Pollock: Age Determination Methods for Northwest Atlantic Species
How to use sectioned otoliths to age Pollock
Pollock, Pollachius virens
K.G. Andrade, C.P. Smith, and W. Duffy
Pollock is an amphiboreal gadoid found on both sides of the North Atlantic. In the western North Atlantic, it ranges from Labrador to Georges Bank, but is most abundant on the southwestern Scotian Shelf and in the Gulf of Maine. Pollock are highly migratory and extensive movements have been documented (Steele 1963, Neilson et al. 2003). Stock structure has not been elucidated in the Northwest Atlantic.
Pollock begin to spawn in October, and spawning peaks in December and January. Most individuals attain sexual maturity in 3 or 4 years, although some pollock may not mature before around age 6. Pollock are comparatively long-lived, attaining a maximum age of 23 years, and may reach lengths of 130 cm (51 in) and weights of 16 kg (35 lbs) (Collette and MacPhee 2002). No significant differences have been observed in growth rates between sexes (Hoberman and Jensen 1962, Steele 1963, Clark et al. 1978, Collette and MacPhee 2002).
Scales and otoliths have been used for age determinations of pollock. However, otoliths have become the preferred ageing structure because of the difficulty in distinguishing outer annuli on scales. Therefore, scales are no longer collected. Otoliths are stored dry until processing. The ageing method using otoliths was validated by Steele (1963) using micro-increment analysis of the otolith edge. This method was also validated by Neilson et al. (2003), using mark/recapture. They were able to tag small pollock at an assumed age of one or two (based on the size and date of capture). Once recaptured, time at large was used to determine "known" age. Otoliths were then extracted and read to confirm the age of the fish (Neilson et al. 2003).
Pollock otoliths were traditionally processed using an Isomet low-speed saw to cut transverse thin-sections (0.17 to 0.28 mm thick) exactly at the nucleus in the dorsoventral plane (as described in Methods and Equipment). However, because of the need to provide more age data for expanded stock assessments, it was necessary to increase processing efficiency. For otolith sectioning, a high-speed saw system is now used: Otoliths are embedded in polyester resin in aluminum molds, with sulcus of each otolith aligned for sectioning. Six to ten otoliths are aligned in each row, with six rows per mold. The blocks of resin are then cut using a high-speed saw with one metal-bonded diamond blade. Two cuts are made for each row of otoliths, yielding a thin strip (0.30 mm thick) of resin in which the sections are embedded. These six strips are then glued onto a Plexiglas slide.
Age determinations are made by placing the Plexiglas slide under a binocular microscope and viewed using 20–30X magnification under reflected light. Annual zones on a pollock section are composed of a white opaque zone representing fast, summer growth and a dark translucent zone representing slow, winter growth. The annulus is defined as the translucent zone marking the end of a year of growth, i.e., the winter growth zone. These zones are easily distinguishable in fish up to 6–8 years of age. Deposition of translucent material begins as early as October in some fish and is clearly evident in almost all specimens collected by December and January (Figure 1). Opaque material may be deposited as early as December, but is typically found May–September (Steele 1963) (Figure 2).
By convention, a 1 January birth-date is used; therefore, a translucent zone forming on the edge of the otolith is counted as an annulus on 1 January, even though the zone is not complete.
Age determinations are usually made by counting translucent rings from the center to the edge. The first annulus is rarely located close to the nucleus. Spacing between the opaque and translucent rings is important in locating settling checks. This settling check consists of a series of light translucent lines more widely spaced than the translucent lines comprising a true first annulus. The overall shape of the settling check is more elliptical and has fewer, if any, undulations than the true first annulus. The settling check in some otoliths encircles a dark area, but for others, it is barely visible. An opaque space usually marks its location (Figures 2 and 3).
Age determinations may be made on several areas of the otolith. The proximal side near the nucleus is optimal for an initial reading, and the side closer to the dorsal end sometimes is easier to read because the annuli tend to be more separated in this region. Rings are also well defined on the dorsal end, but an age reader should carefully follow the rings around to the distal and proximal sides to insure that no splitting has occurred. For older fish, the dorsal end can be very helpful, particularly when searching for the most recently formed annulus. Older specimens taken in summer (July–September) can be particularly difficult, because for most fish little opaque summer growth is visible. The dorsal end generally magnifies the size of the annuli and clarifies differentiation. The second, third, and fourth annuli are usually broad and frequently paired or split. The fifth and sixth annuli may also be split. Split rings must be carefully followed around the otolith. Split translucent rings commonly occur for one or two successive years and may continue for up to four years (Figures 4 and 5).
Several criteria must be considered when evaluating split translucent rings. Spacing is most important. Usually, the opaque increment between the split rings is narrower than the opaque increment between the outside split ring and the next translucent ring towards the edge. This next annulus, if not split, is usually darker with more clearly defined boundaries than the split ring. If two closely spaced translucent rings, which appear suspect, merge as the reader follows them around to the ventral/proximal side, and/or if they merge at the sulcus, they should be recognized as a split ring and counted as a single annulus (Figure 6).
Annuli become narrower and more crowded towards the edge of the otolith and are occasionally difficult to read. In such cases, an age reader should count back toward the center to locate a strong translucent ring. It may be followed down toward the dorsal end until an area is located that has more distinct translucent rings. Repeated counts on each otolith are necessary, especially for older fish. Otoliths from specimens over 10 years of age should be examined several times until a consistent determination is reached (Figure 7).
Although pollock otoliths cause the age reader many problems because of settling checks and split annuli, there is a close correspondence between age and length in smaller and medium sized fish. However, pollock measured between 70–100 cm can be anywhere from 6 to 20 years old. Pollock age readers should show caution and use fish length only as another diagnostic to determine the right age.
Clark, S.H., L. Cleary, and T.S. Burns. 1978. A review of the Northwest Atlantic pollock resource. Int. Coun. Explor. Sea C.M. 1978/G:61 33 p.
Hoberman, J.M., and A.C. Jensen. 1962. The growth rate of New England pollock. Trans. Am. Fish. Soc. 91: 227-228.
Klein-MacPhee, G. 2002. Pollock Pollachius virens (Linnaeus 1758). pp 247–252 in Fishes of the Gulf of Maine. B. Collette and G. Klein-MacPhee, editors. Smithsonian Institution Press, Washington, D.C.
Neilson, J.D., W.T. Stobo, and P. Perley. 2003. Age and growth of Canadian east coast pollock: comparison of results from otolith examination and mark-recapture studies. Trans. Am. Fish. Soc. 132:536-545.
Steele, D.H. 1963. Pollock (Pollachius virens (L.)) in the Bay of Fundy. J. Fish. Res. Bd. Can. 20(5):1267-1314.