OF STOCKS AND MULTISPECIES ASSESSMENT PROGRAM:
Development of New
Age-Structured Stock Assessment Model
Development of a new age-structured stock assessment model is underway
in the Status of Stocks and Multispecies Assessment Program. In addition
to describing the dynamics of each age group in a stock on an annual time
scale, as current models already do, the new model also describes the intra-annual
and spatial dynamics of the stock. The new model is based on the Kalman
filter, in which the true system state (i.e., the number of fish in each
age/area combination) at each time step is assumed to be a linear function
of the true system state at the previous time step, modified by random
natural variability. Observations of the system state (e.g., official
catch estimates, survey and commercial catch per unit effort
at each time step are assumed to be a linear function of the true system
state, modified by random observation error. The likelihood function defined
by the Kalman filter is especially useful because it reflects, in a statistically
rigorous way, the change in precision that takes place as data are acquired
over the course of the time series. Other features of the new model include
1) Fish movement between areas is modeled as a trendless random walk, thus
eliminating the need to estimate a matrix of migration rates which might
not be constant. In principle, the spatio-temporal scale of the model
can be arbitrarily fine, potentially allowing assessment results to be
presented on scales useful to studies of the dynamics of predator species
such as the Steller sea lion. By embedding the spatio-temporal dynamics
of the stock in a full assessment model, random fluctuations in CPUE data
can be distinguished more easily from true differences in abundance.
2) Selectivity, weight, and maturity at age are described by functional
forms that assure a smooth transition between individually modeled age
groups and the age-plus group (the older age groups that are pooled for
convenience in most stock assessment models). Both the abundance and average
individual weight of fish in the age-plus group are described analytically
as functions of the fishing mortality rate. In its simplest form, where
the age-plus group consists of all recruited fish, the new model collapses
into biomass dynamic form. Thus, the new model provides a rigorous interface
between biomass dynamic and explicitly age-structured models.
3) Three measures of uncertainty are estimated: the scale of random variability
in future biological processes (e.g., future recruitment), imprecision
in the estimate of current stock status, and imprecision in estimates of
model parameters (e.g., the natural mortality rate). These estimates of
uncertainty are then incorporated into an optimization procedure that maps
a given level of risk aversion into a recommended harvest level. For any
positive level of risk aversion, greater estimates of uncertainty result
in a lower recommended harvest level, with the magnitude of the harvest
reduction varying directly with the level of risk aversion.
4) A Bayesian approach is adopted in which prior distributions are specified
for all model parameters. One advantage of this approach is that the model
should still function appropriately regardless of the size of the data
set or the imprecision of the observations. In principle, this could eliminate
the need for the present tier system used in the Bering Sea-Aleutian
Islands (BSAI) and Gulf of Alaska (GOA) Groundfish FMPs to define the harvest
control rules for the overfishing level and acceptable biological catch,
because all of the desired quantities can be estimated regardless of the
amount of data available. The precision of those estimates is, of course,
much less when data are extremely sparse or variable, but such imprecision
is formally incorporated into the harvest recommendation process in the
risk-averse manner described above.
5) The fishing mortality rate and spawning biomass associated with maximum
sustainable yield (MSY) are explicit parameters of the model. Most existing
models do not attempt to estimate these quantities. Other models estimate
them in a separate maximization routine as functions of other model parameters
such as those governing the stock-recruitment relationship. However, fishery
biologists tend to have more prior knowledge of the MSY parameters than
of the stock-recruitment parameters, so it is more convenient to treat
the MSY parameters explicitly.
A simplified version of the model was developed, which was subjected to
peer review through the Council of Independent Experts in July 2001. The
full model is expected to be ready for use by summer 2002. It is anticipated
that the models first application will be in the stock assessments for
Pacific cod in the BSAI and GOA.
By Grant Thompson.
Atka Mackerel Tagging
AFSC scientists chartered a fishing vessel from 29 July to 3 August
for the purposes of tagging and releasing Atka mackerel in the Seguam Pass
area of the Aleutian Islands, Alaska. The cruise represents the third
consecutive year of work by the AFSC on Atka mackerel in Seguam Pass and
is part of an ongoing study to determine the efficacy of trawl exclusion
zones to maintain prey availability for endangered Steller sea lions.
Trawl exclusion zones were established around sea lion rookeries as a precautionary
measure to protect critical sea lion habitat, including local populations
of prey such as Atka mackerel. Localized fishing may affect Atka mackerel
abundance and distribution near sea lion rookeries. T-bar tags (small
brightly colored external plastic tags) were used to estimate Atka mackerel
abundance and movement between areas open and closed to the fishery.
Approximately 1,000 Atka mackerel were tagged and released. Fish will
be recovered by the fishery in the open area and by a chartered vessel
in the closed area. Recoveries of fish tagged in 2001 will be used to
estimate abundance and movement between open and closed areas. Continuing
recoveries of fish tagged in earlier years (1999 and 2000) will also contribute
to these estimates. Analyses of tag recoveries to date show that over
the course of short 3-5 day fisheries in September, with catches less than
6,000 t of Atka mackerel, fish did not appear to move from inside the trawl
exclusion zone to outside. Caution should be used in applying these results
to other areas, each with resident Atka mackerel populations and fisheries
of different size and distribution.
By Elizabeth Logerwell.
Trawl Data Retrospective Study
There has been considerable scientific research and discussion regarding
decadal-scale variability in climate and circulation patterns over the
North Pacific Ocean and their possible effects on biological systems.
Climate variation is important to AFSC research programs, both as a source
of annual variability in stock size and as a suspected factor in the decline
of the number of Steller sea lions. We undertook this study to look at
long-scale time trends in groundfish biomass in the BS and GOA and to see
how these patterns are related to published series of changes in various
The RACE survey database includes records of all research surveys in the
BS and GOA since the NMFS survey programs began and includes some older
data collected by the International Pacific Halibut Commission, the State
of Alaska, and other agencies. We are thus in a unique position to put
together a long-term retrospective of bottom-trawl data. Previous efforts
to compare older surveys with new data were hampered because of changes
in trawl gear used over the years. A gear comparison study recently completed
by the RACE Division provided information needed to prorate trawl catch
volumes from the older nets on to a comparable basis with more recent surveys.
Using these results, REFM and RACE staff collaborated on the trawl data
Time-series of bottom trawl catch, using a robust estimate of the median
CPUE, were prepared for three study areas in the BS, three in the western
GOA, and two in the central GOA. In order to look at patterns of groundfish
biomass over space as well as time, index sites approximately 200 km
by 200 km were used rather than basin-wide estimates. Basic results of
the study for both regions were presented in poster format at the American
Fisheries Society meeting in August 2001. A more detailed examination
of the results will be presented at the October 2001 PICES meeting.
The most striking result of the study is a very obvious set of changes
in biomass for a number of different species and taxa in the BS, beginning
in approximately 1980. Large increases in biomass are seen for
gadid fishes (walleye pollock and Pacific cod) and a number of flatfishes
(rock sole, arrowtooth flounder, and halibut). Steep increases also occur,
however, in trawl catch of cartilaginous fishes (primarily rays) and benthic
invertebrates (starfish, sponges, ascidians, and mollusks). Many of the
individual stock assessments had already noted an increased biomass in
the late 1980s and 90s relative to the 1970s. This study, however, presents
trends for all of the different species groups together and suggests that
a system-wide restructuring of food webs in the EBS may have occurred.
Results from sites in the GOA do not show strong patterns over time,
but show clearly the spatial variation in trawl catch from east to west
across the gulf.
By Elizabeth Connors.
RESOURCE ECOLOGY AND ECOSYSTEM MODELING PROGRAM
Stomachs collected totaled 6,781 from the EBS and 4,101 from the
analysis was performed on 1,496 groundfish stomachs from the
EBS, 315 from
the GOA, and 484 from the Washington-Oregon-California region. Two observers
returned groundfish stomach samples during this quarter.
Canadian Workshop on Objectives and Indicators For Ecosystem-based Management
The Canadian Department of Fisheries and Oceans (DFO) sponsored the Canadian
National Workshop on Objectives and Indicators for Ecosystem Based Management
from 27 February-2 March 2001 in Sidney, British Columbia. The workshop
was designed to identify ecosystem-level objectives, with associated indicators
and reference points that could be used in managing ocean activities.
Participants included DFO scientists, fisheries managers, ocean managers,
and habitat managers. Invited experts from other Canadian government departments
and academia along with scientists from other nations attended the workshop.
Alaska Fisheries Science Center scientist Pat Livingston was invited to
attend and relate information pertaining to Alaska ecosystem-based fisheries
management and development of the ecosystem considerations document that
accompanies Alaskan groundfish stock assessment advice to fishery managers.
The full report of the workshop by Jamieson and OBoyle can be obtained
on the World Wide Web at
By Pat Livingston.
SOCIOECONOMIC ASSESSMENTS PROGRAM:
Results of recent research by Ron Felthoven were presented to the North
Pacific Fishery Management Councils Groundfish Plan Teams. The methods
of analysis and results are discussed in a working paper titled, Effects
of the American Fisheries Act on the Harvesting Capacity, Capacity Utilization,
and Technical Efficiency of Catcher-Processors. Some general results
from this analysis are 1) the vessels that were decommissioned as a result
of the American Fisheries Act (AFA) were on average less technically efficient
than AFA-eligible vessels; and 2) for the fishing companies that voluntarily
idled AFA-eligible vessels in 1999 or 2000, the vessels idled typically
had exhibited the lowest historical levels of technical harvesting efficiency
or capacity utilization of the AFA-eligible vessels in each fishing company
and lacked meal plants.
Felthoven collaborated with Rita Curtis (NMFS Office of Science and Technology)
on The Effects of Temporal Aggregation in Fishery Supply Models. The
paper identifies an assumption commonly made about production technologies
that may give rise to biased estimates. The assumption concerns the ability
to aggregate a vessels daily fishing production cycle over individual
fishing days to the trip level. To conform with temporal aggregation requires
that at given output prices, production over the course of a trip is characterized
by no search or learning effects and no stock effects (fish are distributed
uniformly over time). Furthermore, if vessels fish more than one site,
temporal aggregation requires that fish be uniformly distributed across
fishing sites. Temporal aggregation would then fail, for example, if fishers
employ multiple fishing strategies during a trip, such as fishing offshore
for cod and then move inshore to a different ecological niche to harvest
yellowtail flounder because stock distributions are not uniform across
In an application to the Hawaii longline fishery, aggregation consistency
was rejected for all models, and tests on the technology using aggregated
data were found to be biased. In particular, results obtained using aggregated
data suggest that production is joint and that fishermen are either not
able to target specific species or have implausible targeting strategies.
In contrast, daily results identify plausible targeting strategies and
suggest that several species, including swordfish, have separate production
functions. Importantly, this implies that swordfish, which comprises a
high percentage of fishery catch and revenue, can be managed independently.
Foregone Catch and Gross Product Value in 2000 Due to Steller Sea Lion
Estimated annual total catch of groundfish in the BSAI and GOA groundfish
fisheries ranged from about 1.6 to 2.1 million t in 1995-99 and averaged
1.9 million t. Total catch in 2000 was almost 1.8 million t or about 7.4
percent less than the 5-year average. Estimated annual retained catch
(total catch - discarded catch) ranged from about 1.5 to 1.8 million t
and averaged 1.7 million t. The retained catch in 2000 was about 3.2 percent
less than the 5-year average.
The estimated annual gross product value of the groundfish catch after
primary processing ranged from $1.1 to $1.4 billion in 1995-99. Despite
lower than average catch in 2000, the gross product value exceeded $1.3
billion and was almost 10 percent greater than the 5-year average of $1.2
A combination of factors, including regulatory, market, and biological
changes, contributed to the lower than average catch and the higher than
average gross product value in 2000. Some of those changes offset the
gross product value foregone due to the restrictions that were in place
in 2000 to provide additional protection for Steller sea lions. It is
estimated that the foregone gross product value resulting from the restrictions
in place in 2000 may have exceeded $100 million. This is more than 7 percent
of the estimated gross product value of groundfish for 2000. The cost
data necessary to estimate the effects on product value net of harvesting
and processing costs are not available.
The foregone gross product value has a variety of economic effects. They
include the following: 1) decreased employment and income for those involved
in harvesting and processing groundfish; 2) decreased expenditures on goods
and services by the harvesting and processing sectors; 3) decreased expenditures
on goods and services by those who receive income from the harvesting,
processing and support sectors; and 4) decreased tax revenues in fishing
communities. Given the importance of the groundfish fishery to some fishing
communities, the direct, indirect, and induced income effects of decreased
groundfish catch and gross product value can be significant. However,
estimates of these effects are not available.
A variety of restrictions have been placed on the Alaska groundfish fisheries
in order to provide additional protection for Steller sea lions and their
habitat. These restrictions closed the AI pollock fishery and forced fishermen
to change the timing and location of their fishing operations in other
groundfish fisheries. The closure directly reduced catch and the gross
value of the resulting seafood products. By preventing fishermen from
fishing at their preferred times and locations, the other restrictions
adversely affected harvesting costs, vessel safety, product quality, product
mix, and bycatch rates. These effects have in turn decreased catch and
gross product value while increasing the harvesting and processing costs
per unit of catch. Estimates are provided below for five of the major
elements of the foregone catch and gross product value.
The estimates of foregone gross product value do not account for the product
price increases that probably occurred as the result of the foregone catch.
This source of an upward bias in the estimates of foregone gross product
value is offset to an unknown extent by the failure to fully account for
the decreases in gross product value per unit of catch that occurred due
to the sea lion restrictions. Estimates of the increases in harvesting
and processing costs and decreases in vessel safety are not available.
Prior to the closure of the AI pollock fishery, usually 95-100 percent
of the annual total allowable catch (TAC) (quota) was caught. If the pollock
fishery had not been closed, the TAC probably would have been set equal
to the allowable biological catch (ABC) of 23,800 t, and catch in the pollock
fishery during the roe season probably would have been about 22,000 t with
a gross product value of about $24 million in 2000.
The TAC apportionment for the roe season was reduced from 45 percent of
the annual pollock TAC to 40 percent to provide additional protection for
sea lions during the winter. With a TAC of 1,139,000 t in 2000, this shifted
about 57,000 t of catch from the roe season to the nonroe season and decreased
product value by $26 million because gross product value per metric ton
of catch was about $460 more during the roe season.
To the extent that pollock roe recovery rates are higher inside the Steller
Sea Lion Conservation Area (SCA), the restrictions on the amount of pollock
TAC that can be taken inside the SCA decrease gross product value per metric
ton of catch. Based on the percent of the pollock catch that occurred
in the SCA in 1998, it is estimated that this limit redistributed 166,000
t of pollock catch from inside to outside the SCA. The At-Sea Processors
Association estimated that the difference between the gross product value
per metric ton of catch for these two areas was $235 in 2000. Based on
that estimate and the 166,000-t redistribution of catch between the two
areas, the estimated foregone gross product value was $39 million. NMFS
has not yet verified this estimate.
The limits on the percent of Atka mackerel TAC that can be taken inside
the SCA and the injunction-imposed trawl closure of Steller sea lion critical
habitat prevented the Atka mackerel TACs from being fully utilized in the
central and western Aleutian Islands in 2000. Based on catch data from
1995-99, it is estimated that 100 percent of the central area TAC and 95
percent of the western area TAC would have been taken in 2000 in the absence
of these restrictions. This compares to actual utilization of TAC of 90
percent and 35 percent in the central and western areas, respectively.
Due to the high bycatch rates for rockfish outside the
SCA, NMFS closed
the western area Atka mackerel fishery well before the Atka mackerel TAC
was taken. The 2000 TACs were 24,700 t and 29,700 t; therefore, the catches
foregone were about 2,470 t and 17,820 t by area for a total of 20,290
t. The associated loss of gross product value was about $11 million.
However, the early closure of the Atka mackerel fishery resulted in more
effort, catch, and gross product value in the yellowfin sole fishery.
It is estimated that about half of the foregone gross product value in
the Atka mackerel fishery was offset by increased product value in the
yellowfin sole fishery. The net effect of the early closure of the Atka
mackerel fishery was, therefore, about a $5.5 million reduction in gross
Smaller Atka mackerel with a lower value per metric ton of catch account
for a larger percent of the catch outside the SCA in the central and western
areas. In addition to being a factor that prevented the TACs from being
fully utilized, this decreased the gross product value per metric ton of
Atka mackerel catch. The data required to estimate the effect on gross
product value are not available.
The seasonal apportionments of the GOA pollock TACs and the seasonal rollover
rules, the limits on the percent of the TACs that can be taken inside Steller
sea lion critical habitat, and the injunction-imposed trawl closure of
critical habitat prevented the pollock TACs from being fully utilized in
the central and western Gulf of Alaska in 2000. Based on catch data from
1995-99, it is estimated that 100 percent of the central and western area
TACs would have been taken in 2000 in the absence of these restrictions.
In 2000, 21,435 t (over 23%) of the TACs were not caught. The gross product
value foregone probably exceeded $8 million.
By Joe Terry.
Age and Growth Program
Estimated production figures for
1 January to 30 September 2001
|Northern rock sole
|Pacific ocean perch
|Light dusky rockfish
Total production figures were
27,538 with 5,047 test ages and 170 examined and determined
to be unageable.
U.S. North Pacific Groundfish Observer
During the third quarter of 2001, 206 observers were trained, briefed,
and equipped for deployment to fishing and processing vessels and shoreside
plants in the Gulf of Alaska, Bering Sea, and Aleutian Islands. They sampled
aboard 228 fishing and processing vessels and at 16 shoreside processing
plants. These observers were trained or briefed in various locations.
The University of Alaska Anchorage (UAA) Observer Training Center trained
32 new observers and another 102 observers with prior experience were briefed
at this site. The AFSC Observer Program in Seattle briefed 49 observers
who had prior experience. In addition, 23 observers were excused from
briefing, because they had just completed a cruise successfully and were
returning immediately to the field. The third quarter 2001 observer workforce
thus comprised 16 percent new observers and 84 percent experienced observers.
The Observer Program conducted a total of 120 debriefings. One debriefing
was held in Dutch Harbor, one in Kodiak, 51 in Anchorage, and 67 were held
Groundfish Program Third Quarter Highlights
The North Pacific Groundfish Observer Program hosted a contracting workshop
from 30 July through 3 August 2001. The workshop was conducted by staff
from the NMFS National Observer Program in Silver Spring, Maryland. The
objective of the workshop was to bring together contracting specialists
and representatives from all NMFS observer programs in order to develop
contracting standards and improve contract management.
Workshop participants included contracting specialists from NOAAs Western
and Eastern Administrative Support Centers, officers from three NMFS regional
offices, a contracting specialist from the ADF&G, and other NMFS observer
program staff who are involved in writing statements of work or evaluating
bids for contracts.
The emphasis of the workshop was on development of performance-based contracts
that address the primary goals of an observer program. These goals were
identified as: observer coverage, data quality, observer well-being, and
observer program integrity. A template for a statement of work with measurable
performance standards was drafted by the workshop participants.
By Bob Maier.
quarterly Jul-Sept 2001 sidebar
Auke Bay Lab