Early survivorship of juvenile coral reef fishes

Coral Reefs (1988) 7:117-124

�9 Springer-Verlag 1988 o

Early survivorship of juvenile coral reef fishes

Peter F. Sale * and Douglas J. Ferrell

School of Biological Sciences, University of Sydney, Sydney, NSW 2006, Australia

Accepted 3 May 1988

Abstract. Data on early survivorship of newly settled reef fish were collected by monitoring individuals which recruited to 30 small lagoonal patch reefs over three summers. Preliminary survivorship curves spanning the first 45 days after settlement were derived for 17 species. Most species showed greatest rates of mortality in the first 1-2 weeks in the reef environment however there were substantial differences among species in the extent and the temporal pattern of this. In six species, 75% of individuals survived the 45 days, while in 5 others, 20% or fewer survived that long. In eight species, mortality was negligible after the first 14 days. In the other 9, significant mortality occurred in subsequent weeks. Patterns of survivorship did not appear to differ substantially among years in five of the six species for which data were adequate. In particular, survivorship did not appear to be different among years even when levels of recruitment varied greatly.

Introduction

The majority of species of fishes inhabiting coral reefs have an obligate pelagic stage early in the life cycle. In many species, the eggs themselves are pelagic, and are re- leased in mid-water, often on down-current sides of reefs and/or on falling tides. In other species, although the eggs are demersal and cared for, they hatch into larvae which are pelagic (Sale 1980; Thresher 1984).

Because of this life history feature, assemblages of fishes on coral reefs are established and maintained by a continuing process of settlement of new individuals following the pelagic stage, rather than through reproduction within local sites. Since their habitat is patchy, and most species become sedentary following settlement, this process of continuing immigration from the plankton is the prime factor determining which species will occur in

* Present address: Zoology Department, University of New Hampshire, Durham, New Hampshire 03824, USA

which local site (Talbot et al. 1978; Sale and Douglas 1984).

Considerable attention has recently been directed to- wards the spatial and temporal variation in rates of recruitment of particular species of reef fishes (Doherty 1983 a; Victor 1983 a; Eckert 1984; Sale et al. 1984). In at- tempting to account for these fluctuations, authors have emphasized the very high rates of mortality during the pelagic stage, and the presumably highly variable patterns of dispersal which occur during that time (Doherty 1983 a). Such important factors can lead to pronounced spatial and temporal variation in rates of settlement from plankton to reef. In at least some times and places, rates of recruitment are insufficient to saturate resources required by fish on reefs, and these variations may play the major role in determining the structure of fish assemblages (Doherty 1983 b; Victor 1983 a, 1986).

By contrast, little attention has yet been given to the rates of survivorship of fish on reefs, particularly young fish during the first few days following recruitment. If the rate of early post-settlement mortality is high, and varies in space and time, it is potentially as important as larval mortality in determining the variation measured in rates of recruitment to older age classes.

Because newly settled fish arrive into an environment of older and larger resident fishes, of their own and other species, the pattern of early post-settlement mortality may be shaped by predictable biotic interactions involv- ing these fish. Predatory and competitive interactions might play very important roles in determining the structure of reef fish assemblages (Smith and Tyler 1975; Smith 1978; Shulman et al. 1983). Some authors (e.g. Tal- bot et al. 1978) suggest that predatory interactions play a major role in determining which juvenile fishes are able to persist in any given site.

Doherty and Sale (1986) explored the impact of predation on newly recruited reef fishes of a number of species. Their approach, which used cages to control levels of predation, required relatively high rates of settlement and subsequent recruitment to detect any treatment

118

effects. While they obtained useful data on early survivorship of several species, experimental artifacts masked effects of predation for other species. Still other species recruited in such small numbers that analysis of their survivorship was not possible.

In this study we observed juvenile fish on small isolated patch reefs. It was assumed that the expanses of sand between patch reefs ensured that juvenile fishes, once settled, remained close to their settlement sites. The study was non-manipulative to minimize the chance that measured rates of survivorship were influenced by our activities. While we cannot assign losses to particular causes of mortality, we present preliminary estimates of survivorship for a number of species of fish recruiting to patch reef habitat on the southern Great Barrier Reef.

Methods

This study was done over three successive summer settlement seasons in the main lagoon of One Tree Reef, southern Great Barrier Reef (23 ~ 30'S. lat., 152 ~ 06%. long.). During the summers of 1981-1982, 1982-

1983 and 1983-1984, a group of 30 small patch reefs was monitored on a regular basis for juvenile reef fishes. Reefs were visited every second day during the period November 19, 1981 to February 20, 1982, and during the period November 17, 1983 to February 16, 1984, and every third day during the period December 2, 1982 to February 13, 1983.

The 30 patch reefs varied in size from 2.7 to 23.8 m z surface area, were nonemergent, and rested on a sandy lagoon floor 2-4 m deep (low tide). They varied in the extent and diversity of live coral cover but were predominantly composed of coralline limestone covered with an algal turf. All contained non-manipulated assemblages of reef fishes, varying in age and species composition. Twenty of the reefs comprised a group that has been monitored since 1977 in a continuing study of the structure of small reef fish assemblages (Sale and Douglas 1984; Sale and Steel 1986).

On each visit, the diver would search carefully over the entire reef looking for all juvenile fish which had previously been observed, and looking for any newly settled individuals which had arrived since the last visit. When an absence was noted, a thorough search for that individual was carried out on that day and on at least two subsequent censuses.

We assumed that fish were seen on the first day they arrived on a patch reef (Day 0), that is, that settlement and recruitment occur within 24 h. Some species of reef fish are known to remain hidden in the inter- stices of the reef for periods o f several days after settlement (Victor 1983 b), but we have ignored this possibility in evaluating our data. We also assumed that a juvenile of a given species seen at the same location

DATE:

FISH MON TUE WED THU FRI SAT SUN 1 A X

2 A 3 A 4 A 5 A - - K 6 A--.X 7 8

DAY:

FISH 0 1 2 3 4 5 6 1 A X 2 A 3 A .X 4 A X 5 2. X 6 2 ~ 7 A I 8 A X I

MON TUE WED THU I

1

X I

I

A i A "l[ I

A= arrival day X= mor ta l i ty

7 8 9 I

I

I

I l

DAY: 1 2 3 4 5

n 8 8 7 6 6 Px 6 / 8 4 / 6 1 1 2 / 3 lx Method 1 6 / 8 4 / 8 3 / 7 3 / 6 2 / 6 lx Method 2 .75 .50 .50 .50 .33

081 0.7 0.6 0.5

.~ 0.4 0.3 0.2 0.1 �9 , �9 , �9 , ,

0 2 4 6 8 Day

6 7 8 9

5 5 3 2 1 /2 1 1 i 1 /5 1 /5 I / 3 1 /2 .17 .17 .17 .17

-0- Directly from population -o- From 1-[ point mortality

i

lO

Fig. 1 a--d. Example of method used to derive survivorship curves. a A graphical representation of our census technique with hypothetical data based on 11 days of daily censusing. b A single age cohort is generated for calculation of lx and Px curves. Note decline in cohort size as age is increased. See text. e Table ofp~ and lx calculations. In method 1, survivorship (Ix) is taken directly from the population under census (n), whereas in method 2 survivorship (Ix) is the product of individual px's. d Graph of the two lx curves generated by methods 1 and 2. Note the fluctuation in the curve taken directly from the sample population

119

over several days is the same juvenile. If it ceased to be present at that site, it was assumed to have died. This approach assumed that each species being monitored remained sedentary after settlement and that the individuals were likely to be found in the same vicinity from day to day. It was also necessary that rates of recruitment were not so great that many individuals of similar age were present in the immediate vicinity of one another. Were this to occur, it would not have been possible to discriminate reliably among individuals, and their survivorship could not have been measured. A few common species were mobile at an early age and in these cases, when it was no longer possible to reliably identify individuals on single reefs, observations were terminated. The species which caused us the most problems in this regard were: Escenius mandibularis, Coris variegata, Petroscirtes fal lax and Scarus spp. (several species of juvenile scarids which we did not at tempt to distinguish). These factors limited the range of species we were able to monitor, and the length of time following settlement that monitor ing could be continued for each species.

The data for any single species comprised a number of observations of each of a small number of individuals. Where at least 25 individuals were observed over the three summers, we dealt with the data for each summer separately, but for most species it was necessary to pool data obtained over the three years. Because individual fish settled at various times during the summers, the data obtained at any given census of reefs represent information on the survival to different ages of each fish present. Figure 1 demonstrates our t reatment of the census data with a fic- titious data set. In this example censuses began on a Monday (Fig. i a) and continued daily for 10 days. Two fish not seen on the Monday had arrived by the next day. Their survivorship, and that of any subsequent arrivals was noted for the duration of censusing, resulting in the matrix of Fig. 1 a. We rearranged the data matrix as shown in Fig. 1 b, generat- ing a cohort of equal-aged individuals to determine pattern of survivorship. Note that the starting size of this cohort becomes smaller as we con- sider survivorship to increasing age because only small numbers of individuals settle early in the season and can potentially be followed for many weeks (Fig. 1 c). The assumptions contained in our procedure will be discussed below.

One important consequence of the variable cohort size which our procedure generates, is that lx values are increasingly subject to error at greater ages because they are based upon survivorship within smaller and smaller cohorts of fish (Fig. 1 c). Indeed, if calculated directly as probability of survival f rom day 1 to day n, for each n, they can yield survivorship curves in which survivorship appears to increase with age after an initial decline. For example, in Fig. 1 b, c it can be seen that, after three days, only seven of the originaI eight fish can be included in any calculation of survivorship (1,). If l, is taken directly as a proport ion of those seven fish, then the l, curve rises at that point (Fig. 1 d). To avoid this, our lx values are calculated indirectly f r o m p , (the probability of sur- viving to age x from age x- l ) as outlined below:

/'/x p , = and l~= l l P ~ "

Dx-I x = l

Where P0 = 1.

Results

By the end of the three summer periods, we had collected data on 465 individuals belonging to 56 species. Many of these species settled only rarely to our reefs, and we restricted analysis to the 17 species for which we had accu- mulated data on at least 10 individuals (a total of 362 fish). Apart from Ostracion cubicus and Scarus spp., these 17 species belong to the families Pomacentridae, Blen- niidae and Labridae, and these three families together ac- counted for 379 of the 465 individuals monitored. We monitored the patch reefs for 96 days in 1981-1982, and detected 229 recruits. In 198~1983 we monitored for 74 days, and detected 70 recruits. There were 166 recruits detected during the 94 days of monitoring in 1983-1984.

Our data do not represent the total recruitment offish to these small patch reefs. Certain species, particularly among the Apogonidae, but including most eleotrids and gobiids, recruited in such large numbers and/or were so secretive in habit that it was not possible to monitor survival of each individual. All pomacentrids and wrasses, and all blenniids except Petroscirtes rnitratus recruited sparsely, and were monitored. On some reefs, however, recruits appeared simultaneously in groups of 4-5 fish, and individuals could not be separately monitored in such locations.

The size range at which individuals from any given species were first seen was very restricted and in most species, individuals differing by as little as two to four days in age could easily be distinguished on the basis of size. In no species monitored did individuals first appear at a size or stage of development that suggested they were not newly settled from the plankton or obviously several months old. Migration of older juveniles was very rare and of the species considered, only three Coris variegata and two Chromis nitida juvenile migrants were detected. All of these fish were more than double the length of new recruits of their species and were estimated to be at least two months old.

Of the species we monitored, the majority could be successfully identified as known individuals until we terminated censuses at the end of each summer period. This was possible either because they were sedentary, were rare on our patch reefs, or both. Fish of some species were sedentary enough throughout this early period of juvenile life that they were regularly re-sighted in the same sites each day. Fish of other species, although they be- came mobile enough with time to use a large part of, if not the entire, reef, recruited rarely enough that reefs did not contain more than one or two juveniles at the same time, and those juveniles present differed measurably in size/age.

The 17 species discussed here were generally very easily observed. One index of this was the incidence of scor- ing an individual as missing on one census and re-sighting it the next. Less than 2% of the pomacentrids and 6% of the wrasses recorded missing were subsequently re- sighted. Less than 10% of the blenniid absences were incorrectly recorded as missing, except for Atrosalarius fuscus, which along with Scarus spp. and Ostracion cubicus, were incorrectly recorded as absent 20% of the time.

Figures 2 4 display survivorship curves for the 17 most abundant species in our data. These curves were computed by pooling data over the three summers, and then caeulatingpx values for each three days following recruitment (= first sighting). Values for l x were then computed from the set of Px values obtained. The curves in Fig. 2 are primarily for species in the family Blennidae although a plot for the ostraciid fish, Ostracion cubicus is included. Figure 3 plots survivorship for the several Po- macentridae, and Fig. 4 shows the plots for the labroid fishes.

120

1.0 4

0 . S "

I 0.6"

0.4"

0.2

Blenni ids plus Osta'acion

-~ A. fuscus -4- E. mandibularis - - P. fallax -~- M. lineatus

- ~ ~ ~ - - - -~O. cubicus

10 20 30 40 50 Day

S t a r t i n g C o h o r t fo r day:. 0 15 30 45

33 30 23 16 48 43 38 20 14 14 14 8 15 15 13 11 13 10 2 2

Fig. 2. Survivorship of four blenniid species and Ostracion cubicus pooled from censuses of three recruitment seasons. The blenniid species are: A trosalarius fuscus, Escenius mandibularis, Petroscirtes fallax and Meiacanthus lineatus. The cohort sizes on which the survivorship curves are based are shown for four nominated ages on the right of the figure

1.0

.~ 0.8 az

o 0.6

0.4

0.2

Pomacentrids

o

i . . . . . '" ; ; ; =" = = ": : ~ P. wardl 27 ~ . . . . . . . . . ~ ~ P. ambotnensis 48

~ C. rgttida 22 . . . . . . . . . . -~- D. aruanus 25

~ Pom. spp. 12 -~- P. pavo 15

0 10 20 30 40 50

Starting Cohort for day: 15 30 45

27 24 1 42 24 7 22 22 22 22 18 11 12 10 5 12 6 3 15 i0 9

Day

Fig. 3. Survivorship of seven pomacentrid species pooled from censuses of three recruitment seasons. The pomacentrids are: Pomacentrus wardi, P. amboinensis, P. molluccensis, P. pavo, Chromis nitida, Dascyllus aruanus and Pomacentrus sp. [cf. Allen (1975, p. 214)]. Cohort sizes as in Fig. 2

Labroids

1.0

0.8 ~ ~ . . . . . . . .

0 . 6

0.4 ] ~ : :

0.2 ~ ' ' ' - ? '

0 10 20 30 40 50 Day

Starting Cohort for da~. 0 15 30 45

"~ T. lunare 19 14 7 "~ C. variegata 27 22 11 -x- H. melapterus 10 9 4 "~" S. strigiventor 10 8 6 - - Scarus sp. 10 10 10

6 Fig. 4. Survivorship of five labroid 7 species pooled from censuses of three 3 recruitment seasons. The labroids are: 4 8 Thalassoma lunare, Coris variegata,

Hemigymnus melapterus, Stethojulis strigiventor and Scarus spp. Cohort sizes as in Fig. 2

The graphs showed considerable variation among species in mortality during the first weeks on the reef and in the timing of those losses which occur. Per capita rates of loss (Table 1) appeared to be highest during the first week or two in all pomacentrids except Chromis nitida and in all labrids except Coris variegata L., Pomaeentrus wardi and Pomacentrus sp. (Fig. 3) showed negligible mortality after the first week, while Pomacentrus molluccensis (Fig. 3), Hemigymnus melapterus, Thalassoma lunare and Searus spp. (Fig. 4) had two weeks of high mortality before the rates dropped to a low level. Chromis nitida (Fig. 3), and Coris variegata (Fig. 4), and all the blen- nids (Fig. 2), showed approximately constant per capita rates of loss for at least 6 weeks on the reef. The patterns of mortality are such that after 30 days there is a clear di-

chotomy within the 17 species. Stethojulis strigiventer, T. lunare, and five of the seven pomacentrid species considered all have survivorship of 75% or greater while all of the rest have survivorship less than 60% of original numbers.

Figures 5 and 6 present Ix curves calculated separately for each summer for the six species for which we have most data. These data provide little evidence of a year-to- year variation in rates of survivorship. Survivorship of Ecsenius mandibularis appeared greater in 1981-1982 than in 1983-1984. However, other cases of apparent dif- ference among years involve cohorts far too small for us to have confidence in the lx curve obtained (Coris variegata and E. mandibularis, 1982-1983 versus other years). No species for which the 1982-1983 cohort included

121

Table 1. Per capita rates of mortality as calculated for each species in blocks of 6 days following settlement. Means are not weighted

Days

1 to 6 7 to 12 13 to 18 19 to 24 25 to 30 31 to 36 37 to 42

Cromis nitida 0.29 0.09 0.28 0.20 0.50 0 0.17 D. aruanus 0.04 0 0 0.:17 0 0 0 P. amboinensis 0.11 0.04 0.05 0.04 0 0 0 P. molluccensis 0.27 0 0.25 0 0 0 0 P. pavo 0.07 0 0 0 0 0 0 P. wardi 0.07 0 0 0 0.06 0 0 Pomaeentrus spp. 0.25 0 0 0 0 0 0

Mean for pomacentrids 0.16 0.02 0.08 0.06 0.08 0 0.02 C. variegata 0.20 0.09 0 0.11 0.42 0 0.50 H. melapterus 0.32 0 0.20 0 0 0 0 S. strigiventor 0 0 0 0.16 0 0 0 T. lunare O. 18 0 O. 11 0 0 0 0 Scarus spp. 0.50 0 0.33 0 0 0 0

Mean for labroids 0.24 0.02 0.13 0.05 0.08 0 0.10

A. fuscus 0.19 0.15 0.24 0.13 0.08 0 0 E. mandibularis 0.19 0.06 0.32 0.12 0.21 0.29 0.25 M. lineatus 0.14 0.25 0.57 0.33 0 0 0 P. fallax 0.47 0 0.20 0 0.67 0.75 0

Mean for blenniids 0.25 0.11 0.33 0.15 0.24 0.26 0.06

O. cubicus 0.13 0 0.50 0 0 0 0

more than two to three fish had survivorship higher that year than in the other 2 years. Survivorship soon after settlement appears not to be inversely related to cohort strength.

Discussion

In this study, we have reported preliminary estimates of early survivorship following recruitment to a patch reef habitat by 17 species of coral reef fish. The data suggested that there were substantial differences among species in rates of mortality, and in temporal patterns of mortality during the first six weeks of juvenile life (Table 1). For example, Pomacentrus wardi, Dascyllus aruanus (Fig. 3) or Stethojulis strigiventer (Fig. 4) showed negligible mortality after settling to the patch reefs, while in Meiacanthus lineatus (Fig. 2), Chromis nitida (Fig. 2), or Coris variegata (Fig. 3), no more than 20% of fish arriving on patch reefs survived the first 2 months.

Our method of determining survivorship curves was subject to several possible sources of error and the small samples we obtained precluded repeated calculation from which confidence intervals might have been obtained. Nevertheless, we believe this method to be the only method of real utility for obtaining such information for the bulk of coral reef species. These fish tend to be un- common, and to recruit in small numbers over several weeks or months each year. The more conventional approach in which survivorship is followed for a cohort which recruited at the same time and location is not fea-

sible for providing short-term estimates of survivorship (but see Eckert 1987).

Our estimates of mortality would have been biased upwards if fish, having been seen once, were subsequently not seen because they were cryptic; or dispersed, but did not die. While we could not quantify the extent of this error, we believe it to be slight. Given the small proportion of fish incorrectly scored as missing and subsequently seen, the probability of missing individuals on multiple censuses was very small, indicating that fish repeatedly recorded as missing had indeed disappeared permanently from that reef. Furthermore, the consistently very nar- row size range at which each species was detected for the first time suggested fish were not dispersing to neighbour- ing patch reefs. The patch reefs monitored were all isolated from other reef habitat by 5 m or more of open sand. Young fish present on such isolated patch reefs are almost entirely resident individuals which confine their activities to the single reef for long periods of time (Tal- bot et al. 1978; Sale and Douglas 1984; Eckert 1987). This alone makes it unlikely that a significant number of juveniles recorded as missing had moved away to other locations, but the method may be less reliable in other habitats.

We have also assumed that while rates of mortality may have varied with time since recruitment, they did not vary from one time of year to another or between years. We created cohorts of same-age fish by pooling data on individuals which recruited onto patch reefs over periods of several weeks during the peak summer recruitment

122

A t r o s a l a r i u s f u s c u s

S t a r t i n g cohor t for day:

0 1 5 3 0 4 5

-~- 1981-2 8 8 8 4 -~- 1982-3 6 3 2 2 4 - 1983-4 19 19 14 10

q

-,-'1

03

l o ii;. 0.8'

0.6"

0.4 ' - ~ _

- - m

0.2 , , , ,

Coris variegata

Sta r t i ng cohor t for day:

0 15 3O 45 -m- 1981-2 iO 9 6 3 - ,- 1982-3 2 i i i -m- 1983-4 15 12 4 3

1 . 0 . ; - ; " - " . . . . . . . .

\ . . . . . . . . �9 - - : : " "~ ---. -= ~" ~" .~ -2. .~ : -

0 . 6 -

0.4

0 IO 20 30 40 50

D a y

D a s e y l l u s a r u a n u s

S t a r t i n g cohor t for day:

0 1 5 3 O 4 5

-a- 1981-2 11 10 8 5 § 1982-3 9 7 6 3 -m- 1983-4 5 5 4 3

Fig. 5. Survivorship of A trosalarius fuscus, Coris variegata, and Dascyllus aruanus for each of summers 1981-1982 th rough 1983-1984. Cohor t sizes as in Fig. 2

seasons of 3 years. Fish which recruited early spent their first week on the patch reef in November, while those which recruited later did so in January or February. The only way we could have avoided pooling in this way would have been to increase the number of fish detected during a single short period and this was not possible with the man-power available.

Data presented in Figs. 5 and 6 suggest that rates of survivorship were consistent from year to year for five of the six species considered. This result must remain tenta- tive until additional data are gathered, but suggests that pooling across years to obtain preliminary survivorship curves may be appropriate. Our results, which are for early survivorship, contrast with those of Eckert (1987) who demonstrated that annual survivorship of cohorts of young fish was different among years.

Although these data are preliminary, they are the first such data to be presented in a comparative way for a se- lection of coral reef fishes. They indicate that for the types of fish represented here, survivorship during the first week following recruitment may be as high as 90%,

but can often be as low as 50%-70%. In all the species with relatively high mortality rates in the first week, rates of loss dropped substantially by the end of the first month. This result confirms our general impression that for some reef species survivorship is close to 100% after the first week or so in reef environments.

We have emphasized the relatively high survivorship characteristic of some of the species we monitored following the first critical weeks in the reef environment. This is in contrast to other kinds of fish such as apogo- nids on lagoonal patch reefs (P. Sale personal observa- tion) or some planktivorous pomacentrids (Doherty and Sale 1986) or wrasses (Eckert 1987) which appear to expe- rience high mortality throughout juvenile life.

Other studies on the survivorship of newly recruited reef fish have demonstrated patterns similar to those reported here: that mortality is highest immediately after settlement and that survivorship thereafter may vary greatly among species. Doherty and Sale (1986) did not obtain separate data for any of the species we examined, but rates of survivorship in the reef slope habitat for the

0 . 8 ~

0.6

0.4 ~

0.2 , , �9 �9 ,

Escenius mandibularis

Starting cohort for day:

O 15 30 45 -B- 1981-2 18 17 15 3 -r 1982-3 5 1 1 1 -~ 1983-4 25 25 22 16

123

1 . 0 " ~

0.8"

~ 0.6"

O

"~ 0 . 4 -

m 0.2 , , , ,

Pomacentrus amboinensis


0 15 30 45 -~ 1981-2 27 27 19 3 -~ 1982-3 16 11 2 2

1983-4 5 4 3 2

1 . 0 " ~

0.8'

0.6-

0.4-

0,2 , , , , ,

0 10 20 30 40 50 D a y

group of juveniles most similar to the species we examined were approximately 65% for the first 30 days of life. Most mortality occurred in the first 5 days after recruitment. Doherty (1983c) found that two week old Po- macentrus wardi had very low mortality within 2 or 3 days of being transplanted, similar to our finding for similar aged P. wardi. Victor (1986) demonstrated that survivorship for Thalassoma bifasciatum was less than 45% for the first 30 days, with the highest daily mortality during the first days on the reef.

Most who have studied survivorship of juvenile fish have not been able to census with the same frequency we achieved (but see Victor 1986) and/or were unable to fol- low fish as individuals. Both of these factors were important in detecting the high rates of mortality experienced by the species discussed here in the first few days after settlement. By censusing every second or third day, we presumably did not detect a proportion of recruitment and subsequent mortality. Victor (1986) censused daily and found the highest rates of mortality on the first day after detection. It seems likely then, that our estimates of

Pomacentrus wardi


0 15 30 45 -~- 1981-2 8 8 6 I -a- 1983-4 19 19 18 0 Fig. 6. Survivorship of Escenius

mandibularis, Pomacentrus amboinensis and P. wardi for each of summers 1981- 1982 through 1983-1984 plotted separately. Cohort sizes as in Fig. 2

very early mortality from 1982-1983 were underestimates relative to the other 2 years and that other studies using less frequent censuses have also underestimated early mortality.

There remains one final proviso. These data have been obtained from patch reef habitat on the southern Great Barrier Reef. They must be considered habitat- and location-specific and it is likely that species life his- tories will vary both among habitats and among locations (Aldenhoven 1986). For example, while Pomacen- trus molluccensis (Fig. 3) showed a loss of about 50% in the first 2 weeks in this study, followed thereafter by good survivorship, its early survivorship was much better in patches of lush coral habitat within the same lagoon (Sale et al. 1986; B. Mapstone personal communication).

Rates of recruitment of many species at One Tree Reef appear to be substantially (1-2 orders of magnitude) below those recorded for the same species at Lizard Is- land, 1000 km further north (H. Sweatman personal ob- servation; R.Pitcher personal communication), which

124

seems to have unusually high rates of recruitment of reef fish (of. Luckhurst and Luckhurst 1977; Molles 1978; Sweatman 1983; Sale 1985). Yet standing crops of reef fish (at least in patch reef habitat) were, if anything, lower at Lizard Island than at One Tree Reef (Sale unpub- lished), suggesting a markedly different pattern of post- recruitment survivorship at the two locations.

Acknowledgements. The censuses were done with able assistance from A. McDiarmid, W. Douglas, and A. Fowler, but we also thank P. Doherty, T. Jones and Graeme and Wendy Russell, who, by assisting with other field tasks facilitated the field work reported here. A. W. Meats provided helpful demographic comment and B. Mapstone, A. Fowler and two an- onymous reviewers gave helpful criticism. Funds were provided through a Marine Science and Technology grant to PFS. This paper is a contribu- tion from the University of Sydney's One Tree Island Field Station.

References

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Doherty PJ (1983 b) Diel, lunar and seasonal rhythms in the reproduction of the tropical damselfishes: Pomacentrus flavieauda and P. wardi. Mar Bio175:215-224

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Documents

Early survivorship of juvenile coral reef fishes