Indo-Pacific seagrass beds and mangroves contribute to...

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MARINE ECOLOGY PROGRESS SERIES Mar Ecol Prog Ser Vol. 302: 63–76, 2005 Published November 4 INTRODUCTION Throughout the tropics, shallow-water habitats such as seagrass beds and mangroves are favoured as juve- nile habitats by marine fish, and it is assumed that they function as nursery areas (reviewed by Pollard 1984, Parrish 1989, Beck et al. 2001, Sheridan & Hays 2003). It is thought that these habitats can contribute to fish communities on the coral reef through migration of adults or subadults from these nurseries. Results from various studies in the Caribbean support this hypo- thesis and show that the presence of seagrass beds or mangroves significantly influences the composition of the fish community on adjacent coral reefs (Nagel- kerken et al. 2000a, 2002, Dorenbosch et al. 2004, Halpern 2004, Mumby et al. 2004). There is a long-standing debate on whether man- groves and seagrass beds in the Indo-Pacific region function as important juvenile habitats for reef fish (Birkeland & Amesbury 1988, Thollot & Kulbicki 1988, Williams 1991). Many studies suggested the nursery function of mangroves in the Indo-Pacific region to be © Inter-Research 2005 · www.int-res.com *Corresponding author. Email: [email protected] Indo-Pacific seagrass beds and mangroves contribute to fish density and diversity on adjacent coral reefs M. Dorenbosch, M. G. G. Grol, M. J. A. Christianen, I. Nagelkerken*, G. van der Velde Department of Animal Ecology and Ecophysiology, Institute for Water and Wetland Research, Faculty of Science, Radboud University Nijmegen, Toernooiveld 1, 6525 ED, Nijmegen, The Netherlands ABSTRACT: There is a long-standing debate whether mangrove and seagrass habitats in the Indo- Pacific region function as nurseries for coral reef fishes. We studied the use of all major shallow-water habitat types by juvenile coral reef fish using visual census surveys at 4 islands along the Tanzanian coast (East Africa) and at the island of Grande Comoros (Comoros archipelago). We investigated the value of mangroves, seagrass beds, coral reefs, macroalgae and intertidal flats as a juvenile habitat for fish by studying density distribution patterns of juveniles and adults of 76 reef fish species in these habitats. We assessed (1) which part of the reef fish community used mangrove–seagrass habitats as juvenile or adult habitats, (2) whether adult fish densities and diversity on adjacent reefs were related to the presence of these shallow habitats, and (3) whether adults of species that use these habitats when juvenile were less abundant on coral reefs situated far away from these juvenile habitats. Sea- grass beds and coral reefs were the most important juvenile fish habitats. Ontogenetic migrations between seagrass beds and reef habitats possibly occur, since several species showed their highest juvenile densities on seagrass beds, whereas adults showed their highest densities on reefs adjacent to these seagrass beds. The presence of areas with seagrass beds positively influenced adult densi- ties of many reef fish species on adjacent coral reefs. Of the 36 fish species whose juveniles were observed in seagrass beds along the Tanzanian coast, 32 species were absent from or showed low densities on coral reefs of the island of Grande Comoros (lacking seagrass beds or mangroves). On reefs far from seagrass beds and mangroves along the Tanzanian coast, 25 of these 36 species were absent or showed low densities in comparison with reefs adjacent to these habitats. KEY WORDS: Coral reef fish · Seagrass beds · Mangroves · Juveniles · Habitat connectivity · Indian Ocean Resale or republication not permitted without written consent of the publisher

Transcript of Indo-Pacific seagrass beds and mangroves contribute to...

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MARINE ECOLOGY PROGRESS SERIESMar Ecol Prog Ser

Vol. 302: 63–76, 2005 Published November 4

INTRODUCTION

Throughout the tropics, shallow-water habitats suchas seagrass beds and mangroves are favoured as juve-nile habitats by marine fish, and it is assumed that theyfunction as nursery areas (reviewed by Pollard 1984,Parrish 1989, Beck et al. 2001, Sheridan & Hays 2003).It is thought that these habitats can contribute to fishcommunities on the coral reef through migration ofadults or subadults from these nurseries. Results fromvarious studies in the Caribbean support this hypo-

thesis and show that the presence of seagrass beds ormangroves significantly influences the compositionof the fish community on adjacent coral reefs (Nagel-kerken et al. 2000a, 2002, Dorenbosch et al. 2004,Halpern 2004, Mumby et al. 2004).

There is a long-standing debate on whether man-groves and seagrass beds in the Indo-Pacific regionfunction as important juvenile habitats for reef fish(Birkeland & Amesbury 1988, Thollot & Kulbicki 1988,Williams 1991). Many studies suggested the nurseryfunction of mangroves in the Indo-Pacific region to be

© Inter-Research 2005 · www.int-res.com*Corresponding author. Email: [email protected]

Indo-Pacific seagrass beds and mangrovescontribute to fish density and diversity on adjacent

coral reefs

M. Dorenbosch, M. G. G. Grol, M. J. A. Christianen, I. Nagelkerken*,G. van der Velde

Department of Animal Ecology and Ecophysiology, Institute for Water and Wetland Research, Faculty of Science,Radboud University Nijmegen, Toernooiveld 1, 6525 ED, Nijmegen, The Netherlands

ABSTRACT: There is a long-standing debate whether mangrove and seagrass habitats in the Indo-Pacific region function as nurseries for coral reef fishes. We studied the use of all major shallow-waterhabitat types by juvenile coral reef fish using visual census surveys at 4 islands along the Tanzaniancoast (East Africa) and at the island of Grande Comoros (Comoros archipelago). We investigated thevalue of mangroves, seagrass beds, coral reefs, macroalgae and intertidal flats as a juvenile habitatfor fish by studying density distribution patterns of juveniles and adults of 76 reef fish species in thesehabitats. We assessed (1) which part of the reef fish community used mangrove–seagrass habitats asjuvenile or adult habitats, (2) whether adult fish densities and diversity on adjacent reefs were relatedto the presence of these shallow habitats, and (3) whether adults of species that use these habitatswhen juvenile were less abundant on coral reefs situated far away from these juvenile habitats. Sea-grass beds and coral reefs were the most important juvenile fish habitats. Ontogenetic migrationsbetween seagrass beds and reef habitats possibly occur, since several species showed their highestjuvenile densities on seagrass beds, whereas adults showed their highest densities on reefs adjacentto these seagrass beds. The presence of areas with seagrass beds positively influenced adult densi-ties of many reef fish species on adjacent coral reefs. Of the 36 fish species whose juveniles wereobserved in seagrass beds along the Tanzanian coast, 32 species were absent from or showed lowdensities on coral reefs of the island of Grande Comoros (lacking seagrass beds or mangroves). Onreefs far from seagrass beds and mangroves along the Tanzanian coast, 25 of these 36 species wereabsent or showed low densities in comparison with reefs adjacent to these habitats.

KEY WORDS: Coral reef fish · Seagrass beds · Mangroves · Juveniles · Habitat connectivity · Indian Ocean

Resale or republication not permitted without written consent of the publisher

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relatively minor, and have concluded that there is lessinteraction between mangroves and other coastalhabitats than in the Caribbean (Quinn & Kojis 1987,Robertson & Duke 1987, Thollot & Kulbicki 1988,Kimani et al. 1996, Laroche et al. 1997, Huxham et al.2004). Other studies, however, did report the presenceof juvenile coral reef fish in mangrove areas (Lal et al.1984, Little et al. 1988, Robertson & Duke 1990, Wak-wabi & Mees 1999, de Boer et al. 2001). Hence, theimportance of Indo-Pacific mangroves for juvenile fishremains unclear.

Unlike mangroves, seagrass beds in the Indo-Pacificregion are commonly used by juvenile coral reef fish(Jones & Chade 1975, Kimani et al. 1996, Kochzius1997, Gullström et al. 2002, Nakamura & Sano2004a,b). This means that seagrass beds may have agreater value as juvenile habitats for reef fish thanmangroves. So far, however, there have been nostudies in the Indo-Pacific comparing the value ofmangroves, seagrass beds and coral reefs as juvenilehabitats for reef fish using a uniform methodology.

The weakness of the majority of the studies on thevalue of mangroves and seagrass beds as juvenile fishhabitats is that they have examined only a singlehabitat or a few habitats simultaneously. This is animportant limitation, as in the absence of one shallowhabitat, another may take over its function. Moreimportantly, hardly any published study compared fishdensities of juveniles and adults in various shallow-water habitats with those on adjacent coral reefs inorder to determine whether these shallow-water habi-tats could contribute to adult fish populations on thereef. Only Blaber & Blaber (1980) made such compari-son between estuaries and adjacent offshore sandyhabitats, and Nakamura & Sano (2004b) between sea-grass beds and a coral reef. High densities of juvenilereef fish in mangroves and seagrass beds alone do notprove their importance as nurseries, because (1) thesehabitats could merely function as sinks of juveniles ifadults or subadults do not migrate to adjacent coralreefs (Beck et al. 2001), and (2) other shallow-waterhabitats may harbour similar or higher densities ofjuveniles. Various studies have shown that other shal-low habitats can also be used as juvenile habitats byreef fish that are normally known to occur in highdensities in seagrass beds or mangroves (Jenkins &Wheatley 1998, Nagelkerken et al. 2000a, Rossier &Kulbicki 2000, Dorenbosch et al. 2004).

Studying the importance of mangroves and seagrassbeds for juvenile fish requires that all available majorshallow-water habitats are investigated simultane-ously, while densities of fish in the adult habitat(i.e. adjacent coral reef) should also be determined,using a uniform, standardised sampling methodology.Such an approach was used in the present study off the

coast of several islands along the East African coast totest the hypothesis that mangroves and seagrass bedsfunction as important habitats for reef fish in the Indo-Pacific. The main research questions were the follow-ing: (1) To what extent do juvenile and adult coral reeffish use shallow-water habitats present in an area,including mangroves, seagrass beds and coral reefs?(2) Are the densities of coral reef fish populations onadjacent reefs related to the presence of mangrove–seagrass habitats? (3) Are adult coral reef fish that usemangrove–seagrass habitats in high densities as juve-niles less abundant on coral reefs situated far awayfrom these juvenile habitats?

MATERIALS AND METHODS

Study areas. The present study was carried out offthe islands of Mafia, Pemba, Zanzibar and Mbudyaalong the coast of Tanzania (East Africa) and off theisland of Grande Comoros in the Comoros archipe-lago in the western Indian Ocean (Fig. 1). Nine majorshallow-water habitat types were distinguished there(Table 1): mangroves within marine bays (mg); shallowseagrass beds (depth <5 m) situated in marine bays ata distance <500 m from mangroves (bay sg); shallowseagrass beds (depth <5 m) outside bays and adjacentto coral reefs (reef sg); deep seagrass beds (depth>5 m) outside bays and adjacent to coral reefs (deepreef sg); an intertidal flat with patches of small coral,rubble, sand and seagrass beds (intertidal flat); sandflats covered by beds of macroalgae (algae flat); coralreefs adjacent to areas with extensive seagrass bedsand mangroves (sg-mg reefs); coral reefs situated faraway from seagrass beds and mangroves (far reefs);and fringing coral reefs lacking these shallow-waterhabitats off the island of Grande Comoros (Comorosreefs). The average minimum distance from the farreefs to areas with extensive seagrass beds and man-groves (sg-mg areas) was 17.2 km (SD ± 5.8). Far reefsoff the islands of Mbudya, Pemba and Zanzibar wereisolated from sg-mg areas and sg-mg reefs by waterwith a depth >45 m (Fig. 1). Far reefs off the island ofMafia were isolated by a large bare sandy seabed witha maximum depth of 25 m.

Off the islands of Mafia, Zanzibar and Pemba, wesampled mangroves, seagrass beds and coral reefs.Although present, seagrass beds and mangroves offthe island of Mbudya could not be surveyed because ofdifficult access in the field. Coral reefs on GrandeComoros were selected because mangroves and sea-grass beds were completely lacking, allowing thisisland to serve as a true control site to study the effectof the absence of mangrove/seagrass habitats on theadult reef fish population. Algae flats and intertidal

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flats were examined only off Zanzibar. All mangrovesstudied were non-estuarine, normally fell dry duringlow tide, and were dominated by Sonneratia alba. Allsurveyed seagrass beds were dominated by Thalasso-dendron ciliatum (>60% cover), sometimes inter-mingled with Enhalus acoroides, Thalassia hemprichiior Cymodocea rotundata. Algae flats consisted of amixture of the macroalgae Turbinaria sp. and Sargas-sum sp. Sg-mg reefs were predominantly patch reefs

surrounded by sand, seagrass beds or macroalgae. Farreefs consisted of fringing and patch reefs surroundingsand banks. Underwater visibility in seagrass beds andmangroves and on coral reefs close to these habitats(sg-mg reefs) was generally poorer (4 to 12 m) than vis-ibility on the far reefs or Comoros reefs (10 to 30 m).Spring tidal range is approximately 3.6 m for Mafia,Mbudya, Pemba and Zanzibar and 3.3 m for theGrande Comoros (Richmond 2002).

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Fig. 1. Overview of the study area (island of Zanzibar, latitude 6° 10’ S, longitude 39° 10’ E; island of Grande Comoros, latitude 11° 40’ S, longitude 43° 10’ E) and locations of the investigated habitat types

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Field sampling and study design. Reef fish specieswere selected based on our ability to identify them inunderwater visual censuses, as well as on their com-mercial value for fisheries (Jiddawi & Stanley 1997).All species belonging to the families Gerreidae,Haemulidae, Lethrinidae, Lutjanidae, Monodactylidae,Mullidae, Nemipteridae, Plotosidae, Scaridae andSiganidae were included, as well as 2 species ofAcanthuridae (Acanthurus leucosternon and Nasounicornis), 2 species of Chaetodontidae (Chaetodonmellanotus and C. auriga), 4 large-sized species ofLabridae (Cheilinus fasciatus, C. trilobatus, C. undu-latus and Cheilio inermis) and 1 species of Zanclidae(Zanclus cornutus).

Data of the selected reef fish species in all dis-tinguished habitat types were sampled by underwatervisual census using SCUBA and a stationary point-countmethod (Polunin & Roberts 1993, Watson & Quinn 1997).Although Rhizophora mangroves have dense, large proproot structures that diminish efficiency of underwatervisual census of the habitat, Sonneratia alba is charac-terized by a large central stem and short stumpy uprightpneumatophores. The habitat is therefore very acces-sible for observers and underwater visual censusdata can be collected in the same way as in other habitattypes due to its relatively open character. Becauseunderwater visibility differed between the variouslocations, we surveyed 5 × 5 m quadrats in areas with avisibility between 5 and 8 m (mg, bay sg, reef sg, deepreef sg, intertidal flat, algae flat and sg-mg reefs) and8 × 8 m quadrats in habitats where the visibility was>8 m (Comoros reefs and far reefs). Data of each habi-tat were collected simultaneously by 3 observers whoindependently surveyed different quadrats.

Depending on quadrat size, a single rope with alength of 5 or 8 m was used as a visual reference for the

quadrat size. After placing the line, the observerwaited 3 min in order to minimize fish disturbance. Alltarget species within or passing through the quadratwere then counted over a period of 10 min. During thefirst 7 min, the observer waited on the edge of thequadrat, while during the last 3 min, the observermoved through the quadrat to search for small juvenilefish hiding behind corals or other structures. Care wastaken not to count individuals or groups of fish thatmoved in and out of the quadrat more than once.Fish were classified into 2.5 cm size classes using anunderwater slate. Surveys were conducted between09:00 and 16:00 h at either high or low tide, whenwater movement was minimal. Shallow habitats withdepths between 0 and 4 m (mangroves, shallow sea-grass beds, algae flat, intertidal flat and shallow partsof coral reefs) could only be surveyed at high tide.Species identification and quantification were firstthoroughly practiced simultaneously by the 3 ob-servers until their results were similar. Estimation ofsize classes was trained by repeatedly estimating thesizes of 50 pieces of electrical wire of known lengthsplaced at random underwater and representing all fishsize classes. Training was continued until the differ-ences in size estimation were minimal (i.e. a deviationof no more than ±2.5 cm from the actual length forobjects <20 cm). Practicing quantification and sizeestimation was repeated regularly during the visualcensus period.

Habitats were surveyed during the northeast mon-soon in the period September 2003 to February 2004.All habitats at a particular location were sampled untileither the total area of a habitat had been surveyed(if the habitat area at the location was small), or arepresentative part had been surveyed (if the habitatarea was very large).

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Habitat Zanzibar Pemba Mafia Mbudya Grande Total sampledComoros area (m2)

Mangroves 9 (1) 37 (2) 36 (1) – – 2050Bay sg 123 (4) – 101 (2) – – 5600Reef sg 121 (4) 31 (2) 74 (2) – – 5650Deep reef sg 65 (2) – – – – 1625Intertidal flat 262 (2) – – – – 6550Algae flat 125 (2) – – – – 3125Sg-mg reefs 359 (10) 79 (2) 128 (4) 44 (1) – 17941Far reefs 377 (6) 101 (3) 68 (2) 40 (1) – 37504Comoros reefs – – – – 97 (2) 6208

Table 1. Overview of the total number of sampled quadrats per habitat type per island. Numbers in parentheses show the numberof sampled locations per island. Numbers in bold print relate to quadrats with a surface area of 64 m2, other quadrats were 25 m2.Bay sg: shallow seagrass beds (depth <5 m) situated in a bay with mangroves; Reef sg: shallow seagrass beds (depth <5 m) adja-cent to coral reef; Deep reef sg: deep seagrass beds (depth >5 m) adjacent to coral reef; Sg-mg reef: coral reefs adjacent to areascontaining extensive seagrass beds and mangroves; Far reefs: coral reefs situated far from seagrass beds and mangroves;

Comoros reefs: coral reefs on the island of Grande Comoros

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Coral reefs in the study area were divided into patchreefs and fringing reefs. Patch reefs consisted mainlyof 1 coral zone with a relatively constant depth, whilefringing coral reefs consisted of a shallow reef flat, adrop-off and a reef slope. Quadrats on coral reefs weresurveyed in all coral zones that could be distinguished.Coral cover below a depth of 20 m was generally low.Since pilot surveys had revealed that >90% of the indi-viduals of the target species were present betweendepths of 0 and 20 m, deeper reef habitats were notsurveyed. Table 1 lists the total number of sampledquadrats per habitat type per island.

Since fish densities are often correlated with thedegree of habitat complexity (Luckhurst & Luckhurst1978, Öhman & Rajasuriya 1998, Garpe & Öhman 2003),the total coral cover (living and dead stony corals, andliving soft corals) and the maximum height of the corals(in decimetres) were visually quantified for eachquadrat. To estimate coral cover, a quadrat was dividedinto 4 quarters, after which coral cover was estimatedfor each individual quarter and then averaged for thewhole quadrat. When diving circumstances were goodenough, coral reef rugosity was also measured for eachquadrat. A 5 m rope was attached to the substratum inthe quadrat and stretched to obtain a horizontal line.Rugosity of the reef was measured by draping a chainover the corals along the 5 m line. The chain length(in decimetres) was then used as a measure of reefrugosity. The estimations of coral cover, maximum coralheight and reef rugosity in each quadrat were madeimmediately after the visual census. Because the waterdepth of the various coral reefs varied and depth mayinfluence the occurrence of reef fish (Green 1996, Ault& Johnson 1998), the water depth (in metres) of eachquadrat was recorded using a diving computer.

Data analysis. Fish were recorded as juveniles whenthey were smaller than one-third of the maximumspecies length (Nagelkerken & van der Velde 2002).Maximum lengths of species were obtained fromFishBase World Wide Web (Froese & Pauly 2003).Specimens of species with a maximum length >90 cmwere recorded as juveniles when they were <30 cm.For those species whose maturation size was known,the one-third-of-maximum-length rule was found tobe suitable to define juveniles. This was tested forChaetodon auriga, Diagramma pictum, Chlorurussordidus, Lethrinus lentjan, L. nebulosus, Lutjanus ful-viflamma, L. kasmira, L. lutjanus, Parupeneus macro-nema and Scarus psittacus.

In all statistical analyses, each individual quadratwas used as a replicate. For each habitat type, meandensities of juveniles and adults were calculated perspecies (based on all locations per habitat type). Basedon these mean densities, species were classified into7 species groups: seagrass residents (sg residents),

nursery species, seagrass generalists (sg generalists),generalists, reef generalists, reef residents and rarespecies. Criteria used for this classification were arbi-trarily chosen and are defined in Table 2. Although theterm nursery species is often subject of discussion(Beck et al. 2001), we here used the term to describespecies that show high juvenile densities in seagrassbeds and high adult densities on reef habitats.

We also calculated the mean total fish density (pool-ing observed species of all size classes per quadrat)and mean total species richness for each habitat type.Because quadrat sizes varied (25 or 64 m2), speciesrichness could not be directly compared between thesequadrats. To compare habitat types, quadrat size wasstandardized by pooling fish counts of five 25 m2

quadrats (total area 125 m2) or two 64 m2 quadrats(total area 128 m2). Differences in total fish density andspecies richness between the 6 non-reef habitats (mg,bay sg, reef sg, deep reef sg, intertidal flat and algaeflat) and between the 3 reef habitats (sg-mg reefs, farreefs and Comoros reefs) were tested using a non-parametric Kruskal-Wallis test followed by a DunnetT3 post-hoc comparison test (Sheskin 1997). Fish den-sities and species richness of standardized quadratswere used as replicates.

The influences of cover, height and rugosity of thecorals and of the quadrat depth on adult fish densitywere tested by means of multiple linear regressionsfor each species. Juvenile densities were generallytoo low to allow multiple linear regressions. For eachmultiple linear regression, adult fish density perquadrat was used as the dependent variable, andmean coral cover, maximum coral height, reef rugos-ity and quadrat depth as regressors. Possible multi-collinearity between the 4 regressors was investi-gated by 6 additional simple linear regressionsbetween coral cover (regressor) and maximum coralheight, coral cover (regressor) and coral rugosity,quadrat depth (regressor) and coral cover, quadratdepth (regressor) and maximum coral height, quadratdepth (regressor) and coral rugosity and betweenmaximum coral height (regressor) and coral rugosity.Linear regressions were significant between coralcover and maximum coral height (p < 0.001;R2 = 0.04), quadrat depth and coral cover (p = 0.001;R2 = 0.01) and maximum coral height and coralrugosity (p < 0.001; R2 = 0.06). Since R2 values werevery low, multicollinearity between these variablesis considered insignificant.

The influence of the presence of sg-mg areas onadult fish density on coral reefs (sg-mg reefs, far reefsand Comoros reefs) was tested using a general linearmodel (GLM). This was done using 2 analyses: oneGLM compared adult fish density between sg-mg reefsand far reefs, while a second GLM compared adult fish

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density between sg-mg reefs andComoros reefs. These 2 GLMs wererun for each of the 7 species groups(pooling all adult fish species perquadrat) except ‘rare species’(Tables 2 & 3), and for all fish speciesseparately. To increase normality ofthe data set, fish data were trans-formed by y = 122222(0.5 + x)222 . Adult fishdensity was used as the dependentvariable in both GLMs and eachindividual quadrat was used as areplicate. The presence or absence ofsg-mg areas was used as a fixed factorin both models. Because multiple lin-ear regressions showed depth to be asignificant factor for 41% of the spe-cies (see ‘Results’), quadrat depth wasused as a covariable in each GLM.Type III sum of squares was used inboth models. For each performedanalysis, model assumptions werechecked. If the assumptions wereviolated, the analysis was not per-formed. GLMs were not performed forthe ‘rare species’ group. All analyseswere performed using SPSS version11.5.

RESULTS

Species groups and habitatutilisation

Both total fish density and speciesrichness were significantly higher onreef sg than in all other non-reefhabitats (Fig. 2, Table 3). This wasalso true for bay sg, except that nosignificant difference in species rich-ness was found with intertidal flatand algae flat. A comparison of thehabitat types in the mangrove–seagrass bed–coral reef gradient(see Fig. 1) shows a clear spatialtrend (Fig. 2). Both total fish densityand species richness increase frommangroves to reef sg habitats anddecline from sg-mg reefs to Comorosreefs. In this spatial gradient, the bayand reef habitat types located mostclosely to each other (i.e. reef sg andsg-mg reefs) showed the highesttotal fish densities and species rich-

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Species group Occurrence of juveniles Occurrence of adults

Seagrass residents PJD sg-mg > 70% of TJD PAD sg-mg > 70% of TAD

Nursery species PJD sg-mg > 70% of TJD PAD reef > 70% of TAD

Seagrass generalists PJD sg-mg > 70% of TJD Both PAD sg-mg and PADreef between 30–70% ofTAD

Generalists Both PJD sg-mg and PJD Both PAD sg-mg and PADreef between 30–70% of reef between 30–70% ofTJD TAD

Reef generalists Both PJD sg-mg and PJD PAD reef > 70% of TADreef between 30–70% ofTJD

Reef residents PJD reef > 70% of TJD PAD reef > 70% of TAD

Rare species Mean total fish density (pooling juveniles and adultsper quadrat) < 0.20 individuals 100 m–2 (based on allhabitats together)

Table 2. Criteria used to classify fish species into 7 species groups; sg-mg: allseagrass (both shallow and deep) and mangrove habitats, excluding intertidalflats and algae flats; reef: both sg-mg reefs and far reefs, excluding Comorosreefs. PJD: mean partial juvenile density; TJD: mean total juvenile density;PAD: mean partial adult density; TAD: mean total adult density. In partial den-sity, mean fish density (juveniles or adults) of sg-mg habitats or reef habitatsis expressed as percentage of mean total fish density (juveniles or adults).

See Table 1 for habitat abbreviations

Non-reef habitats:Kruskal-Wallis test: p < 0.001Dunnet T3 post-hoc comparisons:

Bay Reef Deep reef Intertidal Algaeseagrass seagrass seagrass flat flat

Mangroves A <0.001 <0.001 NS NS NSB <0.001 <0.001 0.006 <0.001 0.017

Bay seagrass A 0.020 <0.001 <0.001 <0.001B 0.009 0.023 NS NS

Reef seagrass A <0.001 <0.001 <0.001B <0.001 <0.001 <0.001

Deep reef A NS NSseagrass B NS NS

Intertidal flat A NSB NS

Reef habitats:Kruskal-Wallis test: p < 0.001Dunnet T3 post-hoc comparisons:

Far Comorosreefs reefs

Sg-mg reefs A 0.010 <0.001B 0.049 <0.001

Far reefs A <0.001B <0.001

Table 3. Results of statistical comparison (expressed as p-values) of mean totalfish density (A) and mean total species richness (B) between the 6 non-reefhabitats and the 3 reef habitats (Kruskal-Wallis test followed by a Dunnet T3

post-hoc comparison, see also Fig. 2). NS: not significant

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ness, whereas the bay and reef habitats located fur-thest away from this transition (i.e. mg and Comorosreefs) showed lowest total fish densities and speciesrichness.

In total, 76 species of the selected fish families wereobserved in the quadrats in the study area, and thesewere classified into the 7 distinguished species groups(Table 4). The number of species per species groupthat were observed on islands where both reef andnon-reef habitats were sampled (Zanzibar, Mafia andPemba) did not differ much between these islands.Reef residents were represented by the largest numberof species (n = 32, 42% of all observed species,) whilethe other groups were represented by considerablysmaller numbers of species (ranging from 4 to 11 spe-cies). For 36 species (48% of all observed species) thatwere observed in the reef quadrats either as juveniles

or adults, more than 30% of the total juvenile density(see criteria in Table 2) was observed in sg-mg habitats(i.e. all species of seagrass residents, nursery species,seagrass generalists, generalists and reef generalists,see Table 4). According to the criteria listed in Table 2,18 of these species (24% of all species) had their high-est juvenile densities in sg-mg habitats (i.e. species ofsg residents, nursery species and sg generalists), whilethe other 18 species showed similar juvenile densitiesin sg-mg habitats and reef habitats (i.e. species ofgeneralists and reef generalists).

Differences in juvenile densities between seagrassand other habitats were largest for sg residents, nurs-ery species and sg generalists. These species groupsall had their highest juvenile densities in seagrassbeds, and were only observed in low densities in reefhabitats (Fig. 3a). Reef residents, on the other hand,had much higher juvenile densities in reef habitatsthan in non-reef habitats. Although generalists andreef generalists also had high juvenile densities inshallow sg habitats, differences with juvenile densitiesin other shallow-habitat types and in reefs habitatswere considerably smaller. Sg generalists were theonly species group with relatively high densities ofjuveniles in mangroves. Juvenile densities on Comorosreefs were only high for the reef residents.

As regards adults, only sg residents showed muchhigher densities in sg-mg habitats than in reef habitats(Fig. 3b). The nursery species, reef generalists and reefresidents, on the other hand, had much higher adultdensities in reef habitats than in sg-mg habitats. Just asfor juveniles, adult densities on Comoros reefs were onlyhigh for the reef residents. For all other species groups,adult densities on Comoros reefs were low or zero.

Effect of the presence of sg-mg habitats on reef fishdensities

Multiple linear regressions between adult fish den-sity and the 3 coral complexity variables and quadratdepth showed a significant relation for coral cover(20 species), maximum coral height (16 species), coralrugosity (5 species) and quadrat depth (31 species). Allspecies groups except rare species contained speciesfor which multiple regressions were significant. How-ever, in all cases where a significant relationship wasobserved, both the part correlations (correlation of thedependent variable with one of the regressors, wherethe effect of the other regressors on the dependentvariable is controlled) of the individual variables andthe R2 value of the complete model were very low(most part correlations were <0.15, while the highestobserved part correlation was 0.21, and almost all R2

values for the complete model were <0.1).

69

a)

0

50

100

150

200

250

Man

grov

es

Bay sg

Reef s

g

Sg-m

g re

efs

Far r

eefs

Comoro

s ree

fs

Deep re

ef sg

Intert

idal fla

t

Algae f

lat

Mea

n fis

h de

nsity

+ S

E (1

00 m

–2

)M

ean

spec

ies

richn

ess

+ S

E (1

00 m

–2

)

A

A

B

B

C

C

3

2

2

4

1

1

3 3 3

3323

2

Reef habitats

Non-reef habitats

b)

0

2

4

6

8

10

12

14

16

18

Fig. 2. (a) Mean total fish density and (b) mean total speciesrichness in all investigated habitat types, based on standard-ized quadrat sizes (see text). Mangroves, bay sg, reef sg, sg-mg reefs, far reefs and Comoros reefs are arranged accordingto a distance gradient. Mean total fish densities and meanspecies richness are compared for non-reef habitats and forreef habitats using a Kruskal-Wallis test and Dunnet T3post-hoc comparisons (see also Table 4). Different numbersindicate significant differences between non-reef habitats;different letters indicate significant differences between reef

habitats. See Table 1 for habitat abbreviations

Page 8: Indo-Pacific seagrass beds and mangroves contribute to ...penyu.nl/wp-content/02204_Dorenbosch_2005.pdfABSTRACT: There is a long-standing debate whether mangrove and seagrass habitats

Mar Ecol Prog Ser 302: 63–76, 200570

Spec

ies

Mea

n ad

ult f

ish

dens

ity

on s

g-m

g re

efs

Mea

n ad

ult f

ish

dens

ity

Sign

ific

ant m

ulti

ple

GLM

ana

lysi

sgr

oup/

spec

ies

and

far

reef

sa , per

isla

nd (i

nd. 1

00 m

–2)

(ind

. 100

m–2

)lin

ear

regr

essi

onsg

-mg

reef

ssg

-mg

reef

s vs

.vs

. far

ree

fsC

omor

os r

eefs

Zan

ziba

rM

afia

Pem

baM

budy

aSg

-mg

reef

sFa

r re

efs

Com

oros

ree

fsco

vm

axru

gde

pC

ovar

Fixe

dC

ovar

Fixe

d

Seag

rass

res

iden

ts6

66

21.

80.

3(0

)<

0.00

10.

001S-

MN

ON

OC

alot

omus

spi

nide

ns0.

4/0.

0//

0.3/

0.0

0.3

•N

ON

ON

ON

OC

heili

o in

erm

is1.

0/0.

30.

3/0.

40.

8/0.

20.

2/0.

20.

70.

3•

0.00

40.

049S-

MN

ON

OG

erre

s oy

ena

0.3/

0.2

••

•N

ON

ON

ON

OLe

ptos

caru

s va

gien

sis

0.8/

0.1/

0.5

NO

NO

NO

NO

Leth

rinu

s va

rieg

atus

/0.0

0.1/

0.1

/0.0

0.0

0.0

NP

NP

NO

NO

Scar

us s

pp.

0.6/

0.1

NO

NO

NO

NO

Nur

sery

spe

cies

87

77

18.2

12.2

2.4

(3)

0.00

2 <

0.00

1S-M

NS

<0.

001S-

M

Che

ilinu

s un

dula

tus

2.6/

0.0

0.8/

0.0

0.1/

0.1

/0.0

1.7

0.1

0.0

••

NS

<0.

001S-

MN

PN

PH

ippo

scar

us h

arid

0.3/

0.1

3.6/

1.1

0.1/

0.5

/0.1

0.9

0.3

••

NS

NS

NO

NO

Lutj

anus

mon

osti

gma

0.5/

0.1/

0.3

0.3/

0.3

0.0

••

<0.

001

NS

NO

NO

Mon

odac

tylu

s ar

gent

eus

7.7/

0.1

4.5

0.1

NP

NP

NO

NO

Mul

loid

icht

hys

flav

olin

eatu

s0.

6/0.

23.

5/21

.60.

2/3.

40.

6/1.

13.

22.

4•

NS

0.00

8far

NS

NS

Plec

torh

inch

us fl

avom

acul

atus

0.3/

0.1

0.3/

0.1

0.1/

0.1/

1.0

0.2

0.1

•0.

038

0.00

4S-M

NO

NO

Scol

opsi

s gh

anam

4.7/

2.9

6.7/

10.6

15.6

/1.4

26.2

/8.9

8.1

3.9

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S<

0.00

1S-M

NO

NO

Siga

nus

suto

r1.

7/5.

70.

9/6.

30.

3/0.

70.

4/1.

34.

50.

0•

•N

SN

SN

PN

P

Seag

rass

gen

eral

ists

44

43

5.3

1.9

0.0

(1)

NS

0.00

2S-M

NP

NP

Leth

rinu

s ha

rak

1.1/

1.9

4.1/

4.1/

0.8

6.1/

2.5

1.4

0.0

•<

0.00

1N

SN

PN

PLu

tjan

us a

rgen

tim

acul

atus

0.2/

0.0

0.8/

0.1/

0.1/

0.3

•N

ON

ON

ON

OL.

ehr

enbe

rgi

0.3/

/1.0

0.1

0.2

NP

NP

NO

NO

Scar

us g

hobb

an1.

8/0.

15.

7/2.

20.

7/0.

31.

8/2.

50.

4•

••

NS

<0.

001S-

MN

ON

O

Gen

eral

ists

77

65

6.5

6.2

2.4

(4)

NS

NS

0.00

1<

0.00

1S-M

Chl

orur

us s

ordi

dus

2.8/

3.7

6.5/

11.3

0.1/

4.1

0.7/

6.7

3.1

4.9

1.5

••

NS

<0.

001fa

rN

PN

PLe

thri

nus

mah

sena

0.4/

0.0

/0.1

/0.0

0.5/

0.3

0.3

0.1

NS

0.00

4S-M

NO

NO

L. n

ebul

osus

1.1/

0.0

0.2/

0.1

0.7

0.0

0.3

NP

NP

NP

NP

Paru

pene

us b

arbe

rinu

s0.

9/0.

62.

4/1.

42.

2/0.

81.

3/0.

11.

40.

70.

1•

•N

S<

0.00

1S-M

NP

NP

P. p

leur

osti

gma

0.2/

0.3

2.3/

1.1

/0.4

/0.5

0.6

0.4

0.5

••

<0.

001

<0.

001S-

M<

0.00

10.

035S-

M

P. r

ubes

cens

0.1/

0.0

0.8/

0.3/

0.0

0.2

0.0

NS

0.00

5S-M

NO

NO

Scar

us s

cabe

r0.

3/0.

10.

2/0.

30.

1/0.

1/0

.20.

20.

1•

0.00

2N

SN

ON

O

Ree

f ge

nera

list

s11

1110

830

.322

.05.

5(9

)<

0.00

1<

0.00

1S-M

<0.

001

<0.

001S-

M

Cal

otom

us c

arol

inus

0.5/

0.3

0.3/

0.2

/0.0

0.7/

0.3

0.4

0.2

0.2

••

NS

0.00

7S-M

NP

NP

Che

ilinu

s tr

iloba

tus

0.9/

1.1

0.6/

0.2

4.8/

0.6

2.6/

1.5

0.9

0.1

•<

0.00

1N

SN

PN

PC

hlor

urus

str

ongy

loce

phal

us1.

6/0.

60.

4/0.

3/0

.63.

0/0.

41.

20.

51.

0•

•N

S0.

005S-

MN

SN

SLe

thri

nus

lent

jan

1.2/

0.2

0.9/

2.3

0.2/

0.9

0.4

••

<0.

001

<0.

001S-

MN

ON

O

Tab

le 4

.(1)

Sp

ecie

s co

mp

osit

ion

(n =

76)

of t

he

7 sp

ecie

s g

rou

ps

(cri

teri

a li

sted

in T

able

2);

(2) n

um

ber

of o

bse

rved

sp

ecie

s (b

old

) per

sp

ecie

s g

rou

p a

t eac

h is

lan

d (a

ll s

amp

led

hab

itat

s an

d a

ll q

uad

rats

poo

led

, sh

own

in

par

enth

eses

for

Gra

nd

e C

omor

os I

slan

d);

(3)

mea

n a

du

lt f

ish

den

siti

es p

er s

pec

ies

on s

g-m

g r

eefs

an

d f

ar r

eefs

per

isl

and

(al

lq

uad

rats

poo

led

); (

4) m

ean

ad

ult

fis

h d

ensi

ties

on

sg

-mg

ree

fs a

nd

far

ree

fs (

all

isla

nd

s p

oole

d)

and

Com

oros

ree

fs (

all

qu

adra

ts p

oole

d);

(5)

res

ult

s of

mu

ltip

le l

inea

rre

gre

ssio

n b

etw

een

ad

ult

fis

h d

ensi

ty, c

oral

com

ple

xity

an

d q

uad

rat

dep

th; a

nd

(6)

res

ult

s of

Gen

eral

Lin

ear

Mod

els

(GL

M)

wh

ere

the

effe

ct o

f th

e p

rese

nce

of

sg-m

g a

reas

is a

nal

ysed

on

ad

ult

fis

h d

ensi

ties

on

ree

fs.

For

th

e re

gre

ssio

n a

nal

ysis

per

sp

ecie

s, (

•) s

how

s w

hic

h f

acto

r(s)

res

ult

ed i

n a

sig

nif

ican

t re

gre

ssio

n (

p <

0.0

5) w

ith

ad

ult

fis

hd

ensi

ty; a

ll o

bse

rved

R2

valu

es o

f th

e co

mp

lete

mod

els

wer

e <

0.1

0, a

nd

the

hig

hes

t ob

serv

ed p

art c

orre

lati

on w

as 0

.21.

GL

Ms

wer

e p

erfo

rmed

for

6 sp

ecie

s g

rou

ps

and

thei

rin

div

idu

al s

pec

ies.

Res

ult

s of

the

GL

M a

nal

ysis

are

sh

own

for

the

cova

riab

le ‘d

epth

’ an

d th

e fi

xed

fact

or ‘p

rese

nce

of s

g-m

g a

reas

’, w

her

e sg

-mg

ree

fs a

re c

omp

ared

wit

h fa

rre

efs

and

Com

oros

ree

fs r

esp

ecti

vely

. a : d

ensi

ty s

g-m

g r

eefs

/den

sity

far

ree

fs; N

O: s

pec

ies

not

ob

serv

ed a

nd

GL

M n

ot p

erfo

rmed

; NP

: GL

M n

ot p

erfo

rmed

bec

ause

mod

elas

sum

pto

ns

wer

e vi

olat

ed;

NS

: n

o si

gn

ific

ant

dif

fere

nce

; S

-M:

adu

lt f

ish

den

sity

sig

nif

ican

tly

hig

her

on

sg

-mg

ree

fs;

far :

adu

lt f

ish

den

sity

sig

nif

ican

tly

hig

her

on

far

ree

fs;

Com

: ad

ult

fis

h d

ensi

ty s

ign

ific

antl

y h

igh

er o

n C

omor

os r

eefs

; cov

: cor

al c

over

; max

: max

imu

m c

oral

hei

gh

t; r

ug

: cor

al r

ug

osit

y; d

ep: q

uad

rat

dep

th; c

ovar

: cov

aria

ble

‘dep

th’;

fixe

d: f

ixed

fac

tor

‘pre

sen

ce o

f sg

-mg

are

as’

Page 9: Indo-Pacific seagrass beds and mangroves contribute to ...penyu.nl/wp-content/02204_Dorenbosch_2005.pdfABSTRACT: There is a long-standing debate whether mangrove and seagrass habitats

Dorenbosch et al.: Contribution of Indo-Pacific habitats to coral reef fishes 71

L. m

icro

don

0.0/

00.

0.0/

0.1

0.0

0.0

0.6

•N

PN

PN

S0.

001C

om

L. o

bsol

etus

0.4/

0.5

0.4/

0.2

0.1/

0.3

/0.9

0.3

0.4

0.0

•0.

001

NS

NP

NP

Lutj

anus

fulv

ifla

mm

a14

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2.3

46.2

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04.

9/17

.91.

0/4.

918

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4<

0.00

1<

0.00

1S-M

NP

NP

Paru

pene

us in

dicu

s0.

1/0.

00.

10.

0•

NP

NP

NO

NO

P. m

acro

nem

a3.

3/5.

013

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2.2/

1.9

1.0/

6.6

5.2

4.9

2.5

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031

NS

NS

0.00

4S-M

Scar

us p

sitt

acus

0.8/

0.9

3.4/

16.3

2.6/

0.6

0.5/

0.2

1.6

2.6

0.2

•N

SN

SN

PN

PSi

ganu

s st

ella

tus

0.1/

0.2

0.9/

0.2

/0.2

0.9/

0.3

0.3

0.2

0.5

NS

0.01

5S-M

NS

NS

Ree

f re

side

nts

3232

3025

29.1

32.1

30.2

(19)

<0.

001

NS

<0.

001

NS

Aca

nthu

rus

leuc

oste

rnon

1.1/

0.5

0.2/

1.4

/1.0

/0.9

0.7

0.7

2.8

••

NS

NS

0.01

5<

0.00

1Com

Cet

osca

rus

bico

lor

0.1/

0.3

0.0/

0.2

/0.1

0.1/

0.0

0.1

0.3

0.5

0.00

1<

0.00

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NS

<0.

001C

om

Cha

etod

on a

urig

a1.

2/0.

53.

4/1.

74.

2/0.

62.

9/0.

42.

20.

70.

5•

•0.

001

<0.

001S-

M<

0.00

10.

042S-

M

C. m

ella

notu

s0.

9/0.

62.

2/1.

00.

6/0.

60.

8/0.

81.

10.

70.

3•

NS

0.00

6S-M

NS

0.01

0S-M

Che

ilinu

s fa

scia

tus

0.6/

0.4

0.2/

0.1

/0.1

0.2/

0.0

0.4

0.3

0.00

10.

012S-

MN

ON

OC

hlor

urus

atr

ilunu

la0.

3/0.

41.

1/4.

7/0

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.30.

41.

0N

S0.

009fa

rN

ON

OC

. jap

anen

sis

/0.1

/0.1

0.1/

0.0

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0.0

0.1

••

•N

S<

0.00

1far

NO

NO

Dia

gram

ma

pict

um0.

1/0.

01.

4/1.

10.

2/0.

00.

2/0.

00.

40.

1N

PN

PN

ON

OG

nath

oden

tex

aure

olin

eatu

s0.

0/1.

48.

6/17

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82.

912

.4•

NS

NS

NS

<0.

001S-

M

Gym

nocr

aniu

s gr

ando

culis

0.1/

0.0

/0.1

0.1

0.0

0.0

•N

PN

PN

PN

PLe

thri

nus

eryt

hrac

anth

us0.

1/0.

00.

0/0.

2/0

.10.

10.

1•

0.00

5N

SN

ON

OLu

tjan

us b

ohar

0.6/

0.0

0.1/

0.3

/0.9

/0.2

0.4

0.2

0.1

NS

NS

NP

NP

L. g

ibbu

s0.

2/0.

20.

4/9.

4/0

.20.

21.

2N

S0.

004fa

rN

ON

OL.

kas

mir

a0.

4/2.

50.

8/0.

10.

41.

60.

0•

0.01

8N

SN

PN

PL.

lutj

anus

8.5/

12.2

/12.

2/3

1.3

5.0

11.4

••

<0.

001

NS

NO

NO

Mac

olor

nig

er2.

4/0.

00.

5/1.

0/0

.01.

50.

1•

0.03

5<

0.00

1S-M

NO

NO

Mon

otax

is g

rand

oloc

us0.

6/0.

45.

3/0.

3/1

.00.

9/2.

01.

50.

62.

1•

NS

0.00

4S-M

NS

NS

Mul

loid

icht

hys

vani

cole

nsis

1.4/

2.8

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Mar Ecol Prog Ser 302: 63–76, 2005

The GLM comparison between sg-mg reefs andComoros reefs of the effect of the presence of sg-mgareas on the adult fish densities per species groupshows that densities of nursery species, generalists andreef generalists were significantly lower on Comorosreefs (Fig. 3b, Table 4). Sg residents were not observedon Comoros reefs at all, and densities of sg generalistswere almost zero (not enough observations to perform

a GLM). Also, total fish density was significantly loweron Comoros reefs (Fig. 2). The GLM comparison ofadult densities of species groups between sg-mg reefsand far reefs shows that densities for all species groupswere significantly higher on sg-mg reefs than on farreefs, except for generalists and reef residents (Fig. 3b,Table 4). Also, total fish density was significantly loweron far reefs (Fig. 2).

72

a)

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Nurseryspecies

Seagrassgeneralists

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Mea

n d

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ty o

f juv

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Seagrassgeneralists

Generalists Reefgeneralists

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Mea

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ults

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00 m

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Fig. 3. Mean total densities of (a) juveniles and (b) adults per species group per habitat type. Significantly different adult densities(p < 0.05, tested with GLM) between sg-mg reefs and Comoros reefs are indicated by different letters, while significant different

densities between sg-mg reefs and far reefs are indicated by different numbers. See Table 1 for habitat abbreviations

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Dorenbosch et al.: Contribution of Indo-Pacific habitats to coral reef fishes

The GLM comparison of adult densities between sg-mg reefs and Comoros reefs at the level of individualspecies shows that 32 of the 36 species whose juvenileswere observed in sg-mg habitats (sg residents, nurseryspecies, seagrass generalists, generalists and reef gen-eralists) were either absent from the Comoros reefsor were observed in very low densities (not enoughobservations to perform a GLM) or significantly lowerdensities than on sg-mg reefs (Table 4). Only 1 speciesfrom the nursery species group (Mulloidichthys flavo-lineatus) occurred on the Comoros reefs in comparabledensities as on sg-mg reefs. Compared with densitieson sg-mg reefs, only 2 species of reef generalists(Chlorurus strongylocephalus and Siganus stellatus)occurred in comparable densities, and 1 species(Lethrinus microdon) in higher densities on theComoros reefs. All other species of generalists and reefgeneralists were not observed or occurred in low den-sities on the Comoros reefs (Fig. 4). Although manyspecies of reef residents were not observed or onlyoccurred in low densities on the Comoros reefs, thisspecies group included the highest number of speciesfor which no density differences wereobserved between sg-mg reefs and theComoros reefs, or for which densities onthe Comoros reefs were significantlyhigher.

The GLM comparison of adult densi-ties of all individual species between sg-mg reefs and far reefs shows that all spe-cies of sg residents were not observed,occurred in low densities (not enoughobservations to perform a GLM) or insignificantly lower densities on the farreefs (Fig. 4, Table 4). The other speciesgroups whose juveniles were observedin sg-mg areas (nursery species, sea-grass generalists, generalists and reefresidents) consisted of some species thatshowed significantly higher densities onfar reefs than on sg-mg reefs and/or spe-cies that showed comparable densitiesbetween reefs. However, of the 36 spe-cies whose juveniles were observed insg-mg habitats (sg residents, nurseryspecies, seagrass generalists, generalistsand reef generalists), the majority ofthese species (25 species, 69%) wereeither absent from far reefs, wereobserved in low densities (not enoughobservations to perform a GLM) orsignificantly lower densities than onsg-mg reefs. Although the reef residentsalso included species with significantlyhigher densities on sg-mg reefs, most

species did not show differences between reefs or hadsignificantly higher densities on far reefs. Althoughmean adult fish densities on sg-mg reefs or far reefs onZanzibar, Mafia, Pemba and Mbudya were not similarbetween the islands, the patterns of occurrence of spe-cies on either sg-mg reefs and far reefs as describedabove were found on all islands (Table 4).

DISCUSSION

Species groups and habitat utilisation

Of all investigated habitat types other than the reef,shallow seagrass beds harboured the highest fish den-sities. Although mangroves, macroalgae, deep sea-grass beds and the intertidal flat were used by severalspecies of fish, densities were low in comparison withthose in shallow seagrass beds. Although the composi-tion of the species groups varied per island, differencesin species composition between the islands Zanzibar,Mafia and Pemba where mangrove, seagrass as well

73

0

10

20

30

40

50

60

70

80

90

100

% o

f sp

ecie

s in

eac

h ca

tego

ry

Sg-mg reefs S <<

Sg-mg reefs S >>

Present, dens <<

No difference

Not observed

Sgresidents

Nurseryspecies

Sggeneralist

Generalist Reefgeneralist

Reefresidents

far Com far Com far Com far Com far Com far Com

Fig. 4. Overview of GLM results of individual species, per species group (seealso Table 4). Mean adult species densities were compared (1) between sg-mgreefs and far reefs (far) and (2) between sg-mg reefs and Comoros reefs (Com).GLM results were classified as follows: (1) mean adult species density onsg-mg reefs was significantly lower than on far reefs or Comoros reefs (Sg-mg reefs S <<); (2) mean adult species density on sg-mg reefs was signifi-cantly higher than on far reefs or Comoros reefs (Sg-mg reefs S >>); (3) speciesoccurred on reefs (sg-mg reefs, far reefs or Comoros reefs) but adult densitieswere too low to be tested using GLM (Present, dens <<); (4) no significantdifference was observed between species densities on sg-mg reefs and farreefs or Comoros reefs (No difference); (5) no individuals of a species wereobserved on far reefs or on Comoros reefs (Not observed). For each speciesgroup, the bar shows the percentage of species in that particular group thatshowed a possible GLM result relative to the total number of species in the

group. Sg: seagrass; see Table 1 for habitat abbreviations

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Mar Ecol Prog Ser 302: 63–76, 2005

as reef habitats were sampled, were relatively small.Because the observed species community is relativelysimilar between these islands, the observed utilisationof available habitats by juvenile fish is regarded as ageneral pattern, rather than as a specific island effect.For 36 of the 76 selected fish species, juveniles clearlymade use of areas where seagrass beds and man-groves were present, and for 18 of these species (all sgresidents, nursery species and sg generalists) >70% oftheir juvenile density was found in these areas. Sincejuvenile fish densities of all species groups whose juve-niles were found in these areas were highest in theseagrass habitats, it is likely that seagrass habitatsfunction as the most important juvenile habitats inareas outside the reef. In contrast to the high densitiesin seagrass habitats, the low fish densities and speciesrichness in mangroves quantitatively support the sug-gestion that mangroves in the Indo-Pacific region arehardly used as a habitat by reef fish (Quinn & Kojis1987, Robertson & Duke 1987, Thollot & Kulbicki 1988,Kimani et al. 1996, Laroche et al. 1997, Huxham etal. 2004).

A study in the Caribbean that used an approachcomparable to that of the present study indicated that30 of the 85 selected coral reef species used seagrassbeds and mangroves as an important juvenile habitat(Nagelkerken et al. 2000a). In comparison with thenumber of species that used seagrass and mangrovenurseries in the latter study, the present study showed36 of the 76 selected reef species used seagrass beds asan important juvenile habitat. The present study thusindicates that seagrass beds in the Indo-Pacific can, insome cases, function as equally important juvenilehabitats for coral reef fish as seagrass beds andmangroves in the Caribbean.

In addition to seagrass habitats, juveniles of speciesof generalists and reef generalists are clearly able touse other available habitats, in particular reef habitats.These species might therefore be regarded as nurseryopportunists, rather than as species that are highlydependent on 1 habitat type for their juveniles (Lenan-ton & Potter 1987). Therefore, as regards the entirecommunity of the selected reef fish species, the presentstudy shows that shallow seagrass beds and reef habi-tats are the most important juvenile habitats for reeffish in a landscape presenting a mosaic of coastalhabitats.

Of all species groups, adult fish densities on reefsclose to areas with seagrass beds were highest fornursery species and reef generalists. For nursery spe-cies, juveniles were predominantly found in seagrasshabitats. Although juveniles of the reef generalistswere observed in a variety of habitats, relatively highjuvenile densities were observed in seagrass beds too.Although there is no direct evidence supporting con-

nectivity between these seagrass and reef habitats,ontogenetic migrations from the seagrass habitat to thereef habitat are likely, since adults of these 2 speciesgroups showed much higher densities on reefs close toareas with seagrass beds.

Effect of the presence of areas with seagrass beds onadult fish densities

The absence of extensive areas of seagrass nurs-eries at Grande Comoros most likely explains theabsence or low adult densities of all species groupsthere, except reef residents. Individual species withinthe various species groups mostly show the same pat-tern. This may also be the most likely explanation forthe observations on far reefs. Here, all species groupswhose juveniles were observed in seagrass areasshowed reduced adult densities, except generalists.The majority of the species in all groups, except reefresidents, were absent from the far reefs or hadhigher densities on reefs adjacent to seagrass areasthan on far reefs. These observations support thehypothesis that the presence of areas with importantjuvenile habitats, such as seagrass beds, has a positiveeffect on the densities of adults on adjacent coral reefsof species that use these habitats as juveniles. Thishas also been suggested for coral reef species that useseagrass–mangrove nurseries on Caribbean islands(Ley et al. 1999, Nagelkerken et al. 2000a, 2002, Ley& McIvor 2002, Dorenbosch et al. 2004, Halpern 2004,Mumby et al. 2004). Hence, the utilisation of seagrasshabitats as a juvenile habitat by coral reef fishes insome regions of the Indo-Pacific can be as importantas in the Caribbean.

Although the effect of the absence or presence ofseagrass areas on adult fish densities is clearly visiblein comparisons with both the Comoros reefs and the farreefs, various individual species with juveniles foundin the seagrass areas did not show this pattern. Thesespecies occurred in comparable or even higher adultdensities on the Comoros reefs (Chlorurus strongylo-cephalus, Lethrinus microdon, Mulloidichthys flavolin-eatus and Siganus stellatus) or the far reefs (Cheilinustrilobatus, Chlorurus sordidus, Hipposcarus harid,Lethrinus harak, L. obsoletus, Lutjanus monostigma,Mulloidichthys flavolineatus, Parupeneus macronema,Scarus scaber, S. psittacus and Siganus sutor). Themost likely explanation for this distribution pattern isrecruitment and survival of juveniles on reefs (Nagel-kerken et al. 2000a,b, 2002, Dorenbosch et al. 2004).Another possible explanation for the presence of theiradults on far reefs would be the migration of adults orsubadults of these species from sg-mg areas via sg-mgreefs to far reefs. Tulevech & Recksiek (1994) and

74

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Dorenbosch et al.: Contribution of Indo-Pacific habitats to coral reef fishes

Chapman & Kramer (2000) only showed very limitedmovements of coral reef fishes between reefs that wereseparated from each other (several metres to 1 km).Since the distance between the far reefs and sg-mgreefs in the present study is large (on average 17.2 km)and the far reefs are disconnected from the sg-mgareas by deep waters, migration is most likely to below.

In conclusion, the results of the present study showthat in an area featuring various coastal habitats in thewestern Indian Ocean, both shallow seagrass and coralreef habitats are favoured by juvenile fish over otheravailable shallow-water habitats. For nursery speciesand reef generalists (see species list, Table 4) juveniledensities are very high in seagrass beds and consider-ably lower on coral reefs, while adult densities showthe opposite pattern. For these species, seagrass bedsfunction as an important juvenile habitat, and onto-genetic migrations to adjacent coral reefs are likely. Intotal, of the 36 species that used seagrass beds as ajuvenile habitat, 25 showed higher adult densities oncoral reefs adjacent to these areas than on far reefs. Forthese species, it is likely that the presence of seagrassbeds contributes to the fish populations on adjacentcoral reefs, suggesting an ecological connectivitybetween these habitats. However, eleven of these 36species that used seagrass beds as a juvenile habitatwere also able to sustain adult populations in equaldensities on reefs situated far from these habitats, sug-gesting that these species do not entirely depend onthe presence of seagrass beds as nurseries. As a contri-bution to the debate regarding the importance of Indo-Pacific mangroves and seagrass beds as a habitat forjuvenile coral reef fish, it can be stated that in the pre-sent study seagrass beds had high value as a juvenilehabitat, whereas the value of mangroves was low.

Acknowledgements. This study was funded by Nufficthrough the ENVIRONS–MHO programme, by Quo Vadisfund of the Radboud University Nijmegen, by PADI’s Awareproject and by Sensation Divers, Nungwi (Zanzibar). Sensa-tion Divers provided facilities and technical advice on theisland of Zanzibar, while logistic support was provided bySwahili Divers on the island of Pemba, and by the MafiaMarine Park on the island of Mafia. Kisandra Divers providedlogistic support on Grande Comoros. We thank the manage-ment and staff of the Institute of Marine Science in Zanzibarfor the use of their facilities and for their support. Earlierversions of the manuscript benefited from comments of 3anonymous reviewers. This is publication No. 359 of theCentre for Wetland Ecology.

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Editorial responsibility: Otto Kinne (Editor-in-Chief), Oldendorf/Luhe, Germany

Submitted: October 20, 2004; Accepted: May 3, 2005 Proofs received from author(s): October 11, 2005