Distribution and Community Structure of Butterflyfishes ( Pisces : Chaetodontidae ) in Southern Mozambique

Even though Mozambique has the greatest reported diversity of butterflyfishes (24 species) of the continental states of the Western Indian Ocean region, aspects of the ecology and distribution of this group in Mozambique are poorly documented. The distribution, diversity and community structure of butterflyfishes were studied on nine reefs in southern Mozambique using the point count method. Nineteen species from four genera were identified. Three with generalist feeding habits (i.e. Chaetodon auriga, C. guttatissimus and C. interruptus) were the most abundant with a wide distribution range. The butterflyfish communities of intertidal reefs at Inhaca Island differed from those of the offshore, subtidal reefs, as shown by uniand multivariate analysis of abundance and diversity data. This is attributed to differences in habitat structure and food availability. However, most species occurred on both reef types. The butterflyfish diversity of the area is considered high and comparable to other high latitude reef areas in the world. These results emphasize the high biodiversity of the region and constitutes a latitudinal biodiversity peak. These findings also highlight the need for effective conservation measures.


INTRODUCTION
Butterflyfishes (Pisces: Chaetodontidae) are conspicuous and important members of the tropical and subtropical fish fauna that live, with few exceptions, in close association with coral reefs (Williams & Hatcher, 1983;Allen et al., 1998).While most butterflyfishes are omnivorous, some species are highly dependant on corals as their main food source (Hiatt & Strasburg, 1960;Hobson, 1974;Harmelin-Vivien & Bouchon-Navaro, 1983).For these reasons, a number of authors have suggested the use of butterflyfishes as bioindicators of reef health (Reese, 1981;Crosby & Reese, 1996;Öhman et al., 1998).Allen et al. (1998) recognized 116 species in ten genera and a diversity peak in the Indo-Pacific (specifically in the central SE Asia region).

Study sites
Southern Mozambique comprises 86 km of coastline between Cabo de Santa Maria (26°05' S; 32°58'E) in the north and Ponta do Ouro (26°51'S; 32°58'E) in the south (Figure 1).The coast is straight and exposed, consisting primarily of extensive sandy beaches.The sandy beaches are interspersed with occasional rocky headlands (Hatton, 1995;Robertson et al., 1996).The continental shelf narrows in the study area, and extends only few kilometers offshore.The study area is strongly influenced by the warm Agullhas Current, which flows in a southerly direction, reaching mean peak velocities of 1.4 m/s (Lutjeharms & Ruijter, 1996).Inshore, northwards flowing, counter-currents are also common, but normally only reach velocities of less than 0.25 m/s (Schumman, 1988).Swell direction is predominantly from the south, attaining heights in excess of 5 m (Schumman, 1988).The annual mean sea surface temperature for the area is 24°C, the tidal cycle is semi-diurnal and the tidal range is between 1.8 and 2.4 m (Robertson et al., 1996).
The offshore, subtidal reefs at Ponta Malongane are classified as patch reefs.The coral communities grow as a thin veneer on late Pleistocene sandstone, which originated from submerged coastal sand dunes (Ramsay, 1994) and are not derived from biogenic accretion.Reefs run parallel to the coastline 1 to 2 km offshore and, as in Kwazulu-Natal (Riegl et al., 1995), they do not reach the surface while lacking most geomorphological characteristics typical of true coral reefs.None of the usual features of true coral reefs (reef crest, or steep reef slopes) are thus present, resulting in homogenous topographic conditions over most of the reef area.The major topographic features are gullies and associated small drop-offs, usually perpendicular to the dominant SE swells.
Reefs are generally small; the width varies between 10 to 600 m and the length between 50 to 1500 m.Soft corals, predominantly Lobophytum, Sinularia and Sarcophyton, are the most abundant biota on the shallow reefs, while hard corals are relatively more abundant in deeper areas.Live coral cover varies from 29 to 70% (Pereira, 2003).The reefs have not been studied in detail but initial surveys have identified 376 species of reef fishes (Pereira et al., 2004), 19 genera of hard and 10 of soft corals (Robertson et al., 1996).Further details of the structure of coral communities of these reefs are provided by Pereira (2003).
The two intertidal fringing reefs at Inhaca Island have been previously studied (Salm, 1976;Nestler et al., 1984;Kalk, 1995).Barreira Vermelha is about 2 km long and 50 m wide, while Ponta Torres is smaller, with a maximum length of 1 km and width of 30 m (Salm, 1976).In contrast to other reefs of the study area, these are dominated by hard corals, with Acropora, Porites, Montipora and Pocillopora being the most abundant genera (Salm, 1976;Kalk, 1995;Perry, 2003).

Field methods and data analysis
Butterflyfish abundance and diversity was estimated using point counts (Bohnsack & Bannerot, 1986) between April 2001 and December 2002.Butterflyfishes encountered within a 7 m radius (154 m 2 ) and up to 5 m above the substratum were counted over a period of three minutes.A total of 256 point counts were performed (Table 1).Fish counts were made between 0700 and 1700 hours to avoid the diurnalnocturnal fish community shift (Halford & Thompson, 1994).After diver entry a period of ten minutes was allowed, prior to fish counts, to allow fishes to resume normal behaviour (Carpenter et al., 1981).
Fish density data was converted to fish/100m 2 prior to analysis.Data were analysed using Kruskal-Wallis ANOVA (Zar, 1999) in the software package Statistica 6.0.A multivariate clustering technique (hierarchical cluster analysis using a Bray-Curtis matrix) was performed on double square root transformed abundance data.

RESULTS
A total of 19 species representing four genera were observed: Chaetodon (15 species), Forcipiger (1 species), Hemitaurichthys (1 species) and Heniochus (2 species).Chaetodon auriga, C. guttatissimus, C. interruptus (previously C. unimaculatus cf.Allen et al., 1998) and C. meyeri displayed a wide distribution ranges, occurring on all reefs (Table 2).Other widely distributed species were C. kleinii, C. lunula and C. trifascialis, which occurred on eight of the study reefs.Species with wide distribution ranges were also the most abundant (Table 2).Three species, C. falcula, C. melannotus and H. monoceros, had a restricted distribution and were only found on the northern reefs (i.e.Baixo Danae, Barreira Vermelha and Ponta Torres; Table 2).
Chaetodon auriga, C. guttatissimus, C. interruptus and C. kleinii were the most abundant overall (Table 3) while C. auriga was the most commonly observed butterflyfish in both intertidal reefs at Inhaca Island (i.e.Barreira Vermelha = 2.0 individuals/100 m 2 and Ponta Torres = 5.3 individuals/100 m 2 ).C. interruptus dominated Creche (1.2 individuals/100 m 2 ), Shallow Malongane (2.0 individuals/100 m 2 ) and Texas (1.9  3).Barreira Vermelha, Ponta Torres and Techo 1, had the most speciose butterflyfish community, each with a total of 14 species identified.The number of species observed on all the studied reefs was similar, with the exception of Techo 2 where only nine species were identified (Table 2).The differences in butterflyfish community structure of the two intertidal reefs at Inhaca Island (Barreira Vermelha and Ponta Torres) and the other offshore, subtidal reefs was confirmed by hierarchical multivariate cluster analysis (Figure 2), showing a clustering of these two reefs at a dissimilarity level of 22%.

DISCUSSION
This study was carried out in southern Mozambique, a remote area that has received very little attention from marine scientists.While other studies have provided species lists (e.g.Robertson et al., 1996;Pereira et al., 2004), this constitutes the first quantitative assessment of butterflyfish distribution and diversity in southern Mozambique.
The butterflyfish communities were dominated both in abundance and distribution range by three species: Chaetodon auriga, C. guttatissimus and C. interruptus (Tables 2 and 3).These species have a wide distribution range throughout the Indian Ocean, and occur as far south as Durban, as reported by Fennessy et al. (1998).Notably, all species are facultative coral feeders (= generalist feeders) and their diets include hard and soft corals, sponges, polychaetes, crustaceans, anemones and algae (Hiatt & Strasburg, 1960;Hobson, 1974;Harmelin-Vivien & Bouchon-Navaro, 1983;Allen et al., 1998).Harmelin-Vivien & Bouchon-Navaro (1983) and Fowler (1990) also reported a dominance of generalist feeders within butterflyfish populations working in Moorea -French Polynesia and One Tree Reef -Great Barrier Reef.The relatively low hard coral cover typical of the offshore, subtidal reefs studied (Pereira, 2003), probably explains the dominance of generalist feeders, suggesting that food availability (Bouchon-Navaro & Bouchon, 1989;Öhman et al., 1998;Williams, 1991) might be an important factor in the butterflyfish community  C. guttatissimus 1.9 ± 0.5 0.4 structure in southern Mozambique.This has been shown to be the case in several studies elsewhere (e.g.Carpenter et al., 1981;Harmelin-Vivien & Bouchon-Navaro, 1983;Bouchon-Navaro & Bouchon, 1989;Öhman et al., 1998).Furthermore, reefs with relatively higher hard coral cover (e.g.Ponta Torres, Techo 1 and Barreira Vermelha; Pereira, 2003;Perry, 2003) had higher butterflyfish species richness and/or abundance (Tables 2 and  3).Further studies relating to diet, food partitioning and niche breadth of the butterflyfish assemblages in this area should be conducted to clarify the relation with coral community structure.Nineteen species of butterflyfishes were identified from a total of 24 species recorded in Mozambique (Pereira, 2000;Pereira et al., 2004).Heniochus diphreutes and Chaetodon bennetti were previously recorded in the area (Kalk, 1995;Pereira et al., 2004), however, neither species were recorded during our dives.At least three other species (i.e.Chaetodon dolosus, C. marleyi and C. zanzibarensis) also occur in the area and they have been recorded on the Maputaland reefs further south (Chater et al., 1993).In general, the diversity of butterflyfishes in southern Mozambique is high and comparable to other high latitude reef areas.For example, Lecchini et al. (2003) reported 22 species from the Ryukyu Islands -Japan and Righton et al. (1996) reported 17 species for the Red Sea.The findings of this study emphasise the high biodiversity of the southern Mozambique region, which, constitutes a latitudinal biodiversity peak attributable to the admixture of tropical and temperate fauna (Schleyer, 1999).Currently the reefs do not benefit from any formal protection and there is need to implement conservation measures in the area.This makes them vulnerable to anthropogenic activities that include irresponsible tourism, illegal fishing and use of destructive fishing gear.

Fig. 2 .
Fig. 2. Clustering (UPGMA) of reefs from southern Mozambique obtained by classification of butterflyfish abundance.The Bray-Curtis dissimilarity index was applied to double square root transformed data.Dotted line shows 22% dissimilarity level.Reef codes as in Figure 1

Table 1 . Details of the butterflyfish sampling effort, location and other characters of the study reefs in southern Mozambique. Reef codes are as in Figure 1. PC = number of point counts
individuals/100 m 2 ).At Kev's Ledge and Baixo Danae, C. kleinii proved to be the most abundant.Finally, C. guttatissimus was the most abundant butterflyfish at both Techo reefs (Table