samples making up groups x and y, and is shown at the top of each table. The second and third columns list the average abundance of each species (averaged over all samples comprising a particular group). The next column, dbar(i), shows the average contribution, in descending order of importance, of each species to the overall variation between the two groups. This column is based on calculation of the dissimilarities for each species between pair-wise comparison of all sites comprising the two groups. The fifth column shows the standard deviation of the Bray-Curtis dissimilarity values for the cross-sample pairing of each species [SDdbar(i)]. The sixth column is the ratio of the fourth and fifth columns:

799

generally, if a species has a high dbar(i) value and a low SDdbar(i) (and therefore a high ratio), it is an important and consistent contributor to the variation between the two groups in question. As discussed by Clarke (1993). consistency refers to the fact that a fairly high abundance of a particular species was recorded in most, if not all, samples (sites) making up a group, and thus that species appears consistently in the cross-sample comparisons between members of the two groups. Conversely, if a species has (for example) a high abundance at just one site of a collection of sites forming a group, it is likely to be a poor discriminator between groupspoor discriminators often have a high SDdbar(i) and a low ratio. The final column is the cumulative percentage contribution of each species to the total Bray-Curtis variation between the two groups. (Often a small percentage of the total species contributes to most of this variation.)

Manuscript received 30 May 1995; revised 15 Febmary and 9 May 1996; accepted 9 May 1996