Anita. Behav., 1985, 33, 1332 1337

Patterns of spacing in a coral reef fish in two habitats on the Great Barrier Reef

M I C H A E L S U T T O N * Department of Zoology, School of Biological Sciences, University of Sydney, New South Wales, Australia

Abstract. Patterns of spacing in the butterflyfish Chaetodon tr!fasciatus (Pisces, Chaetodontidae) were studied in two habitats on the southern Great Barrier Reef. Pairs offish were found to defend territories at reef slope and lagoon study sites. Territories were 2.7-3.7 times larger in the lagoon than on the reef slope, where population density and food abundance were greater. No significant changes occurred in the sizes of territories in either habitat over one year, but patterns of spacing were generally more variable in the lagoon habitat. This was interpreted as a response to low population density and poor habitat quality.

The pattern of dispersion of a population reflects a species' response to its environment, and thus has important ecological consequences. Both intraspe- cific behavioural interactions and the distribution and abundance of crucial resources may have proximate influence on the spacing of individuals (Brown & Orians 1970; Village 1983). Attention has focused on differences in spacing patterns and behaviour among populations of species which exploit a variety of environments (Stenger & Falls 1959; Glas 1960; Weeden 1965; Schoener 1968; Holmes 1970; Simon 1975; Newton 1980; Hooper et al. 1982).

Chaetodon trifasciatus (Pisces, Chaetodontidae) occurs on coral reefs throughout the Indo-Pacific region (Carcasson 1977). The basic social structure of this butterflyfish has been described by Reese (1973, 1975, 1977, 1981), who found it to be strongly heterosexually paired, 'home ranging', and an obligate coral feeder. This species was selected for study due to its relative abundance, ease of individual recognition, well-defined pri- mary social unit (heterosexual pairs), quantifiable food resources (scleractinian corals), and compara- tive site attachment. This paper examines the spacing patterns in populations of C. trifasciatus in two different habitats on the southern Great Bar- rier Reef, Australia.

M E T H O D S

Fieldwork was conducted at Heron and One Tree Reefs, Great Barrier Reef, Australia, approxima-

* Present address: U.S. Fish and Wildlife Service, 847 N.E. 19th Avenue, Suite 225, Portland, Oregon 97232 U.S.A.

tely 23~ 152~ A site at One Tree Reef was selected to investigate the behaviour of lagoon populations, and for logistic reasons, an area at nearby Heron Reef was chosen to study fishes living on protected outer reef slopes. Because individual C. trifaseiatus normally do not rise more than 1 m above the substratum, it was possible to study their space-related behaviour in a two- dimensional framework. At each study site, an area of approximately 8000 m 2 was surveyed into a reference grid system of 10 x 10 m quadrats. The grid line intersections were marked with coded subsurface buoys. Censuses of all C. trifasciatus living on the grids were carried out to obtain baseline population data. Food supply was quanti- fied by line transect sampling for percentage live coral cover in nine randomly-chosen quadrats on each grid. Observations of feeding were conducted to determine any preference for particular species of corals.

Three pairs of C. trifasciatus at One Tree Reef and eight pairs at Heron Reef were observed between February 1979 and February 1980. Home range sizes were measured using a modification of the technique developed by Odum & Kuenzler (1955). Observers on SCUBA watched pairs of fish from distances of 1~20 m for up to 4 h at a time. During observation periods, divers dropped con- secutively-numbered markers at the location of the fish at 3-min intervals. Afterwards, the position of each marker was recorded in relation to the nearest grid buoy, and the data plotted using a Hewlett- Packard 9862A graphics plotter. The outermost points of observation were connected, and the area of the resultant convex polygon was calculated as an index of home range. A comparison of home

1332

Sutton: Spacing in a coral reef fish 1333

range areas and the duration of observation for 30 each pair revealed the point at which enough location data had been obtained to yield an ~ 25 accurate index of their extent of movements.

Observations o fbehav iour towards conspecifics ~ 20 were also made while collecting location point o data. Agonistic behaviour was defined to include ~ ~5 both lateral displays (dorsal spines flared, head down body position) and active chases. Experi- ~ ,.0 ments, in which individual C. trifasciatus were

0"5 captured, confined to a clear perspex case, and placed within the home range of a conspecific pair, were conducted to test for territorial behaviour. Control experiments were performed using an empty case.

/ /

0"5 1'0 2"0 3"0 4.0

HOURS OF OBSERVATION

Figure I. Observation-area curve for one pair of C. triJasciatus at the One Tree lagoon study site, April 1979.

R E S U L T S

Forty pairs of C. trifasciatus were censused on the reef slope grid at Heron Reef in April 1979. In contrast, only seven individuals (three pairs and one subadult) were found on the lagoon grid at One Tree Reef. Two subsequent surveys over the fol- lowing year revealed no change in either popula- tion, probably due to low levels of recruitment and mortality.

The distribution and abundance of food differed between the two study sites. The percentage bot- tom-cover o f live scleractinian corals was used as a direct index of food availability, since C. trifascia- tus was not observed to prefer any particular species; 95~o confidence limits for percentage coral cover were 53-0_+23'89~ at the reef-slope study site and 11-0_+ 6.20~ at the lagoon site. The food supply was more variable between quadrats at the

reef-slope study area due to the uneven distribution of large patches of staghorn coral (Acropora sp.).

At least 1 h of observation was necessary to obtain an accurate map of the home range for each pair of C. trifasciatus (Fig. 1). The patterns o f spacing in populations at the two study sites differed in several respects (Table I). Individual pairs used significantly more space at the One Tree lagoon study site than their conspecifics living on the outer slope grid at Heron Reef (Table II). Variability in home range size among pairs in the lagoon habitat was consistently greater than that on the reef slope site (Fig. 2). The use of space within home ranges also differed between the two study areas. The distribution of location points on maps of outer slope home ranges appeared regular or random, while location points within lagoon home ranges were clumped.

The variation in use of space over time was also

Table I. Mean home range sizes (in m z) for three randomly-selected pairs of C. trifaseiatus each at One Tree and Heron Reefs over 1 year

Pair Autumn 1979 Winter 1979 Spring 1979 Summer 1979-80

Heron Reef (outer reef slope habitat) HCTI 137-19 100-97 159.78 138.62 HCT2 77.17 76.86 79.62 73.71 HCT4 100.30 124-06 101.35 93.45

One Tree Reef (lagoon habitat) OCT1 225.38 242.82 326.25 163.93 OCT2 387-49 433-40 525.48 611.32 OCT3 259.19 352.26 163.24 348.73

1334 Animal Behaviour, 33, 4

Table II. Two-way analysis of variance in size of C. trifasciatus home ranges between habitats over 1 year

Source Sum of of variation squares df Mean squares F

Between habitats 321 185.52 1 321 185.52 24.65* Over time (1 year) 5117-82 3 1725.94 0-13 Habitats/time

interaction 5923.08 3 1974.36 0.15 Residual (error) 208435.32 16 13027.21

Total 540 721.75 23

3OO

25O

z 2oe

) 15o =

Ioc

MAR APR MAY JUN JUY AUG SEP OCT NOV DEC JAN

1979

Figure 2. Home range dynamics for C. trifasciatus at Heron and One Tree Reefs, 1979-1980, with 95% confi- dence limits.

examined (Fig. 2). No significant changes in home range size or position occurred seasonally at either study site (Table II). However, there was consider- ably more seasonal variability in patterns of space use at the lagoon site than on the outer slope grid. In the lagoon, home range boundaries constantly shifted, as did the centres of activity within home ranges.

Agonistic behaviour between pairs of C. trifas- ciatus was observed at both study sites. There were 0.14 encounters/h at Heron Reef ( N = 2 9 h) and 0.03 encounters/h in the One Tree lagoon ( N = 30 h). Encounters invariably took place along home range boundaries in small zones between home ranges, These observations suggested that both members of a C. trifasciatus pair defend their home range as a territory. This was confirmed by experi-

ments in which individual C. trifasciatus were confined to a transparent perspex case and placed at the approximate centre of the home range of a conspecific pair. In both habitats, the residents swam up to 5 m in a straight line to attack the enclosed intruder. Defending pairs repeatedly rammed their dorsal spines into the walls of the case containing an intruder, but did not respond to an empty case in control experiments. Ehrlich et al. (1977) elicited similar behaviour when they pre- sented plywood models of C. tr!fasciatus to conspe- cific residents.

Groups of 6-8 individual C. trifasciatus were observed on the reef slope on two occasions and at the lagoon study area once. These groups were composed of several known pairs of fish, and were marked by agitated behaviour and lateral displays. The fish characteristically milled around rapidly with fins erect for a few minutes, then broke up into pairs and returned to their respective territories.

D I S C U S S I O N

Territorial Behaviour

Most coral reef fishes are highly 'site-attached' as adults (Sale 1978; Sutton 1983). The butterflyfishes (Chaetodontidae) are no exception (Fricke 1973; Reese 1973, 1975; Ehrlich et al. 1977). Reese (1973) stated that C. trifasciatus 'did not establish territor- ies but rather swam about in an area of the reef, apparently avoiding other species, especially other pairs of C. trifasciatus'. He observed agonistic encounters between pairs, but attributed them to violations of 'personal space' rather than territorial

Sutton: Spacing in a coral reef fish 1335

defence. Reese (1975, 1978) has noted that agonis- tic behaviour concerned with territorial defence is often ritualized into subtle displays, hence actual combat may be rare. While attacks and chases were infrequent in C. trifasciatus, lateral displays were relatively common, though many appeared to be related to the maintenance of pair bonds.

That C. trifasciatus pairs defended their home ranges against conspecifics became evident only after long periods of observation. The case for territoriality in C. trifasciatus at the two study sites rests on four major points: (1) observations of agonistic displays and encounters along home range borders; (2) the results of experiments with artificially-introduced intruders of the same spe- cies; (3) the occurrence of buffer zones between home ranges; and (4) observations of groups of known C. trifasciatus displaying behaviour analo- gous to 'visiting' in territorial damselfish (Poma- centridae) (Clarke 1970; Keenleyside 1972; Sale 1978). 'Visiting', or 'clustering', is a behavioural mechanism which facilitates non-agonistic contact between neighbours and is presumably adaptive in reducing overall levels of aggression among terri- tory holders. Buffer zones have been found in birds (Williamson 1956; Stenger & Falls 1959), and apparently serve to prevent overlapping of adja- cent territories. All of the major territorial encounters observed between pairs of C. trifascia- tus occurred in these zones, which strongly suggests that the fish recognized the boundaries of their territories. The vigorous attacks on introduced intruders by resident fish could not be interpreted as a simple agonistic response to a violation of individual space.

Patterns of Spacing

The significant disparity in use of space by C. trifasciatus in the two habitats can be considered a response to differences in features of the environ- ment. Animals of the same species often display differences in spacing patterns under different environmental circumstances. For example, a spe- cies of bird may be territorial in some situations and colonial in others (Wynne-Edwards 1962; Horn 1968; Lack 1968), or may simply exhibit different territorial sizes under different conditions (Stenger & Falls 1959; Weeden 1965; Holmes 1970; Hooper et al. 1982; Village 1983). Most of these variations in spacing behaviour have been seen as direct responses to differences in environmental

factors, usually the distribution and abundance of food (Brown & Orians t970).

An enduring controversy exists as to what factors are of greatest influence in determining territory size in animals. Many studies have shown negative correlations between food availability and territory size (Schoener 1968; Watson & Moss 1970; Brown 1975; Simon 1975; Hooper et al. 1982), fostering the hypothesis that animals adjust the sizes of their territories primarily in accordance with the local distribution and abundance of their food resources. Other studies have demonstrated negative correlations between population density and territory size in a wide variety of animals (Brown 1975; Wilson 1975). This has given rise to an alternative hypothesis, that territories are analo- gous to 'elastic disks' (Huxley 1934) and their compressible sizes are adjusted largely in response to pressure from conspecific neighbours (Krebs 1971). These hypotheses are not mutually exclusive (Brown 1975).

Actual dispersion patterns reflect the influence of both proximate and ultimate factors (Brown & Orians 1970). The distribution and abundance of live coral food is probably of ultimate importance in determining the extent of movements of indi- vidual C. trifasciatus. Reef-building corals are an energy-rich, constantly regenerating, highly de- fensible source of food, exploited by many reef fishes (Benson & Muscatine 1974; Randall 1974; Harmelin-Vivien & Bouchon-Navaro 1983; Sutton 1983). Live corals could be or have been in short supply for coral-feeding fishes in some habitats, thus giving rise to territoriality. However, beha- vioural interactions with conspecifics are probably of primary importance, in a proximate sense, in regulating patterns of spacing in C. trifasciatus. The agonistic compression of territory size seems to be the mechanism by which high population densit- ies are maintained and variability in spacing is reduced at the reef-slope study site. Within a territory, however, the distribution of food resources may have an important influence on the use of space. For example, the clumped dispersion of location points in territories at the lagoon study site probably reflects a greater within-quadrat patchiness of live coral.

Seasonal variation in spacing patterns has been found and examined in a number of species (Brown 1975; Wilson 1975). In most cases, changes in space use came in conjunction with the breeding season, particularly in animals which displayed extensive

1336 Animal Behaviour, 33, 4

courtship and parenta l care. Seasonal migra t ion has also been shown to alter pat terns of spacing. It is thus not surprising to find a lack of seasonal dynamics in the use of space by C. triJi~sciatus and many other reef fishes (Sale 1978). Cont inua l m o n o g a m y in this species precludes the need for e laborate courtship behaviour each year. As a pelagic spawner, C. triJasciatus does not become involved in any form of parenta l care. The relative constancy of envi ronmenta l condi t ions on the coral reef obviates the necessity of migra t ion and facili- tates the high degree of site a t t achment found in mos t adul t reef fishes.

A C K N O W L E D G M E N T S

P. J. Doher ty , B. G. & A. M. Hatcher , L. Leum, W. Wiley, D. McB. Williams, and M. Wunder l ich generously assisted with the fieldwork. M. Godsey, P. A. Medvick, E. S. Reese, P. F. Sale, and R. Thresher critically reviewed the manuscript . Facilities and equipment were provided by the One Tree Island Field Stat ion (Universi ty of Sydney) and the Board of the Heron Island Research Station. The au thor was suppor ted dur ing this study by the Universi ty of Sydney, the Grea t Barrier Reef Commit tee , and the Grea t Barrier Reef Mar ine Park Author i ty .

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(Received 25 July 1984; revised 15 November 1984," MS. number: A4357)