The development of sampling strategies for population studies of coral reef fishes. A case study

Coral Reefs (1987) 6:49-58 Coral Reefs �9 Springer-Verlag 1987

The development of sampling strategies for population studies of coral reef fishes. A case study

A.J. Fowler

Department of Zoology, School of Biological Sciences, University of Sydney, Sydney, Australia

Accepted 3 December 1986

Abstract. Fish ecologists need to do pilot studies to develop accurate, precise and efficient sampling strategies. This paper presents a case study of the pilot investigations carried out to achieve this for three species of butterfly- fish Chaetodon rainfordi, C.plebeius and Chelmon rostratus, at One Tree Reef. The effects of different transect sizes and methods on density estimates and their precision for each species were assessed. Varying transect dimensions (25, 50, 75 and 100 m length x 1, 2 and 3 m width) did not significantly affect the density estimates. However for C. rainfordi the precision of estimates was variable with the smallest transect length and width producing the least precise results. Higher density estimates were obtained for juvenile fish over a 1 m strip width than a 2 m width. A significant effect of disturbance on the densities of C.rainfordi and Chelmon rostratus was caused by the laying out of the transect tape. Con- sequently a technique of simultaneously censusing and delimiting transect boundaries was tested and found to give consistently higher density estimates. The potential effect ofcensusing at different times of the tide was examined and found to be insignificant for two species but quite complex for the other. Finally, a pilot sampling program was done at seven localities across One Tree Reef to determine the most efficient way of allocating sampling effort, for future census work.

Introduction

Since Brock (1954) first used visual transects in an ecological study of coral-reef fish, their use has become wide- spread (Chave and Eckert 1974; Hobson 1974; Sale 1974; Jones and Chase 1975; Doherty 1980; Robertson and Lassig 1980; Sale et al. 1984). Despite this, there still remain inherent problems in the use of transects, especially where many species are simultaneously included in the census (Russell et al. 1978).

To date, reef ecologists have given little consideration to the accuracy and reliability of the results obtained

from visual census techniques. In cases where this has been examined, definite biases in estimates of density obtained for some species have been recognised (Sale and Sharp 1983). Brock (1982) compared data from transects with that from a quantitative collection from the same area and concluded that the transect technique under-estimated the most cryptic as well as the most abundant species, but gave good estimates for the diurnally active species. Sale and Douglas (1981) did total counts on small patch reefs which yielded only 82% of the species and 75% of the individuals present at the time of censusing. They therefore concluded that visual census techniques are not 100% accurate. Jones and Chase (1975) checked their transect data by doing random counts in the same area and by so doing increased their species count by up to 30%.

A wide range of factors may interact to affect estimates of density gained from strip transects (refer to Sale and Sharp 1983 and Keast and Harker 1977 for more de- tails). These include:

1. the sampling technique with such considerations as transect number and size;

2. the behavioural characteristics of each target species; 3. the sensitivity of the observer towards particular spe-

cies or size-classes; 4. the number of species included in the census and their

relative abundances, since it should be more accurate to count a small number of fish than a large number;

5. the topography of the areas being sampled.

The potential interaction of these factors was demonstrated by Sale and Sharp (1983) who found a negative association between density of fish and width of transects, for five unrelated species or species groups. They concluded that the magnitude of this effect was influ- enced by the conspicuousness of morphology or behaviour of the target species.

The best sampling procedure will depend not only on characteristics of the individuals to be counted and the habitat they are found in, but also on the question being

50

asked. I f the s tudy is a communi ty- level one, where a variety of species are to be sampled s imul taneously , an al- ternative to t ransect-based techniques may be available with the species-time techniques such as Rap id Visual Censuses ( G B R M P A 1978; Jones and T h o m p s o n 1978; bu t see DeMar t i n i and Rober ts 1982) and the Visual Fas t C o u n t (Kimmel 1985). However, for popu la t ion studies, where an accurate descript ion of changes in density over space and time are required, t ransect-based visual census techniques still remain the best (Sanderson and Solonsky 1980, K i m m e l 1985). However, p re l iminary sampl ing should be carried ou t so that the t ransect technique can be applied to give the mos t accurate and precise results to ensure the effective use of time, money and energy. The choice of appropr ia te t ransect d imensions and number s of replicates should be justified and the statistical procedures for doing this are well established (Unde rwood 1981). Ecologists work ing in other env i ronment s have employed these techniques and have expended consider- able effort in exploring biases in sampl ing and have consequently developed techniques suited to their target species (Anderson and Pospaha la 1970; Robine t te et al. 1974; Caughley et al. 1976; Kenne l ly and U n d e r w o o d 1984).

This paper summarises the pre l iminary sampl ing employed to develop an effective sampl ing strategy for a popu la t ion study of several species of butterflyfishes (family: Chaetodont idae) on one reef on the Grea t Bar- rier Reef. As such it may serve as an example or s tar t ing po in t for workers p l ann ing popu la t i on studies on other demersal fishes. The specific aims addressed were:

1. to explore the effects of varying transect sizes on the result ing estimates of densi ty and the precision of these estimates;

2. to evaluate the possible inf luence of diver effects, the inf luence of tide and the visibility of different sizes of fish as potent ia l biases to the accuracy and precision

of density estimates; 3. to design a sampl ing technique to overcome the effects

of any biases detected in 2. above, and to test the accuracy of this technique;

4. to determine the mos t efficient way of d is t r ibut ing sampl ing effort, to make compar i sons of the dis tr ibu- t ion and a b u n d a n c e at several spatial scales and across

time.

Methods

General

The study was carried out at One Tree Reef, on the Great Barrier Reef (27~ 152~ The various localities used are shown on Fig. i. The species censused were Chaetodon rainfordi, C.plebeius and Chelmon rostratus, the most widely distributed and abundant chaetodontid fishes on this reef.

The study was divided into sections, each being a separate investigation of the effectiveness of different procedures. Each investigation in- volved the running of a balanced set of transects, in which the order of treatments was randomised and always initiated from some haphazardly-chosen area of reef. Data recorded (sometimes all the fish from the

three target species, and sometimes only a single size-class) were usually examined using analysis of variance. When data were found to have het- erogeneous variances they were usually transformed by square root of (x + 1) unless otherwise stated (Snedecor and Cochran 1967).

Where the investigations explored spatial patterns of distribution the terms "locality" and "site" are used. A "locality" was considered an area of reef supporting a relatively uniform habitat type, geographically isolated from other such localities whilst a "site" was a haphazardly chosen area within a locality, through which the random transects were run. Where possible each study was done within a locality to minimise the effects of topographic complexity presented by different habitat types. All sampling was done between 08.30 hours and 16.00 hours to minimise the effects of time-related activity patterns.

Preliminary work was done using the following strip transect technique (referred to as the "sequential" sampling technique). A PVC tape was swum through the site to mark out the transect. A swim was then made along this line recording the fish occurring within a prescribed distance on both sides of the line. The width was estimated by the use of a T-bar, a T-shaped aluminium rod carried by the diver (Sale et al. 1984). The stem of the T, 75 cms long, was held out ahead, pointing the direc- tion of the swim. The cross-piece, 1.5 m long, was used to measure the width of the transect on both sides of the line, thus enabling a transect width of up to 3 m to be measured. A scale on the cross-piece allowed the accurate measurement of distance, to facilitate measuring the smaller transect widths.

Transect dimensions

To explore the effect of different transect sizes on the variability and precision of the estimates of density, the following study was done. At Lo- cality "A" (Fig. 1) transects of four lengths (25, 50, 75 and 100 m) were done in all combinations of three widths (1-3 m) using the "sequential" transect technique. Four replicates of each combination of dimensions were done.

Densities were standardised to number of fish. 25 m-2 and analysed by a two factor analysis of variance (lengths x widths) for each species. Variability and precision were assessed by examining the Standard Er- rors and the Coefficients of Variation (SD/mean). These were calculated from the four replicate estimates of fish. 25 m- 2 for each combination of dimensions.

Diver effects

An attempt was made to determine whether the use of the "sequential" transect technique could disturb chaetodontid fishes sufficiently to bias density estimates. This could occur if the laying out of the transect line through the study site frightened fish from the transect area, caused them to move into hiding holes or attracted inquisitive fish. The study was done by varying the elapsed time between when the transect line was laid out, and when the transect was censused. The increasing time period was to allow fish that had been disturbed, time to resume normal activity and for normal densities to be re-established. Three arbitrarily chosen elapsed times were used: 1 rain, 5 rain and 15 min. At Locality "A" four transects per treatment were done using transects of 75 x 2 m (see Results: Effects of transect size).

Comparison of sampling techniques

The aim of this study was to determine which of two sampling techniques was the more suitable for chaetodontid fishes. The two techniques tested w e r e ~

a) the "sequential" technique, of initially laying a transect line through the study site and then subsequently doing the census;

b) the "simultaneous" technique of delimiting the transect boundaries whilst simultaneously doing the census. Here the tape was unwound whilst swimming and eensusing and the 75 m mark depicted the end of the transect. This technique allowed the site to be censused on the first swim thus avoiding any possible disturbance effects associated with laying out the tape as with the "sequential" technique.

4/

~" LOCALITY"A"

51

".5, LOCALITY"D'"

LOCALIT~

, , C ~

Fig. 1. Distribution of "localities" across One Tree Reef. Localities "A" to "E" are boxed whilst the seven localities sampled in the pilot sampling regime are indicated by SR

- /

x x /

The use of this "simultaneous" method precluded using the T-bar to measure transect width, as it could not be carried while also carrying the tape and slate. Consequently, for both techniques, the width was estimated by extending an arm perpendicular to the line of swim to the position where the fish was initially observed. When this position fell within the arm's length the fish was included in the census, when it did not it was excluded.

The study was done at Localities "A" and "B" (Fig. 1) where six sites were sampled at each locality, with one transect per technique at each site. The number of fish per transect for each species were compared by paired t-tests, independently for each locality.

Comparisons of sampling techniques for juveniles

Juveniles tend to be more cryptic then the adults, and may be more dif- ficult to see across a wider transect band. Here, the effects of two variations of the "sequential" sampling technique on the estimated densities of the juveniles, were compared. The two variations were:

a) the "sequential" transect technique using transects of 75 x 2 m, where the width of strip searched was 2 m;

b) a modified "sequential" technique, in which only one side of the transect line was searched at a time. Here, the width of the strip searched was only 1 m, allowing a more comprehensive search to be done. In order to search the same area as in the previous technique, the transect length was swum twice, along one side of the transect line and then back along the other. The results had to be considered in the light of this dou- bling of effort.

One transect was censused by each technique at six different sites within Locality "E" (Fig. 1). The results for the total number of fish across the three species and the number of all chaetodontial species were analysed by paired t-tests.

Effect of tidal phase on effectiveness of sampling In the lagoon of One Tree Reef, much of the contiguous reef, reef-crest areas and large micro-atolls (large lagoonal patch reefs), consist of vertical walls that rise to the low tide level, giving way to a horizontally flat- tened top. It is only on the rising, falling and high tides that the tops of these reefs are covered with water, giving fish access. The tops of the micro-atolls consist of a reef rim approximately two metres wide, which then drops away vertically to the sandy floor of the shallow, hollowed- out top.

The aim of this study was to determine if there was a change in distribution of fish between vertical wall and horizontal top areas, that was related to the daily changes in the height of the tide. Such a phenomenon would have important implications for the timing of sampling these areas.

The study was done on micro-atolls randomly chosen from Locali- ties "C" and "D". The "tops" and "walls" were sampled at low and high tides, using three replicate transects. At low tide, transects were done on the "tops" of the reefs, around the inside vertical rim, extending down to the inside sandy floor of the reef. At high tide the transects on the "tops" of the reefs included the horizontal reef rim. The "simultaneous" method was used with transects of dimensions of 75 x 2 m. Densities were analysed independently, for each species by a three factor analysis

52

of variance with the three factors being, height of the tide, habitat on the reefs ("tops" and "walls") and sites nested within these habitats.

Test of accuracy of density estimates

Visual census data were compared with collected samples, to evaluate the accuracy of the sampling technique. The visual census estimates were obtained for the three target species, using the "simultaneous" census technique, on four large isolated micro-atolls (range of radii = 28-47 m). The estimates of the "real" number for each target species occupying these reefs, were obtained by collecting the fish over a number of spearing sessions.

It became evident during the course of the spearing exercise that it would be impossible to remove all fish, as those remaining became in- creasingly wary as densities were reduced. After five spearing sessions on each reef, using 2-4 divers, spearing was stopped. To document the number of fish remaining on the reefs, a second lot of transects were run.

The "real" density for each target species was calculated as the sum of the number of fish removed and the number remaining per 150 m - 2. This was obviously still an estimate, since the second part of the function was derived using the very technique that the study was testing. How- ever, since the numbers at this stage were so reduced, the few fish remaining were easy to notice, so that the estimates probably reflect close ap- proximations to the real densities,

In order to calculate accurately the "real" density it was necessary to calculate the area of each of the reefs used by each species. For C. rainfordi this area was only the vertical wall, for Chelmon rostratus and Chae- todon plebeius it was the area of the vertical wall plus the reef rim. These decisions were made on the basis of behavioural observations and the results from the study of the effects of tidal phase on sampling.

Each micro-atoll used was approximately circular. The total perimeter of the reef wall was measured, as was the depth of the wall ev- ery 50 m. An estimate of the vertical wall area was calculated as the prod- uct of the perimeter of the reef, and its mean depth. The area of the reef rim was calculated as

A(r im ) : 7~r~rim ) - - ~ ( r - - 2 ) 2

since the rim was approximately 2 m in width. These areas were then used to calculate the "real" densities for each

species. The "estimated" densities from visual counts, and the "real" densities from the removals were compared by a paired t-test for each species.

Determination of sampling regimes

When sampling is done using a nested or hierarchial sampling design, cost-benefit procedures can be used to maximise the efficiency and reliability of subsequent sampling effort (Snedecor and Cochran 1967). These procedures assign greater sampling effort to the hierarchial level which obtains the largest variance, thus achieving maximum precision for the effort expended. The aim here was to apply these procedures to results from such a pilot sampling study in order to identify the regime of sam- piing intensity that most effectively distributed effort among and within sites to census localities adequately, for each target species.

The nested sampling design used in this pilot study consisted of three random transects within three haphazardly chosen sites, within each of seven localities (Fig. 1). The cost-benefit procedures used both an estimate of the variance due to each of these three levels and an estimate of the time to perform each of the levels of work, to predict the recommended sampling regime. The estimates of variance were calculated from the mean square term from the analysis of variance for each species. Data were initially analysed by a two factor analysis of variance (localities and sites). The time taken to do each level of sampling was estimated in the field. These times were defined as follows:

1. ct = the time required to perform logistic tasks to sample a locality e.g. motoring to the locality;

2. c2 = the time required to perform logistic tasks to sample a site e.g. moving from site to site and anchoring the boat;

3. c3 = the time to census a transect.

The estimates for cl , c2 and c3 used in this study were respectively 30 min, 10 min and 15 rain.

The estimates of variance and costs (in terms of time) were incorpo- rated into the appropriate equations (Underwood 1981) to give the sampling regime of maximum precision for minimum effort for each species.

Results

Effects of transect size

Most densities of C. rainfordi detected by the various transect combinations fell within the small range of 0.25- 0.50 fish. 25 m -2 except two combinations which gave slightly higher estimates (Fig. 2). The analysis of these data showed no significant effect of transect length or width or an interactive effect of these factors.

Also for this species most of the Standard Errors fell within the small range of 0.08-0.177. However, the highest Standard Errors, i.e. the least precise transects, were obtained by the shortest transect length (25 m) and the narrowest transect width (1 m). The high Standard Errors obtained for the 25 • 1 and 3 m transects caused the Coefficients of Variation from these combinations to be higher then all others, showing that these combinations produce the least consistent results. In contrast, the 75 • 2 m transects gave the lowest Standard Error and Coefficient of Variation, thus producing the most precise results.

The densities of C. plebeius varied little, ranging between 0-0.3 fish. 25 m-2 when compared among all transect combinations. There was no significant effect of transect size on density. However, it is worth noting that the four replicates of both the two smallest transect sizes i.e. 25 • 1 and 25 x 2 m, failed to detect any individuals between them, indicating that these transect sizes were in- appropriate for this species. The pattern of Standard Er- rors for all combinations of sizes followed the patterns of means, and ranged only between 0-0.157. The Coef- ficients of Variation were generally high in comparison to both the other species. The 25 x 3, 50 x 2, 75 x 1 and 75 x 3 m transects gave the highest Coefficients of Varia- tion because the density estimates from these were the lowest.

The densities of Chelmon rostratus were not significantly affected by the transect sizes. Densities were again low, 0-0.375 fish. 25 m- 2, with the smallest transect combination (25 x 1 m) failing to detect any individuals. The Standard Errors ranged only between 0q). 161 with there being no transect size giving particularly variable results. The Coefficients of Variation also occupied a small range, 0.40-0.86, except for the 25 x 3 and the 50 x I m transects that gave Coefficients of Variation of 1.15, both because of high Standard Error values relative to the means.

1.0

.8

- r u9 .~ la_

Ci .5 Z Z .4 <

. 2

.1

.9

.8

.Z tr~

,~, .6 "l- tO ,-r .5 c~ Z .4

Z < .3

.7

-~..5 "l- go ,-r .4

d Z .3 Z < .2

.1

.5

Chaetodon density rainfordi

/ * \ .4 / \

/ z \

I i I t

25 50 75 100

Chaetodon density .4 plebelus

.3 u.J ,'5

.2

/ .

i / x \ .~'~ ,~

25 50 75 100

Chetmon density rostratus .3

uJ go

.2 o �9 .,, ,, /X./

' o ,

/ \�9

0/ i , I

25 50 75 100

"~ 2.5 \ Standard

error \ �9 \ 2.0

\ \ 1,5

\ o , , \ \ 1.0

V ",. o 5

I 1

2'5 5'o z~ ,00

Standard 2.0 error

1.5

d 1.0

/ .

/ ~ ' \ / .o �9 / . ~ . , / . . - - .5

~.- , ~ , J , 25 50 75 100

Standard error 1.5

d 1.0

I I I 25 50 75 100

TRANSECT LENGTH

Coefficient of variation

\ . \

I I

~ qs A '..//\


I I I I

25 50 75 100


. . . . X / "

I I I I

25 50 75 100

53

Fig. 2. Summary of results from the transect dimension study for each species. The graphs show the density (mean no. fish. 25 m - 2) the Standard Errors of these means and the Coefficients of Variation (SD/mean) graphed against transect length for each transect width. Note, there are no CV: estimates for C.plebeius available from the 25 x 1 and 25 x 2 m transects or for Chelmon rostratus from the 25 x 1 m transects since the mean estimates from these transects were zero. o - - - - - - o 1 m; o . . . . . o 2 m; �9 � 9

The 75 x 2 m transects gave the most precise estimates of the mean for C. rainfordi whilst simultaneously giving estimates of density of acceptable precision and consis- tency for C.plebeius and Chelmon rostratus. Con- sequently, this combination was chosen as the preferred transect dimensions for all future work.

Effect of divers

For C. rainfordi and Chelmon rostratus there were significant differences between the numbers of fish recorded after the different time periods (Fig. 3). For C. rainfordi there were significantly more fish in the transects after 15 min than after 1 min (Student Neuman Keuls-test P = 0.05). For Chelmon rostratus there were more fish de-

tected after 15 rain than after the shorter time periods, between which there were no differences. For C.plebeius there were no differences in numbers among the three elapsed times.

Comparison of sampling techniques

Apart from the one exception of Chelmon rostratus at Lo- cality "A", the "simultaneous" technique gave higher estimates of abundance then the "sequential" technique (Table 1). This difference was significant for C. rainfordi at Locality "B" (P = 0.05). Considering the small sample sizes compared in these analyses, these results suggest strongly that the "simultaneous" technique produces higher estimates then does the "sequential" technique.

54

Table 1. Comparisons of the "sequential" and "simultaneous" techniques for the indicated species. The table includes the mean number of fish per transect (i.e. per 150 m-2) , the within-cell confidence limits obtained by botk.techniques and the probability for each comparison and its significance (at P=0 .05 )

Species or group Locality "A" Locality "B"

Sequential Simultaneous Result Sequential Simultaneous Result

C. rainfordi 3.83 __+ 3.0 5.83 • 3.35 P = 0.29 2.33 ___ 1.58 3.67 ___ 1.43 P = 0.01 * C. plebe• 0.33 • 0.54 1.67 • 1.95 P = 0.10 0.50 + 0.57 2.00 ___ 2.39 P = 0.21 Chelmon rostratus 3,17• 2.17+1.81 P=0 .31 0.67__+0.86 2.17__.1.55 P=0 .11

6.O 4.13 ff-- E o 5.0

"0 4.0 o

~ 3,0

x~ 2,C E

95 %C.L

S.E.I I

. . . . . . . . . . . . . . . .- . . . . . . . , " "

a b - - ' " ' " ' " ~ 1 7 6 95%C.L .D ......... S,E. 1"

I . . . . . . "" 1 a

~ , ~ 9 5 % C i .

~ S.E. I O~

0" - - - - - - - ~-~"~ ~ - - 0 ~ ~ . . . . ..0

1 5 ll5 Elapsed Time (Minutes)

Fig. 3. Results from the study of laying out the transect tape on density estimates. Location of significant differences between means for C. rain- ford• and Chelrnon rostratus are indicated by letters. The standard errors and confidence limits for pooled data are shown for each species. C) ............ �9 Chaetodon rainfordi; Q . . . . . (~ Chaetodon plebe• 0 - - - - - - 0 Chelmon rostratus

5.0

%-

4.0 E

O 3.0

o 2.0

1.0

\ \ \ \

i

1 Transect Strip Width metres)

Fig. 4. Results comparing the effectiveness of sampling juveniles over one and two metre strip widths. Significant differences between means are indicated by letters. Within-ceil confidence limits are shown, e - - - - - - e total number of juveniles; o - - - - - - o total number of species

Comparison of sampling techniques for juveniles

The number of juveniles detected was very low, preclud- ing analysis for any single species. Consequently, data were pooled across all species (Fig. 4). The number ofju-

a

o

E

"o

o

o

3.C

2.13

1.0

3.0

2,0

1.0

"Walls" >"Tops" C. rainfordi

/i High Low High Low Tide Level

"Tops . . . . Walls" Habitat

C. plebelus

i i i1 High Low High Low Tide Level


"E

o o

c %

o

4.0

3.0

2.(3

1.C

Chelmon rostratus a

b b H

1 2 1 2 - - 1 2 1 2 Sites High Low High Low Tide Level


Fig. 5. Results showing the distribution and abundance of each target species on micro-atolls at low and high tides. Standard errors are calculated from pooled data. Because of a significant site effect individual sites are shown for Chelmon rostratus but data pooled across sites are shown for Chaetodon rainfordi and C.plebeius. Significant differences between means detected for Chelmon rostratus are indicated by letters and are dis- cussed in the text

veniles detected by the modified "sequential" technique was higher, although only marginally significant (P= 0.07). However, the total number of species was significantly higher (P= 0.03). In spite of the small sample sizes used, these results suggest that searching for juveniles over the 1 m wide transect produced a higher, and hence more accurate estimate for this size-class.

To search for juveniles over a 1 m wide transect whilst maintaining the standardised 75 x 2 m transect, required the length of the transect to be swum twice. Yet, to avoid biased estimates of this size-class, and to ensure appropriate sampling of the whole reef-associated population, the sampling technique is obliged to incorporate this second swim despite the extra expenditure of time and effort.

Effect of tidal height on the effectiveness of sampling

There was a significant difference in the numbers of C. rainfordi found on the "tops" and the "walls" (Fig. 5 a). Consistently lower numbers occurred on the "tops", regardless of the height of the tide. No change occurred in the relative abundance of this species between the two habitats, between changes in height of the tide.

For C.plebeius the numbers obtained were variable but generally low. There was no significant effect of tide on the distribution and abundances (Fig. 5 b). These data suggest that this species uses both the "tops" and "walls" equally at both heights of the tide.

The analysis of variance for Chelmon rostratus gave a significant site effect and an interaction between habitat and the level of the tide. Results from the Student Neu-

55

man Keuls-test (Fig. 5 c) showed a significant increase in abundance to the "top" of site 2 at high tide. This however, was not associated with a reciprocal reduction in the abundance of fish on the "walls" at this time, suggesting that there may have been a permanent population living amongst the interstices of the coral on the "top" which only emerged at high tide. There was no parallel situation to this at site 1, where abundances remained relatively constant between heights of the tide, on both the "tops" and "walls".

Accuracy tests

No significant difference was obtained between the "real" and "estimated" densities for any of the target species (Table 2).

Notwithstanding, for Chelmon rostratus the "estimated" densities were consistently lower than the "real" densities particularly at Locality "C". For the other two species, the "estimated" and "real" means were sufficiently close, to conclude that the sampling technique produced an accurate estimate of the true density of the population.

Sampling regimes

Table 3 summarises the results from the cost-benefit 'analyses. Included are the mean squares used to calculate the number of transects and sites that are recommended to be censused within each locality, derived from the analysis for each species. For both C. rainfordi and Chel-

Table 2. Results from the tests of accuracy of the "simultaneous" sampling technique. The table contains the estimated areas of the micro-atolls (sq. metres) and the "estimated" and "real" densities (per 150m -2) for each species on each reef. Also shown are the t-values obtained from the comparisons and the probabilities of each t-value

Locality Wall Wall plus C. rainfordi C. plebeius Chelmon rostratus and reef no. area reef rim

Est. mean Real mean Est. mean Real mean Est. mean Real mean

"C"/reef 1 599 1 180 10.0 9.3 "C"/reef 2 333 667 5.0 6.5 "C"/reef 1 785 1434 4,0 3.8 "C"/reef 2 659 1068 6.5 4.6

t = 1,8562 P > 0 . I ns

1.0 1.3 2.5 4.4 0,5 0.5 1.5 4.5 0,5 1.0 3.0 3.3 0 0.7 4.0 4,5 t=0.9029 t=2.2567 P > 0 . 4 n s P > 0 . 0 5 n s

Table 3. Summary of analyses from the pilot sampling program. The table shows mean square values from the analyses of variance used to calculate estimates of variance for use in cost-benefit equations, and the probabilities of F-ratios calculated from these mean squares. Where these probabilities are significant (P = 0.05) they are indicated by *. The table also shows the reconamended number of transects and sites for each species

Species Data Mean squares and probabilities for Residual Recommended trans- mean number formed square Localities Sites

Transect Sites

C. rainfordi Y 0.66 ( > 0.05) 0.26 (> 0.35) 0.23 4 1 C. plebeius N 4,04 (> 0.20) 2.52 ( < 0,05)* 1.11 2 3 Chelmon N 5,22 (<0.01)* 0.98 (>0.10) 0.64 2 1

56

mon rostratus the recommended number of sites per locality was 1 whilst for C.plebeius the recommended was 3. This is because the former two species demonstrated less variation in abundance at the spatial scale of "sites", then did C.plebeius for which the differences were significant. In contrast the analysis recommends that 4 transects per site be censused for C. rainfordi whilst only 2 are necessary for the other two species. This indicates that at this smaller within-site spatial scale the former species demonstrated more variability, thus requiring greater sampling effort, to give an accurate estimate of the true density at each site.

Discussion

Transect-based visual census techniques are an important research tool for coral-reef fish ecologists. Yet, when used for this purpose they have given biased estimates for different species (Stone et al. 1979; Brock 1982). These biases have been related to the behavioural nature, the morphological characteristics and the relative abundances of the various species considered (Brock 1982; Sale and Sharp 1983).

Recently, there has been greater emphasis on studies of coral-reef fish populations (de Boer 1978; Doherty 1983; Victor 1983; Jones 1986). Sampling in population studies is often required to give accurate estimates of density to allow spatial and temporal comparisons. Strip transects remain the best non-destructive method for achieving this (Kimmel 1985). To avoid biases and to achieve maximum precision, transect techniques should be adapted to the particular species and the general area to be sampled. This study was done to achieve this for three species of Chaetodonts at One Tree Reef. Follow- ing is a description of the technique and regime that were developed, prefaced by a discussion of the considerations that led to their development.

Sampling technique

Dispersion patterns of fish species and the conspicuousness of different sized individuals can be expected to affect the estimated densities obtained from different sized transects. Yet in this study, varying transect dimensions did not significantly alter these estimates or, above a cer- tain minimum, the precision around them and ultimately only subtle differences allowed a choice between the transect sizes. Apart from the smaller transects of 25 m length and the one metre width strips that produced more variable densities then the larger transects, there was little consistent effect due to different transect sizes. That transect width did not significantly influence the densities of chaetodonts in this study is surprising since Sale and Sharp (1983) documented a significant effect of transect width on the abundances of chaetodonts in this same locality (Locality "A", Fig. 1) on this same reef. They documented a decrease in the total abundance of the eight

most numerous species as transect width was increased. However, they had restricted their sampling to 100 m of reef whereas in this study the transects were run at haphazardly-chosen sites along a wall of reef approximately 1.5 km long. Possibly the patchiness in the occurrence of individuals across this larger spatial scale swamped any effect of transect size. The small effect of transect dimensions on results was also shown by McCormick (1986) for the temperate reef species Cheilodactylus spectabilis. He obtained little variation in the Standard Errors from different sized transects as predicted from computer simula- tions designed to predict the optimum transect size.

Most of the chaetodonts detected in the above study were sub-adults and adults. It is now known that the locality where the study was done (Locality "A", Fig. 1) is a poor site for the recruitment of chaetodonts (personal observation). Because of the lack of juveniles in this area, there was no apparent advantage conferred by sampling over a 1 m width then 2 or 3 m. Yet, when the visibility of the juveniles was compared over 1 and 2 m transects, in a locality where juveniles had been noticed to occur in greater densities (Locality "E", Fig. 1) the advantages of using the I m transect became obvious (Fig. 4). Because transects of 75 x 2 m gave greatest precision for sampling sub-adults and adults (Fig. 2) and that transects of only 1 m width gave higher density estimates of juveniles (Fig. 4), this established a conflict that had to be resolved to minimise any size-class biases when sampling. This will be considered further in the discussion on sampling technique design.

Transects are often done by the "sequential" sampling technique by ecologists sampling coral-reef fish (Chave and Eckert 1974; Sale 1974; Jones and Chase 1975; Sale et al. 1984; Eckert 1985). Yet in this study when the effect of laying out the transect line on the densities of fish was investigated, there was found to be a significant effect for two of the target species (Fig. 3). For both these species the results are consistent with qualita- tive observations of behaviour as the fish have been fre- quently observed to retreat to holes in the reef when disturbed or frightened. These results also suggest that their resumption of normal activity after such a scare is time- dependent. For the third species, C.plebeius, only four individuals were detected in the 12 transects, so it is unlikely there was an opportunity for any disturbance effect to be manifested. Obviously a sampling technique for Chaeto- donts would have to consider the bias introduced by this method of sampling. To achieve this, the "simultaneous" technique was developed, tested and found to give consistently higher estimates of density then the "sequential" technique (Table 1). Considering the effects of the laying out of the transect tape on the density estimates of C. rainfordi and Chelmon rostratus, obviously the "sequential" technique caused disturbances sufficient to bias the density estimates. The "simultaneous" technique avoided this problem by sampling on the first swim through the site allowing fish to be detected before they were frightened away.

57

The results of the study of the effect of tide on the distribution of the three species (Fig. 5) had important implications for the general census program. "Walls" can be censused both on the high and low tides without there being a tidal bias, and the resulting data can be compared with that from other localities obtained at other times of the tide. However, "Tops" should be censused on the high tide primarily because Chelmon rostratus may be under-estimated if censuses are not done at this time (Fig. 5). Secondarily, transects are much easier to do in this habitat on the high tide.

These results were taken into consideration to develop the following sampling technique. The necessary equipment simply consists of a cord of 75 m wound onto a hand-line fishing reel, and a formatted slate for the recording of data. Prior to doing the sampling swim, the cord is fixed at one end to the substratum to mark the start of the transect. The swim is then made through the study site unwinding the cord and simultaneously recording all adults and sub-adults occurring within a 2 m wide path ahead of the diver. The path swum is either a straight line or along a depth contour. When the cord is fully unwound the 75 m cord lies stretched across the reef marking the path that has just been swum, and the area censused. This exact same area is then censused for new recruits and site-attached juveniles by swimming down one side of the cord, and then back along the other, searching over a one metre wide strip. This allows coral heads, small crevasses and holes to be searched carefully. The technique effectively combines elements of the "simultaneous" procedure for larger individuals and the "sequential" procedure with narrower transect width, for the smallest, site-attached size-classes. The technique allows adults to be censused over a 2 m strip width, the juveniles and recruits to be censused over a 1 m strip width while avoiding any disturbance from swimming out a transect line prior to censusing.

The accuracy of this new sampling technique was tested by comparing density estimates for populations of each species occupying four large micro-atolls, with true mean estimates derived from collected samples from these reefs (Table 2). For C. rainfordi and C.plebeius the "estimated" and "real" means were similar enough to conclude that the sampling technique gave estimates of acceptable accuracy. For Chelmon rostratus, although the "estimated" and "real" means were not significantly different (P=0.05) there were worrying differences between these means for both reefs at Locality "C". The statistical test used to compare these means was not strong (df= 3) and would only detect large differences between samples. At Locality "C", reef 1 the "estimated" mean was only 57% of the "real" mean whilst at reef 2, the "estimated" mean was only 33% of the "real" mean. However, at Locality "D" the "estimated" mean was approximately 90% of the "real" mean for both reefs. This suggests there was a differential rate of detection either between the two localities or between the sampling occa- sions. Therefore the results from this sampling technique

for Chelmon rostratus cannot be considered as reliable or accurate as for the other two species.

Sampling regime

Cost-benefit procedures (Snedecor and Cochran 1967; Underwood 1981) were applied to optimise the distribution of sampling effort between different levels within the hierarchial sampling design, so as to maximise precision when comparing among localities. The pilot sampling study across the seven localities produced estimates of variability for each of three levels in the nested regime, i.e. within sites, between sites and between localities. Since the cost-benefit procedures only utilised these single sample estimates of variance, they did not incorporate any seasonal variability. It was therefore decided to best use the recommended sampling regimes (Table 3) as a conserva- tive guide in deciding upon a sampling regime for the species for a seasonal sampling program. It was decided to sample seasonally, at a number of localities across the reef, by censusing two haphazardly chosen sites by three random 75 • 2 m transects at each site. Consequently each locality will be censused by six random transects on each sampling occasion. It is considered that this sampling regime is a compromise regime for the three species, being the best allocation of sampling effort and time available. This will provide estimates of density for each species per locality per season which can be compared statistically. However, it is not known what the power of the regime to detect real differences among populations will be or how much populations will have to change before significant changes will be detected.

No transect-based visual census technique is suitable for all species of coral-reef fish, or to sample in all habitats. Consequently, there is a responsibility on coral-reef ecologists to ensure that the sampling methodology is chosen to maximise accuracy, whilst efficiently using re- sources. To do this requires the preliminary investment of field time in small pilot studies (Sale et al. 1985), to avoid adopting sampling procedures which are inadequate to answer the questions posed. Presumably the amount of time invested initially, should be related to the aims of the sampling program. The use of such pilot studies and the application of such techniques as cost-benefit procedures to determine how sampling effort is best distributed should help to improve the accuracy, precision and confidence in the resulting estimates of density.

Acknowledgements. I thank Bruce Mapstone for his assistance during the conceptual stages of this work and Gina Mercer for her willing assistance during the field work stages. I thank in particular Peter Sale and Geoff Jones and also Neil Andrew, Tim Jones, Uschi Kaly, Marcus Lincoln- Smith, Ricardo Otaizo, Warren Steel, Laura Stocker and two anony- mous referees whose comments helped to improve various drafts of this paper. The study was supported by a grant to Peter Sale from the Great Barrier Reef Marine Park Authority and also by the University of Sydney, whilst I was supported by a Commonwealth Postgraduate Re- search Award. This is a contribution from the One Tree Island Field Sta- tion.

58

References

Anderson DR, Pospahala RS (1970) Correction of bias in belt transect studies of immotile objects. J Wildl Manage 34:141-146

Boer BA de (1978) Factors influencing the distribution of the damselfish Chromis cyanea (Poey), Pomacentridae, on a reef at Curacao, Neth- erlands Antilles. Bull Mar Sci 28:550-565

Brock VE (1954) A preliminary report on a method of estimating reef fish populations. J Wildl Manage 18:297-308

Brock RE (1982) A critique of the visual census method for assessing coral reef fish populations. Bull Mar Sci 32:269-276

Caughley G, Sinclair R, Scott-Kemmis D (1976) Experiments in aerial survey. J Wildl Manage 40:290-300

Chave EH, Eckert DB (1974) Ecological aspects of the distributions of fishes at Fanning Island. Pac Sci 28:297-317

DeMartini EE, Roberts D (1982) An empirical test of biases in the rapid visual technique for species-time censuses of reef fish assemblages. Mar Biol 70:129-134

Doherty PJ (1980) Biological and physical constraints on the populations of two sympatric damselfishes on the southern Great Barrier Reef. PhD thesis, Sydney University

Doherty PJ (1983) Tropical territorial damselfishes: is density limited by aggression or recruitment? Ecology 64:176-190

Eckert GJ (1985) Population studies on Labrid fishes on the Southern Great Barrier Reef. PhD thesis, Sydney University

GBRMPA (1978) Great Barrier Reef Marine Park Authority workshop on reef fish assessment and monitoring. Workshop series no 2. GBRMPA, TownsviUe, Australia

Hobson ES (1974) Feeding relationships of Teleostean fishes on coral reefs in Kona, Hawaii. Fish Bull 72:915-1031

Jones GP (in press) Competitive interactions among adults and juveniles in a coral reef fish. Ecology (in press)

Jones RS, Chase JA (1975) Community structure and distribution of fishes in an enclosed island lagoon in Guam. Micronesica 11:127- 148

Jones RS, Thompson MJ (1978) Comparison of Florida reef fish assemblages using a rapid visual technique. Bull Mar Sci 28:159-172

Keast A, Harker J (1977) Strip counts as a means of determining densities and habitat utilization patterns in lake fishes. Environ Biol Fish 1:181-188

Kennelly SJ, Underwood AJ (1984) Underwater microscopic sampling of a sublittoral kelp community. J Exp Mar Biol Ecol 76:67-78

Kimmel JJ (1985) A new species-time method for visual assessment of fishes and its comparison with established methods. Environ Biol Fish 12:23-32

McCormick MI (1986) Spatial and temporal pattern of abundance of Cheilodactylus spectabilis (Pisces: Cheilodactylidae). MSc thesis, Auckland University

Robertson DR, Lassig B (1980) Spatial distribution patterns and coexis- tence of a group of territorial damselfishes from the Great Barrier Reef. Bull Mar Sci 30:187-203

Robinette WL, Loveless CM, Jones DA (1974) field tests of strip census methods. J Wildl Manage 38:81-96

Russell BC, Talbot FH, Anderson GRV, Goldman B (1978) Collection and sampling of reef fishes. In: Stoddart DR, Johanneg RE (eds) Monographs on oceanographic methodology, vol 5: Coral reefs: research methods. UNESCO, Norwich, UK, pp 329-345

Sate PF (1974) Mechanisms of co-existence in a guild of territorial fishes at Heron Island. Proc 2rid Int Coral Reef Symp 1:193-206

Sale PF, Douglas WA (1981) Precision and accuracy of visual census technique for fish assemblages on coral patch reefs. Environ Biol Fish 6:333-339

Sale PF, Sharp BJ (1983) Correction for bias in visual transect censuses of coral reef fishes. Coral Reefs 2:37-42

Sale PF, Doherty P J, Eckert G J, Douglas WA, Ferrell DJ (1984) Large scale spatial and temporal variation in recruitment to fish populations on coral reefs. Oecologia 64:191-198

Sale PF, Jones GP, Choat JH, Leis JM, Thresher RE, WiUiams DMcB (1985) Current priorities in ecology of coral reef fishes. Search 16:270-274

Sanderson SL, Solonsky AC (1980) A comparison of two visual census survey techniques for fish populations. Pac Sci 34:337

Snedecor GW, Cochran WG (1967) Statistical methods, 6th edn. Iowa State University Press, Ames, Iowa

Stone RB, Pratt HL, Parker RO Jr, Davis GE (1979) A comparison of fish populations on an artificial and natural reef in the Florida Keys. Mar Fish Rev 41:1-11

Victor BC (1983) Recruitment and population dynamics of a coral reef fish. Science 219:419-420

Underwood AJ (1981) Techniques of analysis of variance in experimen- tal marine biology and ecology. Oceanogr Mar Biol Annu Rev 19:513-605

Documents

The development of sampling strategies for population studies of coral reef fishes. A case study