Pacific Halibut

The halibut fisheries in the northeast Pacific Ocean are governed by a treaty entered into between the United States and Canada in 1923. Responsibility for conducting halibut population studies and establishing fishing quotas is that of the International Pacific Halibut Commission "IPHC". The Commission consists of six members, three each appointed by the United States and Canada. The offices of the IPHC and its staff are located on the campus of the University of Washington.

As can be seen in the graph to the right halibut catches dropped precipitously in the mid-1970's and early-1990's and increased rapidly in the mid-1980's and in 1997. The decline in halibut catches in the 1970's is related to a period of sustained bycatch by the foreign trawl fleets beginning in the 1960's and halibut growth rates began declining in the late 70's which is attributed to a cyclical but abrupt change in the climate of the North Pacific Ocean which became known as the "Pacific Decadal Oscillation".

The rapid increase in 1997's catch is most simply explained by the IPHC's decision to raise the catch limit by 36% - to 66,200,000 pounds from 48,660,000 pounds in 1997. The underlying explanation for this increase was based on "a new assessment of the halibut resource" driven by the logic that because the growth rate of halibut had significantly declined the population was much higher than previously estimated under the old assessment model which had been employed for many years.

"Since the climate regime shift of 1976-77 in the North Pacific, the individual growth of halibut (Hippoglossus stenolepis) has decreased dramatically in Alaska but not in British Columbia. Recruitment has increased dramatically in both areas. The decrease in age-specific vulnerability to commercial longline gear resulted in a persistent underestimation of incoming recruitment by the age-structured assessment method (CAGEAN) that was used to assess the stock. This problem has been corrected by adding temporal trends in growth and fishery selectivity to the assessment model. . . .

In Alaska, where the trends in commercial catch rates closely matched survey results but the change in size at age was dramatic, the increase resulted mainly from relaxing the assumption of a constant age-specific commercial selectivity. Recent recruitment, which had appeared to be plummeting, now appears to be soaring (Fig. 6).

At this point we cannot say which model is more appropriate. The staff’s quota recommendations for 1997 were based on the age-specific model mainly because the biomass estimates were a little lower and we preferred to err on the low side. It is somewhat ironic that with all the information about the halibut stock that is available to us, dating back to the 1920s, we are unsure of our assessment because we do not know how to interpret the survey data. In a few years, of course, when the recently recruited year-classes in Alaska have passed through the fishery, we will be better able to estimate their size and in turn find out which assumption about survey selectivity is more likely to be true." Decadal changes in growth and recruitment of Pacific halibut (Hippoglossus stenolepis) William G. Clark, Steven R. Hare, Ana M. Parma, Patrick J. Sullivan, and Robert J. Trumble

Even as these significantly higher harvest quotas were being advocated under the new model, its proponents acknowledged that the increased harvest could not likely be sustained over a long term:

"The 1987 year class of halibut, although small in individual 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. . . . [emphasis added]

Curiously, IPHC's staff noted in this same report that it had "caution[ed] the industry that the new assessment method need[ed] further testing and that additional research surveys [were] needed to confirm trends in abundance and the relative distribution of halibut among regulatory areas." . . and that it 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". http://www.iphc.washington.edu/halcom/pubs/annmeet/1997/Proposals1997.htm

The IPHC staff had proposed a thorough peer review of its new stock assessment method for the spring of 1998. Because of the dramatic changes implied by the results, the review was moved up to the week of September 29, 1997. The three panel members were all senior assessment scientists: Dr. Joseph Horwood of the fisheries laboratory at Lowestoft in the United Kingdom, Dr. Victor Restrepo of the University of Miami and the National Marine Fisheries Service, and Mr. Stephen Smith of the Department of Fisheries and Oceans in Halifax. Below are excerpts from the report submitted by the panel.

INTRODUCTION . . . . . .

"It was evident at an early stage that there was significant complexity in: the biology of the halibut and its environment; the development of the halibut fishery and its regulation; the collection and analysis of data; and modeling and assessments. Consequently the Review Group considered it was not possible, in the time available, to develop an adequately broad and deep understanding to enable a full critique of all aspects of the work undertaken by IPHC . . . [Emphasis added]

MODELING AND ASSESSMENT The annual and developmental movements of the halibut are relatively complex. Adults from all management and assessment areas congregate to spawn on winter spawning grounds, and juvenile recruitment is dependent upon ocean transport systems. The Review Group was told that once fish had recruited to a management area then they were relatively faithful to that area.

The protection of fish on the winter spawning grounds removes one element of complexity . . . . , but bycatch of juveniles in northern groundfish trawl fisheries may have had an impact upon recruitment to other areas. . . . There is also some indication that fishing can be concentrated on the boundary of management areas, and this may pose a problem for assessment. . . ..

With respect to within-data-set results, it is apparent that there exist data-model inconsistencies. . . . Because of the complexity and high parameterization of the model, it is unreasonable to expect that fit diagnostics alone would aid in determining whether one assumption about survey selectivity is superior to the other, or superior to competing assumptions that were not examined.

The size at full selection for the commercial gears points to a possible inconsistency between how selectivity and size are perceived (and modeled) to interact, and the information contained in the data sets: For area 2AB, this size at full selection is substantially lower than it is for the other areas, and is close to the minimum size of 81 cm. Thus the argument that selectivity has changed due to changes in growth and that this has made young fish less available does not seem to hold equally for all areas. There seems to be an inconsistency between data sets.

Another peculiarity highlighted in the comparisons between data sets (Figure 1) is that of the estimated catchabilities for the commercial gear: The catchability for area 3B is about four times larger than it is for other areas. Since the CPUE data for areas 3A and 3B are roughly similar, the consequence of the difference in catchability estimates is that the biomass in area 3B is perceived to be much lower than that of area 3A. The Review Group was unable to reconcile these differences in light of all the information presented. Several aspects could play a role, notably migration between areas 3A and 3B and the lack of survey CPUE data for area 3B.

SYNTHESIS . . . . The previous assessment model (CAGEAN) fitted a standard fisheries population model to data on catch-at-age and commercial catch-rates (CPUE). This model gave results which, it was argued (via a inspection of "retrospective" analyses), tended to underestimate stock biomass.

Further, a large decrease in length and weight at age is observed in recent times. It was postulated by the IPHC that this would have an effect on the selectivity of fish at age. The CAGEAN model assumed that selectivity at age remained constant over the time period of the analyses.

Driven by these two arguments the IPHC developed a new model which attempted to describe the change in selectivity-at-age over time. It was assumed that smaller fish-at-age were less susceptible to capture. Consequently a given catch-at-age indicated more fish in the sea as fish became smaller over time. It is this key feature that gives the main difference in the estimated magnitude of the population between the two models.

The Review Group identified some reasons why the new model should be used with caution. [Emphasis added]

The argument for a change in selectivity is based on a presumption, and there is limited empirical evidence for such change. Although the "retrospective" patterns from CAGEAN are far from ideal, a unique source of the patterns has not been established. . . . . The model incorporating growth changes and changes in selectivity is highly complex and the estimation procedure has to fit a large number of parameters (about 250). This complexity makes it difficult to understand the basic behaviour of the model, and risks the model being driven by noise in the data. . . . . Inspection of the results from the new model has revealed some peculiarities. Catchabilities (the likelihood of capture of the older ages) and selectivities vary significantly between areas. There are significant trends in other parameters described above. The result that young fish in area 2A/B are fully selected at a young age goes counter to the argument that selectivity has changed and young fish are less available.

The results from the new model thus pose taxing questions, which are elaborated further in the Advice sub-section.

COMMUNICATION The Review Group was asked to comment, as possible, on communication of IPHC results. . . . .The Review Group considered that communication to the Commission and user groups was an important consideration, especially at a time when assessment methods and results had been significantly changed. . . . The Review Group had received papers produced for the Commission. Many of these were very technical and the key issues can easily be buried in the complexity. At the same time, the detail produced was insufficient for a highly technical evaluation of the models and estimates. . . . .

The complexity of the current model, and the general evolution of more complex models, presents a problem of communication amongst experts. A key issue is the sensitivity of the models to a large array of "what if" assumptions. Any reviewer would wish to have available such examinations which are conducted as a matter of course by IPHC staff. The orderly documentation of these examinations, as they are conducted, would aid future examinations.

ADVICE TO IPHC Based upon the above approach and cognizant of the limited nature of the review the Review Group offers the following opinions:

The model development by IPHC staff has been innovative and fully appropriate in responding to concerns about their original model. However the new model is extremely complex in both its structure and estimation procedures. There appear evident concerns about the model's results as they indicate between and within area differences that are difficult to reconcile. Consequently the new model should be used with caution. [Emphasis added] . . . .

It follows that the IPHC staff need, as a matter of urgency, to develop a form of advice that adequately reflects, and is robust to, the current weaknesses in the alternative model approaches.

A key issue is the assumption that selectivity has changed with size or age. It is difficult to envisage that the new model can be fully accepted without external validation, by experiment or otherwise, of this key assumption." [Emphasis Added] SCIENTIFIC PEER REVIEW OF [1997] PACIFIC HALIBUT STOCK ASSESSMENT

Sure enough the IPHC soon concluded that its new model suffered from serious flaws as can be seen from its own summary of "Recent Changes in Assessment Methods and Harvest Policy" published in 2001 which appears below. To the right is a graph showing how the IPHC's estimates of exploitable biomass (EBIO) varied by area from model to model.

" Appendix B. Recent changes in assessment methods and harvest policy.
1982-1994: stock size was estimated with CAGEAN, a strictly age-structured model fitted to
commercial catch-at-age and catch-per-effort data. Because of a decrease in growth rates
between the late 1970s and early 1990s, there were persistent underestimates of incoming
recruitment and total stock size in the assessments done in the early 1990s.
Until 1985, allowable removals were calculated as a proportion of estimated annual surplus
production (ASP), the remaining production being allocated to stock rebuilding. In 1985 the
Commission adopted a constant harvest rate policy, meaning that allowable removals are
determined by applying a fixed harvest rate to estimated exploitable biomass. This harvest level
is called the Constant Exploitation Yield, or CEY. The fixed harvest rate was set at 28% in 1985,
increased to 35% in 1987, and lowered to 30% in 1993.
1995: a new age- and length-structured model was implemented that accounted for the
change in growth and was fitted to survey as well as commercial catch-at-age and catch-pereffort
data. The new model produced substantially higher biomass estimates. In Area 3A this
resulted from accounting for the change in growth schedule. In Area 2B, where the change in
growth had been much less than in Alaska, it resulted from fitting the model to survey catch-pereffort,
which showed a larger stock increase since the mid-1980s than commercial catch-pereffort.
Quotas were held at the 1995 level to allow time for a complete study of the new model
and results,
1996: differences in estimated selectivity between British Columbia and Alaska led to the
consideration of two alternatives for fitting the model, one in which survey selectivity was a
fixed function of age and the other in which it was a function of length. Spawner-recruit
estimates from the new model resulted in a lowering of the target harvest rate to 20%. Quotas
were increased somewhat, but not to the level indicated by the new biomass estimates.
1997: setline surveys of the entire Commission area indicated substantially more halibut
in western Alaska (IPHC Areas 3B and 4) than the analytical assessment. Biomass in those areas
was estimated by scaling the analytical estimates of absolute abundance in Areas 2 and 3A by the
survey estimate of relative abundance in western Alaska. CEY estimates increased again, and
quotas were increased again, but still to a level well below the CEY’s.
1998: the working value of natural mortality was lowered from 0.20 to 0.15, reducing
analytical estimates of biomass in Areas 2 and 3A by about 30%. At the same time setline survey
estimates of abundance in Areas 3B and 4 relative to Areas 2 and 3A increased, so biomass
estimates in the western area decreased by a smaller amount.
1999: setline survey catch rates in the 1990s were adjusted downward to account for the
effect of changing to all-salmon bait when the surveys resumed in 1993. This reduced biomass
estimates by 20-30%.
2000: the bait adjustment applied in 1999 was removed, which increased biomass
estimates by 30-40%, approximately back to the level in the 1998 assessment. In addition, a
purely age-structured model was adopted in place of the age- and size-structured model used in
1995-1999. The 2000 model produced similar estimates of present biomass but lower estimates
of historical biomass. . . . .

In summary, both of these models [1997/2000] assume constant age-specific survey selectivity, but they differ in implementation. The 1999 model was more rigid in its treatment of selectivity, and it attempted to predict size at age as well as catch rates at age. It now appears that these features caused some problems: the catch at age was predicted incorrectly, the estimated length-specific survey selectivity in recent years in Area 3A was not very credible (Clark and Parma 2000, p. 122), and in some cases the size at age was poorly fitted. [emphasis added] In contrast the 2000 model is more flexible and simpler. Its estimates of historical abundance are in close agreement with the catch-at age data, and its estimates of present abundance, while they may or may not be correct, are at least not affected by the simultaneous fitting of growth parameters."Assessment of the Pacific halibut stock in 2000, pp.29-30, 3 William G. Clark and Steven R. Hare

In 2006 the IPHC began what appears to be a process of re-allocating the catch of halibut from Area 2 to Area 3 based on a finding that not just juvenile halibut emigrate to Area 2 from Area 3 :

"For many years the staff has assessed the stock in each regulatory area by fitting a model to the data from that area (Appendix B). This procedure relied on the assumption that the stock of fish of catchable size in each area was closed, meaning that net migration was negligible. A growing body of evidence from both the assessments (Clark and Hare 2007) and the ongoing mark-recapture experiment (Webster and Clark 2007) shows that there is probably a continuing eastward net migration of catchable fish from the western Gulf of Alaska (Areas 3B and 4) to the eastern side (Area 2). The effect of this migration on the closed-area stock assessments is to produce underestimates of abundance in the western areas and overestimates in the eastern areas. To some extent this has almost certainly been the case for some time, meaning that exploitation rates have been well above the target level in Area 2 and a disproportionate share of the catches have been taken from there. http://www.iphc.washington.edu/halcom/pubs/rara/2006rara/2k6rara04.pdf, p. 97

"The closed-area assessments overestimate present abundance in Area 2 because in effect they include fish that are migrating to Area 2 from areas to westward. It could be argued that these really are Area 2 fish, so some degree of disproportionate harvest is appropriate. And to some degree it appears to be feasible. According to the present estimates, it would mean taking 25% of the coastwide yield from Area 2, which contains 16% of the coastwide biomass. This would not be a conservation issue for the stock as a whole. The fishery has been prosecuted in that fashion for decades, and it is probably sustainable, although harvest rates in the western areas (the source of the migrating fish) have been higher since 1996 than in previous years." http://www.iphc.washington.edu/halcom/research/sa/papers/sa06.pdf, p.2

In 2007 the IPHC began speaking more bluntly in terms which were no longer defensive of Area 2's historical catch patterns and it began to institute the reallocation of catch of halibut away from Area 2 to areas further west according to the conclusions it reached the prior year.

[A] disproportionate share of the harvest has been taken from Area 2 for decades, so some level of disproportionality was clearly sustainable by the stock with the exploitation pattern that prevailed during that period. Increasing catches from the western portion of the stock in the last decade have altered the exploitation pattern, so the historical high levels of removals from Area 2 may no longer be sustainable". http://www.iphc.washington.edu/halcom/research/sa/papers/sa07.bb.pdf, at p.4

It should be noted that the reallocation is taking affect more slowly than if the IPHC had immediately instituted a new model called the "Coastwide Assessment with Survey Apportionment" which was developed in 2006 and is advocated by its staff. Implementation of this new model would have accelerated the rate at which the reallocation would have occured in 2008 by further reducing the harvest quota in Area 2C from 6.21 million pounds to less than 4 million pounds - a 45% reduction in one year. Although adoption of quotas based on the new model would also have reduced Area 3A's quota by an additional 2 million pounds this would still amount to just a 15% overall reduction in that area.

Clearly, adoption of quotas based on the new " Coastwide Assessment with Survey Apportionment" model would not have been politically feasible because not only would they have resulted in cutting British Columbia's share to far less than half of what it was in 2007, it would have resulted in even more significant quota reductions in all areas except those west of Kodiak Island. Besides, just as in 1997, this new model does not withstand critical peer review.

C. Francis, on behalf of the University of Miami, was quick to point out the model's re-allocative effect and immediately questioned its propriety:

3.5 Area apportionment. Is the area apportionment procedure correct?

"For me, this was the weakest part of the assessment. I do not mean this statement to be a criticism of the IPHC staff, because the approach they took had a logic to it that is hard to escape. They started with the model that had been used previously (the closed-area model) and found that this was inconsistent with the recent information about migration (from the PIT tags). There were also some internal inconsistencies (e.g., the “2C/3B paradox” – see above). Their decision then to switch to a coast-wide model seems utterly reasonable (I doubt that it would have been possible to develop a more complex model, like that in Section 3.2.1, within the time available). Having done that, they needed a method to apportion the total exploitable biomass, as estimated from the coast-wide model, amongst the regulatory areas. The choice they made was the obvious one – to use the fishery-independent
survey data – and this required some assumption about the survey catchability in each area. The usual scientific approach is to apply Occam’s razor. That is, to make the simplest assumption that is consistent with the data. In this case that assumption is that the survey catchability is the same in all areas. This is not because there was strong (or even much) evidence of equality. Rather it is because there was no plausible way to estimate separate catchabilities by area. What is the best decision from a scientific point of view is not necessarily the best to use in managing a fishery. I think the Commissioners were prudent to reject the new assessment because it would have led to a substantial change in the allocation of quota amongst regulatory areas. While there were reasonably good grounds to believe that the previous method of allocation (using the closed-area assessment) was flawed, the evidence to support the new method was weak. In such a situation a pragmatic approach is to stay with something like the status quo until the scientific picture becomes clearer. I have four reasons to doubt the assumption of area-independent survey catchability, though none is strong, and none leads to a clear alternative assumption. The first derives from the area-specific recapture rates for PIT tags in the 2006 survey (bottom part of Table 3, sa06.pdf). A simple chi-square test applied to this table shows a highly significant departure from the assumption of equal catchability (P = 0.005) [I could not understand the assertion that this was only “marginally significant” (bottom of p. 9, sa06.pdf)]. The second reason is the omparison between setline and trawl CPUE in Areas 3A, 3B, and 4A (Fig. 3, prospect.pdf). This was used by IPHC staff to reject the closed-area model, which implied that setline survey catchability was much lower in Area 3A than in 3B and 4A (Fig. 1, prospect.pdf). However, the same data also rejects the assumption of area-independent catchability because, for example, it indicates that setline catchability in Area 3B was 50-60% of that in 4A (the fact that the plotted 3B points in Fig. 3 are lower than those for 4A in all eight length classes makes this difference statistically significant regardless of the associated confidence intervals). I do not consider this strong evidence, because it requires the assumption that trawl catchability is the same in 3A, 3B, and 4A, and I am less willing to make that assumption than the IPHC staff seem to be. The third reason is that the recent Area 2 exploitation rates estimated using the assumption of area-independent survey catchability are very high (see Fig. 6, p. 157, 2k6rara04.pdf), particularly in Area 2B.
It seems hard to believe that the effects of such high exploitation rates would not have been noticed before now, particularly by fishers. Finally, I note that catchability is a function of both fisher behaviour and fish behaviour. Much is done to ensure that fisher behaviour is the same throughout the survey area, but it is not possible to standardise fish behaviour. As the environment, biotic and abiotic, changes from Oregon via the Pacific coasts of British Columbia and Alaska to the Bering Sea, I would expect halibut behaviour to change in response, and this change may well affect its catchability.

. . . . While there is clear evidence that the previous area apportionment procedure (using the closed-area assessments) is flawed, there is little evidence that the new procedure is correct, and some grounds to believe that it is not. [emphasis added] http://www.iphc.washington.edu/halcom/research/sa/papers/francis.review.pdf, pp. 16-18, 26

If you aren't already convinced that high catch limits in Area 3 over the past few years are responsible for reduced catch limits in Area 2 read this:

"Area 2
Area 2A, 2B and 2C indices are illustrated in Figures 7, 8 and 9, respectively. Total removals
have been very steady in all three areas for the past decade. In Area 2A, both commercial catch
and sport catch increased by a factor of two between 1996 and 2007 while bycatch declined by
more than 50% over the period. Commercial fishing effort tripled in 2A over the past decade,
remained level in 2B and has increased substantially in 2C the past few years. All three areas show
similar trends in the biomass indices. Commercial CPUE has declined 20-30% and survey CPUE
has declined around 50% in all three areas since the mid 1990s. The coastwide assessment with
survey partitioning estimates an exploitable biomass decline of 27% in 2A, 45% in 2B and 55%
in 2C. The closed area assessments in Areas 2B and 2C estimate an exploitable biomass decline
of 26% and 17%, respectively.

All the indices are consistent with a picture of a steadily declining exploitable biomass in Area
2. The reasons for the decline are likely twofold. The first is the passing through of the two very
large year classes of 1987 and 1988. Every assessment over the past decade has shown that those
two year classes were very strong in comparison to the surrounding year classes. Now that those
two year classes are 20 years old, their contribution to the exploitable biomass and catches has
sharply declined and the drop in biomass is to be expected as they are replaced by year classes of
lesser magnitude. A second factor relates to the relatively recent finding that eastward migration
apparently continues beyond the age of recruitment to the fishery. Prior to the mid 1990s, the
westward regions were relatively lightly fished. However, now that area 3B and westward are
fully engaged, there is a decrease in the number of fish that migrate into Area 2. Taken together,
the decline in exploitable biomass in Area 2 is understandable and is not cause for undue alarm.
However, under a constant exploitation harvest strategy, removals by the fishery must come down
as the biomass declines. Our present view of Area 2 is that harvest rates have been much higher
than the target rate of 0.20 over the past decade (Figure 6). Such a high target rate was sustainable
over decades but was possible only because of the low exploitation rates to the west. As that
situation has changed, it has become paramount that harvest rates be brought down to the target
harvest rate in Area 2." http://www.iphc.washington.edu/halcom/pubs/rara/2007rara/2k7rara03.pdf at p. 279

and if you're not convinced a reallocation is in the works read this:

"20. Will the change from closed-area assessments to a coastwide assessment with survey
apportionment have a significant effect on capital values?

The estimates of coastwide abundance from the two procedures are about the same, but survey
estimate of biomass in Area 2 is only about 15% of the coastwide total, whereas Area 2 has been
receiving about 30% of the coastwide total according to the closed-area assessments. A complete
implementation of proportional harvest according to the survey apportionment would therefore
reduce the yield associated with Area 2 shares by about half, with yield for shares in Area 3B and
4 increasing in value. However, we also estimate that the use of a constant harvest rate policy
in all areas would result in an increase of biomass in the eastern portion of the stock, so that the
current decreased proportion of the stock in the eastern portion would be only a transitory effect of
a survey-based apportionment. Historically, changes in yield associated with shares do not have a
direct relationship with capital value because of the change in ex-vessel price per pound that may
accompany any changes in yield per share. Increases or decreases in ex-vessel price per pound
associated with supply and demand can act to offset changes in yield per share.

21. What apportionment methods other than the survey method could be used?

The setline survey data are the best information available for estimating the distribution of
biomass among areas. Trawl survey data would be a possibility if we had comparable data in all
areas, but we do not now and never will, because some areas like 2C and 4B are untrawlable.
Commercial CPUE is available for all areas, but the comparison of commercial and survey CPUE
shows that commercial catchability varies greatly among areas. Commercial CPUE is ten times
survey CPUE in Area 2A, about three times in Area 2B, about the same in Area 2C, and so on.
These differences do not result from differences in survey catchability; they result from the fi shery
targeting good grounds more or less effectively while the survey covers the whole area.

The staff has examined several other methods of apportionment, including the historical
recruitment distribution as estimated by the closed-area assessments and historical fishery shares.
However, none of these other metrics for apportionment incorporates the objective standardization
of the survey metric. Historical recruitment estimates are subject to the same errors resulting
from migration as the closed area assessments. Historical fishery shares reflect the distribution
of fi shing effort and are subject to severe biases resulting from the distribution of fishing effort.
Using survey data for apportionment is not perfect, as we have noted, but it represents the most
objective measure currently available.

Over the long term, we believe yield should be distributed among areas in proportion to
biomass. Proportional harvest is standard practice in fi shery management for good reasons. It
protects the stock against disproportionate harvest of sensitive sub-components of the stock (e.g.
behavioral groupings), about which there may be little or no knowledge, but departures can and do
occur. The Commission has temporarily assigned catch limits that resulted in non-target harvest
rates when there have been signifi cant changes in either assessment methodologies or harvest
policies, as a transition to new harvest regimes. However, it now appears that a disproportionate
share of the halibut yield has been taken in Area 2 for some time resulting in very high exploitation
rates and lower biomass than would result from harvesting at the target rate.

22. How about estimating biomass distribution using a mixture of survey and commercial
CPUE?

The IPHC staff had considerable discussion on this proposal. The strongest objection to using
commercial data for apportioning biomass is that the raw data consistently show strong differences
in commercial and survey catchabilities among areas. The ratio of the two indices varies from 0.48
– 0.99 among areas and is consistent within areas, over time. Introducing commercial data into
the apportionment process will embed these biases. However, it can be argued that incorporating
some consideration of the commercial data could offset any temporal bias inherent in the survey
data, which are collected over only a short portion of the year in each area. On balance, the
strongly biased relationship of commercial and survey data convinces the staff to decline the use
of commercial data for this purpose.

23. Will an apportionment of yield based on stock distribution at the time of the survey really
achieve proportional harvest when fish migrate before, during, and after the survey?

Yes. The concern here is that if we estimate the correct stock distribution at the beginning
of the year and allocate yield accordingly, it will be necessary to fish a lot harder in source areas
of migration than in destination areas to catch the quotas, because fish will be leaving the source
areas and entering the destination areas during the year. That line of reasoning is correct, but the
disparity in fishing mortality rates that would result is small.
With a survey apportionment, we do not estimate stock distribution at the beginning of the
year but in the middle of the year. Results of emigration and immigration are therefore refl ected
in survey CPUE, and the rate of fishing mortality is the same in all areas. A full analysis of these
effects is posted at http://www.iphc.washington.edu/halcom/research/sa/papers/proportional.pdf.

24. The last three years of data in both closed-area and coastwide fits show declining CPUE
that is not fitted well. What’s the problem?

The coastwide fit actually tracks survey and commercial CPUE quite well, including the last
three years. It is true, however, that survey catchability declined in 2005 and again in 2006. We
can see this in model fi ts where the rate of fishing mortality in 2006 is fixed at various levels and
the corresponding series of survey and commercial catchabilities are estimated year by year. In
all cases survey catchability is seen to be quite variable among years and to decline in 2005 and
2006.

25. What is the desired distribution of spawning biomass? Will proportional harvest achieve
that distribution, or should it modified in some way?

Absent other compelling information, the desired distribution of spawning biomass would be
something akin to its distribution absent fishing. Simulation modeling across a range of fishing
and migration rates was conducted and reported in the 2007 RARA. The results showed that
proportional harvest, i.e., the same constant harvest in all regulatory areas maintained nearly the
same spawning biomass distribution as in the unfished state. The unbalanced harvest rates we now
believe to have been in effect for at least the past decade – 50% of the target rate in the western
areas and 150-200% in the eastern areas – leads to a substantial change in the distribution of
spawning biomass. Specifically, the contribution of the eastern areas to the distribution is greatly
decreased. At an annual migration rate of 0.06 and instantaneous fishing mortalities in the range
of 0.20-0.30, the contribution of areas 2B and 2C to the spawning biomass change from 44% in
an unfished state to an equilibrium value of 23-26% when the above described unbalanced harvest
rates are applied.

26. To what extent is migration influenced by fishing? In particular, are we seeing migration
from west to east because higher exploitation in the east has reduced densities there and
created openings for migrants?

The question of density-dependent exclusions has not been investigated for large-scale
population distributions. Certainly, studies of territorial fish in both tropical and temperate climates
show dominance-based hierarchies of occupation of prime feeding or breeding habitats. The
evidence of site fidelity seen in recoveries of PIT-tagged halibut from survey stations provides the
potential for such a spatially-explicit behavioral process in halibut. The ubiquity of competitive
exclusion as a biological process in populations suggests that higher densities of halibut repetitively
occupying the same spatial niches would result in shifts in recruitment patterns relative to periods
of lower population densities. Densities of halibut in the central Gulf of Alaska have been at
record levels over the past decade, also evidenced by lower growth rates. In conjunction with
higher exploitation rates in the eastern portion of the stock, it is reasonable to expect that migration
to this eastern region may be higher than it would be under either conditions of lower density in the
central Gulf, or lower exploitation rates in Area 2.

27. The survey apportionment assumes that halibut habitat is the same proportion of total
bottom area in all areas. Is that true?

The survey apportionment makes no assumption about halibut habitat, which is not well defi ned
in any case. The only assumption about habitat involved in the apportionment is that the survey
samples each habitat in proportion to its presence. Survey stations are distributed uniformly in all
areas, so they can be expected to sample different kinds of habitat in proportion to their occurrence
in each area. An area consisting entirely of good habitat will produce a high CPUE at all stations
and therefore a high average CPUE. An area consisting of half good habitat and half poor habitat
will produce a high CPUE at half the stations and a low CPUE at half the stations, so its average
CPUE will be much lower than that of the good area. Habitat differences are therefore reflected in
survey CPUE.

28. How about developing a model with explicit migration in which all of the area-specific
data are fitted with area-specific parameters?

Such a model would obviously be the ideal way to accommodate movement of fish. However,
it is critically dependent on precise knowledge of the rates of migration by all sizes of fish, at all
times, among all areas. Further, if there were any temporal or biomass dependence in such rates,
they would have to be estimated continuously. This would be a very large project that would
present a number of significant technical difficulties, but the main drawback is that we would not
be able to estimate the migration rates internally and the results would depend entirely on what
rates we assigned externally. Given the evident difficulties in generating reliable estimates for
all sizes of fish, it is highly unlikely that these rates could be known with precision sufficient for
making catch limit recommendations.

29. The Commission should recognize that allocation is not a purely biological issue and deal
with it by developing an allocation framework that considers both biological and policy
issues.

The Commission does recognize that allocation is a subsequent process to biomass estimation.
It has traditionally based catch limits on proportional harvest of the estimated biomass for each
area. While alternate policy-based allocation formulae are possible, the staff believes that they
would have to be consistent with the sustainable yield of the stock and, if the formulae were to
have an equitable basis, then they would have to be consistent with the sustainable yield for each
regulatory area as well. The staff does not believe such a policy-based approach will be functional
unless it has this sustainable basis.

Existing policy-based allocation formulae (e.g. the allocative Catch Sharing Plan (CSP) of
the Pacific Fishery Management Council for Area 2A) are implemented after the conservation
(sustainability) decision has already been made (i.e., the CSP works entirely within the catch limit
adopted external to the CSP). Ultimately, conservation and allocation can be separated but in the hierarchy of decisions, conservation and sustainability must be paramount."
http://www.iphc.washington.edu/halcom/pubs/rara/2007rara/2k7rara02.pdf at p. 116

Maybe it's time for all Southeast Alaska halibut user groups to ask S.E. legislators, city councils and borough assemblies why this should be allowed to go on.

Read and listen to what the commercial halibut groups have to say:

http://www.fvoa.org/wwJanuary2008.pdf

http://www.fvoa.org/wwMarch2008.pdf Plan looks pretty good for 3A - up to 45 million pounds by 2013 nearly 20 million more than ever. During same period 2C only gets back to where it was 2 years ago.

http://kcaw.org/modules/local_news/index.php?op=centerBlock&ID=14

For any analysts out there take a hard look at the trends in Area 3B's catch limits over the past 12 years.

The IPHC characterizes continuing eastward migration of adult halibut as a "recent finding"; however, the excerpt below shows that since 1985 it has estimated that 6.92% of fish aged 8-15 years migrate from Areas 3 & 4 to 2C. Note that the 1985 report says "the effect of including migration in the analysis was to increase biomass in Areas 2A , 2B, and 2C and to lower biomass in Areas 3A, 3B, and 4". In 2006, twenty one years later, the IPHC said just the opposite - that "[t]he effect of this migration on the closed-area stock assessments is to produce underestimates of abundance in the western areas and overestimates in the eastern areas". Which way is it?

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