Journal ?/'Fish Biology ( 199 I ) 39,849-866

Age and growth of three species of tuna baitfish (genus: Spratelloides) in the tropical Indo-Pacific

D. A. MILTON, S. J. M. BLABERANDN. J. F. RAWLINSON CSIRO Division of Fisheries, Marine Laboratories, P.O. Box 120, Cleveland,

Qld 4163, Australia

(Received 12 April 1991, Accepted6 July 1991)

Sprafelloides grarilis (Temminck & Schlegel), S. delirarulus (Bennett) and S. lewisi (Wongratana) live generally less than 4 months. They lived more than 5 months at only three of 10 sites sampled. Growth varied seasonally in S. grocilis(Temminck & Schlegel) at two sites in the Solomon Islands and in S. clelicululus (Bennett) at Helix Reef, near Townsville, Australia. This variation in the growth rate of S. grucilis could not be related to specific environmental patterns, but that of S. delicurulus at Townsville might be temperature-related. Instantaneous growth rates for all species were 0.4-1.7 mm day-' at an age of I month and the range of variation in growth rate was similar for all species. These rates declined to 0.1 mm dayy at 3 months of age, which is the rate reported for other short-lived clupeoids at the same age. The growth rates of fish from coral atoll and coastal lagoons were similar but barrier reef fish grew significantly faster. These results indicate that Sprarelloides have an extremely flexible growth pattern and that biological variation within a site can be as great as variation between sites.

Key words: SprateNoides; growth; tuna baitfish; short-lived; temporal variation; spatial variation.

I. INTRODUCTION

Baitfish are important to the pole-and-line skipjack tuna Kutsuwonis pelamis (Linnaeus) industry in tropical Indo-Pacific island states (Blaber, 1990). This industry relies on adequate supplies of suitable live baitfish to attract the tuna. These bait species are predominantly small, schooling pelagic clupeoids particu- larly anchovies (genus Encrasicholina) and sprats (genus Spratelloides). Two Spratelloides species are widely distributed throughout the Indo-west Pacific from the east coast of Africa (Whitehead & Wongratana, 1986) to Fiji and are important components of baitfisheries in both oceans (Dalzell & Wankowski, 1980; Lewis et al., 1983; McCarthy, 1985; Maniku el al., 1990; Rawlinson, 1990). Spratelloides delicatulus (Bennett) is the most widely distributed: from southeastern Africa to Fiji. S . gracilis (Temminck & Schlegel) occurs from the central east African coast to the Solomon Islands and New Caledonia. A third species, S . lewisi Wongratana, is found only in Papua New Guinea and Solomon Islands waters (Wongratana, 1983).

A knowledge of rates of growth is central to providing a sound biological basis for stock assessment (Pauly, 1987), but most growth studies of baitfish (Dalzell & Wankowski, 1980; Dalzell, 1984; Tiroba et al., 1990) have relied on modal pro- gression analysis of length-frequencies to estimate growth parameters. However, since Panella (1971) first recognized that the striations in fish otoliths were laid down daily, analysis of otolith increments has become an increasingly powerful tool in fisheries dynamics. Two such studies of the otoliths of S . lewisi and S .

849

0022-1 112/91/012849+ 18$03.00/0 0 1991 The Fisheries Society of the British Isles

850 D. A . MILTON E T A L .

FIG. I . Map of the Indo-Pacific region showing sampling sites.

delicatulus from Papua New Guinea and Fiji respectively (Dalzell, 1987; Dalzell er al., 1987) reported close agreement between estimates of growth parameters from otoliths and length-frequency analysis. However, a study of growth of S . lewisi from Papua New Guinea (Dalzell, 1987) found considerable differences in estimates of maximum length (L,) between sites, which suggests that area-specific growth estimates are required for stock assessment of this species.

The aims of this study were to compare the growth of S. delicutulus, S . gracilis and S . lewisi from several populations throughout their distribution and to examine whether there was detectable growth variation in these species and if so, whether these variations in growth were related to possible environmental differences.

11. MATERIALS AND METHODS SAMPLING

Samples of up to 100 fish of each of three species of Spratelloides were collected each month from three sites in the Solomon Islands (Fig. 1) between March 1987 to May 1989 and again in October 1989. Samples of S. gracilis and S . delicatulus were collected from the Maldives (Fig. 1) during the same period. Additional samples (Table I) of S . delicatulu.5 were collected from Kiribati (two sites) between January and September 1989 and two sites, in north-eastern Australia between November 1987 and January 1990 (Fig. 1).

The Solomon Island sites and the sampling methods are described in Milton et al. (1990~). Most baitfish samples from the Solomon Islands were obtained from commercial tuna vessels, which use underwater lights at night to aggregate fish (Table I). Commercial catch rates (Table 11) were obtained from baitfish catch composition and catch and effort data (Rawlinson & Nichols, 1990; Rawlinson, 1990). Samples from the Maldives were also obtained from commercial fishermen, who catch their baitfish duringdaylight using minced tuna as an attractant (Maniku et al., 1990). Doherty (1987) used larval light-traps to sample reef fish recruitment in northeastern Australia, we obtained our samples from fish caught in

AGE A N D GROWTH OF THREE SPRATELLOZDESSPP 85 1

his traps. All samples were preserved immediately on capture in 70% ethanol before being transferred to the laboratory.

DAILY AGEING In the laboratory the fish were measured (standard length s.L., mm) and sexed and both

their sagittae and lapilli were removed and mounted on microscope slides under immersion oil. The slides were left to clear for at least 48 h, then examined under a compound micro- scope at x 4O(rlOOO, using image-enhancing, closed-circuit television (Young et al., 1988). Daily ages were calculated from the number of increments (rings) in the otoliths.

The increment pattern in sagitta of Spratelloides was very similar to that described for Surdinella species (Gjosaeter et ul., 1984), so a similar interpretation of presumed daily increments was adopted. All otoliths were counted in longitudinal section as increment spacing was not uniform around the otolith being most widely separated on the long axes. Otoliths were recounted after two or more days by the same person. If counts differed by more than two (fewer than 20% of all counts) the sample was recounted and the median value taken. If consecutive counts differed by more than 5% (approximately 3% of all counts), the data were not included in the analysis.

VALIDATION EXPERIMENTS As Rice (1987) has pointed out, the reliability of otolithic ageing depends on validation of

the periodicity of increment formation. Previous studies (Schmitt, 1984; Milton et id., 1990h) showed that the increments in the otoliths of S. delicutulus and S. grucilis were deposited daily. Otoliths of S. Iewisi were identical in size and shape to those of the other two Spratelloides species and the increment deposition pattern and increment width were also similar. We have therefore assumed that increments in the otoliths of S. Iewisi are also deposited daily.

DATA ANALYSIS As these Spratelloides species have not been reared from the egg, their absolute age is not

known. Thus, temporal and spatial comparisons of growth parameters estimated from within the range of counts, should be more accurate than comparisons of parameters that are extrapolations from the data. Inaccuracies in age estimation may account for some variation in increment counts and depending on the major source of error (length or age measurements), some authors (e.g. Kirkwood, 1983) suggest growth may be better expressed with age as the dependent variable. During initial analysis, residuals from analyses in which length and age were each treated as the dependent variable showed that there was as much variation about the equation with length as the dependent variable as there was with age. To allow comparison with other studies, subsequent analyses were expressed with length as the dependent variable.

The length and otolithic increment data were'plotted to ascertain the form of the curve that best described the relationship. The von Bertalanffy curve, which is most widely used in fisheries science partly because it wasderived from physiological principles (von Bertalanffy, 1938; Longhurst & Pauly, 1987), fitted the data adequately. Where sample sizes were greater than 30 fish, curves were fitted separately for individual samples. The equation is of the form

( 1 )

where L, = length of the fish at age t , L , = asymptotic length, K = growth coefficient and t,, = the age at which L = 0. However, as several authors have noted (e.g. Knight, 1968; Schnute & Fournier, 1980; Ratkowsky, 1986) the estimated parameters L,, K and to either do not have direct biological meaning or are extrapolations from the data (Ratkowsky, 1986). Values of L , in particular are dependent on adequate sampling of fish in the larger length classes so estimates are, by definition, an extrapolation beyond the data and prone to inaccuracy (Knight, 1968). Problems in fitting the von Bertalanffy function have been tackled by reparameterizing the equation in terms of new variables. Ratkowsky (1986) showed that the parameters proposed by Schnute & Fournier (1980) had the best statistical

L, = L , ( I - e-Q-lo) )?

852 D. A. MILTON ET A L .

properties of eight versions examined, especially for comparative studies of populations. Francis (1988) extended the equation of Schnute & Fournier (1980) to derive a new set of parameters L,, L, and L, (his 1,. lx and I,,,), which correspond to the length of the lower, middle and upper limits of any arbitrarily defined age range such that:

L, = L , + (L, - L , ) ( I - r2(' ~ di(w--pl ) )/(I - r2), (2) where r = (L, - L,)/(L, - L,), L, is the length of a fish at age t and L,, L, and L, are the length at the lower, middle and upper limits of two arbitrary ages u, and w: By fitting a curve of this form, extrapolations beyond the data are avoided as the three fitted parameters are chosen to be within the range of the data. In this study, we have set u,= 30 increments and w=90 increments. This equation has the advantage with short-lived fish that the age range to be examined can be chosen by the investigator rather than having to be the largest and smallest age classes found, as in the Schnute & Fournier (1980) equation. These parameters ( L , , Lz and L,) can also be expected to have similar properties to those of Schnute & Fournier (1980) and to not show the high negative correlation found between L , and K (Francis, 1988). However, preliminary analyses showed that L, and L, were often highly positively correlated and highly negatively correlated with L,. The Likelihood Ratio Test of Kimura ( 1 980) was therefore used to test for differences between parameters, as it takes into account the joint probability of the parameters.

Francis (1988) also gave equations for the relationship between his parameters and L , , K and to, which makes it possible to compare our estimates of L , and K with those in the literature. To compare the growth pattern of Spratelloides found in the present study with that found in other studies, the von Bertalanffy parameters L, and Kfrom the least squares equations at each site were used to calculate the growth rate (dL/dt) at L,/2 where

dL/dt = K*L,/2, (311 which takes into account some of the wide variation in growth parameter values between populations and reduce the effect of the strong negative relationship between K and L , This equation was used by Dall ef al. (1990) to compare growth estimates for penaeids., derived from many studies, and across several species.

All parameters were estimated by the least squares method, using the SAS NLIN pro- cedure (SAS, 1985). A measure of goodness-of-fit was obtained by calculating an r2 value: from residual and explained sums of squares derived from the least-squares regression.

111. RESULTS

Details of sample size, method of collection, and the length range examined are: given for each sample (Table I).

INTRASPECIFIC VARIATION Spratelloides delicatulus

Seasonal variation within sites: The relationships between length and number of otolith increments were similar at all sites (Fig. 2), and there was no significant temporal variation in growthexcept at Townsville (Fig. 3). Townsville fishcollectedl in January 1990 grew significantly faster than thosecollected duringearly November and December 1989, and fish sampled in December had grown faster than those sampled in November (Tables 11 and 111; Pc0.05). There were no significant differences in the growth rates of the sexes at any site ( P > O - l ) , although females grew a little bigger than males at some sites. Instantaneous growth rates varied from 1.0 mm day-' at 1 month and 0.5 mm day-' at 2 months (in the November 1989 sample) to 1.75 mm day-' at 1 month and 0.64 mm day-' at 2 months (in the January 1990 sample). There was no significant difference between samples in the growth rate at 3 months (P > 0.1).

TA

BL

E I. D

etai

ls o

f the

sam

ples

of t

hree

spe

cies

of S

prat

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ides

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min

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dy

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ies

Cou

ntry

Si

te

Sam

ple d

ates

Le

ngth

ran

ge

(no.

sam

ples

) (m

m)

Hab

itat

type

M

etho

d

S. d

elic

atul

us

(dis

tribu

ted

from

E

. Afr

ica

to F

iji)

S. g

raci

lis

(dis

tribu

ted

from

E.

Afr

ica

to N

ew C

aled

onia

)

S. l

ewis

i (d

istri

bute

d fr

om

PNG

to S

I)

M

SI

s1

SI

Aus

t. A

ust.

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t. K

K

M

M

SI

Sl

SI

A

ust.

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t. s1

SI

SI

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iyaf

ushi

M

unda

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onav

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gi

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ville

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root

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ylan

dt

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awa

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Apr

. 198

8 (1)

Ju

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t. 19

89 (8

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n. 1

987-

Jan.

198

9 (5)

M

ay 1

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Apr

. 19

89 (6

) D

ec. 1

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3)

Nov

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1)

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7)

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3)

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3)

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1989

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198

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pr.

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1989

(8)

40-5

7 18

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17-5

6 27

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1461

1

65

6

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14-5

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1 00

140

249

197

116

138 78

144

120

130 60

202

158

133 85

100

199

160

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, Pap

ua N

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, Mal

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, Kir

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ish.

854 D. A. MILTON ET AL.

70 60 5 0 40

30 20 10

- Maniyaf ushi

0 30 60 90 120 150 180

’. I 3ot- 1 2 o I T ; I , 1 10

0 30 60 90 120 150 180

50 40 40 30 30

20

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c

E E I I I I I 0 30 60 90 120 150 180 0 30 60 90 120 150 180 c D c Q) J Lizard Island

40

3 0 20

~~

0 30 60 90 120 150 I80

70 60 50 40 30 20 10

0 3 0 60 90 120 150 180

I I 0 30 60 90 120 150 180

70 p r o o t e Eylandt 60

40

30 20

0 30 60 90 120 150 180

No. of increments

FIG. 2. Relationship between fish standard length (mm) and the number of growth increments of 5’ delicatu1u.v at eight sites throughout the Indo-Pacific.

Between sites: The growth pattern of S. delicatulus samples also varied between sites (Table 11; Fig. 3). The fish in the January 1990 sample from Townsville had a larger L, and L, than those from other sites (P<O.OI), which indicates the Townsville fish grew faster up until 2 months (60 days). Growth was more rapid in fish at the two barrier reef sites in Australia (Table 11) than in the three coastal lagoon sites in the Solomon Islands or the other coral atoll sites. Average instan- taneous growth rates were slowest at Maniyafushi (Maldives) and Groote Eylandt

I

AGE AND GROWTH OF THREE SPRATELLOIDESSPP 855

60

50

40

30

20

10

E f 30 c m 3 20

(c 1 .-

- -

- -

0 30 60 90

/ I I 0 30 60 90

No. of increments

FIG. 3. Relationship between fish standard length (mm) and the number of growth increments of S. delicotulus from three samples from Helix Reef, near Townsville, Australia. (a) 1-3 November 1989; (b) 3 4 December 1989; (c) 7-9 January 1990.

(Australia) for fish one to three months old and fastest at Tulagi, Solomon Islands (Table 111).

Spratelloides gracilis Seasonal variation within sites: Growth rates varied seasonally in samples of S.

gracilis from Munda and Tulagi, Solomon Islands [Table 11; Fig. 4(c,e)]. At Munda, fish collected from January to April 1989 grew faster than fish collected in other months between March 1987 and October 1989. At Tulagi, fish collected during 1987 grew faster over the entire length range than fish collected in 1988 (P<O.OOl). No seasonal differences were detected in samples from other sites and there were no differences in the growth rates of the sexes.

Between sites: The growth rates of S. gracifis with 3&90 increments from seven sites were compared (Table 11; Fig. 4). L, was largest from Tulagi (Solomon Islands) and lowest for Lizard Island (Australia). Growth was most rapid in the ' fast ' Tulagi samples (1987) and slowest in the ' slow ' Tulagi (1988) and Munda samples. Mean instantaneous growth rates for fish at 1 month (30 increments) ranged from 0.37 mm day-' for the 1988 (slow) samples from Tulagi (Solomon Islands) to 1.19 mm day-' at Lizard Island. At 3 months (90 increments), fish from Lizard Island were still growing faster than in other populations (Table 111).

Spratelloides lewisi Seasonal and between-site variation: No seasonal or sex-related variation in the

growth rate of S . lewisi was detected at any of the three sites sampled in the Solomon Islands. L, was highest in the Tulagi samples and lowest at Vonavona,

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n.-A

pr.

1989

sam

ples

; Mun

da ‘s

low

’, sa

mpl

esco

llect

ed a

t oth

er ti

mes

bet

wee

n O

ct. 1

987 a

nd O

ct. 1

989;

Tul

agi ‘

fast

’, sa

mpl

esco

llect

ed in

198

7; T

ulag

i ‘sl

ow’,

sam

ples

colle

cted

in 1

988;

M, M

aldi

ves;

SI,

Sol

omon

Isla

nds;

A

usr.

, Aus

rrai

ia; K

, Kir

ibat

i; n,

num

ber o

f fis

h.

x L c-) c =I s

a,

rA c) .-

rn 0 u a, a vl

.-

m - - - - - - * - - - --------- --- 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 00000000000 000000000 000 +I +I +I +I +I +I +I +I +I +I +I +I +I +I +I +I +I +I +I +I +I +I +I o F m m m \ D m b m o m - m w * * w m l l m w * \ D m m m b m - m w e b m m m m - w m - m - m m m 00000000000 000000000 000

m - - - - - - m - - - ---- 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 00000000000 0000 +I +I +I +I +I +I +I +I +I +I +I +I +I +I +I P w - w m m O b P m b P M m a m b m m b b w \ D m m b * * w m 00000000000 0000

m - - - m d - m m m - - - - - - - m e - - --- 0 0 0 0 0 0 0 - 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 00000000000 000000000 000 +I +I +I +I +I +I +I +I +I +I +I +I +I +I +I +I +I +I +I +I +I +I +I w m * m m ~ . - m m o ~ m w e w m o f i m w o m m y q ~ y ~ o o F b P w r - w F b F m m - \ o YO? ooooo-;--0000000000-0 0-0

858 D. A. MILTON E T A L .

40 30 20

70 60 50 40 30 2 0 10

0 30 60 90 120150 0 30 60 90 120 150

I I I I

50 40 30

20 I- .z.:

5 0 30 60 90 120 150 0 30 60 90 120 150 5 P L

Y 70 70 t t f ) Lizard Island 1 60 60 50 50 40 40

30 20

30 20

0 30 60 90 120 150 0 30 60 90 120 150 No. of increments No. of increments

70 2 60 E 5 0

40 p 30

10

0

1 20

( g ) Townsville I

1 30 60 90 120 150 No. of increments

FIG. 4. Relationship between fish standard length (mm) and the number of growth increments of S. gracilis at seven sites throughout the Indo-Pacific. (c) Munda, Solomon Islands ' fast ', January-April 1989; ' slow ', samples collected other times between October 1987-October 1989; (e) Tulagi, Solomon Islands ' fast ', 1987 samples; 'slow ', 1988 samples. Note (a), (b) and (d) are redrawn from Milton ef al. (19906).

but by 3 months (90 increments) fish grew faster at Munda and Vonavona than at Tulagi (Table 11; Fig. 5; P<0.05). Overall growth was faster at Vonavona than other sites (Table 111). The mean instantaneous growth rates of fish 1 month old (30 increments) ranged from 0.53mmday-' at Tulagi to 1,03mmday-' at Vonavona. By 3 months (90 increments) growth rates were similar at all sites (Table 111).

INTERSPECIFIC VARIATION (WITHIN SITES) Comparisons of growth rates between species were possible for samples from1

five sites in the Solomon Islands and Australia (Table 11). No comparisons were

AGE AND GROWTH OF THREE SPRATELLOZDESSPP 859

60 ( a ) ( b ) - 60 -

possible for Spratelloides species from Maldives (no two species were found at the same site) or Kiribati (only one species). The catch rates of Spratelloides species at the three Solomon Island sites (Table 11) indicate that S . lewisi was the most abundant species at all sites and S. gracilis the least abundant species.

I I I I I

Munda The growth rates of the three Spratelloides species at Munda (Table 11) differed

significantly in their first month. The S. gracilis ' fast ' cohort collected between January and April 1989 grew faster than other species, while the ' slow ' S. gracilis sample grew the slowest. There was little difference in growth between S. delicatulus and S. lewisi, but S. lewisi attained a greater L, (P<O.Ol).

I

Vonavona There were different patterns of growth among the three species of Spratelloides

from Vonavona (Table 11). S . gracilis attained the greatest length after the first month ( P < 0.05), while S. lewisi grew faster and had significantly larger L, and L, than the other species (Tables I1 and 111; PcO-001). However, the patterns of growth of S . delicatulus and S. gracilis were similar.

Tulagi The growth rates of the three Spratelloides species at Tulagi was compared. The

1987 (fast) sample of S. gracilis grew faster than the other species (Tables I1 and 111; P< 0.01). S . lewisi grew faster than S. delicatulus or the S. gracilis ' slow ' samples (1988) and had significantly larger L, and L, (P<O.OOl) , although S. delicatulus attained a greater L,.

860 D. A . MILTON E T A L .

Lizard Island S. delicatulus showed more rapid early growth than S. gracilis at Lizard Island

with significantly larger L, (Table 11; P<O.OOl). This growth rate was not main- tained as s. gracilis attained larger L, and L,. The average instantaneous growth rate of fish with 30 increments (approximately 1 month old) was 0.87 mm day-' for S. delicatulus and 1 * 19 mm day-' for S. gracilis.

Townsville Although the growth of S . delicatulus from Townsville varied seasonally (Table

11), fish in all samples grew faster over the entire range of increments examined than did S. gracilis collected during the same period (Tables I1 and 111; P<0.05). No seasonal variation in growth was detected among S. gracilis samples.

FACTORS INFLUENCING GROWTH Estimates of the von Bertalanffy growth parameters (K , La) from Spratelloides

otolith increment data (Table 1V) varied widely between samples, both at a site and between sites for S . delicatulus and S . gracilis. The results, however, were similar to those obtained from length-frequency analyses of the same species at the same sites. Growth rates at L,/2 varied less and were in the range 130-458 mm year-' for all species (Table IV). The mean growth rates for each species (205-4 f 23.2 for S. delicatulus; 200.3 k 14.5 for S . gracilis; and 190.0 21.4 for S . lewisi) were not significantly different (P>0.2) although S . delicatulus had more of the higher values. There was no difference in the mean growth rate of fish populations from coral atoll sites (dL/dt = 178.0 f 16.0; n= 6) and coastal lagoon sites (dL/dt = 178.0 f 9.0; n = 19), although growth was more rapid at barrier reef sites (dL/dt = 298.0 k 38.0, n = 6; P< 0.005). The growth rate (dL/dt) and L, of S . delicatulus and S . gracilis were also compared with the mean water temperature at each site using Spearman's rank correlation. There was a significant negative relationship with both parameters for S. delicatulus (R,= -0.92, P<O.OOI; R,= -0.83, P < 0.005 respectively) but no relationship for S. gracilis (R, = - 0.15, P> 0.5; R,$=0*05, P>0.9).

Comparison of the commercial catches and growth rates of baitfish in the Solomon Islands (Table 11) did not demonstrate a relationship between the relative numbers of S. gracilis and periods of variable growth nor was there a consistent relationship between relative catch rates and growth rates of Spratelloides at the three sites.

IV. DISCUSSION Previous studies of Spratelloides species have estimated that these species live to

6 months (Dalzell, 1984, 1987; Dalzell et al., 1987) but the growth data presented here indicate that, in the populations studied, most fish live less than 4 months, which makes these species extremely short-lived compared with most marine fishes. Short life-expectancy is usually found in species that experience high mor- tality or unpredictable environmental conditions (Stearns, 1976). Spratelloides are important prey species in Solomon Island coral reef fish communities (Blaber et al., 1990). Their behaviour-such as their schooling and staying in shallow habitats during the day, and living amongst seagrasses (especially S . de1icatulus)-may be adaptations to reduce predation (Major, 1977, 1978; Main, 1987). Milton et al.

TAB

LE IV

. L,

, K

, gro

wth

rate

(K*L

,/2)

and

mea

n w

ater

tem

pera

ture

(” C

) dur

ing

the

life c

ycle

of v

ario

us p

opul

atio

ns o

f thr

ee s

peci

es o

f Spr

atel

loid

es

Spec

ies

Cou

ntry

Si

te

Mea

n G

row

th ra

te

tem

pera

ture

L,

K

(” C

) (m

m)

(yea

r-’)

(m

m y

ear-

.’)

Sour

ce

S. de

licat

ulus

M

aldi

ves

Man

iyaf

ushi

So

lom

on Is

. M

unda

Von

avon

a

Tula

gi

Tow

nsvi

lle ‘

Nov

.’ To

wns

ville

‘D

ec.’

Tow

nsvi

lle ‘J

an.’

Gro

ote

Eyla

ndt

But

arita

ri

Van

ua L

evu

Aus

tral

ia

Liza

rd I

.

Kir

ibat

i T

araw

a

Fiji

seve

ral

S. gr

acili

s M

aldi

ves

Alif

ushi

T

hina

dhoo

Pa

pua

New

Gui

nea

Ysa

bel P

assa

ge

Solo

mon

Is.

Mun

da ‘f

ast’

Mun

da ‘s

low

’ V

onav

ona

Tula

gi ‘f

ast’

Tula

gi ‘s

low

’

Tow

nsvi

lle

S. le

wis

i Pa

pua

New

Gui

nea

Ysa

bel P

assa

ge

Cap

e L

ambe

rt

Von

avon

a Tu

lagi

Aus

tral

ia

Liza

rd I.

Solo

mon

Is.

Mun

da

80

86

66

76

71

105 65

59

67

75

49

93

72

75

73

73

75

85

83

76

78

43

58

76

42

79

52

55

70

68

60

70

2.7

3.3

4.6

4.5

4.5

3.1

7.4

11.4

8.

5 12

.2

7.1

3.4

5.1

4.38

4.

38

4.74

5.

3 4.

1 4.

38

4.3

5.4

7.2

6.8

7.1

6.2

7.1

7.2

5.4

5.4

5.7

8.9

4.3

108

142

152

171

160

163

24 1

336

285

458

I74

158

184

164

160

173

199

174

182

163

21 1

155

197

270

130

280

187

149

189

194

267

151

30.0

30

.1

30.1

30

.7

30-7

29

.2

28.5

24

.8

26.0

27

.1

30.4

30

.3

29.9

27

.3

27.3

27

.3

30.0

30

.5

29.9

29

.9

31.0

29

.4

30.6

28

.4

29.5

27

.5

25.9

29

.9

29.9

30

.2

30-7

29

.3

Pres

ent s

tudy

Pr

esen

t stu

dy

LF;

Tir

oba

et a

f. (1

990)

Pr

esen

t stu

dy

LF;

Tir

oba

et a

l. (1

990)

Pr

esen

t stu

dy

Pres

ent s

tudy

Pr

esen

t stu

dy

Pres

ent s

tudy

Pr

esen

t stu

dy

Pres

ent s

tudy

Pr

esen

t stu

dy

Pres

ent s

tudy

L

F; M

unch

-Pet

erse

n (1

983)

L

F; D

alze

ll et

ul.

(198

7)

Oto

liths

; Dal

zell

et u

l. (1

987)

Pr

esen

t stu

dy

Pres

ent s

tudy

L

F; D

alze

ll &

Wan

kow

ski (

1980

) L

F; D

alze

ll(19

84)

Pres

ent s

tudy

Pr

esen

t stu

dy

Pres

ent s

tudy

Pr

esen

t stu

dy

Pres

ent s

tudy

Pr

esen

t stu

dy

Pres

ent s

tudy

O

tolit

hs; D

alze

ll(l9

87)

Oto

liths

; Dal

zell(

1987

) Pr

esen

t stu

dy

Pres

ent s

tudy

Pr

esen

t stu

dy

L, a

nd K

valu

es in

the

pres

ent s

tudy

wer

e de

rived

from

leng

th-o

tolit

h in

crem

ent d

ata;

LF

= le

ngth

-fre

quen

cy a

naly

sis.

862 D. A. MILTON E T A L .

(1990a) found that the density of baitfish (including Spratelloides species) in the Solomon Islands was not related to the density of zooplankton (their food) or to abiotic parameters such as temperature, turbidity or water currents. Hence, predation rather than environmental unpredictability may be the major source of natural mortality in adult Spratelloides. If so, rapid growth and early maturity would be advantageous because it reduces generation time as well as maximizing the number of spawnings per individual.

Species adapted to high natural mortality (such as Spratelloides) should be capable of sustaining increases in mortality due to fishing. Rapid growth, along with continuous spawning (Milton & Blaber, 1991) enables these species to support high or unpredictable mortality events. Spratelloides are important components of tuna baitfisheries in both the Indian and Pacific oceans (Blaber, 1990) and sustain extremely high catches. Annual bait catches of S. grucilis alone have exceeded 60.5 tonne in Male atoll in the Maldives without detectable effects on the stocks (Maniku et al., 1990).

The least squares estimates of the von Bertalanffy growth parameters using the growth increment data for S. delicatulus were higher than those based on analysis of length-frequencies at the same site (Table IV). Our least squares estimates of the growth parameter ( K ) in the other two species of Spratelloides were also generally higher than those derived from length-frequency analysis. These differences may be partly attributable to the different methods used in estimating these parameters, but too small samples of the larger length classes in the ageing studies could also bias the estimate of L , despite the large overall sample sizes. These problems highlight the difficulties of estimating the original parameters (Lm, K ) accurately, although these problems were largely overcome in this study by using Francis' (1988) equation, as the parameters ( L l , L, and L3) were rarely extrapolated from the data.

Few studies of growth variability have quantified differences in growth between sites or seasons, especially for tropical species. An exception is the recent work on the tropical Australian herring Herklotsichthys castelnaui (Ogilby) (Thorrold, 1989; Thorrold & Williams, 1989), which showed that larval cohorts born only 2 weeks apart have detectable differences in their pattern of growth during the first month. Other studies of larval fishes have highlighted temporal (Jones, 1985; Leak & Houde, 1987; Warlen, 1988) and spatial differences (Nishimura & Yamada, 1988; Karakiri et al., 1989) in growth rates. These differences usually correlate with changes in temperature (Warlen, 1988) or prey abundance (Karakiri et al., 1989), which have been shown experimentally to affect otolith growth and hence somatic growth (Neilson & Green, 1982; Campana, 1984; h c e et ul., 1985; Mosegaard et al., 1988). Previous field studies that demonstrated temporal growth differ- ences were of temperate species that are subject to much greater temperature ranges than the populations sampled in the present study.

Only Spratelloides collected from Townsville (Table I), near the southern extremity of their range, had been subject to a wide temperature range (21-30" C) (P. J. Doherty unpublished). Our data indicate a possible relationship between the growth rate of S. delicatulus at Townsville and water temperature. However, as no measure of the relative abundance of zooplankton was available for the sampling period, growth differences could have been due to the influence of temperature on prey availability rather than directly affecting fish growth. However, no temporal

AGE AND GROWTH OF THREE SPRA TELLOIDES SPP 863

variation in growth of S. gracilis was detected, although this fish has a similar diet (Milton et al., 1990a), and was collected in the same samples.

In contrast, the temperature range was only 3" Cat the two Solomon Island sites where there were temporal differences in the growth of S. gracilis, and there was no correlation between increases in temperature and higher growth rates at either site. Neither the density nor the biomass of zooplankton changed between the periods of rapid and slow growth of S. gracilis at either site (Milton et al., 1990a), which suggests that its growth is not influenced solely by temperature or prey availability.

If the growth of Spratelloides was density dependent then higher relative abundance should be negatively related to growth rates. However, this was not the case for catches at Munda, Vonavona or Tulagi, which suggests that the growth rates of Spratelloides may be density independent.

Both the growth rate and the shape of the growth curve varied widely between sites for all species. The degree of variability between sites was not related to their proximity; rather there was a significant negative relationship between the growth of S. delicatulus at L,/2 and temperature. This relationship was very clear, with the higher growth rates occurring on the Great Barrier Reef, northeastern Australia. Thresher & Brothers (1989) found significant differences between demersal coral reef fish in the Pacific and Atlantic oceans in many early life-history parameters, including larval growth rates and length of planktonic phase. Our data do not indicate similar differences in the growth rates of adults in these pelagic species. Growth varied as much within a site as between sites separated by thousands of kilometres. Reproductive patterns of these species were similarly variable (Milton & Blaber, 1991). We have been unable to relate these differences to specific environmental variables which suggests that no single factor underlies the range of variation (Thresher & Brothers, 1989).

Differences in growth between fish from different habitats were also detected in the instantaneous growth rates between barrier reef sites and the coral reef and coastal lagoon sites. Growth rates of fish at 1 month were between 0.61 .75 mm day-' at barrier reef sites, which was generally higher than the growth rates of most Spratelloides populations from coral reef and coastal habitats at this age (0-5-1.0 mm dayp*). However, these growth rates were not maintained by fish from the barrier reef and decreased less with age in Spratelloides from other habi- tats. Growth rates up to 4 months were similar to those of other short-lived clupeoids such as Encrasicholina purpureus (Fowler) or Anchoa mitchilli (Valenciennes) (Struhsaker & Uchiyama, 1976; Methot & Kramer, 1979; Fiveset al., 1986; Palomera et al., 1988; Dayaratne, 1989) but faster than some others such as Herklotsichthys castelnaui (Ogilby) (Thorrold, 1989) at similar latitudes.

The implications of the observed growth variation for the recruitment and reproductive success of successive generations of each species of Spratelloides and the subsequent influence on stock size will be discussed elsewhere. However, the data indicate that, for such short-lived tropical fishes, the results of growth studies based on short sampling periods at one site should not be extrapolated to generalize about the growth of a species.

We thank Gideon Tiroba, John Leqata (Solomon Island Fisheries Division), Hassan Maniku and Ahamed Hafiz (Maldives Fisheries Research Section) for assistance in collect- ing samples. Peter Deherty and Maria Milicich are thanked for making available fish collected from their light-trap experiments. We thank Ian Somers for helpful discussion

864 D. A. MILTON ET A L .

and his and Aubrey Harris’s comments on an early draft of the manuscript. This work was part of the Baitfish Research Project funded by the Australian Centre for International Agricultural Research (Project No. 8543).

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