Growth of American and European eel leptocephali as revealed by otolith microstructure
MARTIN CASTONGUAY' Migratory Fish Research Institute and Department of Zoology, Universih of Maine, Orono, ME, U.S.A. 04469
Received May 15, 1986
CASTONGUAY, M. 1987. Growth of American and European eel leptocephali as revealed by otolith microstructure. Can. J. Zool. 65: 875-878.
Results of a microstructural study of sagittae of 21 Anguilla spp. leptocephali using a scanning electron microscope are presented. Assuming that one growth increment was formed every day, a growth rate of 0.38 nlm total length/day was calculated. Hatching dates of a subsample of larvae were between 4 May and 26 June. These late dates may account for the small size of the larvae and their atypical geographic location. This sample of larvae apparently grew faster than those in previous studies, for which rates were based on regressions of length on date of capture.
CASTONGUAY, M. 1987. Growth of American and European eel leptocephali as revealed by otolith microstructure. Can. J. Zool. 65 : 875-878.
Cet article prCsente les resultats d'une Ctude au microscope Clectronique a balayage de la microstructure des sagittae de 21 1eptocCphales d'Anguilla spp. En supposant qu'une zone de croissance est formee chaque jour, on obtient un taux de croissance de 0,38 mm longueur totale/jour. Les Cclosions au sein d'un sous-Cchantillon se sont produites entre le 4 mai et le 26 juin. Ces dates tardives expliquent probablement la petite taille des larves et leur position gkographique atypique. La croissance de ces 1eptocCphales semble avoir CtC plus rapide que prCvu d'apres des Ctudes antirieures basies sur des rkgressions de la longueur en fonction de la date de capture.
[Traduit par la revue]
Introduction Leptocephali of the American eel (Anguilla rostrata) and
European eel (A. anguilla) hatch from eggs spawned in the southwestern Sargasso Sea. Unlike the leptocephali of other species, they migrate to freshwater streams of North America and Europe where most of their somatic growth occurs (Schmidt 1925). The biology of leptocephali is poorly known. Schmidt (1925) invoked a difference in growth rate between A. rostrata and A. anguilla to explain the geographic separation of the postlarval stages of the two species. On the basis of Schmidt's (1925) data, Harden Jones (1968) and Tesch (1977) drew spec- ulative growth curves of A. rostrata and A. anguilla that indi- cated a faster growth rate in A. rostrata. However, after re- examining Schmidt's (1922) data, Boetius and Harding (1985) concluded that the growth rate of A. rostrata (5.6 mm total length (TL)/month) is not significantly different from that of A . anguilla (5.3 mm TL/month). Kleckner and McCleave ( 1985) and Wippelhauser et al. (1985) studied growth of A. rostrata by calculating a regression of the total length on the Julian date of collection for a large number of specimens. The calculated growth rate was 0.24 mm TL/day (7.2 mm TL/month).
One of the best methods to study early growth of larval fish is to use the microstructure of otoliths to age fish. Since Pannella (197 1) first documented that growth increments are formed daily in fish otoliths, there have been many studies on the subject (see Campana and Neilson 1985 for a recent review). Most studies in which the periodicity of growth increments of otoliths was experimentally determined found that increments were formed daily (e.g. , Pannella 197 1 :, Brothers et al. 1976; Struhsaker and Uchiyama 1976; Taubert and Coble 1977; Tanaka et al. 198 1 ; Campana and Neilson 1982; Victor 1982; Campana 1983). The apparent nondaily formation of incre- ments reported by some authors (Geffen 1982 ; Laroche et al. 1982; Lough et al. 1982) may have been caused by constant temperature conditions of the laboratory, improper otolith pre-
'present address : Department of Fisheries and Oceans, Institut Maurice Lamontagne, 850, route de la Mer, C.P. 1000, Mont-Joli (Quebec), Canada G5H 324.
paration, and (or) lack of resolution of light microscopy (Cam- pana and Neilson 1985). However, feeding frequency (Neilson and Geen 1982; Campana 1983) and anomalous temperature regimes (Campana 1984) may produce subdaily increments. Nevertheless, it appears that daily increment formation is to be expected in most environments (Campana and Neilson 1985).
The objectives of this study were to evaluate daily growth rates and hatching dates of A. rostrata and A. anguilla lepto- cephali from the southwestern Sargasso Sea. Periodicity of increment formation of otoliths in any fish species should be determined experimentally before examining microstructure of otoliths of wild specimens. Such an experimental determina- tion was not possible in this study because Anguilla lepto- cephali cannot be kept alive for more than a few days in the laboratory. Therefore, I made the explicit working assumption that growth increments were formed daily.
Materials and methods Leptocephali ( r l = 2 1 ) were sampled at three locations in the south-
western Sargasso Sea on three different cruises. The largest specimen was caught at 30N, 76' W on 6 October 1984 and the five smallest specimens were captured at 27' N, 69" W on 24 March 1985. The other leptocephali were sampled at 24"N, 68" W on 5 August 1984. Sampling was accomplished with an Isaacs Kidd Midwater Trawl (IKMT) fully lined with 1800-p.m Nitex netting in 1984 and 500-p.m Nitex netting in 1985. The IKMT had a mouth area of 8.7 m2. Sampling consisted of 30-min stepped oblique tows in the top 100 m at night. Anguilla leptocephali were sorted on board the ship as quickly as possible, measured to the nearest millimetre (TL), and identified to species by counting the myomeres under a microscope. The small 1985 specimens could not be identified to species because counting of myomeres of small leptocephali on board a ship proved unreliable. Damage from freezing prevented counting the myomeres of the 1985 specimens once ashore.
One sagitta (referred to as the otolith) was extracted under a dissect- ing microscope using fine forceps. The otolith was embedded distal (convex) face up in epoxy, which was allowed to polymerize over- night in an oven at 60C. A sagittal section of the otolith was then obtained by hand grinding with very fine grain sandpaper from a micro- tome knife sharpening kit (Fisher Scientific Co.) until the nucleus was reached. Following 2-3 min of etching in 5% disodium EDTA, the
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876 CAN. J . ZOOL. VOL. 65, 1987
FIG. 1. Centre (A) and edge (B) of an otolith of an A . anguilla that was 30 mm TL and had 57 growth increments. Scale bars represent 10 pm. The hatching mark (dark ring, A) delimits a circle with a diameter of 14 pm. The arrow indicates the cue that was used when counting to switch from one photograph to the other.
otolith was coated with 200 A of gold and observed under an AMR lOOOA scanning electron microscope (SEM) at 5 kV. Polaroid photo- graphs of the centre at about 3000x and the edge at about 5000x were taken for each otolith (Fig. 1).
Growth increments were counted from photograhs. Each of !wo independent readers made two independent counts of each otolith. Results of averaged counts are presented. In 18 of 21 cases the variability between the averaged counts of each of the two readers was less than 10% of the overall mean count.
When more than about 90 increments were present in otoliths, increments near the edge were generally too narrow (about 0.1 pm) to be counted. Etching also produced better contrast in the centre of the otolith than at its edge. For these reasons only one otolith from one of the older larvae obtained during October could be used.
Results A prominent check surrounding the core was present in all
otoliths (Fig. I). It was postulated to be a hatch check, as no growth increments were seen inside the circle formed by the
A ros l ra la
A , sp. a
N U M B E R OF I N C R E M E N T S
check and as hatch checks are formed in many fish F,G. 2. Growth of Anguilla leptocephali expressed as a regression (Campana and Neilson 1985). The hatch check was used as a of body total length (mm) of leptocephali on number of growth incre- reference point from which growth increments were counted. ments of otoliths. In order to verify that the check was not formed posthatch, check diameters -of 12 SEM-prepared otoliths (mean 2 SD, 16 + 2 pm) were compared with otolith diameters of four newly hatched Anguilla leptocephali 1 5 mm (29 2 7 pm) from a different collection. The smallest otolith diameter from newly hatched eels was larger than the largest check diameter, indi- cating that the check was formed very early. The check, there- fore, probably was not a source of age underestimation. The four leptocephali 5 5 mm are referred to as newly hatched be- cause artificially reared leptocephali of Japanese eel (A. japonica) reached 5 mm TL about 5 days after hatching (Yamamoto and Yamauchi 1974; Yamauchi et al. 1976). Early growth rate of A. rostrata and A. anguilla is unknown but it is probably similar.
From 2 1 specimens with 13 to 130 growth increments (Fig. 2), the linear regression of body TL (mm) on number of growth increments (x) is TL = 0 . 3 8 2 ~ + 4.103, where x is the pre- sumed age in days. The average daily growth rate is 0.38 mm TL/day, with 95% confidence limits of 0.33 and 0 .44mm
ficantly different from the observed TL at hatching of experi- mentally reared larvae of A. anguilla (2.7 mm) and A. japonica (2.9 mm) (t-test, p > 0.05), obtained by Bezdenezhnykh et al. (1983) and Yamamoto et al. (1975), respectively. The coeffi- cient of determination ( r2 ) of the regression is 0.91, indicating a good model fit to the data. No difference in growth rates and back-calculated hatching dates (Table 1) between A. rostrata and A. anguilla were apparent.
Discussion The growth rates and hatching dates for Anguilla leptocephali
obtained in this sample do not match those in the existing litera- ture very well. The mean growth rates of 0.19 and 0 .24mm TL/day that Boetius and Harding (1985) and Wippelhauser et al. (1985) calculated for A. rostrata are much lower than that found in this study (0.38 mm TL/day). Boetius and Harding (1985) and Wippelhauser et al. ( 1 985) observed variation in growth rates among individual leptocephali and in the date at
TL/day. The predicted TL at hatching, 4.10 mm, is not signi- which leptocephali-were spawned. In this study, variability is
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TABLE 1. Capture dates, total lengths, and back-calculated hatching dates of Anguilla leptocephali
Capture Total length Hatching date* Species (mm) date*
08-05-84 A. rostrata
Anguilla sp. A. rostrata Anguilla spp.
caused only by variation in individual growth rates. The growth rate of 0.38 mm/day, however, appears somewhat high for the following reasons. Anguilla rostrata leptocephali undergo meta- morphosis between 55 and 65 mm (Kleckner and McCleave 1985). A leptocephalus growing 0.38 mm/day would reach metamorphosis length in about 5 months or in July, if February is assumed to be the spawning peak for A. rostrata (Kleckner and McCleave 1985; Wippelhauser et al. 1985). However, metamorphosing leptocephali of A. rostrata first appear only in October (Kleckner and McCleave 1985).
The results support Boetius and Harding's (1985) conclu- sion that there is no differential growth rate in O-group lepto- cephali of A. rostrata and A. anguilla .
Hatching dates of the 1984 specimens are much later than the reported peak spawnings of A. rostrata and A. anguilla, which have generally been assumed to occur in February and April, respectively (Harden Jones 1968 ; W ippelhauser et al. 1985). Interestingly, the 5 August sample (which comprises all the 1984 specimens but one) is atypical in two other ways. Firstly, Anguilla rostrata larvae from Kleckner and McCleave's (1985) report on historical data were on average about 46 mm TL on 5 August, while A. rostruta in this sample had a mean TL of only 34 mm. Secondly, Anguilla larvae commonly occur in the Florida current in August where they join the Gulf Stream system (Kleckner and McCleave 1 982). By contrast, larvae from this sample were about 1200 km southeast of the Florida current. The fact that these larvae were small and so far into the Sargasso Sea so late in the season argues for a late spawning. On the basis of the wide range of lengths of A. rostrata at any one time in the Sargasso Sea, Kleckner and McCleave (1985) estimated that spawning of A. rostrata may occur as late as June. Late-spawned individuals could exhibit a high growth rate, which would explain the discrepancy between my findings and those of Boetius and Harding (1985) and Wippelhauser et al. ( 1 985). However the annual cycle of
secondary production in the Sargasso Sea does not support the hypothesis of a faster growth rate in late-spawned leptocephali. Indeed high levels of secondary production in the Sargasso Sea occur in March or April (Deevey 197 1 ).
Subdaily increments cannot account for the fast growth rate reported in this study. If subdaily increments were present and treated as daily, then the larvae would actually be younger and the true growth rate even faster than what is reported.
One possible source of error in the age determinations is that what I identified as a hatch check may have been a check related to a later transition in the early life history of Anguilla, such as the onset of external feeding or the onset of vertical migration (Castonguay and McCleave 1987). But even if this were true, it would add only a few days to the age of the larvae. Another possible source of error is that some incre- ments were systematically missed because of problems with sample preparation. A last possibility of error is that the fre- quency of formation of increments was less than daily. This possibility has little empirical support in the literature.
Alternatively, estimates of growth rates based on regressions of TL on date of capture (e.g., Kleckner and McCleave 1985) are indirect. Examination of microstructure of otoliths is a better way to study growth rates of fish larvae. It is therefore possible that no errors in age determination were involved and that this sample of larvae really grew faster than what has been previously reported.
This study was a first step that showed the possibilities of microstructural analysis of otoliths in leptocephali. However, an experimental determination of the frequency of increment formation in otoliths of Anguilla is required before any firm conclusions regarding growth rates and hatching dates can be drawn. To accomplish this, techniques will have to be devel- oped...