December
5, 2006
International
Pacific Halibut Commission Staff Preliminary Catch Limit
Recommendations: 2007
Assessment of the status and available
yield from stocks of Pacific halibut is conducted annually
during the fall and considers the results of stock assessment
surveys, commercial fisheries and biological studies conducted
by International Pacific Halibut Commission (IPHC, or
Commission) staff. The
staff then makes catch limit recommendations to the Commission
and the halibut industry, for consideration at the
Commission’s annual meeting in January.
In determining catch limit recommendations for 2007, the
staff considered the results of the analytic assessment, changes
in the commercial and survey indices used to monitor the stock,
recruitment of incoming year classes, and an analysis of an
optimal harvest rate to achieve coastwide management goals.
Ongoing returns of the coastwide PIT (passively
integrated transponder) tagging program have influenced the
choice of an appropriate assessment model and the corresponding
harvest policy.
Commercial catch per unit effort (CPUE)
in 2006 did not exhibit a consistent trend coastwide, although
the majority of regulatory areas showed lower catch rates than
in 2005. Notable
declines occurred in Area 2A, 2C, 3A, 4A, 4C, and 4D.
Commercial fishery catch rates in Areas 2B, 3B, and 4B
were largely unchanged.
The 2006 IPHC setline survey CPUE values
were lower in almost all regulatory areas, compared with those
of 2005. These
declines were generally in the 14-20% range, although Area 2A
exhibited a particularly sharp decrease of 43%.
Only Area 4B showed a higher catch rate, continuing the
modest increase in CPUE noted in 2005.
However, it should be noted that in both Areas 2A and 4B
the interannual variance of catch rates is high.
The 1994-1996 year classes continue to
recruit in relatively strong numbers in most areas, particularly
in Areas 2B, 2C, and 3B. Area
4B continues to show a notably low level of recruitment, in
comparison with the same year classes in other regulatory areas.
Stock Assessment Changes
Information accruing from the PIT tag
recoveries, as well as inconsistencies in the traditional
closed-area stock assessments for some areas, prompted the staff
to explore alternate stock assessment frameworks.
In particular, movements of legal-sized fish among
regulatory areas call into question the assumption inherent in
the individual regulatory area assessments used by the
Commission for the past two decades.
It has been assumed that fish of legal size had largely
completed their migratory behaviour by the time they were
recruited to the exploitable stock in a given regulatory area.
However, the PIT tag recaptures indicate greater movement
among regulatory areas than believed previously.
Accordingly, the staff developed a coastwide assessment
based on a single stock, which does not invoke any assumptions
regarding movement by fully-recruited fish, rather than an
assessment for each regulatory area.
A coastwide stock assessment is also coherent with the
concept of a coastwide spawning stock for Pacific halibut, which
has been verified by genetic analysis.
To conform to a coastwide stock
assessment, the staff also examined the current harvest policy,
to ensure that long-term stock management goals and reference
points were appropriate. In
2005, an optimum harvest rate of 22.5% was adopted for the core
stock areas (Areas 2 and 3A), while lower harvest rates were
used in Areas 3B and 4. For
the 2006 stock assessment, the staff re-examined the limit and
threshold reference points in consideration of a coastwide stock
unit. Movement of
fully-recruited fish from outside of the core areas used in the
original harvest rate analysis results in an underestimate of
the historic minimum spawning biomass used as a basis for these
reference points. Therefore,
the reference point biomass levels are greater and the harvest
rate must be lowered to achieve the same level of protection
from reaching these levels.
The analysis of optimum harvest rates
for a coastwide assessment resulted in a target harvest rate of
20% of coastwide exploitable biomass.
While a 20% harvest rate was determined to be optimal for
the aggregated coastwide biomass, regulatory areas which have
net immigration of fish, such as Area 2, can sustain higher
harvest rates. Accordingly,
the staff recommends using a higher harvest rate of 25% for Area
2. For Area 4B, the
staff continues to recommend a harvest rate of 15% as indicated
by the analysis of productivity conducted in 2005.
Similarly for Area 4CDE, the continued decline in survey
and commercial catch rates supports the continued use of the 15%
harvest rate for this area as well.
The staff also required a framework to
partition the coastwide estimate of exploitable biomass into
regulatory areas. This
is similar to the process the staff has used to obtain a catch
limit for Area 2A, through scaling the Area 2B catch limit
assessment by the biomass distribution between Areas 2A and 2B,
based on the survey. Using
the distribution of biomass as determined by the three-year
average CPUE of legal-sized fish obtained on the stock
assessment survey, the survey results can be used to partition
the coastwide exploitable biomass estimate into regulatory area
biomass totals. However,
such a static depiction of biomass does not capture the impact
of fish movements on the apparent productivity of different
areas.
Catch Limit Recommendations
The staff recommendations totaling 66.56
million pounds for 2007 are presented in Table 1.
The Area 2A recommendation includes all removals
(commercial, treaty Indian, sport) allocated by the Pacific
Fishery Management Council's Catch Sharing Plan.
The Area 2B catch limit recommendation includes the
commercial and sport fisheries, which will be allocated to the
sectors by the Canadian Department of Fisheries and Oceans.
The use of a coastwide assessment, with
differing harvest rates by regulatory area, and partitioning of
coastwide biomass with survey estimates of distribution creates
some substantial changes in constant exploitation yields (CEY)
and recommended catch limits among areas.
The two major components of these differences are the
different distribution of biomass associated with survey
partitioning of a coastwide total biomass, compared with the
traditional closed-area biomass distribution, and the real
declines arising from incorporating recent commercial and survey
data. As noted in
the 2005 stock assessment, the survey distribution of biomass is
more consistent with other estimates of biomass distribution
that are not dependent on the stock assessment.
The recommended 2007 catch limits for
Area 2 are lower than the 2006 catch limits adopted by the
Commission, while Areas 3 and 4 have higher recommended limits.
In partial contrast to these lower recommendations, the
mid-term outlook for Area 2 is positive, with stronger than
average recruitment projected for the 1994-1996 year classes.
The staff continues to recommend a slow
rate of increase in catch limits when biomass is increasing and
a more rapid reduction of catch limits when biomass is
decreasing. For Area
2 the staff recommends catch limits that are lower by 50% of the
difference between 2006 catch limits and the estimated fishery
CEYs for 2007. For
Areas 3 and 4, the staff recommends an increase over the 2006
catch limit equivalent to one-third of the difference between
the 2006 catch limit and the estimated 2007 fishery CEY.
These recommendations, along with
public and industry views on them, will be considered by IPHC
Commissioners and their advisors at the IPHC Annual Meeting in
Victoria BC Canada, during January 16-19, 2007.
These recommendations are preliminary and, as final data
are included in the assessment, may be updated for the Annual
Meeting but are not expected to change significantly.
Proposals concerning changes to
catch limits should be submitted to the Commission by December
31, 2006. Catch
limit proposals are available on the Commission's web page (http://www.iphc.washington.edu/halcom/default.htm)
or from the Commission's office. Additional details about the Annual
Meeting can also be found on the web page.
Table
1. IPHC staff
recommended catch limits for 2007, by IPHC regulatory area
(million lbs, net weight). The
2006 fishery catch limits are included for comparison.
