Frazier, J. G.

Polunin, N. V. C.

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Diving Reconnaissance of 27 Western Indian Ocean Coral ReefsOn a voyage lasting from mid-April to mid-July 1973,from Seychelles to Madagascar (Fig. 1), we SCUBA divedon 27 different coral reefs of which 23 were in remoteu.1-V)Uj/1JyEoRc//4LDACOM ORES^ f/ ^AMfRANTES ^5-BRflFARC/SEYCHELLESMAURITIUSOREUNIONFIG. 1. Sketch-map of the part of the Western Indian Oceantraversed. 1, Mahe. 2, Frigate. 3, Praslin. 4, Poivre. 5, Alphonse.6, Cosmoledo. 7, Asumption. 8, Grande Comore. 9, Moheli.10, Mayotte. 11, Bane du Geyser. 12, Nossi Be. 13, Nossi Mitsio.14, Nossi Iranja. 15, Nossi Radam.oceanic situations. A total of over 50 dives of about anhour each were made—always by the two of us together.Despite the isolation of many of these islands, there wasnevertheless often a strong indication that the reef eco-system had been much disturbed by Man. Thus largepredatory species, particularly of Sharks (Carcharhinidae)and Groupers (Serranidae), were often notably absent, inspite of the fact that in historic times these areas were knownto host vast numbers of such animals (Fauvel, 1909).Furthermore, the larger fishes were often very timid andready to flee from a diver. Human interference with coral-reef ecosystems can be brought about in many ways besidesdirect destruction: well-known instances in this region are,spear-fishing, shark-fishing (cf. W. Travis, Shark for Sale,Allen & Unwin, London, 1961), bottom fishing, and thecollection of marine molluscs—particularly Green Snail(Turbo marmoratus) (cf. W. Travis, Beyond the Reefs,Allen & Unwin, 1959) and pearl oysters (Pinctada spp.).We would like also to record that oil-clots, both small(a few millimetres in diameter) and large (up to several cm),were a consistent feature of the beaches on several of theatolls which we visited—including Alphonse, Cosmoledo,and Aldabra, while on Desnoeufs (Amirante Islands) wefound the remains of an oiled Frigate-bird (Fregata sp.).Only two of the 27 coral reefs investigated were out-standing for their apparent lack of disturbance. One ofthese was the Bane du Geyser, a large horseshoe-shapedreef of which none is exposed above high-tide level, andwhich in origin, together with the lies Glorieuses, is proba-bly part of the Comoros volcanic chain (Guilcher et al.,1965). The abundance there of fish life and, more sig-nificantly, the preponderence of large predators, was quiteremarkable. Sharks (mainly Carcharhinidae), Snappers(Lutjanidae), Groupers, Jacks (Carangidae), and DogtoothTunny (Gymnosarda sp.), were all present in impressivenumbers, and their behaviour, and that of most of the otherfishes associated with be reef, indicated that they had notexperienced any significant contact with Man. The'curiosity' of these fishes, and the closeness to which theyapproached us, were quite uncanny (Fig. 2). On a reeff t - •- •*£»!?••W ''''y'X'i),' .i,,\«' •'"* ' '* •FIG. 2. A group of Unicorn-fish (Naso sp.), averaging ca 60 cmin length, at a depth of approximately 24 metres on a reef offGrande Comore, Comoro Islands. Below to right of middle is seena single caesiodid. Many fishes, such as the Unicorn-fish, werewary of divers on most of the reefs visited in the Western IndianOcean. Photo: N. V. C. Polunin.that has been spear-fished, the fishes 'soon' develop a fearof spear fishermen, and of swimmers in general (Randall,1969). Although it is situated in international waters,Bane du Geyser is afforded a degree of natural protectionby its isolation and its notorious dangerousness to shipping.Our second seemingly unaffected reef is, however, not sucha 'natural reserve,' and so it seems wiser not to mentionits location.While our observations on the prolific fish fauna ofBane du Geyser must be considered in the light of possibleupwellings of nutrient-rich waters, such as occur off the71Environmental Conservation, Vol. 1, No. 1, Spring 1974—© 1974 All Rights Reserved—Printed in Switzerland.https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S0376892900003970Downloaded from https:/www.cambridge.org/core. University of Basel Library, on 11 Jul 2017 at 12:20:22, subject to the Cambridge Core terms of use, available at72 Environmental Conservation• ; « .FIG. 3. A Butterfly-fish (Chaetodon meyeri) about 12 cm longin centre, and a 'forest' of branching corals, at a depth o/ca 7 metresoff Middle Island, Aldabra. A Surgeon-fish (Ctenochaetus cf.striatus) is prominent on right below, and others are visible tothe left. A typical mixture of small fishes is seen in the background.Photo: N. V. C. Polunin.island of Juan da Nova in the Mozambique channel, ourdiscovery of the real paucity of remaining virgin reefs inthis region is nevertheless notable. Clearly the coral reefs,with their great diversity of biota (Fig. 3 above), have atleast a poor degree of homeostasis when it comes to thedepredations of Man. The usual fishing method (a line,a weight, and a hook baited with a strip of fish flesh) by itsvery nature selects predatory species. The reduction inpredators and thus of predation pressure due to continueduse of this method, may have had far-reaching effects onreef ecosystems as a whole. Amongst such effects mayhave been a general reduction in species diversity (Paine,1966). Indeed, in this age of earth-shrinking and earth-stinking, there can be few areas as virgin as these two thatwe were fortunate enough to visit.ReferencesFAUVEL, A. A. (1909). [Unpublished documents on thehistory of the Seychelles Islands prior to 1810.]Seychelles Government Archives, Port Victoria, Mah6.GUILCHER, A., BERTHOIS, L., LE CALVEZ, Y., BATTISTINI,R. & CROSNIER, A. (1965). Les recifs coralliens et leLagon de Pile Mayotte (Archipel des Comores, OceanIndien). Mem. ORSTOM, 8, pp. 1-210.PAINE, R. T. (1966). Foodweb complexity and speciesdiversity. Amer. Natur., 100, pp. 65-75.RANDALL, J. E. (1969). Conservation in the sea: a surveyof marine parks. Oryx, 10(1), pp. 31-8.N. V. C. POLUNINIS Chemin F.-Lehmann1218 Grand-SaconnexGenevaSwitzerland&J. G. FRAZIEREast African MarineFisheries ResearchOrganizationBox 81651MombasaKenya.https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S0376892900003970Downloaded from https:/www.cambridge.org/core. University of Basel Library, on 11 Jul 2017 at 12:20:22, subject to the Cambridge Core terms of use, available at

Diving Reconnaissance of 27 Western Indian Ocean Coral Reefs

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