FBP: Age Determination Methods: Silver Hake

Silver Hake, Merluccius bilinearis

L.M. Dery

Silver hake is an important gadoid ranging from Newfoundland to South Carolina and is most abundant from Nova Scotia to New Jersey (Bigelow and Schroeder 1953). Silver hake are found over a wide range of depths, from shallow waters to depths greater than 400 m (Almeida 1984).

Two genetically distinct stocks have been defined south of Nova Scotia: a northern stock occupying the Gulf of Maine-northern Georges Bank region and a southern stock occurring from southern Georges Bank to Cape Hatteras (Anderson 1974, Schenk 1981, Almeida 1984). Some mixing of the two stocks occurs throughout all or most of the year, perhaps facilitated by the wide temperature tolerance of this species.

Silver hake of the southern stock overwinter primarily along the outer continental shelf from Georges Bank to Cape Hatteras. During spring and summer, these fish move northward and inshore onto the southern and southeast parts of Georges Bank (Almeida 1984). Spawning occurs on the southern slopes of Georges Bank from May to November, reaching a peak in Southern New England and mid-Atlantic waters by May and June. Silver hake of the northern stock overwinter in deep basin areas of the Gulf of Maine, moving into shallower waters in late spring-early summer. Spawning occurs in inshore waters from Cape Cod to Grand Manan Island from June through November, peaking in July and August (Bigelow and Schroeder 1953, Colton and St. Onge 1974, Fahay 1974). Secondary spawning occurs on the north to northwestern slopes of Georges Bank (Sauskan 1964, Sauskan and Serebryakov 1968).

Female silver hake grow faster and live longer than males. Males attain a maximum age and length of about 10 years and 42 cm (17 in), respectively, contrasting with 12 years and 67 cm (26 in) for females (NEFSC unpubl. data). Most silver hake are sexually mature by age 2.

Ageing methods for silver hake, based on the otoliths, remain somewhat controversial despite intensive research (Nichy 1969; Anderson and Nichy 1975; ICNAF 1976, 1977, 1978; Hunt 1980a). All investigators have counted hyaline zones as annuli. The prolonged spawning season of this species, and the variability of growth patterns due to genetic and environmental factors have made accurate identification of the first annulus, and discrimination between checks and annuli, difficult. In addition, age interpretation using whole otoliths may differ significantly from interpretations based on otolith thin sections and or sectioned halves. The edge type on some otoliths, either hyaline or opaque, has appeared to some investigators to be inconsistent with the season of the year, causing confusion with regard to edge interpretation. Age validation studies for silver hake of the Scotian Shelf have been conducted by Hunt (1978, 1979, 1980b). Although relatively few otoliths of these fish have been examined at this laboratory in recent years, some of the growth patterns appear similar to that observed on Gulf of Maine otoliths, and others resemble those of the southern stock.

The methods used at the NEFSC Woods Hole Laboratory have evolved from early studies by Nichy (1969) on the growth of young silver hake, from participation in age and growth workshops (ICNAF 1976, 1977, 1978), and from research on types of silver hake growth patterns and their distribution in the study area (NEFSC unpubl. data). Particular attention has been focused on otolith growth patterns of young fish (age 0+ to 1) to facilitate accurate interpretation of the first annulus, and to avoid assigning earlier hatched fish of the southern stock, and later hatched northern Georges Bank-Gulf of Maine fish, to different year classes. Hunt (1980a) summarized previous research in the literature concerning ageing methods used for this species. He described some aspects of otolith growth patterns as characteristic of given geographic areas but did not present an integrated description of these patterns for different stocks of silver hake. The method of presentation of Hunt's interpretations, in addition to his use of whole otoliths rather than thin sections, makes difficult direct comparisons of his criteria with those used at Woods Hole. Validation of methods at Woods Hole has involved comparisons of modal groups in fish length-at-age data with the modal groups in length frequencies, and monitoring modal progression of prominent year-classes in the fishery on a seasonal and annual basis.

Methods of preparing otoliths have been described by Nichy (1969, 1977), Anderson and Nichy (1975), ICNAF (1976, 1977, 1978) and by Hunt (1980a). Such methods have involved storage of whole otoliths in glycerin or some other medium to "clear" the otolith and enhance the hyaline zones. Other methods include dry storage and soaking for a short period in ethyl alcohol before viewing. At the Woods Hole Laboratory, otoliths are stored dry in coin envelopes. A thin transverse section 0.20-0.23 mm thick is removed at the nucleus and examined under reflected light against a dark background, using a method developed by Nichy (1977). The cut surfaces of the sectioned otolith may be used in addition to, or instead of, the thin-section, depending on the degree of complexity of the growth pattern.

For some fish, whole otoliths, examined in ethyl alcohol, are used to verify age from thin-sections, but are not considered completely reliable. This is because the pattern of early growth on the otolith, which is often difficult to interpret, tends to be obscured by subsequent calcification, despite the use of strong clearing media such as glycerin (Anderson and Nichy 1975). In general, silver hake otoliths become thicker with increasing age relative to an increase in width. Therefore, misinterpretation of early growth, especially of older fish, is more likely. Nevertheless, some whole otoliths exhibit a clearer pattern of annulus formation than do thin-sections, especially if the annular zones are weak or diffuse.

Growth patterns observed on silver hake otoliths tend to support Almeida's (1984) definition of two separate stocks from the Gulf of Maine to Cape Hatteras. Variations in the growth patterns on otoliths with geographic location include size and formation of the first annulus, relative growth increment widths between annuli due to differences in growth rate, formation of checks and split zones, and time of annulus formation. Although characteristic growth patterns can be identified for each stock, some patterns are difficult to classify (due in part to individual variability). Other patterns are intermediate in type, with aspects characteristic of both Gulf of Maine fish and those further south. This may reflect stock intermixture as suggested by Almeida (1984). Seasonal shifts in the distribution of growth patterns also appear to be consistent from year to year and seem to reflect observed migratory movements (NEFSC unpubl. data).

The otoliths of silver hake from the southern stock tend to exhibit moderate to large amounts of opaque edge as early as March or April, indicating that annulus formation is complete by the end of the winter and probably earlier (Fig. 1). By convention, a birthdate of 1 January is used; the hyaline zone evident on the edge of the otolith is interpreted as an annulus whether or not it is complete. As is typical for many fish species, seasonal growth resumption is quite advanced for young fish relative to older individuals (Fig. 2) and age 1 fish otoliths show considerable amounts of "+" growth as early as April (Fig. 3). The timing of annulus initiation in the autumn is somewhat variable. Opaque edge may persist on the otoliths of age 0+ or 1+ and older individuals into autumn (September-October) (Fig. 4); however, most otoliths collected during autumn tend to exhibit a narrow hyaline edge which is not included in the age determination (Figs. 5 and 6).

In spite of variability in size and timing of formation of the first annulus, the characteristically large growth increment (wide opaque zone) between the first and second annuli provides a means of distinguishing between the two zones (Figs. 1 and 6). The differences in mean length at age l and age 2 in the spring months do not fully reflect the magnitude of this growth increment, because of the early growth resumption of age l fish (growth beyond the first annulus). The first annulus frequently appears as a small, dense, but split zone of hyaline material surrounding the nucleus of the otolith (Fig. 2), not evident on the otoliths of 0+ fish (Fig. 5). The annulus may also be a large and/or complex zone, with a significant amount of opaque material formed between the nucleus and the first annulus (Fig. 2). Occasionally, however, there is minimal evidence of this annulus, probably due to later hatching (Fig. 4).

The "pelagic" zone, or settling check, traditionally noted as important in age determination (Nichy 1969; ICNAF 1976, 1977, 1978; Hunt 1980a), was initially described by Nichy (1969) in his study of the growth of small silver hake as a small weak zone of hyaline material surrounding the nucleus that appears to form between the pelagic and demersal stages in the life history of this species. Some investigators including Hunt (1980a) have interpreted the "pelagic" ring as an occasionally large and strong zone of hyaline material that may be formed as late as 5 months of age. According to Fahay (1974), however, the length of the pelagic phase following hatching is about 2 months. Therefore, it is possible that a small first annulus formed close to the nucleus of later hatched or slower-growing fish is occasionally mistaken for the pelagic zone by some age readers. At the Woods Hole Laboratory, the pelagic zone has been conservatively interpreted as a small, usually weak zone, following the criteria of Nichy (1969) and verified by the appearance of this zone on age 0+ otoliths (Figs. 4 and 7). It should be noted, however, that accurate differentiation of the pelagic zone from the first annulus is difficult and remains a significant source of error.

Checks formed between the first and second annuli on otoliths collected from southern stock individuals may confuse interpretation of annular zones. The formation of a spring check on age l otoliths was documented by Nichy (1969) (Fig. 6) and similar checks may also be formed later in the season. A check formed in late summer to early autumn, close to the time at which the annulus begins to form, is characteristic of most silver hake otoliths of the southern stock. Such checks are usually weak and/or discontinuous zones that are not as prominent as annuli in the sulcus area (Figs. 6 and 8). By comparison, the second and subsequent annuli are strong and consistent around the periphery of the otolith, particularly on the proximal (sulcus) and distal sides of the section (Figs. 2, 6, and 8). Typically, annular zones on otoliths of silver hake of this stock are evident as thick dense bands of hyaline material layered on the proximoventral part of the otolith (Figs. 6 and 8). Occasionally, however, these bands are split into several rings that must be followed around the periphery of the section in order to resolve the annular zones (Figs. 9 and 10). Whole otoliths can be especially useful in helping to resolve anomalous zones such as checks and split zones on thin otolith sections.

Subsequent to the second annulus, growth increments tend to be quite narrow on otolith sections due to a decrease in growth rate, so that annuli are located rather close together. This is particularly characteristic of males whose growth rate is slower than females after age 2 (Fig. 6). Because of these narrow growth increments, older fish may be difficult to age, especially if there are strong checks between the annuli. Even where growth increments are relatively wide, annuli may be weak or diffuse (Fig. 10). If growth is "shifted", that is, if there is an unusual amount of growth on one part of the section in contrast to the normal pattern of deposition, interpretation should be focused in the direction of the shift in growth in order to avoid underinterpretation of age. The large amount of accreted material in each growth zone as a result of this shift enhances definition between the annuli and therefore facilitates age interpretation.

Time of annulus formation for the northern stock is later relative to hake further south, and follows a more seasonal pattern, as would be expected for more northerly latitudes (Williams and Bedford 1974). Annuli of these fish are completed in the late-winter to early-spring months with the exception of age l fish, some of whom resume growth during the winter months. Therefore, the otoliths of most fish in March and April may continue to exhibit hyaline edge or a small amount of opaque edge, particularly on the thin sections (Fig. 11), while age l fish may exhibit a larger amount of opaque edge (Fig. 12). By October-November, some hyaline edge is usually evident on otoliths of age 2+ and older fish (Fig. 13), while opaque edge is likely to persist somewhat longer on age 0+ and 1+ fish.

The first annulus on otoliths of fish collected from the Gulf of Maine is somewhat variable in size, reflecting a tendency for some year classes (e.g., 1982, 1984) to have a bimodal distribution of length at age l. The first annulus on small age 1 fish (e.g., 5 cm) may appear as a relatively weak hyaline zone and coincident with the pelagic zone (Fig. 14). It may be difficult to distinguish from the pelagic zone, spring check, and second annulus because the growth increment between the first and second annulus in the Gulf of Maine is generally smaller than further south, and because more of these fish are hatched later in the year. In some cases, the first annulus may not be evident but is assumed near the nucleus because of the large growth increment between the nucleus and the first strong hyaline zone that can be interpreted as an annulus (Fig. 15). While this interpretation may not appear justifiable biologically, it is necessary in order to avoid assigning these fish, and earlier hatched individuals, to different year classes. Figure 16, for example, shows a section from a 13 cm fish sampled in April with no evidence of an annulus. This fish is interpreted as age 1 (late hatched) and not 0+, because spawning in the Gulf of Maine does not begin until the summer.

Otoliths from larger age l fish may exhibit a well defined hyaline zone (first annulus) formed some distance from the pelagic zone, which may be more prominent on these otoliths (Figs. 13 and 17). No marked discontinuity appears to exist between the growth patterns of these large age l fish and age 2 fish with a tiny first annulus. One technique for differentiating large age l fish from small age 2 fish with similar growth patterns involves measurement of the first annulus on otoliths collected from fish identified as age l using length-frequency data. Such measurements can provide an estimate of average and maximum first-annulus width for adult fish so that overinterpretation of age can be avoided in cases where the pelagic zone is prominent.

The weak zone formed around the pelagic zone on some silver hake otoliths can be difficult to identify as either a spring check (typical for silver hake of the southern stock) or a weak first annulus (more characteristic of the northern stock), since these zones form at the same time (April-May) (compare Figures 6, 13, and 14). Some silver hake from the Gulf of Maine exhibit a large first annulus with a very weak or nonexistent pelagic zone (Fig. 18). This type of pattern is more commonly observed on silver hake from the Scotian Shelf. Because of the variations in first year growth patterns observed in the Gulf of Maine, the detail available on the otolith section seems necessary in order to make the most accurate interpretation possible.

The otoliths of Gulf of Maine silver hake are narrower and thicker in cross-section in contrast to those from more southern areas. Subsequent to the second annulus, accurate annulus interpretations of these fish are facilitated by this increased thickness and relatively wide increment widths between annuli resulting from faster growth (Figs. 11 and 14). In addition, the annular zones are quite easy to interpret because of relatively few anomalies (checks and split zones) and because the annular zones are strong and well defined (Figs. 14 and 19). Prominent checks are evident on some Gulf of Maine otoliths, but they are easily recognized by weak formation in the sulcus area in contrast to the annular zones (Fig. 11).

Some fish collected in the Southern New England and southern Georges Bank area exhibit growth patterns that appear to be hybrids of the two basic growth patterns described above. For example, the growth increment between the first and second annulus is intermediate in width, or the growth pattern will exhibit larger numbers of checks than is characteristic for the Gulf of Maine but fewer than typically seen further south (Fig. 20). Other otoliths, especially from southern Georges Bank fish, exhibit numerous strong checks and split zones that make annulus identification difficult (Fig. 21). In general, growth patterns observed among fish collected in the spring from the Southern New England-southern Georges Bank area are rather heterogeneous compared with the greater consistency observed in the mid-Atlantic or northern Georges Bank-Gulf of Maine areas.

In summary, systematic study of the types of otolith growth patterns exhibited by silver hake of various stocks may facilitate consistency of age interpretation of these fish because of their prolonged spawning season and the variability of their growth patterns. Although bias may be created in anticipating an interpretation based on the geographic location of the sample, errors due to inconsistent interpretations could be more serious. Age readers at the Woods Hole Laboratory, having noted the variability of growth patterns on silver hake otoliths, attempt to apply standard criteria for the identification of annuli and checks that are agreed upon as valid by other age readers.


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