|
Pacific Cod:
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Year | Age 1 | Age 2 | Age 3 | Age 4 | Age 5 |
---|---|---|---|---|---|
1977 | 22.0 | 33.0 | 41.0 | 51.0 | 61.0 |
1988 | 24.2 | 34.9 | 40.9 | 50.1 | 58.6 |
1989 | 19.2 | 33.8 | 41.7 | 51.9 | 58.9 |
1990 | 16.9 | 27.2 | 35.9 | 46.8 | 57.2 |
1991 | 19.6 | 28.4 | 36.2 | 47.4 | 55.6 |
1992 | 18.7 | 26.0 | 33.9 | 44.4 | 53.6 |
Pacific cod stock
assessments in both the United States and Canada
have depended largely on length frequency data alone
to model the population age structure because of
ageing difficulties. However, the use of length
frequency data as proxies for age data has proven
problematic. The current length-based model for
Alaska Pacific cod has produced results that are
difficult to explain. One explanation is that
external factors such as ocean conditions affect
somatic growth to such a degree that length-at-age
within the population is highly variable and
difficult to model. Otoliths, on the other hand, are
composed of layer upon layer of daily growth
accumulations and are theoretically permanent
records of growth, more independent of external
factors. Consequently, the Age and Growth Program
initiated a new study in 1998 to reexamine the
otolith ageing structure for Pacific cod. Research
entailed determining the best otolith preparation
method; establishing and justifying reading
criteria; and investigating the 1990-92 shift in
length-at-age. The following article summarizes the
M. S. thesis of Nancy Roberson of the Age and Growth
Program.
Otolith Preparation
At the beginning of the study a decision was made to
use otoliths in conjunction with digital imaging. It
was thought that the approach would yield the
highest quality data for Pacific cod. The first step
in this study was to find a method for preparing the
otoliths that would maximize the clarity of the
annulus pattern of alternating opaque and
translucent zones. (Note: all further references to
"zones" refer to reflected light.) The
traditional break and burn method involves cutting
through the transverse plane of an otolith and
burning it over an alcohol flame. For many
species of fish, burning sufficiently enhances the
zones, allowing the age reader to better
differentiate the opaque and translucent zones.
However, burning does not consistently darken
the rings of Pacific cod otoliths (Figure 1 below).
An alternative method for preparing otoliths is known as “thin sectioning.” This method, used by many laboratories worldwide and particularly for daily growth studies, involves removing a thin cross section of an otolith and encasing it in resin before analysis under a microscope. After trial and error with various resin types and section thickness, a preparation method was chosen that uses a black resin (Technovit 3040) and sections the otolith to a thickness of 0.2mm with a Hilquist thin section machine. This method provides better contrast between opaque and translucent zones and greater uniformity of color within the respective zones, thus making it easier to discern patterns (Figure 2 below). Furthermore, sectioning an otolith creates a polished, two-dimensional surface that facilitates digital measurements and imaging, which the coarse surfaces of break and burn preparations do not.
Reading
Criteria
Reading criteria are guidelines age readers use to
interpret otolith patterns. Reading criteria are
particularly important in the scientific study of
ageing fish because they are the qualitative
information on which age estimates are based. The
criteria used in this study were designed to
instruct the age reader on how to classify the
translucent zones of an otolith pattern. A
translucent zone is a compilation of many closely
spaced daily growth rings, which in theory occur
when there is decelerating or slow somatic growth.
There are two categories of translucent zones:
annuli (formed once a year and normally associated
with the age of a fish) and checks (any subannular
mark).
Pacific cod have a strong tendency to develop
otolith checks at young ages (under 6 years), making
it difficult to age the species to an exact age.
Evidence from tagging studies and surveys
indicates that the species is relatively
short-lived, with a maximum age of approximately 13
years. Differentiating between checks and
annuli, therefore, is crucial in the age
determination of Pacific cod. Checks that are
counted as annuli result in overestimation of age
and underestimation of size-at-age. For this reason,
working with validated reading criteria is critical.
This study followed traditional qualitative ageing
criteria to distinguish annuli from checks:
completeness of a zone around an entire section,
zone darkness, and spacing between zones.
Pacific cod otoliths are confusing to
interpret because their checks frequently have the
same characteristics as their annuli: the checks are
just as dark, and their ring-like pattern just as
complete. This in turn makes it difficult to
differentiate the two, other than by spacing between
zones (Figure 3 below). Fortunately, as Pacific cod
get older it is easier to distinguish spacing
patterns between zones.
In this study,
rings that could be interpreted as both annuli and
subannular marks were classified as checks. In
addition to counting annuli, the annuli and strong
checks were hand-traced with Optimas 6.5 imaging
software, resulting in measurements for ring area
and ring axis. This made qualitative data more
quantitative and enabled the reading criteria to be
statistically evaluated.
A total of 174 otoliths collected during the NMFS
1989 Bering Sea Groundfish Survey served as the
initial study sample to develop the reading
criteria. The 1989 study sample enabled
calibration of age estimates with ages generated
prior to the 1990-92 problem years.
Justifying Reading Criteria
Reading criteria should reflect the actual age of
the fish. To evaluate the accuracy of the
reading criteria developed from the
initial study sample, a second sample of
approximately 106 otoliths from a previous tag and
recovery study was used. The advantage of using
otoliths of tagged fish was that length at release
was available. This information was helpful in
two applications: 1) to determine whether
back-calculated fish lengths based on ring areas are
superior to those based on ring lengths and 2) to
provide an indirect validation of ageing criteria by
comparing predicted growth increment for a given
time at liberty versus the growth increments
actually observed. (This second computation
utilized von Bertalanffy growth parameters
previously estimated from Pacific cod tag recovery
data and the reader age of the otolith at the time
of recovery.)
The study found that fish lengths back-calculated
using ring areas were much closer to observed
lengths than those calculated with ring lengths
(Figures 4 below). Although back-calculations are
typically performed using radial or diametral
measurements, these findings are not surprising in
that ring area is a more comprehensive measure of
otolith three dimensional growth.
The strong
relationship between ring area and fish length was
important to this study because it provided a
quantitative means of evaluating the qualitative
reading criteria through back- calculations. Back-
calculation allows the examination of fish
length-at-age less than the age at capture. However,
back- calculations are valid only when they are
based on true annuli, not on checks. Length-at-age
based on back-calculation can be a test of whether
ideas of true annuli and checks are correct. In
order to perform this test, use was made of the
original 1989 training sample. Using the
previously measured areas, fish length-at-age was
back-calculated twice: once using only what were
considered annuli and then again including checks
(Table 2 below).
Fish Age | 1989 back-calculated fish length (cm) not counting checks | 1989 back-calculated fish length (cm) counting checks | 1977
year-class modal length- at-age (cm) |
---|---|---|---|
1 | 20.7 | 16.6 | 19 |
2 | 32.5 | 26.6 | 33 |
3 | 41.9 | 35.2 | 41 |
The back-calculated fish length-at-age estimates for the 1989 sample using annuli alone were closer to previously validated length-at-age measurements (1977 year class) than the back- calculated lengths using checks. These calculations supported the criteria used to differentiate checks from annuli. Fortunately, these reading criteria could be somewhat validated using the otoliths from the tagged fish. Using von Bertalanffy growth parameters estimated from the tagged fish and their age at recovery (estimated with current ageing criteria), the growth increment for each tagged fish was estimated and compared to the observed growth increment. Estimates calculated with the current reading criteria were roughly the correct ages (Figure 5 below).
A final test of
reading criteria was performed through a more direct
comparison: simply ageing the tagged fish from
length-at-release plus the time at liberty.
Seventy-five percent of these fish were within
1 year of age based on otolith readings, and 94%
were within 2 years.
Validation of reading criteria is most convincing
when achieved through daily growth counts and marked
otoliths in tag and recapture studies.
However, the indirect validation techniques
employed in this project using back-calculations on
ring area measurements and otoliths from tagged fish
provided evidence that criteria used for this study
were accurate.
1990-92 Shift in Age at Length
The final phase of this investigation examined the
decrease in length-at-age of Pacific cod in 1990-92.
One hundred and sixty four otoliths, subsampled from
the 1992 Bering Sea trawl survey, were used in
conjunction with the 1989 sample to compare current
ageing criteria to the criteria used in 1992.
The sample was composed of specimens less than
50 cm, representing young fish, because the shift
was seen in ages 1-4 years old.
The 1992 sample was problematic from the standpoint
of otolith preparation. First, it was impossible to
thin section the otoliths because over the years the
survey sample had been examined so many times that
no whole otoliths were available to section.
As a result, it was necessary to use
previously broken and burned otoliths. Also,
the annuli on the burns were too faint to examine
clearly, and only the outer edge of the otolith
could be measured. Fortunately, edge measurements
represent otolith size at fish age and could be used
in the comparisons as proxies for the ageing
criteria used in 1992.
For each sample, the outside edge of the transverse
plane of the broken and burned otoliths was traced
and the areas calculated. The original 1992 ages
were used. For the 1989 sample, ages and edge
measurements collected earlier in this study were
used. Since both samples had similar season
sampling dates (early summer), it was concluded that
differences in edge growth were negligible.
When 1989 and 1992 otolith areas were compared, the
average otolith size-at-age was smaller in the 1992
fish than in the 1989 fish. The smaller
otolith size alone can largely explain the change in
size-at-age of Pacific cod between 1989 and 1992
(Table 3 below). However, these results cannot
explain whether the smaller ring sizes in 1992 were
due to misread ages (counting checks inadvertently)
of the 1992 sample or real differences in otolith
size-at-age.
Table 3. The difference in whole otoliths areas (mm2), at different ages, obtained from 1989 sections in the current study and break and burns aged in 1992. This difference is converted into differences in fish length by using the otolith area versus fish length relationship: for every mm2 increase in otolith area, fish size increases 3.4 cm (regression of fish length on otolith size using 1989 and 1992 sample.) This gives a predicted difference in fish lengths (cm) between 1989 and 1992. A comparison is made with the observed difference in fish length from the same sample of fish. | |||
Fish age | Difference in otolith area for 1992 and 1989 sample results | The predicted difference in 1992 and 1989 fish lengths using a multiplier of 3.4 cm | Observed sample difference in 1992 and 1989 fish lengths (cm) |
---|---|---|---|
1 | +0.52 | 1.77 | 1.08 |
2 | -1.03 | -3.50 | -4.89 |
3 | -1.28 | -4.35 | -3.94 |
4 | -0.47 | -1.60 | -3.01 |
Table 4. Fish length-at-age (cm) estimated from back-calculations, the original 1992 ages, and a re-ageing of some of the 1992 otoliths using the current ageing criteria. | ||||
Fish age | 1989
back- calculations not counting checks |
1989
back- calculations counting checks |
1992 original ages | 1992 re-ages using ageing criteria from this study |
---|---|---|---|---|
1 | 20.7 | 16.6 | 18.7 | 20.8 |
2 | 32.5 | 26.6 | 26.0 | 31.8 |
3 | 41.9 | 35.2 | 33.9 | 40.2 |
4 | 51.1 | 43.2 | 44.5 | 43.9 |
In an attempt
to resolve this issue, the 1992 samples were read
according to current criteria. This enabled
the comparison of 1992 fish size-at-age using
historic and current ageing criteria (Table 4
above). It seemed clear that the 1989
back-calculated lengths at age, counting checks,
were similar to those associated with the 1992
original reading. It also appeared that the
1992 re-ageing using current ageing criteria had
resulted in size-at-age closer to the 1989
back-calculation without checks. Therefore, it
appears that the original 1992 criteria may have
counted checks. However, there is still some concern
that counting checks might not account for all the
differences in length for 1-, 2-, and 3-year-olds
shown in Table 1 above.
For example, consideration must also be given
to the observation that halibut length-at-age
declined sharply at the same time that Pacific cod
size-at-age was seen to decline. It could be
that both a true decline in length-at-age and the
inadvertent counting of checks both played roles in
the observed decline in the length-at-age of Pacific
cod in 1990-92. This issue will be examined in
greater detail as the investigation of
production ageing of Pacific cod continues.
Conclusion
Thin-sectioning Pacific cod otoliths was
investigated as an alternative preparation method
for ageing Pacific cod to the break and burn method.
The break and burn method is inadequate for ageing
Pacific cod because it fails to provide sufficient
contrast between the translucent and opaque zones on
a consistent basis. Also, it is difficult to create
an “even” burn over the entire surface without
overburning or incinerating parts of the otolith.
Thin sectioning otoliths encased in black resin
seems to maximize the clarity of the viewing surface
by adding contrast to the faint annulus
pattern. Furthermore, the resin preserves the
physical integrity of the otolith so that all axes
of an otolith can be examined. Precision testing
will be conducted in the future to determine whether
this truly is a better preparation method.
This investigation generated indirectly validated
age reading criteria for Pacific cod otoliths
through the use of a thin section preparation method
and otoliths from tagged and recaptured fish.
Back-calculations, based on digital measurements of
what the reading criteria classified as annuli, were
used to estimate size at initial capture of the
tagged fish to show that the reading criteria were
roughly correct. Analysis based on back-calculation
showed that checks should not be counted.
Back-calculated fish lengths based on otolith areas
are superior to those based on otolith lengths.
These results support the use of otoliths to
production age Pacific cod.
The decline of size-at-age observed in Alaska
Pacific cod between 1989 and 1992 also was
investigated. It could be that both an inadvertent
counting of checks and a true decline in
length-at-age played roles in the observed decline
of the length-at-age of Pacific cod in 1990-92.
This issue will be examined in greater detail
as the Age and Growth Program resumes production
ageing of Pacific cod this fall.