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Pacific Cod:
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Table 2. Two separate calculations of the 1989 fish length-at age (cm), estimated from back-calculations counting checks and from not counting checks compared to the 1977 year class modal length-at-age. |
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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).
Figure 5. A comparison of estimated growth
increments and observed growth increments based on
otoliths from tagged Pacific cod. This provides an
indirect validation of ageing criteria because
estimated growth increments are a function of von
Bertalanffy growth parameters estimated from tagged
fish and the otolith age assigned to a fish at the
time of recovery.
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.