Accounting for bycatch in management of the Pacific halibut fishery

International Pacific Halibut Commission
Post Office Box 95009
Seattle, Washington 98145

Manuscript accepted (Mar. 98) by the North American Journal of Fisheries Management

Since the 1960s, fisheries for other species of groundfish have caused an average of about 9,000 metric tons (mt) of Pacific halibut Hippoglossus stenolepis bycatch mortality every year, while directed catches have varied from 13,000 to almost 50,000 mt (both measured in round weight). Around half of the bycatch consists of juvenile fish caught in Alaska, some of whom would otherwise migrate south and contribute to the fishery in British Columbia. These interceptions have long been a difficult issue for the United States and Canada. At recent high levels of juvenile abundance, juvenile bycatch reduces coastwide recruitment by about 10%. The resulting yield loss, plus bycatch of adult fish, reduces yield to the directed fishery by about 11,000 mt per year. Migration modeling indicates that the yield loss due to bycatch occurs almost entirely in the area where the bycatch is taken. In particular, bycatch in Alaska reduces Pacific halibut yields in British Columbia by at most a few percent. During the 1980s and early 1990s, annual quotas in the directed Pacific halibut fishery were reduced by an amount equal to (or sometimes greater than) the amount of total Pacific halibut bycatch mortality, and the quota reduction was distributed among regulatory areas in proportion to Pacific halibut biomass. At present, the Pacific halibut quota in each regulatory area is reduced by the amount of adult Pacific halibut bycatch mortality in that area, and the target exploitation rate is adjusted downward (slightly) to offset the bycatch mortality of juveniles.

The Pacific halibut Hippoglossus stenolepis is widely distributed in coastal waters of the North Pacific from central California to the northern Bering Sea, and on the Asian side south to Hokkaido. Aboriginal peoples in North America have fished halibut for thousands of years, and a commercial longline fishery has been conducted in U.S. and Canadian waters for more than a hundred years. Since 1923 the stock has been studied and managed by the International Pacific Halibut Commission (IPHC), making this one of the longest- and best-studied groundfish stocks in the world. Halibut assessment and management have been described by Skud (1977a), Sullivan and McCaughran (1995), and Hoag et al. (1993).

Pacific halibut bycatch in other fisheries was negligible until the early 1960s, when major fisheries for other groundfish species began in the Gulf of Alaska and Bering Sea (Williams et al. 1989). Since then halibut bycatch mortality in those fisheries has averaged about 9,000 mt per year in round weight (Fig. 1). The actual bycatch is somewhat greater, but all of the halibut taken as bycatch must, by law, be returned to the sea and some survive. Total halibut bycatch and the survival rate of discarded fish are estimated by shipboard observers. This paper will consistently refer to the quantities of halibut that are killed in the bycatch fisheries, and it will consistently refer to both directed catch and bycatch amounts in round weight. (In many IPHC reports, catches and even stock size estimates are usually stated in net weight, meaning headed and gutted. Net weight is 75% of round weight.)

Both the United States and Canada have adopted a number of management measures over the years to limit the bycatch of Pacific halibut in other groundfish fisheries. At present the total annual bycatch mortality in Alaska is capped at 7,000 mt. This total is distributed as bycatch quotas among a number of fisheries and management areas, which are closed when the bycatch quota is reached. (In some years the bycatch quotas have been inadvertently exceeded, but not by much.) Canada first imposed similar (in fact stricter) controls in 1996, which had the effect of reducing annual halibut bycatch in British Columbia from its previous average of about 1,000 mt to only 200 mt. There are as yet no halibut bycatch control measures in the groundfish fisheries off Washington and Oregon.

Annual quotas in the directed Pacific halibut fishery since 1960 have varied from as little as 13,000 mt (in the mid-1970s) to almost 50,000 mt (in the late 1980s). The IPHC has routinely taken bycatch into account when setting quotas, but the process has been complicated and contentious because of uncertainty about the migration of juvenile halibut and the resulting distribution of bycatch impacts among IPHC regulatory areas.

During summer Pacific halibut are distributed over the entire continental shelf of the northeast Pacific, although the distribution is patchy at the northern and southern ends of the range. Adult fish congregate and spawn in winter on spawning grounds in deep water on the continental slope scattered throughout the Gulf of Alaska from the Queen Charlotte Islands to the Aleutians, and into the Bering Sea (Fig. 2). The eggs, larvae, and postlarvae drift to the west in deep currents for about six months, then settle out and metamorphose on nursery grounds in the western Gulf of Alaska (west of Kodiak Island) and the eastern Bering Sea (and possibly along the coast of Russia). The juveniles spend some time on the nursery grounds, then migrate to home areas throughout the Gulf of Alaska and Bering Sea, as far south as Washington and Oregon, arriving by age 7 or 8. Thereafter movement is minimal except for the winter spawning migration (Skud 1977b). Females reach sexual maturity at age 10-12; males sooner. It is not known whether the juveniles that migrate to a given home area are the progeny of that area’s spawners.

The migration of juveniles from the western nursery grounds to their home areas appears to be a unidirectional movement. Juvenile fish marked in the northern Gulf of Alaska have been recovered there, and to the south, but hardly ever from the western Gulf or Bering Sea. Similarly, juveniles marked in British Columbia are almost always recovered in British Columbia (or farther south), and almost never in Alaska.

The commercial Pacific halibut fishery is conducted mainly in spring and summer when the adult fish are feeding on the continental shelf in their home areas. Owing to the selectivity of longline gear for larger fish and the commercial minimum size limit, the commercial catch consists almost entirely of fish eight years old and older, i.e., fish that have completed their migration.

Pacific halibut bycatch in other fisheries includes both adult fish that have completed their migrations and juveniles that may or may not have completed their migrations. The bulk of the trawl bycatch, for example, is made up of fish ages 4-6. Many of the juveniles in the bycatch are therefore fish that have been intercepted in the course of their migration, and their capture has the effect of reducing recruitment to the adult stock not in the area of capture but in the home area that they would have migrated to.

Despite a recent decrease in growth rates, the age at which juvenile halibut complete their migration (7-8) still corresponds approximately to the minimum size limit in the commercial fishery, which since 1974 has been 81 cm. As a reasonable approximation, therefore, bycatch of fish above the legal size limit can be considered to affect only the stock in the area where the bycatch is taken, whereas bycatch of sublegal fish may affect the stocks in other areas as well by reducing recruitment there.

Both directed catches and bycatch were quite stable in the first half of the 1990s. The catches in weight in 1995 are shown as an example in Table 1, and the length frequencies in Figure 3. Coastwide, bycatch in weight was about a third as much as the directed removals, and was almost equally divided between legal sized fish and sublegals, but there were large differences among areas. Legal sized bycatch was more than half as much as directed catches in Area 2A and Area 4, and more than half of the coastwide bycatch of sublegals was taken in Area 4, mainly in the Bering Sea trawl fisheries.

Management of the directed fishery by the IPHC is based on the distribution of legal sized fish in summer, when the fishery takes place. That is, a management stock consists of the adult fish that feed in a given home area in the summer (and remain there year after year, leaving only for a winter migration to a spawning area on the continental slope). Each year the IPHC sets a quota for each of the regulatory areas in Figure 2. In doing so it has to allow for the effect of bycatch in all areas on the stock in each area, which in the case of sublegal bycatch is not obvious. For example, the effect of a given level of bycatch in the Bering Sea on recruitment to British Columbia clearly depends on the age composition of the bycatch, the migration schedule of juveniles, and the abundance of eventual British Columbia recruits relative to eventual Alaska recruits in the Bering Sea. There is a good deal of uncertainty about all of these.

This paper reviews knowledge of the distribution and migration of juvenile Pacific halibut, outlines a model of juvenile migration, and reports area-by-area impacts of juvenile bycatch corresponding to a range of plausible migration schedules. The last section reviews how the IPHC has actually adjusted quotas to account for bycatch in the past, and how it does so at present.

Despite having released and recovered large numbers of marked juvenile Pacific halibut over the years (Trumble et al. 1990), the IPHC staff has not been able to obtain reliable estimates of migration rates from marking data. The most ambitious effort along these lines was the release of some 55,000 marked juvenile halibut in the northern Gulf of Alaska in 1980-81. A thorough analysis of the recovery data (Hilborn et al. 1995) showed that, in statistical terms, migration rates were highly confounded with other factors that affect recovery rates in different areas and years, such as fishing effort, selectivity, tagging mortality, and reporting rates by fishers. For example, there were fewer recoveries than expected from Southeast Alaska (relative to the number of recoveries in southcentral Alaska and British Columbia), but that was probably just a result of having fewer IPHC port samplers on hand in Southeast Alaska to redeem tags. Similarly, there was an unexplained increase in recovery rates in the first few years after the IPHC began to give out coveted tag reward hats instead of modest cash payments when redeeming tags. These factors are indistinguishable from migration in their effect, with the result that migration rates are poorly determined by the recovery data.

While marking has not yielded all the information that was hoped for, it has provided some information, and there are other data sources that reveal some of the general features of juvenile distribution and migration, as summarized below.

The general counterclockwise circulation in the Gulf of Alaska can be expected to result in most Pacific halibut postlarvae settling to the bottom from about Kodiak westward. That is about how far the currents would carry a halibut hatched in the Queen Charlotte Islands (the southeasternmost major spawning area) during the six months that halibut spend in midwater as eggs, larvae, and postlarvae. This hypothesis was corroborated by an ichthyoplankton survey in May-June 1986 in which stations were fished from Dixon Entrance (above the Queen Charlotte Islands) right around the Gulf of Alaska and into the Bering Sea. Significant numbers of halibut postlarvae were caught only at stations around Kodiak and to the west, into the Bering Sea (St-Pierre 1989).

The National Marine Fisheries Service of the United States (NMFS) conducts a trawl survey of the eastern Bering Sea every year, and of the Gulf of Alaska every third year. Along with some large Pacific halibut, the survey trawl catches large numbers of age 2-6 fish, which are not caught by the setline gear used in IPHC surveys, so the NMFS trawl surveys supply information on the distribution of juveniles that is not available in any IPHC data. The relative abundance by INPFC region of age 2-4 halibut in the last four Gulf survey years is depicted in Figure 4. As expected, very few small halibut were caught east of Kodiak Island. This is not to say that none are there; IPHC trawl surveys have found small halibut in the Yakutat, Southeast and Charlotte regions. But the Gulfwide NMFS survey results indicate that the proportion of juveniles east of Kodiak is small.

Another interesting feature of the graphs is that in most years the Bering Sea accounted for only a small part of the total juvenile abundance, although it was long considered to be the major nursery area. The exceptional year was 1990, when the large 1987 year-class was present as 3-year-olds. Overall, the proportion of juvenile Pacific halibut in the Bering Sea for the four survey years was about 40%, but if 1990 is excluded, the proportion drops to 20%. In 1990, about 60% of the juveniles were located in the Bering Sea. It is possible that during periods of high recruitment a higher proportion of juveniles are in the Bering Sea. (It is also possible that periods of high recruitment are precisely the periods when large numbers of postlarvae are swept into the Bering Sea.)

As mentioned above, recoveries of marked juveniles indicate that the migration from nursery grounds in western Alaska to their eventual home areas is unidirectional. Juveniles marked in the Bering Sea (Area 4) have been recovered from all parts of the range. Juveniles marked in the western Gulf of Alaska (Area 3) have been recovered there and to the east and south, but almost never from the Bering Sea or Aleutians (Area 4). Similarly, juveniles marked in British Columbia (Area 2B) have been recovered there and to the south, but almost never from Alaska. It therefore appears that early in life, say at age 2, the eventual recruits to Area 4 are all located there, while the eventual recruits to the management stocks in Areas 2 and 3 are distributed between Area 3 and Area 4 in some proportions.

The eventual Area 2 and Area 3 recruits may be unevenly mixed at age 2 on the nursery grounds in Area 3 and Area 4. For example, it is plausible that at age 2 the future Area 2B recruits, having been spawned in northern British Columbia and southeast Alaska, will be found mostly in the western Gulf rather than the Bering Sea, while a higher proportion of future Area 3 recruits, having been spawned in the northern and western Gulf, will be found in the Bering Sea. If that were true, future Area 2B recruits would be less numerous relative to future Area 3 recruits in the Bering Sea than in the Gulf. Consequently they would be relatively less numerous among recoveries of adult fished marked as juveniles in the Bering Sea than among recoveries of adult fish marked as juveniles in the Gulf. But there is no difference in relative recoveries: releases of marked juveniles in both the Bering Sea (Area 4) and the Gulf (Area 3) resulted in about three times as many recoveries from Area 3 as from Area 2B (Table 2). This implies that future Area 2 and Area 3 recruits are well mixed (i.e., distributed in the same proportions) on the nursery grounds in the western Gulf and the Bering Sea.

Every year the IPHC staff estimates the abundance of Pacific halibut (ages 8+) in each regulatory area by fitting a size- and age-structured model to historical data series from the commercial fishery and IPHC setline surveys. Among other things, the annual stock assessment provides estimates of year-class strength at age 8 in each IPHC regulatory area (i.e., for each management stock), and therefore shows the eventual distribution of Pacific halibut after they have completed their migration from the nursery grounds to their home areas. The assessment does not produce estimates of juvenile abundance (ages 1-7) because fish younger than age 8 are quite rare in both survey and commercial setline catches.

The relative abundance of 8-year-olds among regulatory areas does not appear to have varied greatly since the early 1980s (when there was an increase in abundance in Alaska relative to British Columbia). The most recent estimates, based partly on the assessment itself and partly on survey coverage of lightly fished areas, indicate that Area 4 receives about 30% of the total number of 8-year-olds, Area 3B 20%, Area 3A 30%, Area 2C 10%, Area 2B 10%, and Area 2A 1%.

All of the information above indicates that all but a small proportion of juvenile halibut are in the western Gulf and the Bering Sea at age 2, and that the future recruits to Areas 2 and 3 are well mixed throughout the nursery grounds. (All of the Area 4 recruits are restricted to nursery grounds in Area 4.) The proportion of juveniles that settle out in the Bering Sea appears to be variable, with a range of 20% to 60% and an overall average of 40%. Migration to the summer feeding areas is unidirectional, taking place between ages 2 and 8. The eventual distribution of 8-year-olds among areas is reasonably well known from the annual stock assessment. The timing of the migration of each area’s recruits from the nursery grounds to the summer feeding area is not known.

In order to calculate the impact of bycatch in a given area on the various management stocks, we need to know the stock composition of the bycatch in that area. In the case of the Bering Sea (Area 4) bycatch, for example, we need to know what proportion of the fish in the bycatch are bound to migrate and recruit to British Columbia (Area 2B), what proportion to Southeast Alaska (Area 2C), and so on. But it is more complicated than that, because the impact depends on the size composition of the bycatch, and the Area 2B juveniles in the Bering Sea must have a different size composition from, say, the Area 4 juveniles. That is because the Area 2B juveniles must migrate out of the Bering Sea over time while the Area 4 juveniles accumulate there, with the result that the Area 2B juveniles in the Bering Sea are smaller on average than the Area 4 juveniles, both in the sea and in the bycatch.

To calculate impacts, therefore, we need to know the stock composition of the juveniles in each size group in the bycatch in each regulatory area (or, equivalently, the relative abundance and geographic distribution of each age group of each stock’s juveniles). This is a level of detail far exceeding what we now know or may ever know about the initial distribution of juvenile Pacific halibut and the timing of their migration between ages 2 and 8. Doing the calculations therefore required setting up a range of migration schedules that incorporate the known general features of juvenile migration summarized above, and that bracket a plausible range of values for initial distribution and migration timing. We have used the best information available, but this part of the exercise is inevitably uncertain and sure to remain so.

As an example, Table 3 shows one of the schedules constructed to describe the migration of Area 2B recruits from nursery grounds in western Alaska to Area 2B. Each row of the table gives the proportional distribution of a particular age group among regulatory areas; each row sums to one. Initially, at ages 1 and 2, the bulk of the fish are in Area 4 (32.5%) and Area 3(51.5%). By age 7 most them have reached Area 2B (75%), and by age 8 all are there. The important features of a schedule like this are:

Using similar reasoning, we drew up four alternative location schedules to encompass the plausible range of juvenile migration schedules for each management stock. The four schedules represented the extreme values of two tunable parameters: degree of “Beringness” and speed of migration to the home area (“quickness”). The Beringness parameter was used to set initial proportion by area with the two extreme settings referred to as Bering and Local. At the Bering end, the maximum proportion of 2-year-olds was placed in the Bering Sea and the minimum in the home area. At the Local end, the reverse was true. The quickness parameter was used to set how quickly the juveniles would move from the nursery grounds to their home areas, and ranged from Slow to Fast. At the Slow end, fish linger in the nursery grounds in Area 4 and Area 3, and move slowly. At the Fast end, they get under way earlier and move faster. Calculated out-of-area bycatch impacts are greatest under the Bering Slow schedule, and least under the Local Fast schedule. We also constructed an Intermediate schedule, which was simply the midpoint of what we regarded as the plausible range of the two tuning parameters.

We used the migration model and recent estimates of recruitment at age 8 to each management stock (regulatory area) to calculate stock-specific impacts of the 1995 bycatch. Basically we took a single year-class and simulated its migration from ages 2 through 8, calculating how many juveniles of each management stock would be lost to bycatch in each regulatory if the 1995 bycatch amount and size composition were repeated every year in every regulatory area. This is a fair estimate of bycatch impacts in the early 1990s because neither bycatch nor recruitment varied greatly during those years.

Our migration model allocates bycatch mortality among management stocks by calculating the stock composition of every age group in the regulatory area where the bycatch is taken, and then apportioning the estimated bycatch mortality by age class among stocks in proportion to their abundance in that regulatory area. The age composition of the bycatch is estimated by applying a length-age key to the reported length distribution. Owing to the rapid growth of juvenile halibut, there is little overlap between the length ranges of successive age groups, so in this case a length-age key is reliable.

To keep track of the relative abundance by age of every stock in every regulatory area, we begin with the estimated absolute number of each stock at age 1, distributed according to one of the migration schedules. At each age (1-8) we subtract natural mortality and bycatch mortality in each regulatory area, and then redistribute the fish according to the migration schedule to obtain the absolute abundance at the next age in every regulatory area.

In principle the calculations are straightforward, but in practice they are complicated by the effect of bycatch mortality on both the proportional distribution of age groups among areas and on the relative abundance of the various stocks as juveniles.

The location schedules (e.g., Table 3) developed earlier are reasonable when viewed as examples of how juveniles would migrate in the absence of bycatch. Each schedule implies a set of age- and area-specific migration rates, and using those rates in the absence of bycatch will generate the given location schedule. For example, the location schedule of Area 2B recruits in Table 3 shows that between ages 3 and 4, the proportion of 2B juveniles present in Area 2B itself will rise from 0.19 to 0.30, an increase of 0.11. Under unidirectional migration, this increase will be drawn first from Area 2C, then from Area 3 if needed. Only 0.078 are in Area 2C at age 3, so the migration rate of 2B recruits from 2C to 2B at age 3 is 100%. An additional 0.032 (= 0.11-0.078) must come from Area 3, where 0.47 are located at age 3, so the migration rate of 2B recruits from Area 3 to Area 2B at age 3 is 0.032/0.47 = 6.8%.

But using those same migration rates and allowing for the actual age- and area-specific bycatch will result in a different location schedule. For example, a heavy bycatch mortality in Area 4 at age 5 will affect the distribution of age 6 fish among areas, and that has to be allowed in the calculations. We do that by using the migration rates implied by each location schedule rather than the location schedule itself to calculate the distribution of each stock at each age sequentially.

Estimates of the absolute abundance of each stock at age 8 are available from the annual stock assessment. The absolute coastwide abundance at age 1 at any given level of bycatch can be back-calculated by a simple cohort analysis, but the absolute abundance of each stock at age 1 will depend on the distribution of bycatch impacts among stocks. Because the calculation of bycatch impacts requires an estimate of the absolute abundance of each stock at age 1, the model has to be run iteratively to locate the stock-specific values of abundance at age 1 that will result in the correct values at age 8, given the migration schedule in effect and the distribution of bycatch by area and size. The procedure is similar to (but simpler than) the measurement error model of migratory catch-age analysis described by Quinn et al. (1990).

In effect, the model estimates stock-specific abundance at age 1 by cohort analysis (back-calculating from age 8). A migration schedule that allocates more bycatch mortality to a given stock increases the removals that go into the cohort analysis, and therefore the estimate of abundance at age 1. An important consequence of this part of the calculations is that the estimates of stock-specific bycatch mortality rates turn out to be much less variable (among migration schedules) than absolute bycatch impacts, because higher absolute impacts generate higher stock-specific estimates of abundance at age 1.

The effect of the 1995 sublegal bycatch mortality on recruitment to each stock, for each of the migration schedules discussed above, is shown in Table 4. Coastwide, juvenile halibut suffer a bycatch mortality of 1-3% per year, with the higher rates falling on ages 4-6. The cumulative effect over ages 2-8 is to reduce year-class strength by about1.8 million 8-year-olds, or by about 10% at recent levels of recruitment. The impact does vary substantially among areas, but it is not very sensitive to the migration schedule used for the calculations. Sublegal bycatch reduces recruitment to the Area 4 stock by 15-20%, but to the Area 2 and 3 stocks by only about 5-10%. For every stock except 2C (where bycatch is very low), most of the recruitment loss due to sublegal bycatch results from sublegal bycatch in that area itself, rather than from interceptions of migrating juveniles in other areas. In particular, even under the Bering Slow migration schedule, which maximizes interceptions, the sublegal bycatch in Area 4 only reduces recruitment in Areas 2 and 3 by about 3%. Similarly, sublegal bycatch in all of Alaska (Areas 3 and 4, plus 2C) reduces recruitment to British Columbia (2B) by at most about 5%, while at the 1995 bycatch level, sublegal bycatch in British Columbia itself reduced recruitment by about 8%.

The yield loss to the Pacific halibut fishery resulting from bycatch consists of two parts: the yield that would have been obtained from the sublegals in the bycatch if they had survived and recruited to the fishable stock, and the immediate loss of legal sized fish in the bycatch. Among sublegals growth exceeds natural mortality, so the yield loss in weight is somewhat larger than the bycatch mortality in weight. (The scaling factor depends on the size composition of the bycatch, so it varies among areas and years.) These two components of yield loss, and their total, are shown for each stock in Table 5. The range of values shown for the sublegal component in each area results from uncertainty about the migration schedule and consequently the distribution of the yield loss, not from any uncertainty about the total (coastwide) yield loss. That total (6,136 mt) is the same for all migration schedules, as is the legal sized bycatch in each regulatory area.

Interceptions of migrating juveniles account for at most about 15% of the total yield loss due to bycatch (1,506 out of 11,152 mt), and probably much less. The rest of the yield loss results from bycatch of sublegal and legal sized fish in their home areas. Roughly speaking, therefore, it can be said that around 90% of the yield loss due to bycatch occurs in the area where the bycatch is taken.

Since the early 1980s, the IPHC harvest strategy for the directed Pacific halibut fishery has been to apply a constant exploitation rate (formerly 30-35%, presently 20-25%) to the estimated exploitable biomass in each regulatory area. This produces a recommended level of total removals from each area. Sport and subsistence catches, wastage in the halibut fishery (due to lost gear), and some kind of bycatch adjustment are then subtracted from the total to arrive at the recommended commercial setline quota in each area. (Strictly speaking, this is only the staff’s recommendation. The Commissioners can and do depart from the staff’s recommendations in some cases, but seldom by very much.)

The target exploitation rate is based on estimates of historical spawning biomass and subsequent recruitment of 8-year-olds throughout the Gulf of Alaska (Areas 2 and 3). The recruitment estimates include an upward adjustment for the recruits lost to sublegal bycatch. In other words, the spawner-recruit estimates show what the productivity of the spawning stock would have been in the absence of sublegal bycatch. Until 1997 the choice of a target harvest rate was similarly based on yield calculations that assumed there would be no sublegal bycatch in the future. The bycatch adjustment used in calculating quotas therefore had to correct for the effect of sublegal bycatch on stock productivity as well as the effect of interceptions. It also had to allow for the effect of bycatch of legal sized fish in each area.

From 1981 to 1996, the bycatch adjustment was a deduction from setline quotas of an amount equal to (or sometimes greater than) the current bycatch amount. This adjustment was called “bycatch compensation”, the idea being first to calculate what the quota would be with no bycatch, and then to “compensate” the stock for whatever level of bycatch was taken.

In the early 1980s the bulk of the bycatch was taken in foreign and joint venture trawl fisheries in the Bering Sea. The primary management concern was yield loss, and it was calculated that each kilogram of bycatch (sublegal and legal sized combined) resulted in an eventual loss of 1.58 kg of yield to the setline fishery, so the setline quotas were reduced each year by 1.58 times the current bycatch amount (Quinn et al. 1985). In the long term, the effect of this adjustment was to maintain exploitable stock biomass at the level it would have reached if fished at the target exploitation rate with no bycatch.

The setline quota reduction was calculated as a single coastwide total and distributed among regulatory areas in proportion to estimated exploitable biomass. The Bering Sea was then thought to be the nursery area for the bulk of the juveniles coastwide, and the bycatch consisted mainly of juveniles in the Bering Sea, so the yield loss due to bycatch would be roughly proportional to the biomass in each area. In principle it would have been preferable to use this kind of adjustment only for the sublegal component of the bycatch, and to reduce quotas kg for kg of legal sized bycatch in each regulatory area, but data on the length composition of the bycatch were lacking for a number of areas and fisheries, so this level of detail was not feasible.

At that time it was estimated that about 20% of the exploitable biomass was in Area 2B (British Columbia), so the 2B quota was reduced by an amount equal to about 20% of 1.58 times the coastwide bycatch mortality, even though only about 10% of the bycatch was taken in Area 2B. This substantial and disproportionate impact of U.S. (mainly Bering Sea) bycatch on Canadian setline quotas was a highly contentious issue for the IPHC and the governments.

In 1990, the aim of bycatch compensation changed from compensation for lost yield to compensation for lost egg production. The rationale was that the exploitation strategy was based on maintaining, on average, an optimum spawning biomass and therefore an optimum egg production, so the logical way to compensate for bycatch was to reduce the setline quota by an amount just sufficient to result in the same level of egg production as would have occurred had there been no bycatch. Calculations showed that the reproductive compensation factor was lower than the yield loss factor, averaging 1.00 kg of required compensation for each kg of bycatch mortality, so from 1990 through 1996 the setline quota was reduced by the amount of bycatch mortality kg for kg. The reduction was still calculated as a coastwide total and distributed among regulatory areas in proportion to the exploitable biomass in each area (Sullivan et al.1994). In the long term, the effect of this adjustment was to maintain the spawning biomass at the level it would have reached if fished at the target exploitation rate with no bycatch.

During the first half of the 1990s both countries implemented domestic observer programs that now provide good and reasonably complete information on the length composition of the bycatch. In addition, the lengthening series of NMFS trawl surveys showed that the western Gulf of Alaska rather than the Bering Sea usually held the bulk of juvenile Pacific halibut, and both NMFS and IPHC surveys indicated that total halibut abundance in the Bering Sea/Aleutians region was higher than had been estimated previously by the annual stock assessment. When used in the migration model described above, all of this information indicated that the impact of sublegal bycatch falls largely on the stock in the area where the bycatch is taken.

On the basis of these findings, the IPHC in 1997 adopted a new method of handling bycatch in setting quotas. The bycatch of legal sized fish in each regulatory area is now treated just like other non-commercial removals—as a quota deduction for that regulatory area. The mortality due to sublegal bycatch is now incorporated into the population model that is used to evaluate alternative exploitation rates, so an allowance for sublegal bycatch is contained in the chosen rate. There is no explicit adjustment for sublegal bycatch in the quota setting process.

As mentioned above, the cumulative prerecruit mortality due to sublegal bycatch is now 5-10% for the Area 2 and Area 3 stocks, and 15-20% for the Area 4 stock. The staff’s evaluation of alternative harvest rates is based on spawner-recruit data for Areas 2 and 3 combined, so a working value of 10% was used in developing recommendations for 1997.

As a practical matter, this addition to the population model has little influence on the eventual choice of a harvest rate. Its effect is to reduce the slope parameter of the estimated spawner-recruit relationship by 10%, and a difference of 10% is minor in comparison with the general uncertainty about the form and parameter values of the relationship. Moreover, in the vicinity of the optimum exploitation rate, yield is not very sensitive to the precise value of the exploitation rate, so a rate that is optimum for a prerecruit bycatch mortality of 10% will still perform very well if the true value is really 5% or 20%. This means that the chosen exploitation rate will perform well even if prerecruit bycatch mortality is only roughly estimated or variable among years and regulatory areas (as it is).

Bycatch in other groundfish fisheries has substantially reduced yield to the directed Pacific halibut fishery over the last few decades, and still does. The IPHC staff has estimated the long-term potential productivity of the stock as 30,000-40,000 mt per year, so at recent levels of bycatch the yield loss has amounted to about a third of potential production (11,000 mt/yr). Obtaining good estimates of bycatch mortality, and accounting for it somehow in setting quotas for the directed fishery, are clearly essential to good management.

While the coastwide impact of bycatch is known, the distribution of the impact among regulatory areas is uncertain because the migration patterns of juveniles are uncertain. The bycatch compensation procedures used by the IPHC in the 1980s and early 1990s distributed quota reductions in proportion to biomass. There was some justification for this approach, and it certainly served its purpose for the stock as a whole, but it was quite controversial, mostly because of the disproportionate levy on the Canadian fishery to compensate for bycatch in Alaska. Distributing the quota reductions according to the results of the migration modeling reported above might be less controversial but probably not much less, because the underlying migration schedules are invented and at best they provide a range of values.

Another, more subtle drawback of the compensation procedures was that they were formulated and implemented separately from the harvest strategy, and their effect on yield was not considered when formulating the harvest strategy. In fact, however, the adjustment of quotas for bycatch did affect yield as well as biomass, and therefore was a part of the harvest strategy.

The main advantage of accounting for sublegal bycatch by including that mortality in the population model used to choose the target harvest rate, is that now the treatment of bycatch is an integral part of the harvest strategy. The effects of all sources of mortality on both biomass and yield are considered simultaneously, and the Commissioners can consider both when choosing a harvest rate that achieves the best balance of their management objectives, which include maintaining a healthy level of spawning biomass along with obtaining a high and stable yield. In equilibrium conditions, it can be expected that the addition of sublegal bycatch mortality to the population model would result in the choice of a slightly lower target harvest rate, but that might not happen when the stock is at a high level of abundance, as it is now.

Another advantage of the present procedure is that it does not explicitly reduce the setline quota in one regulatory area to account for bycatch in another regulatory area. The only explicit quota reduction is for the bycatch of legal sized fish within each regulatory area. That avoids some controversy, even though changing the procedure has in no way reduced the yield loss due to sublegal bycatch.

Now that the sublegal and legal sized components of bycatch are being treated separately, the impact of juvenile interceptions in Alaska on yield in British Columbia appears much smaller than it appeared to be during the 1980s, for several reasons. In those days all bycatch was treated as an interception, and there was a tendency to think of it as being all juveniles, even though then as now about half (by weight) was legal sized fish. Also, the migration modeling showed that for a range of plausible migration schedules, the impact of juvenile bycatch was largely confined to the area where the bycatch was taken. These model results were reinforced by recent survey results which indicate that the Area 4 stock is relatively larger than previously thought. In the workings of the model, that shifted more of the large juvenile bycatch in the Bering Sea to the Area 4 stock itself. Finally, the staff’s estimates of coastwide Pacific halibut abundance approximately doubled in 1997 when the assessment procedure was reworked to adjust for the large decrease in halibut growth that occurred between the 1970s and the 1990s. This effectively halved the estimate of the coastwide recruitment reduction due to juvenile bycatch, from 20% to 10%.

Hilborn, R., J. Skalski, A. Anganuzzi, and A. Hoffman. 1995. Movements of juvenile halibut in IPHC Regulatory Areas 2 and 3. International Pacific Halibut Commission Technical Report Number 31. Seattle, Washington.

Hoag, S. H., G. J. Peltonen, and L. L. Sadorus.1993. Regulations of the Pacific halibut fishery, 1977-1992. International Pacific Halibut Commission Technical Report Number 27. Seattle, Washington.

Quinn, T. J. II, R. B. Deriso, and S. H. Hoag. 1985. Methods of assessment of Pacific halibut. International Pacific Halibut Commission Scientific Report Number 72. Seattle, Washington.

Quinn, T. J. II, R. B. Deriso, and P. R. Neal. 1990. Migratory catch-age analysis. Canadian Journal of Fisheries and Aquatic Sciences 47:2315-2327.

Skud, B. E. 1977a. Regulations of the Pacific halibut fishery, 1924-1976. International Pacific Halibut Commission Technical Report Number 15. Seattle, Washington.

Skud, B. E. 1977b. Drift, migration, and intermingling of Pacific halibut stocks. International Pacific Halibut Commission Scientific Report Number 63. Seattle, Washington.

St-Pierre, G. 1984. Spawning locations and season for Pacific halibut. International Pacific Halibut Commission Scientific Report Number 70. Seattle, Washington.

St-Pierre, G. 1989. Recent studies of Pacific halibut postlarvae in the Gulf of Alaska and eastern Bering Sea. International Pacific Halibut Commission Scientific Report Number 73. Seattle, Washington.

Sullivan, P. J., R. J. Trumble, and S. A. Adlerstein. 1994. Pacific halibut bycatch in the groundfish fisheries: effects on and management implications for the halibut fishery. International Pacific Halibut Commission Scientific Report Number 78. Seattle, Washington.

Sullivan, P. J., and D. A. McCaughran. 1995. Pacific halibut assessment: changing strategies for changing times. Pages 129-144 in G. T. Sakagawa, editor. Theme 5: Assessment methodologies and management. Proceedings of the World Fisheries Congress. Oxford & IBH Publishing Company, New Delhi.

Trumble, R. J., I. R. McGregor, G. St-Pierre, D. A. McCaughran, and S. H. Hoag. 1990. Sixty years of tagging Pacific halibut: a case study. Pages 831-840 in N. C. Parker and five other editors. Fish-marking techniques. American Fisheries Society Symposium 7. Bethesda, Maryland.

Williams, G. H., C. C. Schmitt, S. H. Hoag, and J. D. Berger. 1989. Incidental catch and mortality of Pacific halibut, 1962-1989. International Pacific Halibut Commission Technical Report Number 23. Seattle, Washington. Tables

Table 1. Coastwide removals of Pacific halibut in 1995, by IPHC regulatory area, in mt round weight. The "directed catch" includes all directed commercial, sport, and subsistence catches.

IPHC area	Directed catch	Bycatch in other fisheries
		Legal sized	Sublegal

4 (Bering Sea/Aleutians)	3004	1989	3268
3B (Western Gulf of Alaska)	1811	581	478
3A (Northern Gulf of Alaska)	13869	1077	706
2C (Southeast Alaska)	6007	175	35
2B (British Columbia)	6627	425	492
2A (Washington/Oregon)	362	224	37
Total	31680	4471	5016

Table 2. Recoveries of halibut marked as juveniles in the Bering Sea (Area 4) and northern Gulf of Alaska (Area 3) and later recovered as adults in the directed setline fishery.

	Where marked
Where recovered	Area 4	Area 3
Area 4	354	0
Area 3	89	417
Area 2C	19	57
Area 2B	31	136
Area 2A	10	9

Table 3. Sample schedule describing the migration of Area 2B recruits from nursery grounds in western Alaska (mostly) to Area 2B. Each row of the table gives the proportional distribution of that age group among regulatory areas; each row sums to one.

	Proportion of age group located in:
	Area 4	Area 3	Area 2C	Area 2B	Area 2A

At age: 1	0.325	0.515	0.055	0.105	0
2	0.325	0.515	0.055	0.105	0
3	0.263	0.470	0.078	0.190	0
4	0.188	0.425	0.088	0.300	0
5	0.125	0.363	0.100	0.413	0
6	0.087	0.250	0.100	0.563	0
7	0.025	0.175	0.005	0.750	0
8	0	0	0	1.00	0

Table 4. Proportional recruitment reductions due to sublegal halibut bycatch in 1995. "Bering Slow" is the migration schedule that maximizes interceptions, "Local Fast" is the migration schedule that minimizes interceptions, and "Intermediate" is just that. "Stock" refers to a summer feeding stock, so for example the first table says that under the Bering Slow migration schedule, juvenile bycatch taken in Area 4 reduces recruitment to the Area 4 stock by 0.150 (15%) and to the Area 2B stock by 0.032 (3.2%).

	Area where sublegal bycatch was taken
Stock	4	3	2C	2B	2A	Coastwide

Area 4	0.150	---	---	---	---	0.150
Area 3	0.032	0.050	---	---	---	0.082
Area 2C	0.031	0.019	0.002	---	---	0.052
Area 2B	0.032	0.014	---	0.081	---	0.128
Area 2A	0.033	0.015	---	0.009	0.067	0.124

	Area where sublegal bycatch was taken
Stock	4	3	2C	2B	2A	Coastwide

Area 4	0.180	---	---	---	---	0.180
Area 3	0.014	0.053	---	---	---	0.067
Area 2C	0.014	0.014	0.002	---	---	0.030
Area 2B	0.015	0.010	---	0.084	---	0.109
Area 2A	0.015	0.011	---	0.013	0.069	0.107

	Area where sublegal bycatch was taken
Stock	4	3	2C	2B	2A	Coastwide

Area 4	0.204	---	---	---	---		0.204
Area 3	---	0.057	---	---	---		0.057
Area 2C	---	0.008	0.002	---	---		0.010
Area 2B	---	0.006	---	0.086	---	0.092
Area 2A	---	0.006	---	0.011	0.073	0.090

Table 5. Yield loss due to sublegal and legal sized bycatch in 1995, in mt round weight. "Interceptions" refers to bycatch of migrating sublegals before they reach their home area. "Local" bycatch means bycatch within the home area of a stock. The ranges result from using different migration schedules that result in different distributions of impacts among areas; the total (coastwide) yield losses are the same for all schedules..

	Yield loss due to bycatch of sublegals			Yield loss due to legal sized bycatch	Total yield loss
Stock	Interceptions	Local	Combined	(local only)

Area 4	---	2692-3980	2692-3980	1989	4681-5969
Area 3	7-910	1407-1533	1539-2316	1658	3197-3974
Area 2C	43-286	10	53-296	175	228-471
Area 2B	32-277	480	512-757	425	937-1182
Area 2A	9-33	39	48-72	224	272-296
Total	91-1506	4628-6042	6136	5016	11152

Figure 1. Directed catches and bycatch of Pacific halibut, 1962-1995, in round weight. The lower two panels show bycatch by size category, 1974-1995. Legal sized fish are those above 81 cm.

Figure 3. Length frequencies of the directed fisheries and the bycatch in 1995, by IPHC regulatory area. The commercial longline fishery was subject to a 32 inch (ca. 81 cm) size limit.

Figure 4. Relative distribution of age 2-4 halibut in NMFS trawl surveys, by INPFC area. (The Shumagin INPFC area is IPHC Area 4A plus western 3B; Chirikof is eastern 3B; Kodiak is western 3A; Yakutat is eastern 3A; Southeast is 2C.)