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Stock Assessment Improvements
Last year the IPHC staff alerted the industry that methods for
estimating biomass and recruitment needed to be changed, and that
revised estimates could indicate higher biomass levels and perhaps
higher catch limits in 1997. The staff continued to work on a
new method of estimating biomass during 1996 and reviewed the
rate of exploitation that should be used to set catch limits.
Significant progress was made during 1996, and although more work
needs to be done, the staff is confident that the new method is
an improvement over the previous method. Further, the staff is
confident that the biomass and recruitment of halibut are higher
than previously indicated.
The new estimates of biomass and recruitment also required us
to reexamine the exploitation rate used to compute constant exploitation
yield (CEY), because harvest rates depend on the relationship
between biomass levels and future levels of recruitment. Under
the new method, harvest rates must also be reduced due to bycatch
mortality of halibut below the legal size limit (32 inches). An
initial analysis indicates that the rate of exploitation that
is applied to the new biomass estimates needs to be reduced from
the 30% that was previously recommended. We recommend an interim
rate of 20%, but even with this reduced exploitation rate an increase
in catch limits appears justified.
The increase in estimated biomass under the new method can be
attributed to three factors: (1) Halibut size at age information
is better represented in the population model. Growth has declined
and the new method more appropriately accounts for the availability
of young halibut to capture by setline gear; the estimated abundance
of young halibut has increased accordingly, (2) Bycatch mortality
of legal-sized halibut (32 inches or greater) is now included
in the assessment along with other removals such as commercial
and sport; this causes the estimated biomass to increase to account
for the increase in removals, and (3) Catch rates from IPHC setline
surveys are included in the analysis; these data support trends
observed in the commercial fishery that the halibut population
has increased.
The old method not only underestimated the exploitable biomass
of halibut in recent years, but the catch limits that were set
were lower than they might have been if we had realized that biomass
was being underestimated. The estimated exploitation rate using
the new method varies with area but the total for all areas combined
has ranged from 10% to 16% since 1980. This low rate of exploitation
plus above average recruitment of juvenile halibut during the
1980s allowed halibut abundance to increase beyond historically
normal levels. The 1987 year class of halibut, although small
inindividual size, appears to be very abundant. The strength of
this year class has increased current estimates of abundance and
suggests that halibut biomass is likely to stay high for the next
several years. It is doubtful, however, that the current level
of halibut abundance can be sustained indefinitely, and a decline
in halibut biomass is likely over the longer term as harvest levels
increase and recruitment declines.
Catch Limit Recommendations
The staff continues to caution the industry that the new assessment
method needs further testing and that additional research surveys
are needed to confirm trends in abundance and the relative distribution
of halibut among regulatory areas. We therefore recommend a precautionary
approach to setting catch limits in 1997. The staff recommendations
for each area reflect not only the results from the new assessment,
but also a judgment of the relative precision of the assessment
among areas and other information not included in the assessment.
We are proposing to cover all areas with our stock assessment
surveys for, at least, the next five years to extend our knowledge
of stock trend and distribution. As these data are included into
the new stock assessment we expect uncertainty will decrease.
The quotas that the staff recommends are not uniform proportions
of the new estimates, but include all ancillary information from
each area and reflect the different levels of uncertainty in the
assessments in each area.
Estimates of the 20% constant exploitation yield (CEY), the 1996
catch limits, and staff recommendations for 1997 catch limits
are provided below. The staff considers these recommendations
to be conservative and this precautionary approach will allow
the staff time to fully explore technical aspects of the new model
while not putting the stock in jeopardy of overfishing. We further
note that the staff would not object if the fishing industry recommends
lower catch limits than those proposed by the staff in order to
provide for greater stability in the level of harvest.
* Includes sport, tribal, and commercial fishery.
Additional comments on catch limits for each regulatory area are
provided below:
Area 2A: The 1995 stock assessment survey indicates that
7% of the combined Area 2A-2B biomass is in 2A. This was used
in the Area 2A biomass calculation, but is higher than the 4.5%
to 5.5% that has been estimated historically. Future recommendations
will be based on the average from successive surveys.
Area 2B: The fishery CPUE and the stock assessment survey
CPUE have increased over the past ten years, while the previous
assessment model showed a decline in the biomass. The new model
estimates are in line with the CPUE data and support a modest
increase in catch limits.
Area 2C: While the Area 2B estimate of biomass has increased,
Area 2C has remained fairly stable in recent times. Nevertheless,
the higher biomass estimates suggest that a slight increase in
the catch limit is acceptable.
Area 3A: The new assessment shows considerably more biomass
in this area than previously thought. We believe this is accurate,
but being cautious with the new assessment, we recommend a moderate
increase in catch limit.
Area 3B: This area poses a dilemma. The stock assessment
survey and the CPUE and habitat estimates indicate that the biomass
should be approximately 60% of Area 3A. However, current estimates
of biomass in Area 3B indicates about 30% of the biomass in Area
3A. This may be due, in part, to the fishery not covering all
fishing grounds in this area. The stock assessment for this area
is less certain than Area 3A, but we do feel strongly that there
are more halibut in this area than previously estimated. The recommended
catch limit is 2-1/2 times higher than in 1996 and 36% of the
proposed Area 3A catch limit.
Area 4: This area lacks stock assessment survey information.
The biomass estimate is the most unreliable of all the areas.
Including the adult bycatch into the new assessment has increased
the biomass considerably. While we believe the catch limit should
be increased, we advise caution until we conduct a series of stock
assessment surveys that can be incorporated into the assessment.
Fishing Periods
Individual quota fisheries will be in effect for all areas except
Area 2A. The staff is still in favor of a winter closure from
November through March to reduce the interception of fish that
move between regulatory areas during spawning migrations. However,
we have no evidence that any significant interceptions occurred
during the 1996 fishing season, which ran from March 15 through
November 15. The staff will not object to a similar season in
1997 if this is what the industry prefers. In Area 2A, the staff recommends fishing periods similar to those in effect in 1996: a series of 10-hour periods, with fishing period limits to be sure that the catch limit is not exceeded. The size of the fishing period limit will be determined later when more information is available on fleet size.
Area 4 Management
The North Pacific Fishery Management Council has in effect a catch
sharing plan which calls for allocating the Area 4 catch limit
among subareas using the same percentage that was in effect last
year as long as the total catch limit is not increased. These
percentages are: 4A-33%, 4B-39%, 4C-13%, 4D-13%, 4E-2%. In the
event that the total Area 4 catch limit is increased, the first
80,000 pounds would be allocated to Area 4E. The staff recognizes that allocation regimes are important in Area 4 and that some of the subareas were created primarily for allocation reasons. However, we continue to support the management policy of setting catch limits in proportion to biomass in Areas 4A, 4B and a combined Area 4C, 4D, and 4E. Our best estimate of the relative biomass in these three areas is very similar to that in the Council's catch sharing plan: 33.0% in 4A, 35.5% in 4B, and 31.5% in 4C-E.
Proposed Changes To The IPHC Regulations The staff makes the following recommendations for changes to the commercial fishery regulations.
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