L. M. Dery and J.P. Mayo
Black sea bass is an economically important serranid ranging from New England to Florida (Kendall 1977). A protogynous hermaphrodite, sex reversal from female to male occurs for at least half of the population, usually between the ages of 2 and 5 (Mercer 1978). Males are faster growing than females, attaining a maximum length and age of over 60 cm TL (24 in) and 20 years, respectively. Females reach a maximum length and age of 38 cm and 8 years (Lavenda 1949). Female black sea bass are sexually mature by age 2; males may not mature until age 4 (Mercer 1978).
Two stocks of black sea bass have been recognized north and south of Cape Hatteras (Cupka et al. 1973). The northern stock migrates seasonally in response to temperature changes. Most of these fish overwinter along the edge of the continental shelf in the southern part of the Mid-Atlantic Bight. In the spring movement is inshore and northward to depths less than 40 meters for spawning and feeding during the summer months (Musick and Mercer 1977, Kendall and Mercer 1982). Spawning extends from June through October, reaching a peak progressively later further north (Mercer 1978, Kendall and Mercer 1982). The southern stock does not appear to be seasonally migratory, frequenting the live bottom areas south of Cape Hatteras (Kendall and Mercer 1982). Spawning commences in February, and reaches a peak in April or May (Mercer 1978).
Several hard structures have been used for age determination of black sea bass. Lavenda (1949) and Briggs (1978) used cellulose acetate impressions of the scales to age black sea bass from New York and New Jersey waters. They identified zones of closely spaced circuli as annuli on these structures. This technique, however, was not validated and was questioned by other investigators (Cupka et al. 1973, Mercer 1978, Link 1984). In studies of Virginia-South Carolina fish, otoliths were preferred over scales and were found to have valid age marks. Although investigators did not find operculae or vertebrae to be useful, pelvic spine sections and impressions of scales from behind the pectoral fin were cross-validated with otoliths and found to be acceptable alternate ageing structures. 'Cutting-over' marks on scales and hyaline zones on otoliths have been validated as annuli utilizing marginal increment analysis of OTC marked scales and otoliths from fish held in laboratory aquaria fro up to 2 years (NEFSC unpubl. data). Marks on scales were found to form at approximately the same time as the deposition of opaque material on otoliths is completed. The outer edge of the opaque zone has been interpreted as the annulus by other investigators (Cupka et al. 1973, Mercer 1978, Link 1984, Wenner et al. 1986). Both scales and otoliths are used at the NEFSC Woods Hole Laboratory for routine age determinations.
Glycerin has been used as a storage medium to enhance the clarity of hyaline zones on otoliths (Cupka et al. 1973). Mercer (1978) reported that glycerin tended to over clear the otolith's edge, and therefore used glycerin to clear only those otoliths where annuli were obscured by the overgrowth of calcium. Wenner et al. (1986) stored otoliths dry, and viewed them in water. At Woods Hole, otoliths are stored dry and examined in ethyl alcohol to avoid the over enhancement of hyaline zones. Thin transverse sections (0.20-0.23 mm thick) are removed at the nucleus and are examined instead of whole otoliths if the annuli are obscured by later calcification. Otoliths are examined distal-surface-up against a black background at 10-l5x using reflected light.
Five or six scales from behind the pectoral fin are impressed in laminated plastic (Dery 1983) and viewed under a microprojector at 40x.
Annulus formation on otoliths and scales occurs in May or June. The outer edge of the opaque zone is interpreted as the annulus on otoliths and the cutting-over mark as the annulus on scales. By convention, a birthdate of 1 January is used; the annulus forming on the edge of these structures is included in the age whether or not it is completely formed. Formation of opaque material may persist on some otoliths into the early autumn and should be taken into consideration when backcalculating lengths from otoliths. The formation of hyaline zones on otoliths, which normally occurs from June through the following January (Mercer 1978), is unusual because hyaline material, indicating slow growth, generally forms during the colder months of the year. It is possible that the lack of opaque material deposited during the warmest months may be due to shifts in calcium metabolism during onshore movements into very warm coastal water in the summer. The summer flounder (Paralichthys dentatus), a species that has a migration and distribution pattern similar to that observed for black sea bass, shows similar seasonal calcification patterns on its otoliths.
Otoliths show the clearest record of first-year growth. Figure 1 shows the age structures from a 22 cm, age 1+ fish collected in November. A weak hyaline core area formed after hatching occurs close to the center of the otolith (Fig. 1a). Opaque material is deposited around this central core. The deposition of this material is complete by the following spring, forming the first annulus. A wide hyaline zone then forms during the summer and autumn of the second year. This otolith shows an unusual amount of opaque edge for November. Hyaline edge would persist on most adult black sea bass otoliths until January.
A poorly defined first annulus on the corresponding scale is typical for most black sea bass (Fig. 1b). If present it will usually appear as a zone of closely spaced circuli without a cutting-over mark. The first cutting-over or erosion mark representing the second annulus is not yet evident.
Figure 2 shows the two types of age structures for a 40 cm, age 4 fish collected from Nantucket Sound in June, the time of annulus formation. Both show rapid growth typical of more northern areas. The otolith (Fig. 2a) shows prominent hyaline zones separated by wide growth increments. Four opaque zones, including the edge, are formed on this otolith. The scale of this age 4 fish (Fig. 2b) has two clear cutting-over marks representing the second and third annuli. The first annulus is vaguely indicated by the zone of compacted circuli near the focus of the scale. The outer edge of the scale is the fourth annulus.
Clear growth patterns are also characteristic of slower growing black sea bass from more southerly ranges of the northern stock. Figures 3a and 3b show the growth patterns of a 35 cm, age 6 fish collected from Virginia waters in February. Growth increments on these structures are relatively narrow. The last (sixth) annulus on the outer edge of the otolith and scale is not complete because of the February collection date. Nevertheless, we include the edge in the age of the fish because of the 1 January birthdate convention.
Although opaque zones are usually well defined on the otoliths, they may sometimes be bordered by such thin hyaline zones that annuli could be missed in the age interpretation. Figure 4 shows the age structures of a 31 cm, age 3 black sea bass sampled from Nantucket Sound in June. The second hyaline zone bordering the second annulus (opaque zone) is weakly defined on the otolith (Fig. 4a) but is very strong on the scale (Fig. 4b).
Weak annuli are also characteristic of the central region of the otoliths for some older fish, prior to a sharp increase in growth rate after four or five years of slow growth. This pattern occurs in the age structures of a 46 cm, age 8 fish collected from Virginia waters. The second, third, and fourth annuli are clearly formed, although they are closely spaced on the scale (Fig. 5b). On the otolith (Fig. 5a), however, these annuli are very difficult to distinguish without referring to the other structure. The change in growth rate reflected by these structures may be the result of sex reversal or migration.
The formation of strong checks and split hyaline zones (or split cutting over marks on the scale) may make annulus interpretation difficult. Figure 6 shows the age structures of a 30 cm, age 2+ fish sampled from New Jersey waters in November. Both the first and second annuli (opaque zones) (Fig. 6a) are split into two rings, but the relative spacing between them does not identify these rings as "split" zones without reference to the other structure (Figs. 6b). Therefore, based on examination of the otolith alone, an age of 3+ or 4+ could be interpreted. It should be noted that the first hyaline zone is split into two or more rings on many otoliths. Identification of this anomaly is difficult only if there are narrow growth increments between the first several annuli.
Figure 7 shows the difficult to interpret age structures of a 35 cm, age 4(3)+ black sea bass sampled from New Jersey waters in November. If the second annulus is bordered by a weak hyaline zone (Fig. 7a), the age would be interpreted as 4+, otherwise the age would be 3+. The most likely interpretation of the scale impression, however, would be age 3+, recognizing a false cutting-over mark formed between the first and second annuli (Fig. 7b). For such fish, the final age must be determined using the strongest evidence for a particular age.
Annuli may remain relatively easy to interpret at older ages, although increasingly narrow growth increments may cause some confusion. The age structures from a 57 cm, age 10 fish sampled from Virginia waters in February, show the clear annuli typical of most older fish. Annuli on the otoliths may be somewhat obscured by overgrowth of calcium (Fig. 8a), and erosion of the scale may obliterate the annuli close to the central anterior edge of the scale (Fig. 8b). Nevertheless, these structures can still be accurately aged, especially if the otolith is sectioned and the anterior corners of a scale are carefully studied.
In summary, some geographic variation in growth patterns appears to exist. For example, the growth patterns on the structures of some New Jersey fish are especially difficult to interpret because of the formation of strong checks or split zones (Figs. 7a and 7b). Characteristic differences between the northern and southern stocks have not been documented, however.
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