Precision and Bias of Using Opercles as Compared to
Otoliths, Dorsal Spines, and Scales to Estimate Ages of
Largemouth and Smallmouth Bass
V. Alex Sotola, George A. Maynard, Erin M. Hayes-Pontius, Timothy B. Mihuc, Mark H. Malchoff, and J. Ellen Marsden
Northeastern Naturalist, Volume 21, Issue 4 (2014): 565–573
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2014 NORTHEASTERN NATURALIST 21(4):565–573
Precision and Bias of Using Opercles as Compared to
Otoliths, Dorsal Spines, and Scales to Estimate Ages of
Largemouth and Smallmouth Bass
V. Alex Sotola1,*, George A. Maynard1, Erin M. Hayes-Pontius1,
Timothy B. Mihuc1, Mark H. Malchoff
2, and J. Ellen Marsden3
Abstract - Several structures can be used to estimate ages of Micropterus salmoides (Largemouth
Bass) and Micropterus dolomieu (Smallmouth Bass). Otoliths are often employed
for these black bass age determinations, but processing otoliths can be time consuming
and requires an investment in training and equipment. Scales and dorsal spines can also
be analyzed to measure age, but precision and accuracy problems have been documented.
Use of opercles to estimate age in Largmouth and Smallmouth Bass has not been previously
examined. Utilization of both otoliths and opercles requires sacrificing the fish, but
opercles are easier to remove and process than otoliths. In our study, four readers estimated
the ages of the fish using each of the four structures. Opercles had the lowest coefficient of
variation (CV) for both species (Largemouth Bass = 6.31, Smallmouth Bass = 5.23), but
underestimated the ages of Largemouth Bass older than nine and Smallmouth Bass older
than six, relative to otoliths. Opercles proved easier to prepare and read, and the results
showed lower age-bias, higher precision, higher among-reader agreement, and less reader
bias than scales and dorsal spines.
Introduction
Estimates of fish ages provide important demographic information that is a key
component of fisheries research and management (Maceina and Sammons 2006).
Determinations of fish longevity, growth rate, and age at maturity hinge on correct
age information; inaccurate age estimations may hinder sound management of
the fishery resource (Everhart and Youngs 1981). Scales are the most commonly
used structure for estimating ages of many species, including Micropterus salmoides
Lacépède (Largemouth Bass; LMB) and Micropterus dolomieu Lacépède
(Smallmouth Bass; SMB) because they are easy to remove and sample collection is
non-lethal (Besler 1999, Maceina et al. 2007). Other bony structures (e.g., otoliths,
dorsal spines, and opercles) can be used to estimate ages of bass and other fishes,
but the use of opercles is rare in North American studies (Besler 1999, Long and
Fisher 2001, Maraldo and MacCrimmon 1979). Opercles have been successfully
used to estimate ages of Perca fluviatilis L. (European Perch), Morone saxatilis
1Lake Champlain Research Institute, State University of New York at Plattsburgh, 101
Broad Street, Plattsburgh, NY 12901. 2Lake Champlain Research Institute and Lake Champlain
Sea Grant, State University of New York at Plattsburgh, 101 Broad Street, Plattsburgh,
NY 12901. 3Rubenstein School of Environmental and Natural Resources, University of
Vermont, 308D Aiken Center, Burlington, VT 05405. *Corresponding author - vasotola@
gmail.com.
Manuscript Editor: David B. Halliwell
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Walbaum (Striped Bass), Cyprinus carpio L. (Common Carp), Perca flavescens
Mitchill (Yellow Perch), Xyrauchen texanus Abbott (Razorback Sucker), Salvelinus
alpinus L. (Arctic Char), and Morone chrysops Rafinesque (White Bass) (Baker and
Timmons 1990, Bardach 1955, Kilambi and Prabhakaran 1987, Le Cren 1947, Mc-
Carthy and Minckley 1987, Phelps et al. 2007, Soupir et al. 199 7).
Otoliths and scales have been verified to have annual ring deposition in LMB and
SMB (DeVries and Frie 1996, Heidinger and Clodfelter 1987, Taubert and Tranquilli
1982); dorsal spines are also assumed to have annual marks, though their use for
age estimation has not been validated (Hanchin 2011, Maraldo and MacCrimmon
1979). Age estimates using scales and dorsal spines were found to be biased and to
have low precision (Long and Fisher 2001, Maraldo and MacCrimmon 1979). For
this reason, otoliths are often the preferred structure to estimate ages of LMB and
SMB, but otolith analysis requires sacrificing the fish and more preparation time
than for scale analysis (Hoyer et al. 1985, Long and Fisher 200 1).
A 2007 survey conducted by the American Fisheries Society found that, out of
45 American and Canadian management agencies, 75% used scales, 60% used otoliths,
5% used spines, 2% used fin rays, and 0% used opercles to estimate LMB and
SMB age (Maceina et al. 2007). It is important to use a structure that can be analyzed
quickly and provide precise results; however, scales do not yield precise age
estimates for older fish (Besler 1999), and otoliths and spines are costly to prepare
(e.g., time and supplies). Opercles were extracted from LMB and SMB to collect
age data for a lake management plan (MDNR 2006, Winemiller and Taylor 1987),
but the authors did not provide data regarding the presicion of their results, nor did
they compare these results with those obtained from otoliths. Because opercles have
been used for age estimation in other fish species, they warrant consideration for use
in estimating the ages of LMB and SMB.
Methods
We collected LMB and SMB caught during fishing tournaments on Lake Champlain
in Plattsburgh, NY June–September 2012. All fish collected from tournament
anglers were longer than 300 mm. We obtained fish shorter than 300 mm via boat
electrofishing in northern Lake Champlain, using a Smith-Root VVP-15B (Smith-
Root, Inc., Vancouver, WA) boat-mounted electrofisher. We used opercles, sliced
sagittal otoliths, dorsal spines, and scales to age a sample of bass (LMB: n = 63,
SMB: n = 66) that ranged in length from 150 mm to 500 mm; the wide length range
assured that different age classes were present in the sample.
We assigned each fish a unique identification code, then measured its total length
and weight (Anderson and Neumann 1996). We collected scale samples from below
the lateral line and behind the pectoral fin (Besler 1999, DeVries and Frie 1996).
The first 3 dorsal spines were removed from fish using wire cutters. Scale and spine
samples were stored dry in coin envelopes. We removed opercles by making two
cuts—one above and one below the opercle bone—then pulling it off the fish (Le
Cren 1947). Opercles were stored in 70% ethyl alcohol for several days until they
were processed; post-processed opercles were stored dry. We removed otoliths
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from the fish following standard procedures and stored them dry in coin envelopes
(Buckmeier and Howells 2003).
We cleaned and mounted scale samples onto a microscope slide with a cover slip
and observed them at 25x magnification on a Zeiss Stemi 2000-C dissecting microscope
with an overhead light. We set dorsal spines in epoxy and sectioned them using
a Buehler Isomet® low speed saw 11-1280-160 (Buehler, Ltd., Lake Bluff, IL). We
mounted spine sections on a microscope slide, added 3-In-One SAE 20 oil to make
polishing smoother, and then polished them using 600-b-grit sandpaper. Spines were
viewed with transmitted light at 200x magnification under a Leitz-Wetzler compound
microscope. We sliced otoliths in half through the focus using the Buehler Isomet
saw, lightly burned the otolith, mounted the samples in clay with the cut surface parallel
to the clay base, submerged them in water, and viewed them under the dissecting
microscope (Buckmeier and Howells 2003). We boiled opercles for ~30 seconds,
scrubbed them briefly with a wire brush to remove excess flesh and scales, then
viewed them with the naked eye to count annuli. We captured images of all structures
using a Canon Powershot S51S digital camera mounted on a dissecting microscope.
Four readers estimated ages independently using digital images of otoliths,
spines, and scales; opercles were aged with the naked eye. We calculated percent
agreement among the four readers for each structure. We estimated precision of
determinations for each structure by calculating the coefficient of variation (CV;
Campana et al. 1995) as follows:
R
CVj = 100 x (√ [Σ ([Xij - Xj]2 / [R - 1])] / Xj)
i =1
where R is the numbe,r of readers, Xj is the mean age estimated for the jth fish, and
Xij is the ith age estimate for the jth fish (Chang 1982). To determine whether there
were differences in estimated ages among structures, age-bias plots were constructed
for each pair of structures using average age to test if the slope was equal to 1;
t-tests were performed using GraphPad Prism (GraphPad 2012).
Results
Precision and agreement
Age estimation using opercles produced the most precise determinations for
both species —LMB CV = 6.3, SMB CV = 5.2—compared with scales: CV = 14.6
and 12.1, spines: CV = 13.2 and 9.1, and otoliths: CV = 9.1 and 7.4 for LMB and
SMB, respectively (Table 1). Age estimates from opercles also had the highest
between-reader agreement compared to the other structures (Table 2).
Structure bias
When compared to the results from otoliths, age-bias plots for LMB opercles
(P = 0.62, df = 15), spines (P = 0.98, df = 7), and scales (P = 0.30, df = 6), had
a slope that did not significantly differ from 1, indicating there is very little bias
between structures (Fig. 1). Opercles underestimated the ages of fish older than
9 compared to otoliths, and older than 6 when compared to spines; however,
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compared to scales, opercles underestimated ages of younger fish and overestimated
ages of older fish.
Age-bias plots for SMB opercles showed a significant difference from a slope of
1 when compared to scales (P = 0.01, df = 7), which indicated that bias was present.
However, when compared to spines (P = 0.09, df = 8) and otoliths (P = 0.17,
df = 12), there was no significant difference from a slope of 1, indicating no bias
(Fig. 2). Opercles underestimated ages when compared to otoliths and spines, and
underestimated ages of younger fish when compared to scales.
Reader bias
Reader age-bias regressions for LMB indicated that determinations using otoliths
had the least amount of bias (average slope of 0.91) between readers, compared
Table 2. Percent agreement between readers for each of the four age-estimation structures in Largemouth
and Smallmouth Bass.
Between-reader
difference Scales Spines Otoliths Opercles
Largemouth Bass
0 20.2 30.8 31.7 46.3
± 1 53.4 66.1 75.7 84.7
± 2 74.2 84.7 91.8 96.6
± 3 87.9 92.8 96.2 98.4
± 4 93.8 97.5 100.0 99.7
> ± 4 100.0 100.0 - 100.0
Smallmouth Bass
0 17.4 37.4 29.3 43.9
± 1 48.5 73.2 73.0 89.9
± 2 68.7 89.7 91.0 97.5
± 3 81.8 93.9 96.2 99.8
± 4 89.9 97.7 98.7 100.0
> ± 4 100.0 100.0 100.0 -
Table 1. Coefficient of variation (CV) among readers for four age-estimation structures in Largemouth
and Smallmouth Bass.
Reader
Structure 1 2 3 4 Mean
Largemouth Bass
Scales 15.3 11.5 14.9 16.5 14.6
Spines 11.1 15.6 12.6 13.6 13.2
Otoliths 7.2 9.8 9.8 9.6 9.1
Opercles 6.5 6.9 6.3 5.5 6.3
Smallmouth Bass
Scales 11.9 10.1 11.8 14.5 12.1
Spines 7.7 9.7 10.9 8.1 9.1
Otoliths 5.8 8.5 7.7 7.6 7.4
Opercles 5.2 6.2 4.9 4.7 5.2
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to an average slope of 0.44 for scales, 0.62 for spines, and 0.81 for opercles. Results
for all structures except otoliths showed some between-reader bias; all 6 pairwise
comparisons between readers using scales, 5 comparisons using opercles, and 4
comparisons using spines were significantly different from 1 (P < 0.05). No comparisons
were significantly different for analyses of otoliths.
Reader age-bias regressions for SMB indicated that results for opercles (average
slope of 0.84) and otoliths (average slope of 0.91) had the least bias between readers.
Results for scales (slope = 0.36) and spines (slope = 0.59) showed more bias
between readers. All 6 reader pairwise comparisons using scales, 5 comparisons
using spines, and 2 comparisons using opercles significantly differed from a slope
of 1 (P < 0.05), whereas no comparisons dif fered for otoliths.
Figure 1. Age-bias graphs for age estimates from Largemouth Bass for comparisons between
all structures. Solid line indicates a 1:1 agreement in age estimations, error bars
represent one standard deviation around the mean.
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Discussion
Analyses of opercles had higher percent agreement between readers, were found
to be more precise, had higher among-reader agreement, and the structures were
easier to remove and process than spines, scales, and otoliths in LMB and SMB. As
Muir et al. (2008) pointed out, otoliths require removal via dissection, slicing, and
mounting, which can be very costlyand time consuming. Dorsal spines also require
slicing and mounting, but, as Koch and Quist (2007) suggested, there are cheaper
alternatives. Ours is the first study to evaluate age determination using LMB and
SMB opercles; opercles can simply be pulled off fish after two cuts and require only
30 seconds of cleaning to prepare them for age estimation (Le C ren 1947).
Figure 2. Age-bias graphs for age estimates from Smallmouth Bass for comparisons between
all structures. Solid line indicates a 1:1 agreement in age estimations, error bars
represent one standard deviation around the mean.
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Use of otoliths to estimate LMB and SMB age has been validated (Buckmeier and
Howells 2003, DeVries and Frie 1996), therefore, in order to determine if opercles
are a viable age-estimation structure, age estimates must be comparable to those derived
from otoliths. Ages estimated from opercles agreed with those estimated from
LMB otoliths until age 9, and with SMB otoliths to age 6. Based on our results for
reader agreement, precision, and ease of processing, opercles are a viable alternative
to use of otoliths for age estimation, particularly for LMB. However, otoliths yield
more accurate estimates for older fish than counts made from opercles.
High reader precision among age estimates from a given structure indicates
the structure is generally easy to use for age estimation, and the age estimates can
be easily reproduced due to the presence of distinct annuli. The high precision of
opercles and otoliths among the four readers indicates that both structures are valuable
for age estimation of LMB and SMB in Lake Champlain.
Between-reader agreement is the most common method to assess age estimation
structures (Campana et al. 1995) because high agreement indicates higher reliability
and reader confidence. Fisheries-management agencies often rely on inexperienced
seasonal employees to conduct age estimations, so it is advantageous to use a method
that requires little training and has high precision. We found that analysis of opercles
had the highest between-reader agreement and precision, and therefore would be
the most appropriate structure for use by inexperienced readers, but use of opercles
needs further validation through analysis of opercles from known-age fish.
In conclusion, opercles are easier to remove, process, and observe for estimating
SMB and LMB ages than otoliths, scales, and spines; opercles have also been
verified as more accurate than scales in other species (Bardach 1955, Kilambi and
Prabhakaran 1987, Le Cren 1947). Age estimation using scales and otoliths requires
more experienced readers due to the inherent ambiguity in the clarity of the
annuli, whereas opercles can be read by novice readers due to their well-defined annuli.
A high percentage of between-reader agreement, high precision, and little bias
among readers, along with ease of preparation, make opercles an ideal alternative
for estimating the ages of LMB to age 9, and SMB to age 6; however, validation is
required for older fish.
Acknowledgments
We particularly thank Dr. Aude Lochet for the use of her equipment. We thank Luke
Myers, Dr. Jacob Straub, Mark LaMay, and Rory O’Carroll for assisting with age estimation;
the students and staff of the Lake Champlain Research Institute for assisting at fishing
tournaments and with fish collection during electrofishing; and the Lake Champlain Basin
Program and New York State Department of Environmental Conservation for funding for
this project.
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