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North American Journal of Fisheries Management
ISSN: 0275-5947 (Print) 1548-8675 (Online) Journal homepage: http://www.tandfonline.com/loi/ujfm20
Precision of Three Structures for Saugeye Age
Estimation
Jeff Koch, Ben Neely & Bryan Sowards
To cite this article: Jeff Koch, Ben Neely & Bryan Sowards (2017): Precision of Three
Structures for Saugeye Age Estimation, North American Journal of Fisheries Management, DOI:
10.1080/02755947.2017.1394938
To link to this article: http://dx.doi.org/10.1080/02755947.2017.1394938
Accepted author version posted online: 25
Oct 2017.
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Date: 26 October 2017, At: 09:50
Precision of three structures for saugeye age estimation
Jeff Koch* and Ben Neely
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Kansas Department of Wildlife, Parks, and Tourism, 1830 Merchant, Emporia, KS 66801, USA
Bryan Sowards
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*Corresponding author: jeff.koch@ks.gov
Abstract—We evaluated precision of age estimates obtained from sagittal otoliths, scales, and dorsal fin
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spines from 341 saugeye (Walleye Sander vitreum × Sauger S. canadense) collected from six Kansas
reservoirs. Between-reader agreement was greatest for otoliths, followed by dorsal spines, and then
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scales. Coefficient of variation values for between-reader comparisons for otoliths were 1.3 to 3.1;
whereas, those for dorsal spines were 13.8 to 18.2. Between-reader comparisons for scales were most
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variable, with CV estimates between 19.9 and 23.1. Age estimates from otoliths aligned with age
estimates from scales more often than those from dorsal spines; although, age estimate comparisons
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between structures were generally variable. Between-reader agreement for scales and fin rays
decreased with increasing estimated age; however, agreement for otoliths generally remained high (i.e.,
> 80%) through age 10. Given greater precision relative to other structures, we recommend the use of
sectioned otoliths to estimate age of saugeye, especially when sacrificing fish is not a concern.
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Kansas Department of Wildlife, Parks, and Tourism, #3 State Park Road, Sylvan Grove, KS 67481, USA
Introduction
An important consideration of age and growth investigations is precision of age assignments,
which addresses repeatability of individual estimates, regardless of whether they are accurate
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(Campana 2001). Although precision cannot be used as a surrogate for accuracy in the absence of true
validation, precision is useful in assessing the relative ease of assigning age to a structure (Campana
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comparing relative ages of structures obtained from the same fish. Biased age estimates can lead to
erroneous estimates of population dynamics, harvest models, and population viability (Koch et al. 2009;
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Hamel et al. 2016). As such, evaluating precision and the potential for bias in age estimates are
important to fisheries managers.
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Saugeye (Walleye Sander vitreum × Sauger S. canadense) are stocked by natural resource
management agencies to provide angling opportunities and biological control of abundant fishes (e.g.,
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centrarchids and Gizzard Shad Dorosoma cepedianum) in water bodies where Walleye and Sauger do
not perform well (Mosher 2001; Boxrucker 2002; Galinat et al. 2002; Quist et al. 2010). Walleye x
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Sauger hybrids also naturally occur in systems with sympatric populations of parental species (e.g.,
Graeb et al. 2010). Although saugeye are commonly stocked by fisheries managers and are popular
among anglers, there is a relative paucity of age and growth information for this hybrid, especially when
compared to other recreationally important percids. A reason for this lack of information may be
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2001). Additionally, researchers can identify relative biases along a gradient of estimated ages when
difficulties associated with obtaining precise age estimates from saugeye hard structures (i.e., scales,
dorsal spines, otoliths) or a lack of literature examining aging precision associated with these structures.
Since saugeye are commonly used in attempt to balance predator-prey relationships, quality age and
growth information is needed to properly manage saugeye populations and examine their influences on
recreational fisheries.
Several studies have investigated precision of structures used to estimate age of Walleye
(Erickson 1983; Kocovsky and Carline 2000; Isermann et al. 2003), and have shown that otoliths
generally provide more precise age estimates than scales or dorsal spines. Walleye otoliths have also
been validated as an accurate hard structure for age determination to age 4 (Erickson 1983; Heidinger
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and Clodfelter 1987; Spurgeon et al. 2015). Unlike Walleye, Sauger aging structures have not been
validated and relatively few studies have compared precision of their age estimates. Dattilo et al. (2008)
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the Missouri River. Additionally, Williamson and Dirnberger (2010) found age estimates obtained from
whole and sectioned Sauger dorsal spines were similar. Saugeye and naturally occurring Walleye x
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Sauger hybrids have generally been aged with whole or cracked sagittal otoliths (Denlinger et al. 2006;
Graeb et al. 2010); although, Doyle (1999) attempted to age Oklahoma saugeye using scales but noted
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difficulties in obtaining reliable age estimates with this method. To our knowledge, no studies have
evaluated precision and comparative ages of hard structures used to age saugeye. The objective of this
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study was to estimate and compare precision of age estimates of saugeye obtained from sectioned
sagittal otoliths, scales, and sectioned dorsal spines.
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Methods
Saugeye were sampled in October 2016 using night electrofishing and gill nets at six Kansas
reservoirs that varied in surface area from approximately 40 to 1,400 hectares. Study reservoirs and
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indicated that otoliths provided more precise age estimates than scales or dorsal spines for Sauger in
number of saugeye aged from each water included Madison City Lake (n = 9), Chase State Fishing Lake (n
= 89), Geary State Fishing Lake (n = 94), Harvey County East Lake (n = 75), Marion County Lake (n = 23),
and Kanopolis Reservoir (n = 51). Fish were measured for total length (TL; mm), euthanized, and
transported to a laboratory for processing. Sagittal otoliths were extracted and stored in a plastic vial
placed in a numbered scale envelope until further processing. Otoliths were later cleaned of residual
tissue and mounted in epoxy following methods of Koch and Quist (2007). Encapsulated otoliths were
sectioned transversely through the nucleus with a Buehler Isomet low-speed saw operated at 300 rpm
(Buehler, Lake Bluff, IL). The third dorsal fin spine was removed as close to the body as possible with
side cutters and placed in a scale envelope to dry. Dorsal spines were mounted in epoxy similar to
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otoliths and sectioned as closely to the base of the structure as possible. Resulting otolith and dorsal fin
spine sections were approximately 0.6 mm thick and viewed with the aid of immersion oil to improve
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a numbered scale envelope to dry. Dried scales were cleaned of debris if necessary and aged without
further processing. All structures were viewed with transmitted light on a stereoscope (10 – 40×
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magnification) linked to a computer monitor. Structures were examined in their processed state so
readers could manipulate the structure, lighting, or optics.
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Three readers independently aged each structure without knowledge of the fish’s length.
Readers 1 and 2 had several years of experience aging hard structures, including all three types used in
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this study. Reader 3 had minimal experience with all three structures but had previously examined
scales and otoliths. When all three readers’ independent age estimates did not agree, a consensus age
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was agreed upon by all readers. Between-reader agreement of the same structure and agreement of
each reader’s ages of different structures from the same fish were quantified as percent agreement (PA;
when two age estimates were the same) and agreement within one year (PA1; when the absolute value
of the difference between two age estimates was zero or one). Bias in age estimates was evaluated with
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structure clarity. Scales were collected from the area directly posterior to the pectoral fin and placed in
age bias plots using the ageBias function in R (Ogle 2015; R Core team 2016). This function uses onesample t-tests, which are corrected for multiple comparisons, to determine if mean age estimates for
one age (i.e., x-axis) are significantly different from the average corresponding age estimates (i.e., yaxis). These t-tests essentially test for significant departures of mean age estimate values from the 1:1
line of equivalency. Stolarski and Sutton (2013) suggested that bias is evident when deviations from the
line of equivalence occur for more than two successive years. Precision of age estimates (i.e., among
readers and structures) was also evaluated using the coefficient of variation (CV; Chang 1982; Campana
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et al. 1995).
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number of times each fish is aged.
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Results
Sectioned otoliths, scales, and sectioned dorsal spines were collected from 341 saugeye varying
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in TL from 130 mm to 670 mm (Figure 1). Between-reader agreement was generally greatest for
otoliths, followed by dorsal spines and scales (Figure 2). Agreement between readers varied from 96.2%
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to 98.0% for otoliths and CV values for reader combinations were between 1.3 and 3.1. No bias
between readers was detected for otoliths for any age estimate combination. Agreement between
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readers for scale age estimates varied from 64.1% to 68.8% and CV values ranged from 19.9 to 23.1.
Significant deviations between readers occurred at young ages (i.e., ages 0 and 1); however, patterns in
bias thereafter were not evident in older age classes. Between-reader agreement varied from 70.5% to
77.7% for sectioned dorsal spines and CV values were between 13.8 and 18.2. Significant deviations
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where Xij is the ith age determination of the jth fish, Xj is the mean age of the jth fish, and R is the
from equivalence were evident between readers from ages 0 to 4. The three readers agreed on otolith
ages 95.6% of the time with a mean CV of 3.2. No consistent bias was indicated by greater than two
consecutive years between readers for any structure. Most deviations from equivalency between
readers occurred at estimated ages 4 and less.
Dorsal spine age estimate agreement with otolith age varied by reader from 55.7% to 63.6% and
coefficient of variation values for the comparison between the two structures varied from 17.0 to 21.7
(Figure 3). When the age estimates for the two structures differed for presumed younger fish, dorsal
spine age estimates were typically greater than otolith ages; however, for older saugeye, significant
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deviation from the line of equivalency were evident with otolith ages being greater than those from
dorsal spines. The relationship between otolith and scale age estimates was the least variable structure
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relation to other comparisons, varying from 63.5% to 80.7% agreement. When dorsal spine ages were
compared to scale ages, agreement varied by reader from 49.7% to 70.0%. Eight of nine age bias plots
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illustrated bias between structures that occurred for at least two consecutive age estimates; however,
there were never three consecutive years of significant bias in any plots. Thus, persistent bias was not
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observed. In most comparisons for fish estimated to be 0 or 1 yr by otoliths, scales and dorsal spines
had significantly greater ages compared to otoliths. Furthermore, for fish estimated as age 0 and age 1
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by dorsal spines, scales generally had greater estimates as well.
In general, agreement of age estimates was greater for saugeye estimated to be less than three
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years old (Figure 4), and agreement generally decreased for purportedly older saugeye. Percent
agreement among all three readers for structures with consensus ages greater than three years was
79.1% for otoliths, 16.6% for scales, and 45.8% for dorsal fin spines. For saugeye with a consensus
otolith age greater than three years, between-reader agreement varied from 83.3% to 87.5%. For
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comparison in regard to CV values, and the two structures also generally had the highest agreement in
saugeye with consensus scale ages greater than three years, between-reader agreement rate was 20.8%
to 29.2%; however, this value varied from 54.1% to 66.7% for similar comparisons of sectioned dorsal
spines.
Discussion
Biologists must make several decisions when choosing an aging structure, including whether
sacrificing fish is acceptable, removal and processing times of structures, and precision and accuracy of
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resulting age estimates. Our results indicated that age estimates obtained from sectioned otoliths
exhibited no bias between readers and were more precise than those obtained from sectioned dorsal
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spines, especially at young ages, indicating inconsistencies in identifying annuli from early ages.
Although between-reader agreement was greater for scales and dorsal spines at young ages, bias was
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more consistent at young ages, leading to significant deviations from equivalency. Our between-reader
agreement rates were slightly greater than those reported by Isermann et al. (2003) who evaluated
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Walleye scales, sectioned dorsal spines, and sectioned otoliths; however, mean CV values were similar.
Our results were also similar in regard to between-reader agreement reported for scales, sectioned
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dorsal spines, and cracked otoliths from Sauger collected from the Missouri River (Dattilo et al. 2008).
Although CVs from between-reader comparisons were four to ten times greater for scales and dorsal
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spines, collection of these structures is not lethal and requires considerably less removal and processing
time compared to otoliths (Isermann et al. 2003). Although Isermann et al. (2003) indicated that wholeview otolith age estimates were the most time-efficient and precise approach for estimating age of
walleye, cursory examinations of whole saugeye otoliths used in our study did not reveal readily
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spines and scales. Conversely, significant bias was indicated between readers for scales and dorsal
observable annuli. As such, we used sectioned otoliths, whose age estimates were highly precise and
relatively easy to read compared to dorsal spines and scales.
Although our study used sectioned otoliths, our results were similar to those of Isermann et al.
(2003), who reported that scale ages more closely reflected whole-view otolith ages compared to
sectioned dorsal spines. Isermann et al. (2003) asserted that inconsistencies and difficulties associated
with precisely aging sectioned Walleye dorsal spines were associated with identifying the first annulus.
We also noted difficulties identifying the inner annulus on dorsal spine sections, and likely led to
inconsistencies in age estimates between readers. This bias between readers was evident for dorsal
spines at young ages as significant deviations from the line of equivalence was noted in every reader
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comparison for at least one year for saugeye estimated to be age three or younger. A consideration for
future examinations of saugeye dorsal spines is the findings of Williamson and Dirnberger (2010), who
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illuminated Sauger dorsal spines, the side-illumination technique was more time efficient and displayed
outer annuli on older fish better than sectioning. This method may be useful for future examinations of
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saugeye dorsal spines as dorsal spine ages were significantly less than those obtained from otoliths in
purportedly older fish. Futhermore, Logsdon (2007) suggested that side-illuminated Walleye dorsal
younger Walleyes.
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spines provided a reasonably precise and non-lethal approach for replicating otolith ages for age-7 or
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Although sectioned otoliths yielded precise age estimates in our study, we were unable to
assess the accuracy of estimates. Furthermore, no published study has validated the use of any hard
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structure for use in estimating age of saugeye. As such, validation studies of saugeye age estimates are
warranted. Although we cannot assess accuracy of age estimates of saugeye in our study, age estimates
obtained from walleye otoliths have been validated as accurate to age 4 (Erickson 1983; Heidinger and
Clodfelter 1987; Spurgeon et al. 2015). As such, until age estimates of saugeye are validated, biologists
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indicated that although reader agreement and precision were similar for sectioned and whole, side-
must consider options regarding the choice of saugeye hard structure. If fish sacrifice is not acceptable,
scales more closely represented otolith ages (i.e., as evidence by higher percent agreement and lower
variability between structures) compared to sectioned dorsal spines; however, sectioned dorsal spines
yielded more precise age estimates. Regardless, we recommend sectioned otoliths for age estimation
of saugeye if fish sacrifice is not a concern.
Acknowledgements
We thank Paul Stockebrand for preparing aging structures. We also thank John Reinke, Craig Johnson,
Sean Lynott, Tyler Thomsen, Chris Steffen, and Paul Stockebrand for assistance with sampling. This
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paper was improved by comments from Martin Hamel, Michael Quist, and four anonymous reviewers.
Funding was provided by Kansas Department of Wildlife, Parks, and Tourism as well as Sport Fish
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estimate Sauger age. North American Journal of Fisheries Management 30:1016-1019.
Figure 1. Length distribution of saugeye aged using sectioned otoliths, scales, and sectioned dorsal
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spines. Saugeye were sampled from six Kansas reservoirs in October 2016.
Figure 2. Age-bias plot with estimates of mean coefficient of variation (CV), between-reader percent
agreement (PA), and percent agreement within one year (PA1) for three readers of sectioned sagittal
otoliths, scales, and sectioned dorsal spines from saugeye collected from Kansas reservoirs, 2016. Error
bars represent 95% confidence intervals around the mean age assigned by one reader relative to fish
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assigned an age by a second reader. Open circles represent significant deviation from the line of
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equivalency.
Figure 3. Age-bias plots with estimates of mean coefficient of variation (CV), between-reader percent
agreement (PA), and percent agreement within one year (PA1) for three readers of sectioned sagittal
otoliths, scales, and sectioned dorsal spines from saugeye collected from Kansas reservoirs, 2016. Error
bars represent 95% confidence intervals around the mean age assigned by one reader relative to fish
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assigned an age by a second reader. Open circles represent significant deviation from the line of
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equivalency.
Figure 4. Between-reader agreement of three readers (i.e., R1, R2, R3) by consensus age of sectioned
sagittal otoliths, scales, and sectioned dorsal spines from saugeye collected from Kansas reservoirs,
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2016.
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