Haddock is a demersal gadoid found on both sides of the North Atlantic Ocean. In the northwest Atlantic, they range from northern Newfoundland to Cape May and are most common in water temperatures of 2–10°C (36–50°F) and at depths of 45–135 meters (Klein–MacPhee 2002). In U.S. waters, the species is managed as two separate stocks, one from the Gulf of Maine and one on Georges Bank (Clark et al. 1982). Although Georges Bank haddock are relatively sedentary, seasonal coastal migrations are known to occur in the western Gulf of Maine (Begg 1998).
Growth rates for males and females are similar. Haddock become sexually mature at age 1 to 2, with larger females producing up to 3 million eggs apiece (Klein–MacPhee 2002). Spawning occurs from January through June, with peak activity in late March and early April (Brown 1998). A maximum age of 22 years has been observed from NEFSC commercial samples, although fish greater than 10 years of age are uncommon. Among NEFSC survey samples, the maximum length of haddock is 89 cm (35 inches), and the maximum weight is 7.9 kg (17.5 lbs).
Historically, the Woods Hole ageing laboratory used scales to determine ages for haddock under 65 cm (a full description of methods to age haddock using scales can be found in Penttila 1988), while otoliths from larger fish were broken in half. This approach was based on findings by Kohler and Clark (1958) that age determinations based on scales were comparable to those based on otoliths up to about age 7; in older fish, scale readings yielded consistently lower ages than otolith readings. During 1985–1990, scales alone were used for age determinations on survey samples (Van Eeckhaute and Buzeta 1994). In 1991, an international ageing workshop (Buzeta et al. 1992) observed that U.S. age–length keys had become truncated, and recommended that otoliths again be used for ageing larger fish. The Woods Hole ageing lab switched to using thin-sectioned otoliths for all haddock age determinations in 1995.
Methods for ageing sagittal otoliths from haddock were validated by Campana (1997) using bomb radiocarbon. He found the internationally accepted criteria for ageing haddock to be accurate to at least an age of 10 years, and within 2–3 years for fish up to 22 years old.
Otoliths are stored dry before processing. Prior to 2003, otoliths were thin-sectioned with an Isomet low-speed saw (see Equipment and Techniques). Since then, however, we have begun using a mass-sectioning approach. Otoliths are carefully aligned in rows and encased in polyester resin in a specially-made mold. This allows multiple sections to be cut at the same time on a high-speed saw with a single metal-bonded diamond blade. Two cuts are made for each row of otoliths, resulting in a thin strip (0.18 mm thickness) of resin in which the otolith sections are embedded. These strips are then glued onto a Plexiglas plate prior to ageing (Fig. 1).
Age determinations are then made by viewing the sections under a binocular microscope at 15–25X magnification, using reflected light. Sections are moistened with a weak solution of liquid dish detergent in water to enhance contrast. Viewing the otolith halves, as well, is advisable, especially if the section is unclear. The halves are most helpful for distinguishing the edge type (opaque/hyaline), and in cases where the section is not aligned on the nucleus.
Annual zones on a haddock section are composed of a white opaque zone, representing fast summer growth, and a darker hyaline zone, representing slow winter growth. The annulus is defined as the hyaline zone marking the end of a year of growth, usually laid down during the colder months. By convention, a birthdate of 1 January is used in the northern hemisphere.
Age determinations are made by counting the number of hyaline zones from the nucleus to the edge. The entire otolith section should be examined, and an effort should be made to follow annuli around the section as much as possible. Even so, certain axes are generally more reliable for ageing than others. For fish up to about 6 years of age (Fig. 2), the dorsal axis is usually the best. Fish over 8 years (Fig. 3) are most reliably aged in the proximal region near the sulcus, as regular growth continues on this side of the otolith even after accretion largely ceases along the other axes. (See Fig. 5 in Equipment and Techniques for a diagram of the parts of an otolith.)
It is important to age along these axes, as the various axes do not always show similar growth. The proximal axis sometimes shows additional growth relative to the dorsal axis; therefore the general rule of “counting out to the edge” for the first half of the year (January–June) may not always apply. For example, hyaline material may appear on the proximal axis during spring in some fish, nearly a full year before it would be counted. This may be caused by opaque material accreting on the proximal axis earlier than on other sides of the otolith; such material then would extend gradually from the proximal side around to the distal side. This effect is stronger in some years than in others; therefore, the variation in timing should be taken into account when ageing.
Normally, during the first half of the year (January–June), the proximal edge is opaque and the last annulus is just inside this opaque zone (Figs. 2, 4). However, some fish may have a thin layer of hyaline material outside this opaque zone (Fig. 5) on the proximal axis; this should not be counted. In either case, the final annulus is visible on the edge on the dorsal axis (Fig. 4, 5); this is why this axis is best for younger fish. For older fish (Fig. 3), where it is necessary to use the proximal axis, the outermost annulus is considered to be the last hyaline zone inside the edge, although the relative distance between the annulus and the edge may vary year to year.
Fish sampled in the second half of the year (July–December) generally exhibit both opaque and hyaline zones beyond the last annulus (Figs. 6, 7). Occasionally, a thin opaque edge may also be visible on the proximal axis (Fig. 8). This indicates early formation of the next year's annulus, and should not be counted until the next January. On the dorsal axis, in both cases, the last annulus should be inside the edge (Fig. 6, 8) with one opaque and one hyaline zone beyond it; rarely, the last annulus appears as a wide hyaline edge.
Younger fish tend to lay down the outermost annulus earlier than older fish, perhaps due to their faster growth rates. This results in an opaque band beyond the last annulus during the second half of the year. Deposition of opaque material also begins earlier in more southerly portions of the species’ range.
The first annulus is usually rather weak and irregularly shaped (Figs. 3, 6). This annulus has been described as having a “cauliflower shape.” Sometimes there may be a small, oval check before the first annulus (Fig. 7). The second annulus is usually wide and checky (Figs. 4, 9), and is often the first clearly defined zone beyond the nucleus. The first two annuli are usually widely spaced from one another.
After approximately age 5, annuli tend to be less checky (Fig. 10). For fish over age 9 or 10, the outer annuli are closer together but regularly spaced (Figs. 3, 11). This regular spacing may help in detecting the presence of weak annuli or checks. It is often helpful to count annuli both outward from the nucleus and inward from the edge to detect any irregularities in spacing.
Georges Bank fish (Fig. 12) often have a very checky second annulus (Fig. 4), which may sometimes appear as wide as two annuli elsewhere. Growth rates are rapid in this stock, and the annuli are easier to distinguish.
Gulf of Maine fish (Figs. 13, 14) have close, weak annuli and grow more slowly than those on Georges Bank. The first annulus in this stock tends to be small and weak (Fig. 15), followed by a strong second year (Fig. 9).
Variability in length at a given age is greater among older fish and is more pronounced in the Gulf of Maine and among strong year classes. In recent years, growth appears to have slowed versus historical samples, in that fish of a given length may have higher ages than in decades past. This has led, for example, to an increased frequency of fish under 60 cm with ages of 10 years or more (i.e. Fig. 13).
The youngest age classes may be an additional source of confusion. When working with fall samples, expect sections for young-of-the-year fish to have irregular outlines (Fig. 16). Usually no structure is visible inside the edge, but a check is sometimes present. Age-1 fish in fall will show a first annulus inside the edge (Fig. 17), and have a more regular outline than age-0 fish. Spring age-1 fish (Fig. 18) will have an irregular outline and hyaline material in or near the edge; some may also show a check inside the edge.
Other issues which commonly complicate ageing of haddock include off-center sections, shifting, crystallized otoliths, and split annuli. These issues are described further in the next few paragraphs.
When an otolith is cut off-center, the first annulus may appear abnormally small, the second annulus may have the irregular “cauliflower” shape typical of the first annulus, and the annuli may appear excessively checky (Fig. 19). The sulcus also is shaped differently than in properly-sectioned samples. If this is observed, the otolith halves often yield a better view of the otolith. This problem may be avoided by more careful alignment of otoliths when sectioning.
Shifted otoliths (Fig. 20) show a discontinuity in otolith growth, due to movement of the otolith within the sacculus. In many cases, it is still possible to age these fish. However, if it is not possible to decide on an age for a section, it is better to leave it unaged or examine the second otolith from that particular fish. Occasionally, an otolith may be crystallized (Fig. 21), where the accreted material is strongly hyaline and and/or the otolith assumes an unusual shape. These sections are often unageable, but the other otolith from that fish may be unaffected.
“Split annuli” (Fig. 22) consist of two apparent annuli near one another. It may be difficult to distinguish whether each mark is a true annulus or if the annuli alternate with checks. Any such mark should be traced around the otolith to resolve the issue, and affected fish may be better left unaged.
In summary, age determination for haddock is relatively straightforward. The dorsal axis is most reliable for younger fish. Growth continues in the proximal region for the lifespan of the fish, but ageing of this axis is complicated by year-to-year variability in the timing of annulus formation. Close examination of the spacing between annuli, however, should resolve most difficulties.
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