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Butterflyfishes of the Southern Red SeaZekeria, Zekeria Abdulkerim

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Butterflyfishes of the Southern Red Sea 81

Chapter 8

Growth of the Brownface butterflyfish (Chaetodon

larvatus) in the Southern Red Sea

Z. A. Zekeria, S. Weertman, B. Samuel, T. Kale-ab and J.J. Videler

Chapter 8. Growth Patterns82

Abstract

Chaetodon larvatus is the most abundant butterflyfish in the southern Red Sea.Otolith microstructure analysis and length-frequency analyses were used to study the ageand growth of this species. 180 C. larvatus specimens were collected from a reef nearMassawa and their otoliths extracted. Alternating opaque and translucent bands wereobserved in sectioned sagittae of all specimens. An annual pattern of increment formationwas confirmed by marginal increment analysis. Length-at-age data revealed a very fastgrowth during the first year. Growth proceeded at a slow rate during the second and thethird year but fishes older than three years did not grow noticeably. No difference ingrowth between males and females could be detected. The growth parameters obtainedfor the whole population are: the asymptotic standard length (L∞) = 10.64 cm, thegrowth constant (K) = 1.14-1 and the theoretical age at length zero (t ) = -0.30yr. Themaximum-recorded age was 14 years.


Introduction

Age determination and growth studies are essential for understanding the lifehistory of fish species. Several methods are employed to estimate the age and growth offishes. The two most frequently used ageing methods are interpretation of periodicdeposition of calcified tissues and length frequency analysis (Weatherlay & Gill 1987).The first method uses growth bands in the scales, otoliths or vertebrae to estimate fish agewhile the second is based on the length distribution of fish in a cohort of fish andmonitors changes in the distribution with time. Both methods have been widely employedfor growth and ageing studies of temperate fishes and yielded good results. Untilrecently, the techniques were not employed for tropical fish growth studies. Two mainreasons hampered the application of the above techniques for tropical fish studies. Firstly,tropical fish were assumed to lack seasonal growth patterns, which was thought to resultin poorly developed growth marks in the hard parts. This assumption precluded the use ofscales, otoliths, and vertebrae in growth studies in the tropics (Brothers 1980). Secondly,tropical fishes were believed to lack seasonality in their recruitment. Protractedrecruitment would results in the development of skewed and bimodal length distributionsof cohorts (Isely & Nobel 1985). In the absence of a distinct recruitment season, it isdifficult to employ the length frequency method for growth studies.

However, Longhurst and Pauly (1987) pointed out that in most tropical fishesgrowth follows predictable seasonal patterns, which can be detected using length frequentdata, or by the analysis of seasonal bands in the otoliths. Pannella (1971) first discussedthe daily growth rings in the otoliths of fishes. Since then, many workers investigatedgrowth of tropical fishes using the length frequency method and by examining growthrings in the otoliths. Many models have been developed that employ length frequencydata to assess growth patterns of fishes from lower latitudes (Pauly 1987). At the sametime developments in the methodology of otolith microstructure analysis made wide useof otoliths for ageing of tropical fishes possible (Stevenson & Campana 1985). As aresult, in the two decades growth patterns from a number tropical fish species have beenreported (Boesa 1987, Ntiba & Jaccarani 1988, Morales-Nin & Ralston 1990, Acosta &Appeldoorn 1992, Al-Ghais 1993, Choat & Axe 1996, Choat et al 1996, Wilson andMcCormick 1999, Harris & Collins 2000, Labropoulou & Papaconstantinou 2000,Newman et al 2000).

To date, growth studies of coral reef fishes were restricted to a few familiesincluding lutjanids, haemulids, sparids and serranids (Choat & Axe 1996). Theserepresent a limited range of the taxa that make up the reef fish assemblage. Informationon growth rate and life span is scarce for other families of coral reef fishes. For example,we are aware of only one study on the growth of butterflyfishes. In that study, Fowler


(1989) investigated the microstructure of daily growth rings in three chaetodontid speciesfrom the Great Barrier Reef.

Butterflyfishes (Chaetodontidae) are prominent members of most tropical reef fishcommunities. Fifteen species were recorded from the Red Sea of which seven areendemic to the region (see chapter 2). Chaetodon larvatus is endemic to the Red Sea andis the most abundant chaetodontid in the Eritrean coastal waters (see chapters 3 & 4).Studies on the feeding habit and social behaviour of C. larvatus show that adults live inheterosexual pairs and defend feeding territories ranging in size from 24m2 to 66m2. Thefish are obligate corallivores and their abundance correlates with live coral cover (seeZekeria, in press). Despite its high abundance in the Eritrean Red Sea and its outstandingbeauty, C. larvatus has not yet been extensively collected for the aquarium fish trade(anonymous, 1999). This is probably because the species is an obligate corallivore whichis difficult to keep in an aquarium (Wood 1992).

In this paper, we investigate the patterns of growth in C. larvatus. Otolith analysisand length frequency methods are used independently for the growth studies and resultsof the two methods are compared. Mortality rates of young C. larvatus are estimatedbased on abundance data collected by visual census.

Material and Methods

Study site

The study was conducted in the southern Red Sea near Massawa (Fig 8.1). Fishsamples were collected from the reef east of Sheik Said Island while field data on theabundance of C. larvatus were obtained from Resimedri reef. This reef, which reaches amaximum depth of 10 m, is about 2km long and forms a narrow fringe along the coastwhere diverse corals are patchily distributed. Dominant coral genera include Porites,Echinopora, Montipora, and Stylophora. Monotypic coral carpets of branchingMontipora cover some pockets on the reef. Preliminary surveys showed that thesepockets are used as recruitment and nursery grounds by juvenile C. larvatus.

Collection and dissection of fish

Adult C. larvatus were sampled at monthly intervals from July 1998 to July 2000using a barrier net. Juvenile specimens were collected in June and July 1999 using theanaesthetic quinaldine. The fishes were preserved on ice and transported to the laboratorywhere their total length (mm), standard length (mm), and body mass (g) were measured.The fishes were then dissected and the gutted mass measured. The sex of adult fish wasdetermined by macroscopic examination of the gonads.


Extraction and polishing of otoliths

sagittae otoliths were extracted by dissecting the head of the fish dorsally from thetip of the snout to the anterior end of the dorsal fin base. The otoliths were cleaned withbleach and rinsed in distilled water. The mass of 38 sagittae was measured to the nearest0.01g and length measurements were taken along three axes to the nearest 0.01mm (Fig8.2). The length figures were related to the ages of the fish using linear regression.

The sagittae were embedded in epon and polished sequentially using carbonadepapers, alumina slurry, polishing cloth and lapping films. Both sides of the otolith werepolished until a thin transverse section through the core remained. These sections wereexamined with transmitted light using a light microscope at 400X magnification. Foreach otolith, two independent counts of annual bands were conducted. The periodicity ofannuli formation was validated using marginal increment analysis.

The von Bertalanffy growth curve was fitted to the available length-at-age datausing the ELEFAN I program (Pauly & Morgan 1987). Growth rates of males andfemales were determined separately and the results compared. FISAT (FAO-ICLARM1997), a computer aided software, was used for the determination of growth rateparameters and for comparison of growth between the sexes.

Figure 8.1. Study site. FCS = fish collection site; MP = Massawa proper; OS = observation site;RR = Resimedri reef; SSI = Sheik Said (Green) Island; TW= Twalot.


Visual census of juveniles and adults

The abundance of juvenile and adult C. larvatus was estimated using visual censuson Resimedri reef. Preliminary surveys showed that the distribution of juvenile C.larvatus was restricted to areas covered by branching corals whereas adult fish werewidely distributed over the whole reef. Due to the spatial variations in distributionbetween young and adult fish, different methods were employed to estimate the densityof the two age groups.

Density and length of juvenile fishes was estimated by surveying a fixed sitecovered by dense branching Montipora. Five 50 m2 quadrats were selected andpermanently marked using nylon rope and nails. Each quadrat was subdivided usingropes into 10 m2 stripes to assist in the counting process and to avoid double counts. Thesurveys were carried out by diving on the reef between May 1998 and July 1999 at 7-14days intervals. During each survey, the number of fish within the quadrats was countedand their size estimated to the nearest cm. Accurate size estimates could be made becausedata were collected while diving close to the substrate. A cm-scale glued to a PVC slatefacilitated length estimation. Moreover, samples of young fish were occasionallycollected from nearby sites using the anaesthetic quinaldine to validate the size estimates.Comparisons between estimated and actual length showed that the former is one thirdhigher than actual lengths. Thus, length estimates were corrected accordingly.

Visual counts of adult C. larvatus were conducted at monthly intervals from May1998 to July 1999 at each of five 50m line transects. Data were collected by slowlysnorkelling along the permanently fixed line transects and counting the number of C.larvatus observed within 2.5 m on either side of the lines. Total length of the fishes wasestimated to the nearest 2 cm. All counts were made between 8:00 and 13:00 hr and thedata were recorded on a PVC slate. The underwater magnification effect was corrected byreducing the estimated size by one third.

Figure 8.2. Schematic diagram of a transverse section of a Chaetodon larvatus otolith showing thethree axes along which length measurements were made.


Results

Age and growth estimates using otolith bands

The total length of the fish sample ranged from 3.2cm to 11.9cm. Out of the 180specimens, 62 were males, 71 were females, 10 were juveniles (< 6.7cm), and theremaining 37 were not sexed. Relationships between body mass and length measurementsare given in table 8.1. The morphometric measurements were taken from 600 specimens,which were collected for this and other studies. Comparisons of morphometricmeasurements for males and females showed no size variations between the two sexes(P>0.05).

Table 8.1. Relationships between length and mass measurements for both sexes combined, for males,and for females. r = correlation coefficient; N = number of sample; SL = standard length; TL = totallength; BM = body mass; GM = gutted mass.

Both sexes Males Females

Relationship: a b r N a b r N a b r N

SL = aTL + b 0.75 0.02 0.97 559 0.74 0.11 0.98 294 0.77 -0.13 0.98 320

BM = aTLb 3.10 -3.66 0.99 597 3.09 -3.64 0.99 290 3.11 -3.68 0.99 322

GM = aTLb 3.12 -3.80 0.96 563 3.15 -3.86 0.98 270 3.16 -3.90 0.98 299

BM = aGM + b 0.90 0.37 0.99 552 0.94 -0.55 1.00 264 0.87 0.76 0.99 294

All polished otoliths had concentric opaque and hyaline bands. The non-transparentopaque bands were to be wider than the translucent hyaline ones. Observations of themarginal increments of samples taken throughout the year revealed that opaque bandswere deposited during the colder winter months while hyaline bands were formed duringthe warm summer period (fig. 8.3). This confirmed the annual deposition of one opaqueand one hyaline band. In 174 otoliths, the seasonal bands were clearly visible; in 6 thiswas not the case. Daily rings could be counted under higher magnification in the otolithsof juveniles of less than one year.

The same person counted annual bands of the otoliths twice with an interval of atleast 8 weeks. The results were identical in 146 cases. There was a difference of one inthe counts of 23 otoliths. We decided to take the value of the last count in these cases.The difference was larger than one in the remaining 5 cases and these were discarded.

The ages of the fishes in the sample varied from a few months to 14 years (Table 8.2).Eight individuals were smaller than 6.0 cm and they were less than one-year old. Thirteenone-year-old fishes ranged in size from 6.0 to 10.0 cm. The remaining 148 individuals werelarger than 8.0 cm and their ages ranged from 2 to 14 years. This result shows that size is agood predictor of age for individuals smaller than 8.0 cm but for individuals larger than 8.0cm it becomes difficult to estimate the age from the size of the fish.


The Von Bertalanffy growth model was fitted to the length-at-age data for the maleand females separately. The L∞ values obtained for males and females are 10.6 cm and10.7 cm respectively and the corresponding K values are 1.04 y-1 and 1.10 y-1. Statisticalcomparisons show no variation in growth between the two sexes (Table 8.3). Length-at-age data from both sexes were analyzed using FISAT and the resulting growthparameters were used to draw a growth curve (Fig 8.4). The result shows that C. larvatusgrows very fast during the first two years after which growth slows down. Fishes olderthan three years do not change in size noticeably.

Table 8.2. Length at age for 169 individuals of C. larvatus grouped into nine size classes. The totalnumber of fish in each size class is shown on the right end of the table; total numbers in each yearclass are given at the bottom of the table.

Age (yr.)

Length


Table 8.3. Estimates of the von Bertalanffy growth parameters K (yr-1), Loo (TL, cm), and to (y) formales, females and sexes combined C. larvatus. Lower boundary (LB) and upper boundary (UB) forthe 95% confidence interval are also given.

Parameter Estimate LB UBSexes combined L∞ 10.64 10.55 10.72 (n = 164) K 1.14 1.03 1.24 To -0.30 -0.37 -0.23Males L∞ 10.64 10.49 10.74 (n = 62) K 1.04 0.90 1.19 To -0.34 -0.44 -0.24Females L∞ 10.71 10.58 10.85 (n = 71) K 1.10 0.95 1.24 To -0.31 -0.39 -0.22

Otolith length and otolith mass measurements were compared with the age of thefish (Fig 8.5). Regression equations for the relationships between the age of the fish andmorphometric measurements of their otoliths are given in Table 8.4. There is strongcorrelation between the size parameters and age. Otolith mass increases linearly with age.The length parameters are related to age following power curves. All these relationshipsare highly significant.

Table 8.4. Relationships between the age (A) of C. larvatus and morphometeric measurements of theotoliths. Lx, Ly and Lz are lengths of otoliths along three axis’s as shown in figure 8.2. (r =correlation coefficient, p = probability).

Parameters Regression r p

Otolith mass (m) A=0.57m +1.75 0.97


Estimates of growth rates using length frequency data

The distribution of juvenile C. larvatus is restricted to pockets covered by branchingcoral but adults are abundant throughout the reef. Length frequency distribution of adultfish was monitored for twelve consecutive months (Fig. 8.6). Most of the adult fishrecorded from the sites were less than 9.5 cm long and densities fluctuated around 60individuals 1000 m-2 with no apparent seasonal pattern. Neither the size of adult fish northe density changed during the study period.

A cohort of juvenile fish joined the adult population in October 1999 at an averagesize of 6.5 cm. The length of the cohort increased from 6.5 cm to 8.5 cm in eight months.At the same time the density of the young fish decreased gradually from 280 to 30individuals 1000 m-2 (Fig 8.6).

Juvenile length increase is shown in figure 8.7. Recruitment of C. larvatus occurredin June and July. The recruits remained at the recruitment site until October. During thistime, their average size increased from 4.5 cm to 6.5 cm. They reached an average size of8.5 cm in January. From January onwards, the number of young fish remaining in therecruitment sites was so small that it was difficult to follow the growth of the cohort.

Figure 8.4. Growth curve for C. larvatus in the Red Sea. The von Bertalanffy growth curveL t = 10.64 - e -1.14 (t + 0.30) was fitted to the length-at-age data.

0

2

4

6

8

10

12

0 2 4 6 8 10 12 14

Age (yr)

Tota

l len

gth

(cm

)


Figure 8.5. Relationships between length and mass measurements of otoliths and fish age for C.larvatus.

The first part of the Von Bertalanffy growth curve of figure 8.4 is compared withthe average sizes recorded during the visual surveys in figure 8.7. Growth estimatesobtained from the two independent methods are remarkably similarity. In both casesyoung fishes grow fast during the fist 8 months. The rate was very fast from June toOctober but slowed down from October onwards.

Changes in the density of juvenile C. larvatus through the first eight months aredepicted in figure 8.9. The numbers decreased gently during the first three months.During the next two months the abundance decreased sharply. The average size duringthat period was about 6.5 cm. They become too big to hide among Montipora branchesand probably leave the recruitment site to seek shelter among larger corals. A steadydecrease in the density of young fishes at the recruitment sites was recorded fromOctober to January.

r = 0.97

02

46

810

0 2 4 6 8 10 12

Age (y)

Oto

lith

mas

s (m

g)

r = 0.91

1.0

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Oto

lith

leng

th (m

m)

r = 0.86

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Oto

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th, L

y (m

m)

r = 0.91

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leng

th L

z (m

m)


May-98

2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.50

20

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Abun

danc

e per

100

0m-2

Length (cm)

Figure 8.6. Monthly changes in size and abundance of adult C. larvatus in the southern RedSea


Figure 8.8. Changes in the density of juvenile C. larvatus during the first 10 months after recruitment.

0

50

100

150

200

250

300

J J A S O N D J F M A

Months

Fish

abu

ndan

ce (c

ount

100

0m-2

)

Figure 8.7. Comparison between the von Bertalanffy growth curve obtained from length-at-age data(solid line) and length frequency data (scatter plot ) of juvenile C. larvatus.

0

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8

May Jul Sep Nov Jan Mar

Age (yr)

Tota

l len

gth

(cm

)


Jun-98

2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.50

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Figure 8.9. Monthly changes in size and abundance of juvenile C. larvatus in the southern RedSea


Discussion

Growth estimates of tropical fishes have been considered problematic due to thesupposed lack of annual bands in the hard parts and because of the believe in the absenceof a distinct spawning season (Morales-Nin & Ralston 1990). However, Longhurst andPauly (1987) pointed out that the opinion had no scientific basis. Recent studies haveshown that it is indeed possible to determine the age of tropical fishes by reading growthbands in the otoliths (Ntiba & Jaccarini 1990, Fowler 1989, Brown & Sumpton 1998,Morales-Nin & Ralston 1990). Results from the present study revealed the presence ofdistinct growth bands in the otoliths of C. larvatus. This is the first study where otolithshave been used to determine the age of fish in the Red Sea.

Alternative bands of opaque and hyaline bands are deposited yearly in the otolithsof C. larvatus. Opaque bands are formed during the colder season while deposition ofhyaline bands takes place during the summer. This result validates the annual periodicityof band deposition. Annual deposition of an opaque and a hyaline band in the otoliths oftropical fishes has been recorded for Lutjanus kasmira (Morales-Nin & Ralston 1990),surgeonfishes and parrotfishes (Choat & Axe 1996).

Seasonal growth cycles in tropical fish might be related to physiological changesinduced by the influence of temperature, feeding regime and reproductive cycle. Morales-Nin & Ralston (1990) pointed out that a seasonal temperature difference of 2-3°C mightbe sufficient to cause band formation in fish otoliths. In the study area the watertemperature ranges between 27.5°C and 33°C and reproduction in C. larvatus shows aclear seasonal cycle (see chapter 6).

Many workers have reported the deposition of opaque bands in summer (Fowler1990, Choat & Axe 1996, Lopez Cazorla 2000). The present work, however, show thatopaque bands were deposited in the colder season. According to Fowler (1990b), opaqueand hyaline bands are generally deposited during periods of fast and slow growthrespectively. Results from the present work suggest that C. larvatus grow faster in thecolder season. Although high temperature is known to enhance growth, the extreme highwater temperature in summer months (>32°C) could inhibit growth in adult C. larvatus.Moreover, bleached corals, which are common in the study area during the summer(personal observation), could have less nutritional value than healthy corals found duringthe cold season.

A very fast growth was recorded for juvenile fish during the summer. Preliminaryinvestigation of feeding rates showed that juvenile fishes take 50% more bites per unittime than adults. The observed fast growth of juvenile fishes could be due toconsumption of more food. In their experimental studies on the growth of juvenilesablefish (Anoplopoma fimbra), Sogard and Olla (2001) observed faster growth of the


fish at higher temperature. They pointed out that the rapid growth rate at highertemperatures was due to increased food consumption.

The mass and the size of otoliths show strong correlation with the age of the fish.Unlike the length of the fish, which reaches asymptotic values after about three years, theotoliths increase in mass and size with increasing age. Hence, otolith growth is notsynchronous with the growth of the fish itself. Positive correlation between the length andmasses of otoliths was recorded for acanthurids and scarids (Choat 1996), macrouridfishes (Labropoulou & Papaconstantinou 2000), damselfish (Fowler 1990), Bay anchovy(Leak 1986). The very strong relationships between the mass and size of the otolith createthe possibility of ageing C. larvatus using otolith mass or length.

Rapid growth of juvenile butterflyfishes was recorded for a number of butterflyfishspecies. C. Miliaris grew to over 8 cm within the first month (Ralston 1976). Similarly,C. rainfordi and C. plebius grew to about 6.0 cm and 6.5 cm respectively in one-yeartime (Fowler, 1989). Ralston (1976) pointed out that the rapid growth of juvenilebutterflyfish is an adaptation to predator pressure. Predation on adult butterflyfishes isvery low because predators are discouraged from consuming the compressed, spiny adultfishes, which may lodge in a predator's mouth. In contrast, juveniles are poor swimmersand they are easily handled by predators, which make them easy targets for heavypredation (Neudecker 1989).

The length frequency data provided a means to check results obtained from otolithband counts. Recruitment in C. larvatus is seasonal in the southern Red Sea wherejuvenile fishes settle on branching corals in June and July (see chapter 7). These youngfish form a distinct cohort whose growth is easy to follow by monitoring changes in fishsize. In the present study, the growth pattern estimated from otolith bands is similar toresults obtained using length frequency data. Both methods show fast growth of youngfish during the first few months of the fish life.

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