Using marginal increment analysis to validate the periodicity of
annulus formation in Dover sole, Microstomus pacificus
Lisa M. Lysak
Pacific States Marine Fisheries Commission, Cooperative Ageing Project
2032 S Oregon State University Drive Newport, OR 97365
[email protected]
Introduction
Conclusions
Age data derived from saggital otoliths contribute to stock
assessments and management of commercial fisheries. To minimize
inaccuracies, criteria must be developed that will consistently identify
annual growth zones (MacLellan & Fargo 1995).
Marginal increment analysis can be used to validate the frequency
of growth zone formation. If a translucent zone forms once a year,
there should be a clear pattern of periodic growth on the otolith edge
throughout the year (Pearson 1996). When plotted across months,
edge growth should display a sinusoidal cycle with a frequency of one
year in true annuli (Campana 2001).
Limitations of marginal increment analysis include the fact that it is
suited for young and/or fast-growing fish, and relies on a fair amount of
subjectivity (Campana 2001).
This study applies marginal increment analysis to investigate the
periodicity of annulus formation in Dover sole and to validate the
methodology of age determination.
Methods
Otoliths were randomly selected from 6, 7 and 8 year-old age
classes from Oregon commercial samples collected in 1992 and
1994. The original age was estimated using the break and burn
method (Chilton and Beamish 1982).
Opaque zone
Using reflected light, opaque zones
appear as light bands and translucent
zones appear as dark bands. When
determining age for Dover sole, opaque
growth beyond the last translucent zone
Translucent zone
is considered the marginal growth and is
categorized relative to the previous opaque zone.
Otoliths were analyzed with a dissecting microscope (40 - 60x)
under reflected light. Growth on the otolith edge was examined along
the axis that corresponds with the dark/light boundary and was
classified into one of the three following groups:
Group A: Translucent
zone on the edge of the
otolith.
Results
translucent growth.
A total of 648 otoliths were examined. When plotted over
all twelve months, each of the three edge groups shows a
periodic curve with a peak and trough occurring once per
year (Fig. 1).
Group A
Group B
Group C
Group C: More than half
a year’s opaque growth
beyond the last
translucent zone.
Current methodology for age determination in Dover sole
correctly identifies annual growth increments.
60%
40%
Translucent growth is present on the otolith edge in the winter
months. This coincides with the heavy spawning period and
20%
0%
J
F
M
A
M
J
J
A
S
O
N
D
M onth
Figure 1. Percent frequency of each edge type group for all months of the year.
Both sample years and all age classes are combined.
Group A - translucent growth on the edge - is most
abundant in the winter months and is rarely seen in the summer
months. Its peak occurs in February when it accounts for 93.3%
of all edge groups (Fig. 1).
Group B - less than half a year’s opaque growth on the
edge - occurs most frequently in spring but is relatively scarce
the rest of the year. Its highest frequency occurs in May,
accounting for 60.4 % of all edge groups in that month (Fig. 1).
These results are age-specific to 6, 7 and 8 year-old Dover
sole. Future research is necessary to validate increment
periodicity over a broader range of age classes, especially in
young and mature fish.
Additional investigation is required to fully validate the
periodicity of annulus formation in Dover sole. Research should
involve methods that can:
•utilize quantitative rather than subjective data
•determine absolute age of the fish
•verify the age of first increment formation (Campana 2001)
Group C - half a year’s opaque growth or more on the
edge - is the most prevalent edge type from July through
December. Its peak occurrence is in August when it accounts
for 89.1% of all edge groups (Fig. 1).
Table 1. Categorical analysis of variance
Source
DF
Chi-square
p
Intercept
1
4931.22
< 0.0001
Month
11
755.83
< 0.0001
Residual
0
--
--
Categorical analysis of variance (Table 1) performed on the
counts of edge type (Table 2) demonstrates the significant
variation in edge type with month (p < 0.0001). The edge type
follows a consistent pattern as months change through each
year.
Group B
Group C
Month
Group A
January
20
2
4
February
56
2
2
March
30
18
12
April
22
16
May
19
9
32
12
June
10
23
25
July
0
10
August
September
5
18
14
1
11
50
49
October
The percent frequency of each edge group was plotted for all
months of the year. A categorical data analysis was performed with
the CATMOD procedure using SAS statistical software.
from translucent through opaque growth and back to translucent
each year.
growth.
80%
Table 2. Total count of each edge type per month.
Group B: Less than half
a year’s opaque growth
beyond the last
translucent zone.
The consistent pattern of edge type variation through the
months of each year suggests that there is a single transition
The combination of an opaque zone and a translucent zone
constitutes an annulus and corresponds to one calendar year of
100%
Percent frequency
Age estimates for Dover sole
(Microstomus pacificus) are
produced assuming that an
annulus forms once a year
and consists of one opaque
zone and one translucent zone. No validation study has been
published that proves an annulus corresponds to one year of growth.
Dover sole form one opaque zone and one translucent zone
per year. The highest frequency of opaque growth on the edge
of the otolith occurs six months after the highest frequency of
31
2
41
November
16
1
35
December
19
1
30
Literature Cited
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
Chilton, D.E. & R.J. Beamish. 1982. Age determination methods for fishes
studied by the Groundfish Program at the Pacific Biological Station. Canadian
Special Publication of Fisheries and Aquatic Sciences. 60, 102 p.
MacLellan, S.E. & J. Fargo. 1995. Validation of age and growth for English sole
in Hecate Strait, British Columbia. In Recent Developments in Fish Otolith
Research, pp. 341-360. Columbia: University of South Carolina Press.
Pearson, D.E. 1996. Timing of hyaline-zone formation as related to sex, location,
and year of capture in otoliths of the widow rockfish, Sebastes entomelas.
Fishery Bulletin. 94:190-197.
Sampson, D.B. & C. W ood. 2001. Stock status of Dover sole off the U.S. west
coast in 2000. Pacific Fishery Management Council, Portland, OR.
Acknowledgements
I would like to thank: Jim Colbert for invaluable help
with the results & statistics. Michael Schirripa and
Susan Coccetti for advice & opinions. Bob Mikus for
all his experience & expertise on Dover sole. Mark
Freeman for Oregon market sample data. The CAP
ageing lab in Newport for editing.
Dover sole photographs by: Bill Barss of ODF&W and Doug Markle.