A Test of the Re ationship between Otolith Weight and Age for the Pilchard Sardinops neopilchardus

W. J. Fletcher Western Australia Marine Research Laboratory, P.0. Box 20, N. Beach, W.A., 6020, Australia

Fletcher, W. 1. 'B 991. A test of the relationship between otolith weight and age for the pilchard Sardinops nes- pilchardus. Can. ). Fish. Aquat. Sci. 48: 35-38.

A sample of 60 fish revealed a near-linear relationship between otolith weight and the estimated age (determined from scale and otolith annuli) for the pilchard Sardinsps neopilcharders, with little overlap of otolith weight values between age classes. In an additional sarnpk of 61 2 fish where only otolith weight and length were measured, only two clear length modes were found, but five modes using an otolith weight frequency distribution could be distinguished. The means of the five modes were similar to that predicted from the mean weights for each age class in the original sample of 60 fish. These five modes were also present in a subsample of the 61 2 fish, chosen such that their lengths formed a uniform distribution (i.e. no modes). These results were consistent with a direct relationship between otolith weight and age, but further validation is required before this variable can be used as the sole determinant of age.

Un echantillon de 60 poissons a rnontre I'existewce d'rrne relation presque lineaire entre le poids des otolithes et ['Age estirnk (dkterrnin6 selon les anneaux des ecailles et des otolithesf chez la sardine Sardinrsps neopilckarders; il existe peu de chevauchement entre les classes d'iige quawt aux poids des otolithes. Dans un autre kchantillsn comprenant 61 2 poissons OD seuls le poids des otolithes et la longueur ont etk mesures, on ne puvai t distinguer clairement que dekox modes pour ce qui est de la longueur, mais bien cinq modes distincts au rnoyen d'une distribution de frkquence du poids des otslithes. La moyenne des cinq modes etait sernblable A la msyenne prevue 2 partir du poids moyen pour chacune des classes d'ige dans I'6chantillon de depart de 60 poissons. Ces cinq modes ktaient 6galement kvidents dans un sous-echantillon des 612 poissons, ckoisi de f a p n que ies longueurs etaient distribu6es de rnanihe unifsrme (absence de modes). Ces rksultats correspondent i3 I'existence d'une relation directe entre le poids des otolithes et l'sge, mais des ktudes de confirmation s'irnposent avant que cette variable puisse &re employee csmme unique &terminant de I'age.

Received December 8, 1989 Accepted luly 24, 1990 (JAB2

T he accurate and efficient determination sf age in fish is crucial to many of the analyses required for the manage- ment of exploited species. The methods used to assign

ages to what is generally a luge number of individuals are, however, usually subjective, involving the often difficult task of accurately counting the annuli on otoliths or scales. Such methods may require considerable effort to prepare each spec- imen, and subsequent readings are subject to both systematic and random errors in interpretation (Bemish 1979) requiring independent validation. Furthemore, considerable time is gen- erally needed to acquire the skill necessary for consistently interpreting the material. In addition, because the criteria used to age fish are subjective, two or more readings are amsudly needed to 'verify the age assigned to saw individual. Agreement mong readers rarely exceeds 98% and is often much worse (e .g . Sandeman B 969).

Finding an objective measure by which the age of m indi- vidual fish could be estimated would be of considerable value. This would obviate the need for complex preparation and mul- tiple readings md it would allow the easy exchange of infor- mation among research groups, which often differ in their interpretation using the subjective methods (Kimma et al. H 949; Boehlert 1985).

A number of ageing studies have been done basing length- based cohort analyses (e.g. Samb 1988). Such methods, how-

ever, we generally only possible for fast-growing species or for the juveniles of slower growing species because the relative increase in size between age classes becomes less with age thm the variation in size within each age class. Thus, overlap of lengths becomes too large md makes it virtually impossible to age individuaB fish.

It has been suggested that the otolith weight of a fish may be directly related to its age (Brander 1974). This hypothesis is supported by evidence that fish ffom slow-growing popula- tions have been found to have relatively larger otoliths at the s m e length (Spratt 11972) and by experimental tests which show that the increase in size sf otoliths is partidly independent sf the growth in length sf individuals (Rezniek et d. 1989). Finally, Bwhleat (1985) found that a combination of otolith weight and several other objectively measured variab%es could be used to accurately age individuals. In this paper, the rela- tionship between otolith weight md age is further examined.

If there is a relationship, a number of preliminary hypotheses can be tested. First, there should be little variation in otolith weights of individuals of the same age, such that the m g e of otolith weights for fish of different ages should not overlap greatly. Second, in a multiage population with a restricted annual spawning season, a frequency distribution of otolith weights taken from a m d o m sample s f individuals collected at one time should show a series of modes, each of which

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reflects a different year class. The position of these modes should be similar to that predicted from the mean otolith weight- at-age relationship.

In this paper these two hypotheses are tested for the Austra- lian pilchard Sardinops neopikcharde~~. These should ascertain whether further tests of the relationship between age and otolith weight are worthwhile.

Materials and Methods

Test 1: Otolith Weight versus Age

Samples of pilchards were obtained from the Albany region of Western Australia (1 18'E, 35%) where a major purse-seine fishery operates. The stoliths (sagittae) from a total of 68 randomly selected fish over the size range taken by the fishery, 124-1 86 mm length to caudal fork (LCF), were collected during a 3-mo period (June-August 1989). The otoliths were removed from the fish, washed in distilled water, dried in an oven at 6Q0C for 12 h, and then weighed to the nearest 0.0 1 mg . One of each pair of otolith (no systematic differences between left and right otoliths were encountered) was embedded in clear epoxy resin under a glass coverslip on a black persgex slide. The rings (annuli) in each sf the otoliths were mdysed in the manner described by Baird (1970) and Thomas (1985) for South African pilchards to estimate age in yeas for each individual.

In addition, scales from the same fish were also removed, washed, and mounted between glass slides. They were examined using the criteria described for S. raeopt~ehardus by Blackbum (1949, 1958) to corroborate the age ascribed using the otolith method.

The relationship between otolith weight with age and LCF was malysed among age classes using an analysis of covariance and a stepwise multiple regression (hot GEM and Pmc Reg; SAS Institute 1988). The mean weight of otoliths for each age class was also determined to provide a prediction of where the modes of otolith weights should sccur, if this variable is related to age.

Test 2: Otolith Weight Frequency

Otolith were removed from approximately 600 randomly selected individuals, all of which were captured during Sep- tember 1989 in the Albany region. Each fish was measured md the otoliths removed, washed, dried, and weighed as described above. The resultant distribution of otolith weights was exam- ined by utilising the entire sample of fish and also by examining a stratified subset of 380 individuals such that their lengths formed a uniform distribution s f 38 individuals in each 5-mrn size class fmm 138 to 170 m E F (i.e. no modes were pres- ent, see Fig. 2). The distributions (the modes of which were assumed to have lognoma1 distributions) were malysed with the assistance of 'Mix9 ver. 2.3 (Macdsndd and Green 1988), a program designed to aid in the discrimination of frequency distributions.

Results

Test 1

In %he study on the June-August samples, individuals ranged in estimated age horn 2 t s 6 yr old as determined by reading the annuli on the scales and otoliths (for which there was 90% agreement). For each of these ages, the range of fish lengths

L C F h r n ) FIG. 1 . Otolith weights (mg) and lengths (LCF) of individuals for age classes 2(H), 3(0), 4(@), 5 ( 8 ) , and 6($), estimated by the annuli ow otoliths and scales from the June-August I989 samples.

TABLE 1 . Analysis of covariance using LCF and age, with otolith weight as the dependent variable. The interaction between LCH and age was not significant (F = 0.058, P = 0.67). Owe individual of age 6 which ha8 excessive influence on the analysis was excluded.

TYF 1 Type 11% Source MS 8; Significance MS 8; Significance

LCF 49258 666 0.0001 739 I8 0.0028 Age 1631 22 0.0001 1631 22 0.0081 Ewor 3458

TABLE 2. Mean ( s ~ ) weight of stoliths (mg x 100) for individuals in the five different age classes, with age estimated using three methods: (1) the rings on scales and otoliths, (2) m d e s in the total September sample, and (3) modes in the subsample of individuals which formed a miform length distribution. Age e l a s 6 for both modal groups did not include the 0u41ie1-s Bast 2.4 me.

OtoBitWscale Model groups, Modal groups, A S estimate total sample subsample class (JuneAugust) (September) (September)

was large, being generally in excess of 20 mrn with considerable overlap among the different age classes (>50% for all age classes greater than 2; Fig. 1). In contrast, the range of otoliths weights within an age class was relatively small with little overlap in ranges (< 15%; Fig. 1).

An analysis of cov&ance indicated that age still made a significant contribution to explain the variation in otolith weight even when length had been accounfd for (Table I), thereby supporting the hypothesis that otolith weight and age are closely related. Similarly, using a stepwise multiple regression, the mount of variance explained, as m e w m d by the partid regression coefficients, by age and length was 89 md I%, respectively. These results confirmed that age was a stronger predictor of otolith weight than was length.

The mem weights of otoliths for each age class increased in an almost linear fashion with age; importantly, a substantial increase continued even in the older age classes (Table 2). The

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L E N G T H MlDPOBBJf (LCF . mow) FIG. 2. Length frequency distribution of the total sample (solid line, n = 612) and subsample (br~ken line, n = 289) of pilchards used in the September 1989 otolith weight analysis.

OTOLITH WEIGHT

FIG. 3. Otolith weight (mg x 100) fqlaaency distribution of pilchards from the total sample of pilchards caught in September 1989 (n = 612). Arrowheads indicate the psition of the mean value for each mode.

closeness of age classes 3 and 4 probably reflects a variation in the time of spawning between some years. Similarly, the gap between ages 5 and 6 may be an atifact of the low smple numbers.

Test 2

The sample from September 1989 consisted of fish with two partly separated length modes (Fig. 2). The otolith weight fie- quency distribution was significantly different from the length distribution (KS test, D = 0.19, P <0.81). The best fit for the frequency distribution (Fig. 3) for otolith weights was when five modes were assumed (chi-square = 65, P >0.05); all other combinations resulted in the fitted distribution being sig- nificantly different from the observed distribution. The mean values of these modes, as determined by the "Mix" program, were similar to those predicted by the mean values sf the five

OTOLITH WEBBIHT

FIG. 4. Otolith weight (mg X 100) frequency distribution for the uni- form subsample of pilchards (30 individuals in each 5 - m length class fmm 130 to 170 mrn LCF) from the September sample (n = 289). howheads indicate the position sf the mean value for each mode.

age classes of the initial analysis (Table 2). In general, the slightly larger values in the later September smple could be expected because the otoliths should have grown between the two periods.

A further test sf the independence of otolith weight from fish length was made by malysing the otolith weight distribution from the subsample of fish with the uniform distribution of lengths. Despite there being no modes in lengths, the rnulti- m d a l situation persisted for otolith weight (Fig. 4); again, five modes gave the best fit (chi-squre = 56.7, P >Q.Q5). Fur- themore, the mean values of these modes were still consistent with the predicted vdues (Table 2).

Discussion

While there is some relationship between otolith weight md LCF (Fig. 1; Table I), the otolith weight of an individual Aus- tralian pilchard is not total@ dependent upon overall length. If otolith weight was merely dependent upon length, it would be expected that the distributions of these two variables would be similar. Instead, the distributions of otolith weights were sig- nificantly different f r s d the length structures of the smpBes.

The independence sf otolith weight from fish length has been found elsewhere. For the Californian anchovy Engraulis mor- dm, it was found that some slow-growing populations had oto- liths which were relatively bigger than similarly sized fish fmm fast-growing populations (Spratt 1972). This information was, however, only used to help separate the populations. Similarly, Reznick et al. ( 1988) experimentally influenced the growth rate of guppies, Psecilia retieuBatca, and found that slower growing individuals had relatively larger otolitks thm faster growing fish, despite coming from the same population. They cited eight other studies where less rigorous tests showed similar findings. Nonetheless, most of these studies did not elaborate upon a major implication of these results that otolith weight could pos- sibly be used as a determinant of age.

The results of the present study support the hypothesis that there is a relationship betweam otolith weight and the age sf individuals for the pilchard S. neopikchardus. The evidence includes the smdl overlap sf otolith weights among age classes as determined by the annuli. The mean vdues increased sig-

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nificmtly even for the older age classes, which is not generally evident in other structures (e.g. lengths). Moreover, distinct modes in otolith weight could be dischminated even when there was no rndd shwcture in the length distribution sf the fish. The means of these modes were in close agreement with those predicted independently by the ageing study.

The use of otolith weights has already been suggested as an objective method for ageing the rmHish Sebastes diploproa by Boehlert (1 985). In his study, a combination of factors was used in a multiple regression technique to age fish up to 30 yr old, which ageed closely with the ages determined from sectioned stoliths. For S . neopilchrdus, if seems that otolith weight alone may be sufficient to discriminate ages with a similar level of accuracy to scale a d otolith reading. At the very least, it could be used as a check on the ring counts from otsliths and scales.

The findings presented here are only preliminary; before oto- lith weights c m be used as the sole ageing technique, further validation is required. This will involve the validation sf ageing by m u l i and establishing that the modes of otolith weights persist and progress though time. Nonetheless, the results to date are promising a d it may be possible that this objective measure may take ageing out of being what William aafd Bedford (1974) described as more ar t than science.

'3l=mks we expressed to the fishemem and procesors sf Albany for providing the fish. Sampling was done with the expert assistance of R. Tregomning, S. Blight, and M. R~ssbach. Figures were photo- graphed by S. Blight. The manuscript was improved by reviews from M. Moran, R. knmton , N. Caputi, N. Monisey, J, Benn, N. Hall, and two referees.

References

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BLACKBURN, M. 1949. The age, rate of growth, and general life history of the Australian pilchard (Sardimps neopikchardus) in New South Wdes. C.S.I.R.B. Aust. Bull. 242: 86 p.

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B O E H L ~ T , G. W. 1985. Using objective eriteda and multiple regression models for age determination in fish. Fish. Bull. U.S. 83: 103-1 17.

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MACDONAID, P. B. M., AND P. E. J. OREEN. 1988. Users guide to program MIX: an interactive program for fitting mixtures of distributions. Ecthus Data Systems, Ontario, Canada.

R ~ N I C K , B., E. LINDBECK, AND H. BRYGA. 1989. Slower growth results in larger otoliths: m experimental test with guppies (Poecilice rediculata). Can. J . Fish. Aquat. Sci. 46: 108-112.

S a m , B. 1988. Seasond growth, mortality MB recruitment pattern of Sardi- ne& pp~wderensis off Senegal, p. 258-271. In S. Venema, J . MolHer- Christensen, and D. Pauly 1ed.j Contributions to tropical fisheries biol- ogy. F A 8 Fisheries Report 389, Rome.

SANDEMAN, E. J. 1969. Age determination ;and growth rate in redfish, Sebastes sp., from selected areas around Newfoundland. Ent. C o r n . Northwest. All. Fish. Res. Bull. 6: 4%1M.

SAS INSTIIFUE. 1988. SASJSTAT Users Guide. Release 6.03. SAS Institute, C x y , NC. 1028 p.

S P R A ~ , 5. D. 1942. The use of otoiiths b separate groups of northern mcho- vies. Calif. Dept. Fish Game Mar. Tech. Rep. 1: 25 p.

THOMAS, W. M. 1985. Growth rate of the pilchard off South West Africa, 1971- 1983. Invest. Rep. Div. Sea Fish. S. Afr. 128: 41 p.

WILLIAMS, T,, AND B. C. BEDFORB. 1974. The use of otolith for age deter- mination, p. 114-123. In T. B . Bagend led.] The ageing of fish. Unwin Bros . , Surrey.

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