© 2006 International Council for the Exploration of the Sea
Age validation of walleye pollock (Theragra chalcogramma) from the Gulf of Alaska using the disequilibrium of Pb-210 and Ra-226
National Oceanic and Atmospheric Administration, National Marine Fisheries Service, Alaska Fisheries Science Center, Resource Ecology and Fisheries Management 7600 Sand Point Way N.E., Seattle, WA 98115-6349, USA
*Correspondence to C. R. Kastelle: tel: +1 206 526 4266; fax: +1 206 526 6723. e-mail: craig.kastelle{at}noaa.gov.
The walleye pollock (Theragra chalcogramma) is a commercially important species in the North Pacific, and harvest quotas are dependent upon accurate determination of ages. The two techniques (called methods A and B) currently used to interpret the growth zone patterns in walleye pollock otoliths were compared. The age distributions from these two techniques differed; method B produced ages twice that of method A. Validation of ages from walleye pollock has not been done previously. Radiometric ageing based on the ratio of Pb-210/Ra-226 was used to evaluate the accuracy of otolith growth zone counts, and it demonstrated that method A, which produced younger ages between 3 and 8 years, was correct. Walleye pollock grow older than the 3–8 year (method A) age range validated in this study. The experimental design was limited to a maximum method A age of 8 years, because available samples did not provide the minimum of 40 fish required for estimating a radiometric age. Our radiometric ageing study on walleye pollock appears to be the first to use the Pb-210/Ra-226 radiometric age-validation method in a boreal fish species where all samples were potentially young, 8 years or less. In previous studies, radiometric ages often approached 100 years. Also, only one presumed year class was used, which was sampled in successive years. Therefore, Ra-226 sample measurements were averaged to provide lower error.
Keywords: age determination, age validation, Pb-210/Ra-226, radiometric ageing, walleye pollock
Received 25 July 2005; accepted 5 June 2006.
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Introduction |
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 Top Introduction Material and methodsResultsDiscussionReferences |
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Walleye pollock (Theragra chalcogramma) age determination methods have not been validated, and opinion differs as to how otolith growth zones should be interpreted. Radiometric ageing has been successfully used in other species where the potential maximum age is >35 years, and often over 100 years (Campana et al., 1990; Smith et al., 1995; Kastelle et al., 2000; Andrews et al., 2002). In the past, the error estimated with each radiometric age has made it difficult to apply the method to a short-lived species like walleye pollock, where the average age of harvested fish is possibly <8 years. This study appears to be the first use of the radiometric ageing method to validate ages for a boreal species where a large number of fish were potentially very young.
Walleye pollock are widely distributed in the North Pacific, stocks ranging from the Sea of Japan, Sea of Okhotsk, Bering Sea, and Gulf of Alaska, to Puget Sound (Bailey et al., 1999). The species supports one of the world's largest fisheries, 1 542 414 t being harvested from US waters in 2003. In the Gulf of Alaska, commercial harvests peaked in 1984 at 307 401 t, then declined to just 50 666 t in 2003 (Dorn et al., 2004; Ianelli et al., 2004). Management of the stocks relies on age-structured assessment models (Dorn et al., 2004), so allowable biological catch estimates are dependent upon accurately ageing the fish.
Currently, the two methods used for estimating walleye pollock ages from otoliths are in question, because they use a different interpretation, or ageing criteria, to determine age from otoliths (Munk, 2001, 2004). Both methods rely on counting presumed annual growth rings in the otoliths, where a single annual growth ring consists of an opaque and a translucent zone (Chilton and Beamish, 1982). However, interpretation of the zones and the assumed growth patterns in these two methods are not the same. One method, hereafter called method A, usually interprets prominent, evenly spaced, and continuous zones as annual, and disregards other less prominent zones. The other method, hereafter called method B, often interprets zones as annual that are finer, less prominent, but still visible, and often not as continuous.
In both approaches, the burnt-otolith-section method is frequently used to view the growth rings (Beamish and McFarlane, 1995). However, there are differences in the way this is used and applied (Kimura and Lyons, 1991; Kimura et al., 1992; Munk, 2001). Method A views the proximal surface of whole otoliths when clear, typically in younger fish. When the surface is not clear, the burnt-otolith section is used more often in older fish. On the surface, the early growth zones are often seen within the otolith's interior, and subsequent growth zones are visible on the otolith's edge. The burnt-otolith section is used on thick old otoliths when newer material was deposited only on the proximal or distal surface. It is also used on otoliths of any age when the early growth zones are not seen while viewing the proximal surface. Method A uses the theory that, if visible, an equal number of growth zones will be seen on both the proximal surface and burnt-otolith section, so the burnt-otolith sections are viewed when the surface is not clear. A definition of young and old fish is deliberately not made here because the age reader analysing the otoliths makes the decision on each otolith if a burnt-otolith section is necessary. Method B uses only the burnt-otolith section, so the proximal surface is not viewed.
The potential maximum ages estimated for walleye pollock in the Gulf of Alaska are very different for these two methods of interpretation. Method A occasionally estimates ages above 15 years (Dorn et al., 2004). Ages estimated using method B are often above 20 years, and occasionally above 30 years (Munk, 2004).
Age validation for walleye pollock is obviously necessary with the two diverging methods of interpreting the burnt-otolith sections, and the importance of this species in the North Pacific. This need is also addressed in an age-corroboration study by Kimura et al. (2006). Those authors considered the difference between age validation and age corroboration, and suggested that confidence could be developed in unvalidated ages through several complementary age-corroboration procedures. An early validation study by LaLanne (1979) used otolith surfaces and occasionally viewed an otolith's cross-section to locate the first translucent zone. That study used otolith edge type vs. time of year and modes in length frequencies as age-validation tools. However, LaLanne (1979) did not use the burnt-otolith section to view either the growth zones on thick otoliths from older fish (proximal and distal areas), or the older ages suggested by method B. Validation of burnt-otolith-section ages has been inferred in other studies where growth zone counts from other hard structures in walleye pollock were compared with those in otoliths (Lai and Yeh, 1986; McFarlane and Beamish, 1990). These two comparative studies do not provide a true validation (Campana, 2001; Kimura et al., 2006). Sometimes an ageing method which produces the oldest ages is favoured over one that produces young ages because of the implied reduction in estimated natural mortality, even though the age or method is not validated (Beamish and McFarlane, 1995).
The goal of the current study was a radiometric validation of walleye pollock ages using Pb-210 and Ra-226. The radiometric age-validation method uses the disequilibrium between a daughter/parent pair of radionuclides: Ra-226 and its progeny, Pb-210. An explanation of the theories and required assumptions of this method is not undertaken in this paper, but the reader can refer to published literature for this information (Fenton and Short, 1992; Kastelle et al., 1994; Burton et al., 1999; Andrews et al., 2002). The goal of our study had two components that differed by the degree of validation provided. The first was to learn whether method A or B was more accurate. The second was to validate the ages produced by the more accurate ageing method. Radiometric ageing with Pb-210 and Ra-226 is an exceptionally good tool to discriminate between two methods of ageing when the differences are as extreme as described above (Campana, 2001).
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Material and methods |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Methods A and B
Walleye pollock otolith pairs were collected by the Alaska Fisheries Science Center (AFSC) from 1995 to 2002 during annual echo-integration/trawl surveys from Shelikof Strait and the shelf break west of Chirikof Island. The survey takes place in March and April each year, and the data collected are used in walleye pollock stock assessments (Guttormsen, 2004). All otoliths utilized in the study were from Shelikof Strait, except those in P16 and P20 (Table 1), which were from the shelf break area south of Kodiak Island. Fish length was recorded for specimens whose otoliths were collected. Otoliths were stored in a 50% ethanol solution. The otoliths were originally read during routine ageing at the AFSC, using method A as described earlier, then archived for up to 7 years. The age readers had access to catch date and fish length. Otoliths were selected from the archives such that presumably all came from the 1994 year class and were aged 1–8 years using method A. Utilizing the 1994 year class was possible because it was relatively strong (Dorn et al., 2004).
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Specimens aged 3, 4, 6, 7, and 8 years using method A were included in the validation study and processed in the following manner. The otoliths were first re-examined using method A. Depending upon the year of original reading, the re-examination occurred up to 5 years later. In the re-examination process, the specimens were either assigned a new age or the original age, and the 1994 year class was confirmed. During this phase of the study, the age readers re-examining the otoliths were only experienced in using method A. As part of the process, in the case of any specimen where neither otolith was originally viewed with the burnt-otolith-section method, one of the otoliths was cut, and one of the halves was toasted (burnt-otolith-section viewing method). The cut was made transversely (dorsoventral) through the focus of the otolith with a Buhler low speed Isomet1 saw. Toasting of the otolith half was done with a small alcohol burner.
Following re-examination, the samples were made available for ageing using method B (cut otolith halves) and for radiometric analysis (remaining whole otoliths). The method B ageing, as described earlier, was performed by independent age readers experienced in using it. The two halves of the cut otolith were aged using method B regardless of the method A re-examination outcome when at least one intact otolith existed (i.e. if an otolith pair had both otoliths cut in the original reading, or if one of the pair was damaged, it was not made available for method B). The age reader using method B had the option of toasting a second otolith half if only one half had previously been toasted. During method B ageing, fish length data were not made available; only catch date was known by the age reader. At the same time as method B ageing, the remaining intact whole otoliths were made available for radiometric ageing in specimens where the method A age had been confirmed in the re-examination process. Both otoliths may have been cut in the original ageing, or one otolith may have been damaged, so not all samples had an intact otolith. In this way, the samples available for radiometric ageing were a subset of those available for ageing using method B.
Specimens aged 1, 2, and 5 years using method A were not included in the validation study. Owing to a research vessel breakdown in 1999, otoliths from fish 5 years old were not available. Specimens collected in 1995 and aged 1 year in routine ageing were used to estimate R*, the initial ratio of Pb-210/Ra-226 during the first year, a parameter necessary for radiometric ageing (Kastelle et al., 2000). Specimens collected in 1996 and aged 2 years were deemed too young for radiometric ageing.
Radiometric analysis
Estimating a single radiometric age requires a group or "pool" of otoliths, because only the material formed in the first year of life (otolith core) was used and 0.6–1.0 g of material was required for each radiometric age estimation. Each pool contained 40–45 otolith cores from specimens with the same method A age. Not all otoliths re-examined using method A were selected for the radiometric age pools. At some ages, every other otolith was selected for a radiometric pool. This experimental design provided replication of radiometric age pools at method A age (Table 1).
To obtain otolith cores for radiometric analysis, three steps were used. First, the average weight, length, width, and thickness of 1-year-old otoliths, and the weight of 2-year-old otoliths were used as a guide, and later to assess coring error. Second, to excise the core, a Buehler low speed Isomet saw was used initially to remove large pieces of material peripheral to the first year, followed by sanding with a small 120-grit drum on a Dremel tool to produce a concave distal surface on the core, and final grinding to the size and shape of a 1-year-old otolith was done with a Buehler Ecomet grinder with 320-grit wet and dry sand paper. The first translucent zone became easier to see as material was removed in this process. Therefore, measurements taken on the 1- and 2-year-old otoliths, as well as the core dimensions and weight, served as a secondary guide to the visual location of the first translucent zone. The grinding progress was monitored with a dissecting microscope at 25x, so each core was complete when all material peripheral to the first translucent zone was removed. Finally, the cores were cleaned in an ultrasonic cleaner, dried, and weighed.
Specimens aged 1 year with method A and used to estimate R* were not cored. The whole otoliths corresponded to the cores used in other samples, so coring was not necessary. They were re-examined using method A, but not aged using method B. The fish length of 1 year olds was compared with age-independent length data for age confirmation. Only specimens where both otoliths remained intact were used (i.e. not originally cut to view with the burnt-otolith-section method). Right and left otoliths were used in P1 and P2, respectively, to estimate R* with two replicates.
Descriptions of the radiometric ageing process can be found in the literature (Campana et al., 1990; Kastelle et al., 1994, 2000). Lead-210 was measured in each pool using its daughter-proxy, Po-210. Each pool of cores was chemically processed with a Po-209 yield tracer. The Po-209 was a standard reference source (SRM 4326) from the National Institute of Standards and Technology (NIST). The Po isotopes were extracted from a solution of hydrochloric acid containing dissolved otolith cores, by spontaneous deposition onto silver planchets. An alpha spectrometer was used to count the decay of Po isotopes on the planchets and to estimate the activity of Po-210 in each pool (Flynn, 1968). The Ra-226 was measured in each pool using its daughter-proxy Rn-222. The Rn-222 activity in each pool's acid solution was measured with a de-emanation system and Lucas cells (Lucas, 1957; Sarmiento et al., 1976). The efficiency of extracting Rn-222 from the solution, the Lucas cells, and electronic equipment associated with the Lucas cells was measured and calibrated with a Ra-226 standard reference source (SRM 4950E) from NIST. Laboratory processing was done over several years in four batches that each contained 2–5 pools and two reagent blanks (Table 1). Background activities of Po-209 and Po-210 in the alpha spectrometer and Ra-226 in the de-emanation system and Lucas cells were measured before and after each batch or calibration procedure.
To estimate the radiometric age of each pool, a series of calculations was made on the activity measurements of Pb-210 and Ra-226. The average 
over all pools was first estimated. The ratio 
was then calculated from the measured Pb-210 activities in each pool. Finally, the age for each pool was calculated by the equation
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is the pool's age in years, R is the calculated ratio of 
, 
is the decay constant for Pb-210 in units of inverse time ( = 0.03108 y–1), and R* is the estimated initial activity ratio of 
as incorporated into the bone (Kastelle et al., 2000). We used R* = 0 because the results for P1 and P2 estimated small negative numbers (see later). The variance of the age estimation was calculated from Equation (1) using the delta method (Seber, 1982):
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| (2) |
The calculated radiometric age for each pool was adjusted to account for the core size of 1 year and the time between capture and analysis. A core size of 1 year necessitates an adjustment of approximately 6 months, because the core is deposited over a span of a year. Therefore, the mean age of the material sampled is not represented by 0 or 1 year, but by a mean of 6 months. After capture, Ra-226 continues to decay and produce Pb-210, so to make direct comparisons between the radiometric ages and methods A and B ages, the time between capture and radiometric analysis was subtracted. This comparison was made in a plot of method B ages and radiometric ages against method A ages.
Errors associated with each procedure mentioned above were measured and propagated through all calculations. This provided an estimate of var(R). The potential low age range in walleye pollock mandated extra measures to reduce the estimated error in the final age estimates as follows. First, the chosen activity of the Po-209 yield tracer needed to be more than 15 times the activity of the Po-210 in each pool. Second, counting times in the alpha spectrometer need to be long, more than 30 days. Third, an average of Ra-226 specific activity across all pools was used. Finally, long-term measurements of background activities were averaged if a trend was not present. The first two items above reduce error, because actual decay events of the radio-isotopes are counted, and the standard error is proportional to the square root of the number of counts (Sarmiento et al., 1976; Knoll, 1989). Therefore, with higher counts, the coefficient of variation (CV) of an activity measurement is reduced. Of the components that make up var(R), approximately 65% is from variability in counting the decay events, and the remainder is due to laboratory procedures such as yield tracers and pipetting.
In batch 2, there was an extraction problem with the Po isotopes. In P10, the process of measuring the Po isotopes with the silver planchet and alpha spectrometer failed. Therefore, the hydrochloric acid solution containing the dissolved otoliths of P10 was re-examined for Po-210 activity with batch 4 (Table 1).
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Results |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Methods A and B
The ages generated by methods A and B were different. In method A, use of burnt-otolith sections increased from 5.1% at age 3 years to 67.2% at age 8 years, with an overall use of 33.7%. In the re-examination process for ages 3–8 years using method A, 839 specimens were aged. For method B, only 618 specimens were aged. The 221 specimens not aged using method B were deemed too difficult to age because both of the otolith halves were previously toasted. However, these specimens were included in the selection process for the radiometric ageing pools because method B and radiometric ageing were performed at the same time. Of the 618 aged using method B, 69 were considered very difficult but were used in the analyses to keep sample sizes as similar as possible between methods. Up to several years lapsed between the time the otoliths were originally processed (cut and toasted) and viewed by method B. By experimental design, the oldest age from method A was 8 years, but the maximum age estimated using method B was 28 years (Table 2, Figure 1). For method A ages 4 years and older, the estimated method B ages, averaged at each method A age, were approximately twice as old (Figure 1). The percentage agreement was 14.6% and the CV was 35.5% between the two methods, with the only agreement in specimens aged 3 and 4 years by method A.
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Radiometric analysis
The grinding method produced cores that approximated the weight of 1-year-old otoliths. The weight of the 1- and 2-year-old otoliths (aged using method A) was 14.7 ± 0.4 mg and 60.0 ± 01.6 mg (±s.e.), respectively. The core weight averaged by pool ranged from 12.9 ± 0.4 mg to 19.8 ± 3.3 mg (±s.e.) (Table 1). The coring process maintained the curve in the otolith, and the core's shape approximated a 1-year-old otolith.
The fish length of 1 year olds agreed with other age-independent length data. Their average fish length used for R* estimation was 11.8 ± 1.4 cm (±s.e.) in March and April at the beginning of the growing season. This can be compared with a maximum fish length of 10 cm in August and September, at the end of the growing season, for what is known to be age-0 fish (Brown and Bailey, 1992; Wilson et al., 1996). This was also compared with a length mode of 12 cm seen in a biomass-at-length plot from the annual echo-integration-trawl surveys, which is assumed to be 1-year-old fish (Dorn et al., 2004).
Radiometric ages were successfully estimated for 11 pools containing a total of 498 cores. The estimated value of R* from P1 and P2 was –0.058 ± 0.043 and –0.021 ± 0.051 (±95% CI), respectively. Specific activities of Pb-210 and Ra-226 are detailed by pool in Table 1. The minimum radiometric age was 1.45 ± 1.22 years and the maximum age was 8.29 ± 2.31 years (±95% CI; Table 2). A plot of radiometric ages and method B average ages against method A ages is shown in Figure 1. Method B average ages are above the line of agreement by at least a factor of two in all but the youngest age. The radiometric ages are near the line of agreement, but with a low bias averaging 0.80 years (Figure 1).
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Discussion |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Methods A and B
This study confirmed that methods A and B provide dramatically different ages for walleye pollock. This was expected in the light of previous reports in Munk (2001, 2004). Although the same otolith preparation was often viewed, the age-reading criteria in methods A and B obviously lead to different presumed growth zone counts. There is no doubt that method B interpreted finer translucent and opaque zones as annual marks, a difference in interpretation that appeared to increase with age (Figure 1). If samples older than 8 years from method A had been used, the maximum age in method B would probably exceed 28 years. Importantly, we think the difference between viewing the otolith's surface and its burnt-otolith section, as explained earlier, was not the source of the discrepancy. This was confirmed by two observations. First, the use of the burnt-otolith sections in method A increased with age, i.e. the viewing methods in methods A and B became more similar at older ages. Second, the difference between methods A and B ages increased at older ages (Figure 1). The percentage agreement of 14.6% between methods was low compared with what is normally seen for walleye pollock. Kimura and Lyons (1991) reported a percentage agreement between two experienced readers using method A at the AFSC of 72.4% over all ages, and 79.1% for the range of 3–8 years. It is possible that some of the burnt-otolith sections faded during the time delay between method A burning and later viewing by method B age readers (Chilton and Beamish, 1982). Conceivably, some of the low percentage agreement seen here may be due to this fading. This could not be evaluated, because in our study the results also reflect the stronger effect of the two ageing methods. We know of no evidence that suggests a possible doubling of age in this situation.
The results for method A relied on an unusually low percentage (33.7%) of burnt-otolith sections. Gulf of Alaska walleye pollock survey data from 1995 to 2002 show that about 60% of the otoliths in the age range 3–8 years (method A) were aged using the burnt-otolith-section method. This difference was not investigated here, but could stem from different choices made by individual age readers at the time of original processing, based on the clarity of otoliths from different collection years.
Different methods may be used to expose the transverse plane of an otolith for burning. The age readers using method B typically do not use the Isomet saw; instead, the otolith is "snapped" in half by hand through the first year in what may be called the break-and-burn method. In this study, all otoliths were cut with the Isomet saw. The growth zones seen in a cut transverse plane may not appear the same as when viewed in a snapped plane. This may have added to the difficulty for the age readers using method B, because for them this technique was unusual.
Radiometric analysis
This study was unique in two ways. First, it appears to be the first published study to use radiometric age validation with Pb-210/Ra-226 on a species for which all pools were estimated to have young radiometric ages. Here, all samples were between 3 and 8 years old, with a maximum radiometric age of 8.29 ± 1.15 years (±1 s.d.). Our results indicate that radiometric ageing with Pb-210 and Ra-226 can be used for young fish. A study with tropical snappers (Lutjanidae) had potential growth zone counts in the same age range as walleye pollock, but their radiometric results showed a greater age of 24.2 years (Milton et al., 1995). Second, the use of 1 year class, sampled over time, is unique among radiometric age validations with this pair of radionuclides. This suggests the logical use of average Ra-226 values instead of individual Ra-226 measurements for each pool. This unique design, with 1 year class and averaged Ra-226, gave results with lower error.
The radiometric results accomplished both our original goals. Method A ages were closer to the radiometric ages, indicating that it was the more accurate of the two methods, and they were validated by them. The 0.80-year bias discussed later between method A and the radiometric ages does not alter this conclusion. The widely different ages generated using methods A and B helped us come to a definite conclusion. The use of method A did not appear to underestimate age, even at the oldest age of 8 years considered here. This follows the advice of Beamish and McFarlane (1995) to use burnt-otolith sections to avoid any under-ageing. When it was apparent that the otolith was thick, and the surface was not adequate to see all of the growth zones, a burnt-otolith section was used. Using method A should allow the maximum number of otoliths to be processed and read for stock assessments.
The 3–8 year age range validated in this study contains the most common ages seen in Gulf of Alaska survey collections (Guttormsen, 2004). The difference between methods A and B at 2 years old would be less than at older ages, making a distinction with radiometric ageing difficult. Therefore, an age of 3 years, based on method A, was chosen as the youngest age to validate. The maximum age chosen, based on method A and then validated, was 8 years. The few fish older than 8 years in collections from Shelikof Strait made radiometric ageing of older fish non-feasible, because more than 40 otoliths would be needed for an age estimate.
The pattern interpretation used in method A for otoliths up to 8 years old can be used to age Gulf of Alaska walleye pollock that occasionally exceed 15 years. Other areas, such as the eastern Bering Sea, have produced older walleye pollock ages using method A that occasionally go beyond 22 years (Kimura et al., 1992). The maximum method A age from Bering Sea stocks is 31 years (Wespestad and Dawson, 1992). However, it is usually considered inappropriate to confer validation beyond the actual age validated, or from a different location, so this should be done with caution (Beamish and McFarlane, 1983; Campana, 2001). Nevertheless, ages more than 8 years are reasonable and expected using method A, and certainly a necessary piece of information for stock assessment. However, the ageing criteria of method B that produced Gulf of Alaska walleye pollock ages of 28 years, as in the samples used here (Figure 1), or consistently above 20 years and occasionally above 30 years in general (Munk, 2004), were rejected by the radiometric analysis. Unfortunately, the full age distribution or maximum age from either method could not be tested here.
Other data can be used to support the results of radiometric ageing. In Dorn et al. (2004), a progression of modes in catch-at-age data (method A) for the 1994 year class ages corresponds to age-independent modes in biomass at length. These modes in Dorn et al. (2004) concur with the length data from Brown and Bailey (1992) and Wilson et al. (1996), which validate the age of 1 year for the fish used to estimate R*. This may be a weaker age-validation method than radiometric ageing, but it is a strong validation in the younger ages (Campana, 2001). This is useful, because the ages of 1 and 2 years were not validated with radiometric ageing, and it also indicates that the use of 1-year-old fish to estimate R* was acceptable. The previously mentioned study by Kimura et al. (2006) that corroborates ages by different methods is also supportive of the results here.
A bias did exist between method A ages and radiometric ages. The radiometric ages were lower than method A ages by an average difference of 0.80 years. The magnitude of this bias does not affect the conclusions of this study. This bias could have been caused by a number of reasons. First, some fish could have been over-aged by a year with method A. This was probably not the case, as indicated by the progression of length modes discussed above. Second, the coring process may have been inaccurate, leaving material outside of the first year. This was avoided, because the first translucent zone became visible as material was removed in the coring process and served as a coring guide. To produce the bias seen in any one pool, the cores would have needed to be inaccurate, or off-centre, by a substantial amount in most samples. Measurements of the cores indicate that their size and shape was close to expected averages. Bias introduced by coring inaccuracies (core heterogeneity) was discussed by Francis (2003). From that study, we conclude that coring inaccuracies were not the cause of the 0.80-year bias seen here. At ages <8 years, with accurate coring, Francis (2003) predicted only a 0.5% low bias in the radiometric age. Third, if the cores were contaminated by younger particulate material produced in the grinding process, a young bias could have been introduced. To avoid this, ultrasonic cleaning was used to remove any small particles. Finally, a possible cause for the bias could have been from Rn-222 loss out of the core material (West and Gauldie, 1994). If Rn-222 escaped from the otolith's core, the radiometric age would be a low estimate. This issue has been the focus of several studies by Whitehead and Ditchburn (1995), Baker et al. (2001), and Kastelle and Forsberg (2002). Baker et al. (2001) suggested that loss does occur, but is insignificant given the age range and errors associated with the radiometric ageing. Further, old ages typically validated with radiometric ageing suggest that Rn-222 loss is minimal (Smith et al., 1995; Stewart et al., 1995; Andrews et al., 2002). The otoliths were analysed at least 8 years after core deposition in 1994, the analysis for batch 1 starting in June 2002, so it is conceivable that the small bias seen here is a result of a minor Rn-222 loss during this timespan.
The measurement of R* resulted in small negative numbers. Using an R* = 0.0 for age calculations was reasonable, because this parameter must, in theory, be non-negative and some small measurement error is expected. In other studies, when a negative value was measured, zero was used (Kastelle et al., 1994, 2000). If P1 and P2 had been aged here, with an assumption of R* = 0.0, they would have fallen below the line of agreement (Figure 1), similar to the results of Andrews et al. (1999, 2001) for very young fish.
The current study was successful in reducing the error in radiometric ages compared with previous studies in several ways. The most innovative was using a single year class so that the Ra-226 could be averaged across pools. A second was the use of the techniques itemized above, in the Methods section. Another stemmed from the fact that the young ages of walleye pollock are on the steeper part of the Pb-210/Ra-226 activity ratio curve, not on the flat upper part (Andrews et al., 2002). This means that for ratios of Pb-210/Ra-226 with a similar CV, the error in the estimated radiometric age for younger specimens becomes smaller. Finally, the specific activity of Ra-226 in walleye pollock otoliths was relatively high, compared, for example, with Sebastes spp. (Kastelle et al., 2000). This provided a high count rate in the Lucas cells, and therefore a lower error, as explained in the Methods section.
The use of an averaged Ra-226 value for radiometric age estimation can be debated. Averaging the Ra-226 was based upon the assumption that fish caught in Shelikof Strait have first-year cores similar in their Ra-226 content. There has not been a consistent protocol in previous studies, but the tendency has been to use individual Ra-226 measurements for each pool's age. For example, in six rockfish species (Sebastes spp. and Sebastolobus spp.) and Pacific grenadier (Coryphaenoides acrolepis), individual measurements are used for each radiometric age estimate (Andrews et al., 1999, 2002; Kastelle et al., 2000). In redfish (Sebastes mentella) and splitnose rockfish (Sebastes diploproa), an average value was used (Bennett et al., 1982; Campana et al., 1990). The averaging of Ra-226 may be acceptable, or even desirable, if individual measurements can be considered replicates. Using the presumed 1994 year class exclusively suggested that all measurements are replicates, and hence should be averaged.
However, if there is any migration into Shelikof Strait or shelf break areas from different spawning or nursery areas, the true value of Ra-226 for any one age may be unique and might not be considered a replicate measurement. Unfortunately, the migration of walleye pollock is not well understood (Bailey et al., 1999; Dorn et al., 2004; FitzGerald et al., 2004). It is possible that stocks upstream in the Alaska Coastal Current, from Middleton Island or Prince William Sound, could provide migrants into Shelikof Strait and shelf break areas (Bailey et al., 1999; Wilson, 2000) with a different Ra-226 input history. The Ra-226 results indicate that only the youngest two pools, P1 and P2 at age 1, appear different (Table 1). If these two were removed from the Ra-226 average, the radiometric ages would all shift lower by a fraction of 1 year. Importantly, this change would not alter any of the conclusions reached by the general agreement between the method A ages and the radiometric ages.
The definition often given for age validation is a confirmation that the methods used produce accurate ages (Beamish and McFarlane, 1995; Kalish et al., 1995; Campana, 2001). The results of this study indicate that ages from method A were more accurate than those from method B. They also show that the radiometric ages are indeed a validation that method A provided accurate ages.
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Acknowledgements |
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We are indebted to Delsa Anderl and Betty Goetz of the Alaska Fisheries Science Center's (AFSC) Age and Growth Program, for support at all stages of this study. We thank Kristen Munk of the Alaska Department of Fish and Game for providing walleye pollock ages and for comments on this paper, and Martin Dorn and Matt Wilson of the AFSC for comments on the paper. Finally, we thank Steve Campana of the Bedford Institute of Oceanography, and Gregor Cailliet and Allen Andrews of Moss Landing Marine Laboratories, for their perceptive reviews of the draft manuscript.
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Footnotes |
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1 Reference to trade names does not imply endorsement by the National Marine Fisheries Service, NOAA.
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Andrews A.H., Burton E.J., Coale K.H., Cailliet G.M., Crabtree R.E. (2001) Radiometric age validation of Atlantic tarpon, Megalops atlanticus. Fishery Bulletin US 99:389–398.
Andrews A.H., Cailliet G.M., Coale K.H. (1999) Age and growth of the Pacific grenadier (Coryphaenoides acrolepis) with age estimate validation using an improved radiometric ageing technique. Canadian Journal of Fisheries and Aquatic Sciences 56:1339–1350.
Andrews A.H., Cailliet G.M., Coale K.H., Munk K.M., Mahoney M.M., O'Connell V.M. (2002) Radiometric age validation of the yelloweye rockfish (Sebastes ruberrimus) from southeastern Alaska. Marine and Freshwater Research 53:139–146.[CrossRef][Web of Science]
Bailey K.M., Quinn T.J., Bentzen P., Grant W.S. (1999) Population structure and dynamics of walleye pollock, Theragra chalcogramma. Advances in Marine Biology 37:179–255.[Web of Science]
Baker M.S., Wilson C.A., VanGent D.L. (2001) Testing assumptions of otolith radiometric aging with two long-lived fishes from the northern Gulf of Mexico. Canadian Journal of Fisheries and Aquatic Sciences 58:1244–1252.
Beamish R.J. and McFarlane G.A. (1983) The forgotten requirement for age validation in fisheries biology. Transactions of the American Fisheries Society 112:735–743.[CrossRef]
Beamish R.J. and McFarlane G.A. (1995) A discussion of the importance of aging errors, and an application to walleye pollock: the world's largest fishery. In Secor D.H., Dean J.M., Campana S.E. (Eds.). Recent Developments in Fish Otolith Research(University of South Carolina Press, Columbia, South Carolina) pp. 545–565 735 pp.
Bennett J.T., Boehlert G.W., Turekian K.K. (1982) Confirmation of longevity in Sebastes diploproa (Pisces: Scorpaenidae) from 210Pb/226Ra measurements in otoliths. Marine Biology 71:209–215.[CrossRef]
Brown A.L. and Bailey K.M. (1992) Otolith analysis of juvenile walleye pollock Theragra chalcogramma from the western Gulf of Alaska. Marine Biology 112:23–30.[CrossRef]
Burton E.J., Andrews A.H., Coale K.H., Cailliet G.M. (1999) Application of radiometric age determination to three long-lived fishes using 210Pb:226Ra disequilibria in calcified structures: a review. American Fisheries Society Symposium 23:77–87.
Campana S.E. (2001) Accuracy, precision and quality control in age determination, including a review of the use and abuse of age validation methods. Journal of Fish Biology 59:197–242.[CrossRef][Web of Science]
Campana S.E., Zwanenburg K.C.T., Smith J.N. (1990) 210Pb/226Ra determination of longevity in redfish. Canadian Journal of Fisheries and Aquatic Sciences 47:163–165.
Chilton D.E. and Beamish R.J. (1982) Age determination methods for fishes studied by the groundfish programme at the Pacific Biological Station. Canadian Special Publication of Fisheries and Aquatic Sciences 60: 102 pp.
Dorn M., Barbeaux S., Gaichas S., Guttormsen M., Megrey B., Spalinger K., Wilkins M. (2004) Assessment of walleye pollock in the Gulf of Alaska. Appendix B, Stock Assessment and Fishery Evaluation Report for the Groundfish Resources of the Gulf of Alaska pp. 35–130 Compiled by The Plan Team for the Groundfish Fisheries of the Gulf of Alaska. North Pacific Fishery Management Council, 605 W 4th Avenue, Suite 306, Anchorage, AK 99501, USA.
Fenton G.E. and Short S.A. (1992) Fish age validation by radiometric analysis of otoliths. Australian Journal of Marine and Freshwater Research 43:913–922.[CrossRef]
FitzGerald J.L., Thorrold S.R., Bailey K.M., Brown A.L., Severin K.P. (2004) Elemental signatures in otoliths of larval walleye pollock (Theragra chalcogramma) from the northeast Pacific Ocean. Fishery Bulletin US 102:604–616.
Flynn W.W. (1968) The determination of low levels of polonium-210 in environmental materials. Analytica Chimica Acta 43:221–227.[CrossRef][Web of Science][Medline]
Francis R.I.C.C. (2003) The precision of otolith radiometric ageing of fish and the effect of within-sample heterogeneity. Canadian Journal of Fisheries and Aquatic Sciences 60:441–447.
Guttormsen M. A. (2004) Results of the March–April 2004 echo integration-trawl surveys of walleye pollock (Theragra chalcogramma) conducted in the Gulf of Alaska, cruise MF0403. Appendix B, Stock Assessment and Fishery Evaluation Report for the Groundfish Resources of the Gulf of Alaska pp. 511–536 Compiled by The Plan Team for the Groundfish Fisheries of the Gulf of Alaska. North Pacific Fishery Management Council, 605 W 4th Ave., Suite 306, Anchorage, AK 99501, USA.
Appendix A, Stock Assessment and Fishery Evaluation Report for the Groundfish Resources of the Eastern Bering Sea/Aleutian Islands Regions Ianelli J. N., Bareaux S., Walters G., Honkalehto T., Williamson N. (2004) Eastern Bering Sea walleye pollock stock assessment. 37–126 Compiled by The Plan Team for the Groundfish Fisheries of the Bering Sea and Aleutian Islands. North Pacific Fishery Management Council, 605 W 4th Ave., Suite 306, Anchorage, AK 99501, USA.
Kalish J.M., Beamish R.J., Brothers E.B., Casselman J.M., Francis R.I.C.C., Mosegaard H., Panfili J., Prince E.D., Thresher R.E., Wilson C.A., Wright P.J. (1995) Glossary for otolith studies. In Secor D.H., Dean J.M., Campana S.E. (Eds.). Recent Developments in Fish Otolith Research(University of South Carolina Press, Columbia, South Carolina) pp. 723–729 735 pp.
Kastelle C.R. and Forsberg J.E. (2002) Testing for loss of 222Rn from Pacific halibut (Hippoglossus stenolepis) otoliths. Fisheries Research 57:93–98.[CrossRef][Web of Science]
Kastelle C.R., Kimura D.K., Jay S.R. (2000) Using 210Pb/226Ra disequilibrium to validate conventional ages in scorpaenids (genera Sebastes and Sebastolobus). Fisheries Research 46:299–312.[CrossRef][Web of Science]
Kastelle C.R., Kimura D.K., Nevissi A.E., Gunderson D.R. (1994) Using 210Pb/226Ra disequilibria for sablefish, Anoplopoma fimbria, age validation. Fishery Bulletin US 92:292–301.
Kimura D.K., Kastelle C.R., Goetz B.J., Gburski C.M., Buslov A.V. (2006) Corroborating the ages of walleye pollock (Theragra chalcogramma). Marine and Freshwater Research 57:323–332.[CrossRef][Web of Science]
Kimura D.K. and Lyons J.J. (1991) Between-reader bias and variability in the age-determination process. Fishery Bulletin US 89:53–60.
Kimura D.K., Lyons J.J., MacLellan S.E., Goetz B.J. (1992) Effects of year-class strength on age determination. Australian Journal of Marine and Freshwater Research 43:1221–1228.[CrossRef]
Knoll G.F. (1989) Radiation Detection and Measurement(John Wiley & Sons, New York) 754 pp.
Lai H.L. and Yeh S.Y. (1986) Age determinations of walleye pollock (Theragra chalcogramma) from four age structures. International North Pacific Fisheries Commission Bulletin 45:66–89.
LaLanne J. J. (1979) The validity and consistency of age determinations from otoliths of Pacific pollock (Theragra chalcogramma). International North Pacific Fisheries Commission, Annual Report 1977 pp. 99–107.
Lucas H.F. (1957) Improved low-level alpha scintillation counter for radon. Review of Scientific Instruments 28:680–683.[CrossRef][Web of Science]
McFarlane G.A. and Beamish R.J. (1990) An examination of age determination structures of walleye pollock (Theragra chalcogramma) from five stocks in the northeast Pacific Ocean. In Low L.L. (Ed.). Proceedings of the Symposium on Application of Stock Assessment Techniques to Gadids, Bulletin 50(International North Pacific Fisheries Commission, Vancouver, Canada) pp. 37–56 311 pp.
Milton D.A., Short S.A., O'Neill M.F., Blaber S.J.M. (1995) Ageing of three species of tropical snapper (Lutjanidae) from the Gulf of Carpentaria, Australia, using radiometry and otolith ring counts. Fishery Bulletin US 93:103–115.
Munk K. M. (2001) Walleye pollock otolith aging: comparison of the techniques used by two agencies. Regional Information Report No. 5J01–06. Alaska Department of Fish and Game, Juneau, AK. 18 pp.
Munk K. M. (2004) Otolith diagnostic characters used to reveal older age range in walleye pollock. Poster presented at the Third International Symposium on Fish Otolith Research and Application11–16 July 2004Townsville, Australia (available from K. M. Munk, Alaska Dept. of Fish and Game, Box 25526, Juneau, Alaska 99802).
Sarmiento J.L., Hammond D.E., Broecker W.S. (1976) The calculation of the statistical counting error for 222Rn scintillation counting. Earth and Planetary Science Letters 32:351–356.[CrossRef][Web of Science]
Seber G.A.F. (1982) The Estimation of Animal Abundance and Related Parameters(Macmillan, New York, NY) 654 pp.
Smith D.C., Fenton G.E., Robertson S.G., Short S.A. (1995) Age determination and growth of orange roughy (Hoplostethus atlanticus): a comparison of annulus counts with radiometric ageing. Canadian Journal of Fisheries and Aquatic Sciences 52:391–401.
Stewart B.D., Fenton G.F., Smith D.C., Short S.A. (1995) Validation of otolith-increment age estimates for a deepwater fish species, the warty oreo Allocyttus verrucosus, by radiometric analysis. Marine Biology 123:29–38.[CrossRef]
Wespestad V. G. and Dawson P. (1992) Walleye pollock. Stock Assessment and Fishery Evaluation Report for the Groundfish Resources of the Bering Sea/Aleutian Islands Regions as Projected for 1993 pp. 1.1–1.32 Compiled by The Plan Team for the Groundfish Fisheries of the Bering Sea and Aleutian Islands. The North Pacific Fishery Management Council, 605 West 4th Ave., Suite 306, Anchorage, AK 99501, USA.
West I.F. and Gauldie R.W. (1994) Determination of fish age using 210Pb:226Ra disequilibrium methods. Canadian Journal of Fisheries and Aquatic Sciences 51:2333–2340.
Whitehead N.E. and Ditchburn R.G. (1995) Two new methods of determining radon diffusion in fish otoliths. Journal of Radioanalytical and Nuclear Chemistry 198:399–408.[CrossRef][Web of Science]
Wilson M.T. (2000) Effects of year and region on the abundance and size of age-0 walleye pollock, Theragra chalcogramma, in the western Gulf of Alaska, 1985–1998. Fishery Bulletin US 98:823–834.
Wilson M.T., Brodeur R.D., Hinckley S. (1996) Distribution and abundance of age-0 walleye pollock, Theragra chalcogramma, in the western Gulf of Alaska during September 1990. In Brodeur R.D., Livingston P.A., Loughlin T.R., Hollowed A.B. (Eds.). Ecology of Juvenile Walleye Pollock, Theragra chalcogrammaU.S. Department of Commerce pp. 11–24 NOAA Technical Report NMFS 126. 227 pp.
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