Environmental Biology of Fishes 45: 133-140,1996. 0 1996 Kluwer .Academic Publishers. Printed in the Netherlands.

Seasonal patterns in a temperate intertidal fish community on the west coast of South Africa

Kim Prochazka Zoology Department and Marine Biology Research Institute, University of Cape Town, Rondehosch, 7700, South Africa

Received 23.6.1994 Accepted 2.3.1995

Key words: Seasonality, Species composition, Density, Diversity

Synopsis

Seasonality was investigated in an intertidal ichthyofaunal community on the west coast of South Africa. Fishes were collected monthly from 12 to 16 intertidal pools between May 1992 and May 1993, using the ichthyocide rotenone. On each occasion sampling effort was divided equally between four biologically deter- mined shore zones, namely the cochlear, lower and upper balanoid and littorina zones. A total of 5 409 fishes belonging to 20 species and five families was collected. All fishes were intertidal residents. No seasonal trend could be found in the total density of fishes during the year, the densities of individual species, species diversity or evenness. Thus species composition remained stable over the year. The proportion of mature individuals of each species decreased in summer as a result of recruitment of juveniles at this time. This almost complete lack of seasonality was attributed to the absence of transient species from this community.

Introduction

Fish inhabiting intertidal rock pools are more di- rectly influenced by seasonal climatic changes than those that remain subtidally in the relatively stable body of the ocean. In addition, it is well known that latitude has a marked effect on the expression of the seasons, with seasonal extremes increasing with lat- itude. Consequently, intertidal fish communities in higher latitudes would be expected to exhibit stron- ger seasonal patterns than their more tropical coun- terparts. Recent research has indicated that this is indeed the case (Thomson & Lehner 1976, Gross- man 1982, Moring 1986). These seasonal trends in- clude changes in species diversity, abundance and recruitment (Thomson & Lehner 1976, Jones &

Clare 1977, Gibson 1982, Grossman 1982, Beckley 1985a, b, Moring 1986,199O).

Although several studies of intertidal fish com- munities have been conducted in the temperate re- gion of southern Africa (see Prochazka & Griffiths 1992 for review), few have dealt with seasonal varia- tions. Those studies that have considered season- ality were conducted at Port Elizabeth in the east- ern Cape (Beckley 1985a, b), which falls within the warm temperate south coast biogeographical prov- ince (Emanuel et al. 1992, Prochazka 1994a). To date no studies of seasonality in the cooler west coast or Namaqua province of South Africa have been conducted, although the composition of the intertidal ichthyofauna of this region has been doc- umented by Bennett & Griffiths (1984) and Pro- chazka & Griffiths (1992). However, small seasonal

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Table 2. Species composition of the fish collected from 173 in- tertidal pools on the South African west coast.

Species Number Percent composition

Batrachoididae Batrichthys apiatus

Blenniidae Parablennius cornutus

Clinidae Blennioclinus brachycephalus Blennophis anguillaris Cirrhibarbis capensis Clinus acuminatus Clinus agilis Clinus berrisfordi Clinus cottoides Clinus heterodon Clinus rotundifrons Clinus superciliosus Clinus taurus Clinus venustris Mureanoclinus dorsalis Pavoclinus pavo Xenopoclinus leprosus

Gobiesocidae Chorisochismus dentex Eckloniaichthys scylliorhiniceps

Gobiidae Psammogobius knysnaensis

Total

1 <l

19 <1

14 <l 19 <1 12 <1

551 10 882 16

12 <l 631 12 284 5

4 <l 1798 33

2 <l 39 1

709 13 5 <1 3 <1

415 8 8 <l

1 <l 5409

patterns in reproductive activity and juvenile recruitment have recently been observed in this community (Prochazka 1994b). The present study thus aims to further investigate seasonal trends in the composition of an intertidal ichthyofaunal com- munity on the South African west coast.

Methods

Intertidal fish were collected at monthly intervals from rock pools at Seapoint (33”55’ S; 18”23’ E) on the west coast of the Cape Peninsula between May 1992 and May 1993. Previous studies of the interti- dal ichthyofauna of the South African west coast (Bennett & Griffiths 1984, Prochazka & Griffiths 1992, Prochazka 1994a) have indicated little spatial variability in community characteristics, and thus it was not deemed necessary to sample more than one

geographical location on the South African west coast. The ichthyocide rotenone was used, dis- solved in acetone in the ratio 100 g rotenone:ll ace- tone, and all comatose fish were collected using handnets. The shore was categorised into four zones on the basis of the fauna and flora present on emergent rock substrata. These included the co- chlear, lower balanoid, upper balanoid and littorina zones, as described for the South African west coast by Branch & Branch (1981). Three or four pools were denuded in each zone every month, resulting in a total of 12 to 16 pools being sampled on each occasion. The stretch of shore that was sampled measured approximately two kilometres in length. Care was taken to ensure that individual pools were not sampled more than once during the year.

All fish captured were identified to species level and measured and weighed to the nearest 0.5 mm and 0.01 g, respectively. The proportions of mature to immature fish of each species were calculated us- ing the data on sex ratios and size at 50% maturity presented in Prochazka (1994b). Resident species were defined as those which carry out their entire life cycle in intertidal pools, while transient species are those that use intertidal pools only during a cer- tain interval of their life history, or are occasional visitors. Species diversity indices were calculated after Odum (1971) as follows:

Margalef species richness index: d = (S-l)/lnN, Shannon-Wiener overall index: H = - ~(njN)ln(n/N), Pielou evenness index: e = H/lnS,

where S is the number of species, N the number of individuals of all species combined and n, the num- ber of individuals of each species.

Results and discussion

The fish community

A total of 5 409 fish belonging to 20 species were sampled from 173 intertidal rock pools over a stretch of approximately 2 km of shore between May 1992 and May 1993 (Table 1). All species caught were intertidal residents. Although this is consis- tent with the findings of Bennett & Griffiths (1984)

25

;5 20b 15

z 10

5

50

40

10

M J J A S O N D J F M A M

Fig. 1. a - The density (in numbers per metre squared) of interti-dal fish over the whole shore in each month of the year . b - Totalarea of rock pool sampled each month .

and Prochazka & Griffiths (1992) for other sites on

the South African west coast, it contrasts markedlywith the findings of other workers both in SouthAfrica (Christensen & Winterbottom 1981, Beckley1985a, b, Bennett 1987, Burger 1990) and elsewherein the world (Tyler 1971, Wourms & Evans 1974,Thomson & Lehner 1976, Jones & Clare 1977, Gib-son 1982, Grossman 1982, Moring 1986, Yoshiyamaet al. 1986, Moring 1990), who all recorded the pres-ence of transient species .

Five families were represented, although mem-bers of the family Clinidae were numerically dom-inant, comprising 92% of the total number of indi-viduals. The Gobiesocidae contributed a further8% while the families Blenniidae, Batrachoididaeand Gobiidae contributed < 1% each . These find-ings are similar to those previously reported for theCape west coast (Bennett & Griffiths 1984, Pro-chazka & Griffiths 1992) . Only seven species con-tributed > 5 % of the total numbers . These were Cli-nus acuminatus, C. agilis, C. cottoides, C, heterodon,C. superciliosus, Muraenoclinus dorsalis and Chori-sochismus dentex . No seasonal trends were evidentin the density of fish, expressed as numbers of fishper metre squared of rock pool, of all species com-

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bined (Fig. 1a). However fish density appeared tobe an inverse function of the total area sampledeach month (Fig. lb), as smaller pools have relative-ly more cover available and hence relatively morefish present per metre squared than do larger pools(Thomson & Lehner 1976, personal observation) .

Species diversity

The Margalef species richness and Shannon-Wien-er overall indices for the whole sampling period(Table 2) were higher than recorded by Prochazka& Griffiths (1992). The greater number of poolssampled in the present study is likely to have beenresponsible for the greater overall number of spe-cies (20) being collected than the 13 and 14 collectedby Bennett & Griffiths (1984) and Prochazka &Griffiths (1992), respectively. However, those spe-cies which contributed > 5% to the total catch werethe same seven species that contributed > 5 % to thetotal numbers of fish caught by Bennett & Griffiths(1984) and Prochazka & Griffiths (1992) . Thus thegreater number of species caught in the presentstudy is made up by species which are rare in therocky intertidal as a whole, or by species whose ge-ographical distribution does not normally includethe area west of Cape Point . The Pielou evenness

Table 2 . Species diversity indices each month for the intertidalfish community of the South African west coast .

Margalefspeciesrichness

Shannon-Wieneroverall

Pielouevenness

May 1992 1.60 1 .71 0.71June 1 .28 1 .81 0.82July 1 .61 1 .90 0.79August 1 .60 1 .90 0.83September 1 .53 1 .64 0.71October 1 .89 1 .98 0.83November 1 .24 1 .77 0.85December 0.98 1 .59 0.81January 2.10 1 .76 0.67February 1 .56 1 .73 0.72March 1 .59 1 .87 0.78April 1 .62 1 .62 0.71May 1993 1 .82 1 .85 0.75Total 2.21 1 .91 0.64

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Cllnus heterodon

MJJASONDJFMAM

Fig. 2. The densities (m” pool area) of seven species of intertidal fish monthly between May 1992 and May 1993.

index, however, was intermediate between previ- ously reported values for this region (Prochazka & Griffiths 1992). No seasonal trends in the above in- dices were observed.

Species composition

Fluctuations in the densities of individual species over the whole shore during the year are depicted in Fig. 2. Although these fluctuated throughout the year, as did the densities of the whole community, no seasonal patterns in density of the component species were evident. Densities were highest in C. supercifiosus (average = 4.05 fish m-‘), followed by C. ugilis (1.99 fish m-‘), A4. dorsalis (1.60 fish mm*), C. cottoides (1.42 fish mm2), C. acuminatus (1.24 fish m-‘), C. dentex (0.94fish rn-‘) and C. heterodon (0.64 fish m”).

Size structure

The proportion of mature fish of each species in each zone over the whole sampling period are pre-

sented in Table 3. Chi* tests were applied to the nu- merical data from each zone to test if the ratios of mature to immature individuals were significantly different from those found for the total population of each species in all four shore zones combined. Mature individuals of C. acuminatus occurred only in the upper balanoid and littorina zones. Although sample sizes were too small to test for significant differences in the cochlear and lower balanoid zones, the data show that the occasional C. acum- in&us found in these zones were always immature. This is consistent with the vertical distribution of this species, which is known to occur almost exclu- sively in pools from mid-tide upwards on the shore (Bennett & Griffiths 1984, Prochazka & Griffiths 1992).

The ratio of mature to immature C. ugilis was sig- nificantly lower in the littorina zone than over the whole shore. This result is again consistent with the vertical distribution of this species on the shore. Studies at three localities on the South African west coast have indicated that this species is more abun- dant lower on the shore, and never reaches the high- est intertidal levels (Bennett & Griffiths 1984, Pro- chazka & Griffiths 1992).

The relative number of mature C. cottoides was greater in the cochlear zone and lower in the lower balanoid zone than was reflected by the population over all four shore zones combined. The results for this species in the littoria zone were inconclusive as few individuals were captured in this zone. Bennett & Griffiths (1984) have demonstrated that C. cot- toides is more abundant in the mid-shore area than in either the 1.0~ or high shore. The results present- ed here would thus suggest that, although adults may occur in all four shore zones, juveniles reach their peak abundance in the lower balanoid zone.

Ratios of mature to immature C. heterodon re- mained constant in the cochlear, lower and upper balanoid zones, and although sample numbers were too small to allow statistical validity, all individuals found in the littorina zone were immature. Both Bennett & Griffiths (1984) and Prochazka & Grif- fiths (1992) mdicated that this species has a wide vertical distribution within the intertidal zone, oc- curring in all but the very highest intertidal pools.

C. superciliosus had a significantly greater rela- tive number of mature fish in the cochlear zone and a greater number of immature fish in the littorina zone. This ddfferential distribution of size classes has been reported previously for this species (Ben- nett & Griffiths 1984, Prochazka & Griffiths 1992). The greater proportion of mature fish at the lowest intertidal levels is consistent with the fact that the largest individuals of this species occur almost ex- clusively in the shallow subtidal habitat at low tide (Prochazka unpublished data).

M. dorsahs had a greater number of mature indi- viduals in the upper balanoid zone than in the lower

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shore zones, and results for the littorina zone were again inconclusive. Although this species is known to reach peak abundance at the lowest intertidal levels (Bennett & Griffiths 1984, Prochazka & Grif- fiths 1992) it would appear that adult fish predom- inate at higher tidal levels.

The proportion of mature C. dentex was highest in the cochlear zone and was significantly lower in the lower balanoid zone than for the whole popula- tion. This result is to be expected, since it is known that the largest individuals of this species remain in shallow subtidal areas during periods of low tide (Prochazka unpublished data).

The proportion of mature individuals of each species was variable throughout the year (Fig. 3) although some weak seasonal trends could be dis- cerned. This proportion was lower in summer (De- cember-February) in C. acuminatus, C. agilis, C. cottoides and C. superciliosus, and in autumn in M. dorselis, than for their respective populations over the whole year (Chi2; p < 0.05). This is likely to be a result of recruitment at this time (Prochazka 1994b), as the influx of recruits will effectively de- crease the ratio of mature to immature individuals in the population. Significantly greater proportions of mature individuals of C. acuminatus, C. agilis, C. superciliosus and M. dorsalis were detected in late winter and early spring, shortly before the start of recruitment. Small sample numbers rendered the data for C. heterodon and C. dentex unsuitable for statistical analysis in most months.

Table 3. The percentage of mature individuals of intertidal fish in four shore zones. Asteriks indicate where the ratio of mature to immature individuals differs significantly from that for the whole shore and hatches indicate where sample sizes were too small to perform statistical analysis.

Species Cochlear Lower balanoid Upper balanoid Littorina Whole shore

Clinus acumincrtus O# O# 26 28 27 Clinus ngilis 68 61 70 30* 66 Clinus cottoides 45* 24* 40 67# 35 Clinus heterodon 32 23 31 O# 28 Clinus superciliosus 36* 22 18 14* 20 Mureanoclinus dorsalis 84 85 96* loo# 86 Chorisochismus dentex 18 7* 40# O# 16

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MJ JASONDJFMAM

Fig. 3. The percentage mature individuals of seven species of intertidal fish in each month between May 1992 and May 1993. The dotted lines indicate percent maturity of the population as a whole. Asterisks indicate where the ratio of mature to immature individuals differs significantly from that for the whole population (Ch?, p c 0.05) and dashes denote where sample sizes were too small for statistical validity.

Conclusions

No seasonal patterns were evident in either the den- sity (Fig. la) or species diversity (Table 2) of the fish community of intertidal rock pools as a whole, or in the density of component species (Fig. 2). This agrees with the other studies on the west coast of South Africa, which yielded similar results for spe- cies diversity and abundance, despite having been conducted at different times of the year (July to September: Bennett & Griffiths 1984, January and April: Prochazka & Griffiths 1992). However, it is contrary to the findings of authors working else- where (‘Tyler 1971, Wourms & Evans 1974, Thomson & Lehner 1976, Jones & Clare 1977, Gibson 1982,

Grossman 1982, Beckley 1985a, b, Moring 1986, Yoshiyama et al. 1986, Bennett 1987, Moring 1990), who all found marked seasonality in the communi- ties they studied. This seasonality in diversity and abundance has been attributed to two major causes. These include extreme low temperatures during winter which prevent some species from occupying intertidal pools during this time, and sometimes re- sult in winter kills (Thomson & Lehner 1976, Jones & Clare 1977, Moring 1990) and the influx of tran- sient species at certain times of year (Tyler 1971, Wourms & Evans 1974, Gibson 1982, Grossman 1982, Beckley 1985a, b, Moring 1986, Yoshiyama et al. 1986, Bennett 1987).

The west coast of South Africa is an area of

139

strong upwelling. This upwelling occurs in summer, and ensures that sea surface temperatures remain relatively constant throughout the year (Branch & Griffiths 1988, Field & Griffiths 1991) with inshore temperatures fluctuating between a minimum of approximately 8” C and a maximum of approxi- mately 16” C within an upwelling cycle (Zoutendyk 1979, Dieckmann 1980, Field et al. 1980, Carter 1982). Temperatures in intertidal pools are thus sel- dom, if ever, so low as to exclude, or to cause the death of, certain species at specific times of the year.

Studies on intertidal fish on the warm-temper- ature south coast of South Africa have indicated the presence of transient species in intertidal pools in this area (Beckley 1985a, b, Bennett 1987). These are predominantly juveniles of deeper water spe- cies which utilise intertidal pools in summer, pre- sumably as a refuge from predation. Collections of fish from intertidal pools on the west coast of South Africa have failed to capture any transient species (Bennett & Griffiths 1984, Prochazka & Griffiths 1992, this study). Many species whose juveniles move into intertidal pools during summer on the south and east coasts of South Africa do not occur on the west ooast. However, several species, such as the mugilid Liza richardsoni, and the sparids Diplo- dus sargus capensis and Sarpa salpa, whose juve- niles are known to inhabit intertidal pools on the south and east coasts (Christensen & Winterbottom 1981, Beckley 1985a, b, Bennett 1987, Burger 1990) do occur on the west coast (Smith & Heemstra 1986). However, their juveniles do not utilise inter- tidal pools in this region. It is hypothesised that this is brought about by differences in the relative pres- sure by different types of predators on the west coast as opposed to the south and east coasts, such that in the la.tter two regions predation by subtidal predators is relatively more severe than by terres- trial predators, while this situation is reversed on the west coast. Although data concerning predation of rock pool fish by birds are available (P.A.R. Hockey personal communication), unfortunately data on predation by subtidal predators are not.

Acknowledgements

Sincere thanks go to those who helped with collec- tions, especially Lesley Prochazka, Yves Lechan- teur and Lisa Kruger, and to Charles Griffiths for commenting on the manuscript. Financial support was in the form of a postgraduate bursary from the Foundation for Research Development and a Foundation for Research Development Core Pro- gramme Grant to G.M. Branch, J.G. Field and C.L. Griffiths.

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