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Analysis
to date
Good
progress is being made on the analysis of this Program's recapture data.
It is hoped that the Program will continue to receive recapture information
for many months to come, because long-term recaptures will be valuable
when looking for inter-annual trends.
A
summary of work completed during the first 3 years of this Program are
detailed in the June
2006 Final Report, submitted to the Cooperative Research Partners
Program upon the completion of GMRI's original cod tagging contract. Some
of the results by June 2006 are also summarized below. However, the
most recent overview of findings to date is now available in the January
2008 mailing. This mailing has been distributed to ~5,000 stakeholders
in the US and Canada, throughout the Gulf of Maine region.
Realistic
expectations of the Program's data:
Bearing in
mind the Program's aims, experimental design
and the data collected,
the realistic expectations of what this Program can deliver are as follows:
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Provide
region-wide, contemporary information on cod movements and exchange
rates between areas; |
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Yield
growth data to supplement that found in other growth studies; |
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Yield
data (emigration, immigration & growth) to supplement other data
used in stock assessments; |
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Produce
visible and long-term data which is accessible to the public; |
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Ensure
region-wide & international collaboration between industry, science
& management; |
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Provide
recommendations for future research; |
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Provide
useful advice for future study designs (e.g. in outreach methods and
tagging methodology) |
Analytical
tools:
The analytical
tools currently in development address the following:
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Reporting
rates |
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Shedding
rates (both of which are also necessary for weighting the releases
and recaptures) |
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Size
relationships of tagged cod |
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Growth
rates |
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Displacement
(overall and seasonal) |
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Dispersal/exchange
between areas |
Data
summaries by June 2006:
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Collaboration:
The NRCTP has worked with a total of 107 cod tagging vessels throughout
the region, totalling ~250 fishermen. Over 30 scientists have also
been involved and over 1,511 individuals have participated by reporting
recaptures from 684 identified vessels and numerous processor plants;
a total of 980 tags have been reported from unidentified vessels.
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Releases:
A total of ~114,473 tagged cod were released during this Program
(Fig. 1). Of the releases, ~45% were released from trawl gear and
~55% were released from hook gear.
Fig.
1: The distribution of tagged cod releases from March 2003 through
July 2005.
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Recaptures:
Recapture data exists for a total of 5,873 cod - 62% of these have
been reported by fishermen, 27% by processors, observers and port
samplers (Fig. 2), and a further 11% of recaptures were recorded
during tagging trips. The total recapture rate to date is ~5.1%.
GMRI receives on average ~150 returns per month.
Fig.
2: Who is reporting tags?
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High-reward
tags: Since the high-reward tagging began in May 2004 (Fig.
3), a total of ~9.6% high-reward tags have been returned and rewarded
($100 per blue tag).
Fig.
3: The blue high-reward tag.
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Double-tagged
cod: Around 16% of the cod were double-tagged at release
to monitor for tag loss; findings suggest that ~15% of the single-tagged
fish may have lost their tags, and will thus never be reported. This
information is also used when weighting tag releases. |
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Fish
size: A wide size range of cod have been tagged from 38
cm (14”) and larger; the biggest cod tagged to date was 134
cm (53”) during a trip on Jeffreys Ledge by Maine DMR; the
largest cod recorded (but not tagged) was 138 cm (54”). There
is a 10 cm (4”) size difference between the mean size of fish
released and the mean size of fish recaptured (Fig. 4). This could
be due to a combination of factors, including catchability; Fig
4 shows differences in release between hook and trawl gears - trawl
gear has caught larger cod than hook gear in this study. This size
difference is also likely to be associated with growth.
Fig.
4: Size-frequency distributions of released and recaptured tagged
cod.
Fig.
5: Size-frequency distributions of cod released from trawl gear
versus hook gear.
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Growth:
Growth estimates are being generated by comparing the size at recapture
with the size at release; we should be able to estimate growth from
~79% of the recaptures, since this proportion of fish are reported
with a measured size at recapture. |
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Days
at liberty: Many tagged cod have been recaptured within
30 days of being released, and as such, provide somewhat limited
information with regard to growth and movement. Of the recaptures
reported, ~60% have been caught up to year from the time of release,
~18% have spent 1-2 years at liberty and ~3% had been at liberty
for more than two years by the time of recapture; the longest time
at liberty by June 2006 is ~1,140 days (more than three years) (Fig.
6).
Fig.
6: Time at liberty for recaptured tagged cod.
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Distance
traveled: The average distance traveled by to date is ~70
km - the furthest distance travelled is >1,000 km (>620 miles)
(Fig. 6) - this fish was released in the Bay of Fundy and was recaptured
eleven months later in waters off Maryland.
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This
page will be updated as analysis progresses and as publications become
available.
Please keep checking in!
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