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---
title: "1. Stock assessment for eastern Bering Sea walleye pollock"
author:
- name: James Ianelli
corresponding: true
id: ji
orcid: 0000-0002-7170-8677
email: [email protected]
affiliation:
- name: Alaska Fisheries Science Center
city: Seattle
state: WA
url: https://www.fisheries.noaa.gov/alaska/population-assessments/north-pacific-groundfish-stock-assessments-and-fishery-evaluation
- name: Taina Honkalehto
- name: Sophia Wassermann
- name: Nathan Lauffenburger
- name: Carey McGilliard
- name: Elizabeth Siddon
keep-md: true
keep-tex: true
# csl: cjfas.csl
date: 11/03/2023
date-format: "MMMM YYYY"
bibliography: doc/references.bib
header-includes:
- \usepackage{multirow}
- \usepackage{hyperref}
- \usepackage[capitalise,noabbrev]{cleveref}
fig-pos: H
tbl-pos: H
format:
pdf:
number-pages: FALSE
number-sections: TRUE
number-offset: 1
number-depth: 2
includes:
in_header: "doc/preamble.tex"
extra_dependencies: rotating
editor_options:
chunk_output_type: console
---
```{r setup}
#| echo: false
#| warning: false
# source(here::here("R","prelims.R"))
library(here)
library(xtable)
library(gt)
# load(file="doc/septmod.rdata")
# load(file="doc/modtune.rdata")
# names(modlst) <- c("Last year", "Diagonal cov BTS","GenGam","SSB0","SSB1","SSB2","AVO new","AVO full")
source(here("pm23.R"))
# load(file="doc/novmod.rdata")
# names(modlst) <- c("Last year", "Diagonal cov BTS","GenGam","SSB0","SSB1","SSB2","AVO new","AVO full")
mod_names <- names(modlst)
knitr::opts_chunk$set(fig.pos = "H", out.extra = "")
# \label{tab:modcomp}
# #\usepackage{fancyhdr}
# \pagestyle{fancy}
# \fancyhead[C]{Draft}
# \fancyfoot[C]{EBS pollock assessment}
```
```{=tex}
\begin{centering}
Alaska Fisheries Science Center, National Marine Fisheries Service \\
National Oceanic and Atmospheric Administration \\
7600 Sand Point Way NE., Seattle, WA 98115-6349 \\
\end{centering}
```
------------------------------------------------------------------------
This report may be cited as: Ianelli, J. et al. 2023. Assessment of the
eastern Bering Sea walleye pollock. North Pacific Fishery Management Council,
Anchorage, AK. Available [here](https://www.npfmc.org/library/safe-reports/).
------------------------------------------------------------------------
\thispagestyle{empty}
\newpage
# Executive summary
This chapter covers the Eastern Bering Sea (EBS) region---the Aleutian
Islands region (Chapter 1A) and the Bogoslof Island area (Chapter 1B)
are presented separately. A multi-species stock assessment is provided
separately and available
[here](https://apps-afsc.fisheries.noaa.gov/Plan_Team/2023/EBSmultispp.pdf).
A list of this document contents, including tables and figures is provided
in @sec-contents.
### Summary of changes in assessment inputs
Relative to last year's BSAI SAFE report, the following substantive
changes have been made in the EBS pollock stock assessment. This
includes the `r thisyr` NMFS bottom-trawl survey (BTS) covering the EBS
and NBS. As before, these data were treated with a spatio temporal model
for index standardization. Age data from this survey effort was compiled
and included (also with an extensive spatio-temporal model treatment).
The NMFS acoustic-trawl survey (ATS) age composition data was revised from the
preliminary estimates developed in 2022.
The BTS chartered boats also collected acoustic data and the series was updated
this year (AVO). Explorations were presented in @ianelli2023.
#### Changes in the data
1. Observer data for catch-at-age and average weight-at-age from the
`r lastyr` fishery were finalized and included.
2. Total catch as reported by NMFS Alaska Regional office was updated
and included through `r thisyr`.
3. In summer `r thisyr`, the AFSC conducted the bottom trawl survey in
the EBS and extended into the NBS. A VAST model evaluation
(including the cold-pool extent) was used as the main index.
4. We refined estimates of weight-at-age data used to compute spawning
biomass as presented to the Plan Team and SSC in September/October
2023 (see @ianelli2023 for details).
5. We applied a new time series from the acoustic data
collected from the bottom trawl survey covering 2006-2023 (except
for 2020) as presented in @ianelli2023.
6. We applied updated age-composition from the 2022 ATS survey (last
year a preliminary estimate was used based on the BTS age-length
data plus a juvenile sample from the ATS.
### Changes in the assessment methods
The assessment method changes were presented in September 2023
(@ianelli2023). In that document we re-evaluated the relative weights
specified as input variances and sample sizes. We also applied different
tuning approaches which achieved a balance between the observation
errors and model process errors, specifically for the acoustic time
series. We examined additional model sensitivities under development that included
an exploration of using a generalized Gamma distribution for the bottom trawl index,
age-determination errors, and simplified error structures for the bottom trawl index.
We adopted alternative estimates of weight-at-age applied to
the spawning biomass calculations. The modified estimates are intended
to reflect data available closest to the peak spawning season. The Plan Team and
SSC agreed with the data and model changes, and those present the new base case
presented here.
## Summary of EBS pollock results
The results from the 2022 assessment have largely been confirmed: the
2018 year class appears to be one of the most abundant on record.
Nonetheless, the bottom-trawl survey was lower than expected (about 28%
below the long-term mean and the tenth lowest over the 41-year survey
period). The new AVO index (presented in September 2023) expanded the
area covered by acoustics and provided more precision (lower CV in the point estimates)
than in the past. Ancilliary data indicate that the pollock in 2023 are substantially
skinnier than average given their length. The average weight-at-age was about
average for the 2018 year class, but lighter for most other ages.
The following table is based on results from the selected model ("Model
23.0") based on changes presented in @ianelli2023. The ABC
recommendation is based on Tier 3 calculation as a proxy for Tier 1
because of the variability indicated by the very high value based on the
$F_{MSY}$ estimate and the large but uncertain 2018 year class.
```{=tex}
\begin{table}[ht]
\scalebox{0.95}{
\centering
\begin{tabular}{lrr|rr}
\hline
& \multicolumn{2}{c|}{As estimated or $\mathit{specified}$ } & \multicolumn{2}{c}{As estimated or $\mathit{recommended}$ } \\
& \multicolumn{2}{c|}{$\mathit{last}$ year for:} & \multicolumn{2}{c}{$\mathit{this}$ year for: } \\
Quantity & `r thisyr` &`r thisyr+1` & `r thisyr+1` &`r thisyr+2` \\
\hline
M (natural mortality rate, ages 3+) & 0.3 & 0.3 & 0.3 & 0.3 \\
Tier & 1a & 1a & `r M$tier1ab1 ` & `r M$tier1ab2 ` \\
Projected total (age 3+) biomass (t) & `r M$it$a1[1]` t & `r M$it$a2[1]` t & `r M$age3plus1s` t & `r M$age3plus2s` t \\
Projected female spawning biomass (t) & `r M$it$b1[1]` t & `r M$it$b2[1]` t & `r M$ssb1s` t & `r M$ssb2s` t \\
$B_0$ & `r M$it$c1[1]` t & `r M$it$c2[1]` t & `r M$b0s`,000 t & `r M$b0s`,000 t \\
$B_{msy}$ & `r M$it$d1[1]` t & `r M$it$d2[1]` t & `r M$bmsys`,000 t & `r M$bmsys`,000 t \\
$F_{OFL}$ & `r M$it[1,8]` & `r M$it[2,8]` & `r M$FOFL1s` & `r M$FOFL2s ` \\
$maxF_{ABC}$ & `r M$it[1,9]` & `r M$it[2,9]` & `r M$FABC1s ` & `r M$FABC2s ` \\
$F_{ABC}$ & `r M$it[1,10]`& `r M$it[2,10]`& `r (M$fabc1)` & `r (M$fabc2)` \\
$OFL$ & `r M$it$e1[1]` t & `r M$it$e2[1]` t & `r M$ofl1s` t & `r M$ofl2s` t \\
$maxABC$ & `r M$it$f1[1]` t & `r M$it$f2[1]` t & `r M$maxabc1s ` t & `r M$maxabc2s ` t \\
$ABC$ & `r M$it$g1[1]` t & `r M$it$g2[1]` t & `r M$abc1s ` t & `r M$abc2s` t \\
\hline
Status & `r lastyr-1` & `r lastyr`& `r lastyr` & `r thisyr` \\
\hline
Overfishing & No & n/a & No & n/a \\
Overfished & n/a & No & n/a & No \\
Approaching overfished & n/a & No & n/a & No \\
\hline
\end{tabular}
}
\end{table}
```
## Response to SSC and Plan Team comments
### SSC General groundfish stock assessment comments
The following are relevant SSC comments from their December 2022
minutes.
- The SSC recommends that for future Tier 1-3 assessments some
consideration be given as to how best to represent biomass estimates
in the Executive Summary table for each stock (currently, model
total biomass and spawning stock biomass are provided) so that the
relationship of the biomass to the OFL and ABC in the stock status
table is clear. - *We agree. Within the document we include biomass
estimates that are outcomes for ABC and OFL calculations. However,
the estimates involve an application of expected age-specific
selectivity which can be variable.*
- For all assessments using VAST, the SSC requests a figure comparing
the VAST estimate used in the previous assessment to the current
assessment (if new data are added), noting that VAST will refit the
time series when additional data are added and the estimated extent
and directionality of spatial correlation may change. - *We include
model comparisons showing the impact of new (updated) VAST time
series.*
**The SSC suggests that walleye pollock is a good candidate for
considering the impacts of highly variable recruitment on reference
points in the context of the Council's harvest control rules** (see
discussion on working groups in the JGPT report section). For example,
the SAFE authors suggested exploring an explicit harvest control rule
that maintains productivity at the level observed over recent decades
(p. 33). The SSC supports considerations of modified harvest control
rules, particularly for stocks with highly variable and uncertain
recruitment. If the Council chooses, this could include considerations
for stabilizing catches over time or including other economic
considerations in the harvest control rules.
- *For the 2023 assessment we examine the variability of the
biological reference points historically and note that there is
general stability in the* $B_{MSY}$ estimates. The Tier 1 ABC/OFL
calculations can result in highly variable estimates as the stock
approaches $B_{MSY}$ and drops below that value (as happened in the
2009-2010 period).
The SSC had the following additional recommendations for the authors:
- Maturity and growth information from the NBS has not been examined
yet. Given the possible importance of the NBS to walleye pollock and
other species in the future, the SSC suggests this should be a high
priority.
- *These data are being processed and this work is underway*
- The SSC supports efforts to implement recent advances in improving
the statistical treatment of compositional data using the Dirichlet
distribution or other approaches.
- *Tradeoffs in data weighting were pursued in September 2023 and
sought to find a balance between observation error and process
errors. In general, trade-offs in data weighting appear
consistent with sampling levels and include objective approaches
to their specifications.*
- The SAFE document lists a number of research recommendations (p.
36/37). The SSC notes that some of these are at least in progress.
The SSC generally supports these recommendations but requests that
the authors update the list of priorities to clarify to what extent
some of these priorities have been partially or fully addressed.
- *We updated the priorities and listed those that have been
completed or are continued to be underway*
- In particular, the SSC notes that genetic sample collection and
analyses are listed as a research priority across all pollock stocks
and that some work has been completed. The SSC highlights the
importance of additional genetics work and would appreciate an
update on the status of this work either as part of the assessment
or separately.
- *We revisited the stock structure work attached as an appendix
to the 2015 SAFE report chapter and are examining the extent
that this work needs updating. Updated genetics work indicate
that the Bering Sea pollock represent a distinct stock. This
work indicates that GOA pollock seems to be similar to some
Aleutian pollock but some AI pollock are distinct (Spies and
Schaal, pers. comm.).*
- The SSC appreciated the adjustments to weight-at-age in the survey
that was included in this year's assessment and suggests that these
changes may be substantial enough to warrant an examination of their
impact on assessment results.
- *This publication has been completed and in the present
assessment we evaluated the implication of alternative spawning
biomass-at-age assumptions.*
- With respect to the multi-species CEATTLE model, the SSC concurs
with Plan Team recommendations to use the model to inform risk table
discussions and to consider ways in which model outputs, in
particular estimates of predation mortality, can inform
single-species assessments.
- *We included some comments to this effect.*
- The SSC encourages the authors to consider model-based solutions to
uncertain recruitment estimates rather than ad-hoc adjustments. In
particular, reductions in the assumed recruitment variance parameter
may result in less extreme recruitment estimates. Other systematic
approaches to addressing the uncertainty may also be considered.
- *We revisited applying the age-determination error matrix as a
sensitivity as this can impact the recruitment variability and
estimation uncertainty.*
- The SSC suggests that authors include a plot to compare estimates of
recent recruitments as they change over time similar to Fig 3.33
(pg. 88) in the sablefish assessment.
- *We provide a figure of estimated recruitment by year class
(1977 -- 2019) in number of age-1 fish (billions of fish) for
the 2022 and 2023 models.*
- The SSC supports the move across assessment from design-based
estimates of survey biomass to VAST estimates. The SSC recommends
that the design-based estimates be produced as a check on VAST
estimates and as a fallback option if needed, although they may not
need to be included in the assessment.
- *We provide a table showing the design-based estimates and
conduct a model run with those estimates. This may be an
approach to adopt so that bridging across assessment modeling
platforms can be facilitated (most other assessment model
platforms are unable to deal with index time series that have a
covariance matrix)*
- The SSC noted that a consideration of whether the observed sensitivity in the SRR to
prior specification should constitute an increased risk level
specification within the assessment or population dynamics related
considerations should be considered. This could provide a clearer justification for the
use of the Tier 3 calculation as the basis for harvest
specification.
- *We evaluated factors affecting the Tier classification in the
2020 assessment and showed that the priors used reflect the SRR
curve were conservative and justified based on residual patterns
near the origin (as opposed to alternatives that fit data on the
descending slope of the Ricker SRR.*
- The SSC recommends that if the assessment is considered in the
appropriate Tier, buffers should be based on the use of the Risk
Table rather than the continued use of Tier 3 calculations for a
Tier 1 stock.
- *We agree.*
- The SSC also notes that an alternative approach to consider for a
buffer below the maximum permissible would be apply Tier 2 control
rule. This tier uses the SR relationship for stock status and OFL,
but uses the ratio of SPR rates for adjustments when the stock is
below $B_{MSY}$.
- *An examination of Tier 2 as an option resulted in a value of
`r M$Tier2_ABC1s` t (or a hybrid of Tier 1 and 2 of
`r M$Tier1.5_ABC1s` t) for `r nextyr` ABC values. We note that
selecting Tier 2 would require similar reliance on the
underlying productivity estimates (via the stock-recruitment
relationship) and how that affects the reference fishing rate
(*$F_{MSY}$).
------------------------------------------------------------------------
**The SSC had a number of recommendations for additional research
supporting this assessment:**
**From previous requests:**
The SSC also looks forward to estimates of movement and abundance along
the US-Russia EEZ boundary based on echosounders fixed to moorings in
this area.
- *The data evaluation from the moored sounders has been completed and
initial results show that the flux of pollock back and forth over
the maritime boundary is considerable, and appears to be a function
of temperature conditions.*
\clearpage
# Introduction
## General
Walleye pollock (*Gadus chalcogrammus*; hereafter referred to as
pollock) are broadly distributed throughout the North Pacific with the
largest concentrations found in the Eastern Bering Sea. Also known as
Alaska pollock, this species continues to play important roles
ecologically and economically.
## Review of Life History
In the EBS pollock generally spawn during March-May and in
relatively localized regions during specific periods (@bailey2000 ).
Generally spawning begins nearshore north of Unimak Island in March and
April and later near the Pribilof Islands (@bacheler2010). Females are
batch spawners with up to 10 batches of eggs per female per year (during
the peak spawning period). Eggs and larvae of EBS pollock are planktonic
for a period of about 90 days and appear to be sensitive to
environmental conditions. These conditions likely affect their dispersal
into favorable areas (for subsequent separation from predators) and also
affect general food requirements for over-wintering survival (@gann2015,
@heintz2013, @hunt2011, @ciannelli2004). @duffy2016 provide a review of
the early life history of EBS pollock.
Throughout their range juvenile pollock feed on a variety of planktonic
crustaceans, including calanoid copepods and euphausiids. In the EBS
shelf region, one-year-old pollock are found throughout the water
column, but also commonly occur in the NMFS bottom trawl survey. Ages 2
and 3 year old pollock are rarely caught in summer bottom trawl survey
gear and are more common in the midwater zone as detected by mid-water
acoustic trawl surveys. Younger pollock are generally found in the more
northern parts of the survey area and appear to move to the southeast as
they age (@buckley2009). Euphausiids, principally *Thysanoessa inermis*
and *T. raschii*, are among the most important prey items for pollock in
the Bering Sea (@livingston1991; @lang2000; @brodeur2002;
@ciannelli2004; @lang2005). Pollock diets become more piscivorous with
age, and cannibalism has been commonly observed in this region. However,
@buckley2015 showed spatial patterns of pollock foraging varies by size of
predators. For example, the northern part of the shelf region between
the 100 and 200 m isobaths (closest to the shelf break) tends to be more
piscivorous than pollock found in more near-shore shallow areas.
## Stock structure
Stock structure for EBS pollock was evaluated in @ianelli2015.
In that review past work on genetics (e.g., @bailey1999, @canino2005)
provided insight on genetic differentiation.
The investigation also compared synchrony in year-classes and growth
patterns by region. Pollock samples from areas including Zhemchug Canyon, Japan,
Prince William Sound, Bogoslof, Shelikof, and the Northern Bering Sea
were processed and results presented in @ianelli2021.
A group of researchers at AFSC led by Drs. Ingrid Spies and Sara Schaal
have updated the recent genetics study and this is summarized here:
> Adult samples of walleye pollock were collected from 15 locations
> spanning Japan, the Bering Sea, and the Gulf of Alaska and were used
> for genetic analysis. Researchers performed low-coverage whole genome
> sequencing on 547 individuals from these sampling locations, which
> resulted in roughly 2 million polymorphic loci found throughout the
> genome. Although genetic differentiation is subtle in walleye pollock
> compared to other species, there are two strong and notable genetic
> breaks that highlight the genetic stock structure present. The first
> is between all the US samples and Japan (@fig-genetics: the split on
> PC axis 1) and the second is between the Bering Sea and the
> GOA/Aleutian Islands (@fig-genetics: the split on PC axis 2). This
> suggests that Bering Sea walleye pollock are genetically distinct from
> GOA/Aleutian Islands walleye pollock. Walleye pollock in the Aleutian
> Islands and Bogoslof show weak divergence with the GOA. Individuals
> caught in Adak and Atka comprise two genetic groups (@fig-genetics:
> dark green points). One group is not differentiated from the rest of
> the GOA (@fig-genetics : dark green points clustering behind the GOA
> points in pink) and the other shows some divergence along PC axis 1.
> This complex group warrants further investigation to understand the
> genomic regions that may be divergent between these two genetic groups
> present in the Adak and Atka. Additionally, individuals from Bogoslof
> show weak divergence from GOA samples (@fig-genetics: light green
> points). Walleye Pollock are currently managed as four distinct
> groups: 1) GOA, 2) Bering Sea, 3) Aleutian Islands and 4) Bogoslof.
> Our genetic groups mostly align with these delineations. However, the
> Aleutian Islands/Bogoslof stocks show subtle differentiation from the
> rest of the GOA.
For management purposes, the preliminary conclusions from these genetics
results are: 1) there is stock structure in pollock that appears to be
stable through time and 2) Some aspect of stock structure is
latitudinal---Bering Sea pollock appear distinct from fish collected
from the Gulf of Alaska and the Aleutian Islands. The results appear
strong enough that a GTseq panel could be designed in the future to
determine stock of origin of pollock, the scale of which may be
relatively large, such as "Bering Sea" or "GOA".
# Fishery
## Description of the directed fishery
Historically, EBS pollock catches were low until directed foreign
fisheries began in 1964. Catches increased rapidly during the late 1960s
and reached a peak in 1970--75 when they ranged from 1.3 to 1.9 million
t annually. Following the peak catch in 1972, bilateral agreements with
Japan and the USSR resulted in reductions. During a 10-year period,
catches by foreign vessels operating in the "Donut Hole" region of the
Aleutian Basin were substantial totaling nearly 7 million t
(\cref{tab:catch}). A fishing moratorium for this area was enacted in
1993 and only trace amounts of pollock have been harvested from the
Aleutian Basin region since then. Since the late 1970s, the average EBS
pollock catch has been about 1.2 million t, ranging from 0.810 million t
in 2009 to nearly 1.5 million t during 2002--2006 (\cref{tab:catch}).
United States vessels began fishing for pollock in 1980 and by 1988 the
fishery became fully domestic. The current observer program for the
domestic fishery formally began in 1991 and prior to that, observers
were deployed aboard the foreign and joint-venture operations since the
late 1970s. From the period 1991 to 2011 about 80% of the catch was
observed at sea or during dockside offloading. Since 2011, regulations
require that all vessels participating in the pollock fishery carry at
least one observer so nearly 100% of the pollock fishing operations are
monitored by scientifically trained observers. Historical catch
estimates used in the assessment, along with management measures (i.e.,
OFLs, ABCs and TACs) are shown in (\cref{tab:abctac}).
### Catch patterns
The "A-season" for directed EBS pollock fishing opens on January 20th
and fishing typically extends into early-mid April. During this season
the fishery targets pre-spawning pollock and produces pollock roe that,
under optimal conditions, can comprise over 4% of the catch in weight.
The summer, or "B-season" presently opens on June 10th and fishing
extends through noon on November 1st. The A-season fishery concentrates
primarily north and west of Unimak Island depending on ice conditions
and fish distribution. There has also been effort along the 100m depth
contour (and deeper) between Unimak Island and the Pribilof Islands. The
general pattern by season (and area) has varied over time with recent
B-season catches occurring in the southeast portion of the shelf (east
of 170$^\circ$W longitude; @fig-catch).
Since 2011, regulations and
industry-based measures to reduce Chinook salmon bycatch have affected
the spatial distribution of the fishery and to some degree, the way
individual vessel operators fish (@stram2014). Comparing encounters of
bycatch relative to the effort (total duration of all tows) the pollock
fleet had a slight increase in the Chinook salmon bycatch rate
(@fig-fsh_psc_cpue). The nominal catch rate of sablefish in the pollock
fishery continue to be above historical averages (@fig-fsh_psc_cpue)
while for herring, the rate was low compared to 2020.
The catch estimates by sex for the seasons indicate that over time, the
number of males and females has been fairly equal but in the period
2017-2022 the A-season catch of females has been slightly higher and
conversely, in the B-season there has been a slightly higher number of
males taken (@fig-catch_sex). The pattern of catch numbers is impacted
by the magnitude of the quota (e.g., the drop in 2022 when the TAC was
lower) but also in the relative size of fish. For example, in 2020 estimated
absolute numbers of catch were relatively high because fish were smaller
(and younger) than average.
The 2023 A-season fishery spatial pattern had a
relatively more catch around the Pribilof Islands compared to
2021 (@fig-catch_distn_a). The amount of fishing near the Pribilof
Islands was lower than commonly observed in 2022. The 2023 A-season
nominal catch rates were near peak levels for all fleet sectors (middle panel,
@fig-fsh_cpue). Beginning in 2017, due to a regulatory
change, up to 45% of the TAC could be taken in the A-season (previously
only 40% of the TAC could be taken). This conservation measure was made
to allow greater flexibility to avoid Chinook salmon in the B-season.
The pollock fleet as a whole continues to take advantage of this
flexibility (@fig-prop_a_season). This figure shows that the proportion
of the TAC has been consistent over time.
Pollock roe production remains at a low level but increased over 2022
(@fig-roe).
The summer-fall fishing conditions for 2023 were similar to 2022
(@fig-fsh_cpue). The number of hours the fleet required to catch the same tonnage of pollock
was also improved relative to 2020.
In the B-season catches in the
northwestern area increased relative to the previous two years
(@fig-catch_distn_b). We updated our work on
a measure of fleet dispersion: the relative distance or spread of
the fishery in space. Briefly, the calculation computes for a given day,
the distance between all trawl tows (within and across boats). These
distances are then averaged for year and season. Updated to this year,
results indicated that in the A-season dispersion increased slightly but for
the B-season in 2023, the fleet appeared to be less dispersed than
all the other years and since 2000 (@fig-fleet_dispersal).
We continued to investigate the tow specific mean weight of fish. These
provide a direct mean somatic mass (pollock body weight) for pollock
within a tow. The data arise from the sampled total weight (e.g., of
several baskets of pollock) divided by the enumerated number of fish in
that sample. Such records exist for each tow. Summing these by
extrapolated weight of the pollock catch within that tow, and binning by
weight increments (here by 50 gram intervals), allows us to obtain some
additional fine-scale information on the size trends in the pollock
fishery. The annual patterns of these data suggest that the 2023
A-season size was consistent with the expectation of the 2018 year class
predominating the catch (@fig-fsh_wt_freq). However, the
2023 B-season pattern was smaller than expected.
Compiling the data by week we
show that the fish size was consistent with the pattern of fish being consistently
smaller than expected through the B-season (@fig-fsh_wt_freq_week).
The catch of EBS pollock has averaged 1.26 million t in the period since
1979. The lowest catches occurred in 2009 and 2010 when the limits were
set to 0.81 million t due to stock declines (\cref{tab:abctac}). The
recent 5-year average (2019-2023) catch has been 1.304 million t.
Pollock catches that are retained or discarded (based on NMFS observer
estimates) in the Eastern Bering Sea and Aleutian Islands for
1991--`r thisyr` are shown in \cref{tab:catchdisc}. Since 1991,
estimates of discarded pollock have ranged from a high of 9.6% of total
pollock catch in 1991 to recent lows of around 0.6% to 1.2%. These low
values reflect the implementation of the NMFS' Improved Retention
/Improved Utilization program. Prior to the implementation of the
American Fisheries Act (AFA) in 1999, higher discards may have occurred
under the "race for fish" and pollock marketable sizes were caught
incidentally. Since implementation of the AFA, the vessel operators have
more time to pursue optimal sizes of pollock for market since the quota
is allocated to vessels (via cooperative arrangements). In addition,
several vessels have made gear modifications to avoid retention of
smaller pollock. In all cases, the magnitude of discards counts as part
of the total catch for management (to ensure the TAC is not exceeded)
and within the assessment. Bycatch of other non-target, target, and
prohibited species is presented in the section titled Ecosystem
Considerations below. In that section it is noted that the bycatch of
pollock in other target fisheries is more than double the bycatch of
other target species (e.g., Pacific cod) in the pollock fishery.
## Management measures
The EBS pollock stock is managed by NMFS regulations that provide limits
on seasonal catch. The NMFS observer program data provide near real-time
statistics during the season and vessels operate within well-defined
limits. In most years, the TACs have been set well below the ABC value
and catches have stayed within these constraints \cref{tab:abctac}).
Allocations of the TAC split first with 10% to western Alaska
communities as part of the Community Development Quota (CDQ) program and
the remainder between at-sea processors and shore-based sectors. For a
characterization of the CDQ program see @haynie2014. @seung2016 combined
a fish population dynamics model with an economic model to evaluate
regional impacts.
Due to concerns that groundfish fisheries may impact the rebuilding of
the Steller sea lion population, a number of management measures have
been implemented over the years. Some measures were designed to reduce
the possibility of competitive interactions between fisheries and
Steller sea lions. For the pollock fisheries, seasonal fishery catch and
pollock biomass distributions (from surveys) indicated that the apparent
disproportionately high seasonal harvest rates within Steller sea lion
critical habitat could lead to reduced sea lion prey densities.
Consequently, management measures redistributed the fishery both
temporally and spatially according to pollock biomass distributions.
This was intended to disperse fishing so that localized harvest rates
were more consistent with estimated annual exploitation rates. The
measures include establishing: 1) pollock fishery exclusion zones around
sea lion rookery or haulout sites; 2) phased-in reductions in the
seasonal proportions of TAC that can be taken from critical habitat; and
3) additional seasonal TAC releases to disperse the fishery in time.
Prior to adoption of the above management measures, the pollock fishery
occurred throughout each of the three major NMFS management regions of
the North Pacific Ocean: the Aleutian Islands (1,001,780 km$^2$ inside
the EEZ), the Eastern Bering Sea (968,600 km$^2$), and the Gulf of
Alaska (1,156,100 km$^2$). The marine portion of Steller sea lion
critical habitat in Alaska west of 150$^{\circ}$W encompasses 386,770
km$^2$ of ocean surface, or 12% of the fishery management regions.
From 1995--1999 84,100 km$^2$, or 22% of the Steller sea lion critical
habitat was closed to the pollock fishery. Most of this closure
consisted of the 10 and 20 nm radius all-trawl fishery exclusion zones
around sea lion rookeries (48,920 km$^2$, or 13% of critical habitat).
The remainder was largely management area 518 (35,180 km$^2$, or 9% of
critical habitat) that was closed pursuant to an international agreement
to protect spawning stocks of central Bering Sea pollock. In 1999, an
additional 83,080 km$^2$ (21%) of critical habitat in the Aleutian
Islands was closed to pollock fishing along with 43,170 km$^2$ (11%)
around sea lion haulouts in the GOA and Eastern Bering Sea. In 1998,
over 22,000 t of pollock were caught in the Aleutian Island region, with
over 17,000 t taken within critical habitat region. Between 1999 and
2004 a directed fishery for pollock was prohibited in this region.
Subsequently, 210,350 km$^2$ (54%) of critical habitat in the Aleutian
Islands was closed to the pollock fishery. In 2000 the remaining
phased-in reductions in the proportions of seasonal TAC that could be
caught within the BSAI Steller sea lion Conservation Area (SCA) were
implemented.
On the EBS shelf, an estimate (based on observer at-sea data) of the
proportion of pollock caught in the SCA has averaged about 44% annually.
During the A-season, the average is also about 44%. Nonetheless, the
proportion of pollock caught within the SCA varies considerably,
presumably due to temperature regimes and the relative population age
structure. The annual proportion of catch has ranged from an annual low
of 11% in 2010 to high of 60% in 1998--the 2019 annual value was 58% and
quite high again in the A-season (68%). The higher values
in recent years were likely due to good fishing conditions close to the
main port. The recent transition from at-sea observer sampling of many
catcher vessels to a combination of at-sea electronic monitoring and
shore-based observer sampling has resulted in a temporary hiatus in to
associate catches with specific areas. Work has progressed to link the
position information to offloads so that haul records could be used to
evaluate fishing patterns.
The AFA reduced the capacity of the catcher/processor fleet and
permitted the formation of cooperatives in each industry sector by the
year 2000. Because of some of its provisions, the AFA gave the industry
the ability to respond efficiently to changes mandated for sea lion
conservation and salmon bycatch measures. Without such a catch-share
program, these additional measures would likely have been less effective
and less economical (@strong2014).
An additional strategy to minimize potential adverse effects on sea lion
populations is to disperse the fishery throughout more of the pollock
range on the Eastern Bering Sea shelf. While the distribution of fishing
during the A-season is limited due to ice and weather conditions, there
appears to be some dispersion to the northwest area
(@fig-catch_distn_a).
The majority (about 56%) of Chinook salmon caught as bycatch in the
pollock fishery originate from western Alaskan rivers. This was updated
at the June 2022 Council meeting and is activities are monitored and
reported closely at the Council ([at this
website](https://www.npfmc.org/fisheries-issues/bycatch/salmon-bycatch/)).
In summary, additional Chinook salmon bycatch management measures went into effect
in 2011 which imposed revised prohibited species catch (PSC) limits.
These limits, when reached, close the fishery by sector and season
(Amendment 91 to the BSAI Groundfish Fishery Management Plan (FMP)
resulting from the NPFMC's 2009 action). Previously, all measures for
salmon bycatch imposed seasonal area closures when PSC levels reached
the limit (fishing could continue outside of the closed areas). The
current program imposes a dual cap system by fishing sector and season.
A goal of this system was to maintain incentives to avoid bycatch at a
broad range of relative salmon abundance (and encounter rates).
Participants are also required to take part in an incentive program
agreement (IPA). These IPAs are approved and reviewed annually by NMFS
to ensure individual vessel accountability. The fishery has been
operating under rules to implement this program since January 2011.
Further measures to reduce salmon bycatch in the pollock fishery were
developed and the Council took action on Amendment 110 to the BSAI
Groundfish FMP in April 2015. These additional measures were designed to
add protection for Chinook salmon by imposing more restrictive PSC
limits in times of low western Alaskan Chinook salmon abundance. This
included provisions within the IPAs that reduce fishing in months of
higher bycatch encounters and mandate the use of salmon excluders in
trawl nets. These provisions were also included to provide more flexible
management measures for chum salmon bycatch within the IPAs rather than
through regulatory provisions implemented by Amendment 84 to the FMP.
The new measure also included additional seasonal flexibility in pollock
fishing so that more pollock (proportionally) could be caught during
seasons when salmon bycatch rates were low. Specifically, an additional
5% of the pollock can be caught in the A-season (effectively changing
the seasonal allocation from 40% to 45% (as noted above in the
discussion assosciated with @fig-prop_a_season). These measures are all
part of Amendment 110 and a summary of this and other key management
measures is provided in \cref{tab:mgt}.
There are three time/area closures in regulation to minimize herring PSC
impacts: *Summer Herring Savings Area 1* an area south of 57$^\circ$N
latitude and between 162$^\circ$W and 164$^\circ$W longitude from June
15 through July 1st. *Summer Herring Savings Area 2* an area south of
56$^\circ$ 30' N latitude and between 164$^\circ$W and 167$^\circ$W
longitude from July 1 through August 15. *Winter Herring Savings Area*
an area between 58$^\circ$ and 60$^\circ$N latitude and between
172$^\circ$W and 175$^\circ$W longitude from September 1st through March
1st of the next fishing year.
# Data
The following lists the data used in this assessment:
```{=tex}
\begin{table}[ht]
\centering
\label{tab:dataextent}
\begin{tabular}{p{1.5in}p{1.8in}p{2.8in}}
\hline
Source & Type & Years \\
\hline
Fishery & Catch biomass & 1964--`r thisyr` \\
Fishery & Catch age composition & 1964--`r lastyr` \\
Fishery & Japanese trawl CPUE & 1965--1976 \\
EBS bottom trawl & Area-swept biomass and age-specific proportions & 1982--2019, 2021-`r thisyr ` \\
Acoustic trawl survey & Biomass index and age-specific
proportions & 1994, 1996, 1997, 1999, 2000, 2002, 2004, 2006--2010, 2012, 2014, 2016, 2018, 2020, 2022 \\
Acoustic vessels of opportunity (AVO) & Biomass index & 2006--2019, 2021-`r thisyr` \\
\hline
\end{tabular}
\end{table}
```
*Note the 2020 acoustic survey data based on unmanned surface vessel
(USV) transects and age-specific proportions were unavailable in this
year*
## Fishery
### Catch
Biological sampling by scientifically trained observers form the basis
of a major data component of this assessment (as evaluated in Barbeaux
et al. 2005). The catch-at-age composition was estimated using the
methods described by @kimura1989 and modified by @dorn1992.
Length-stratified age data are used to construct age-length keys for
each stratum and sex. These keys are then applied to randomly sampled
catch length frequency data. The stratum-specific age composition
estimates are then weighted by the catch biomass within each stratum to
arrive at an overall age composition for each year. Data were collected
through shore-side sampling and at-sea observers (@barbeaux2005). The three strata for
the EBS were: i) January--June (all areas, but mainly east of
170$^\circ$W); ii) INPFC area 51 (east of 170$^\circ$W) from
July--December; and iii) INPFC area 52 (west of 170$^\circ$W) from
July--December. This method was used to derive the age compositions from
1991--`r lastyr` (the period for which all the necessary information is
readily available). Prior to 1991, we used the same catch-at-age
composition estimates as presented in @wespestad1996.
The catch-at-age estimation method uses a two-stage bootstrap
re-sampling of the data. Observed tows were first selected with
replacement, followed by re- sampling actual lengths and age specimens
given that set of tows. This method allows an objective way to specify
the starting values for the input sample size for fitting fishery age
composition data within the assessment model. In addition, estimates of
stratum-specific fishery mean weights-at-age (and variances) are
provided which are useful for evaluating general patterns in growth and
growth variability. For example, @ianelli2007 showed that seasonal
aspects of pollock condition factor could affect estimates of mean
weight-at-age. They showed that within a year, the condition factor for
pollock varies by more than 15%, with the heaviest pollock caught late
in the year from October-December (although most fishing occurs during
other times of the year) and the thinnest fish at length tending to
occur in late winter. They also showed that spatial patterns in the
fishery affect mean weights, particularly when the fishery is shifted
more towards the northwest where pollock tend to be smaller at age.
@gruss2021 showed cold-pool-extent impacts on the spatial map
of summer condition and relating environmental conditions to fish
condition continues to be an active area of research.
In 2011 the winter fishery catch consisted primarily of age 5 pollock
(the 2006 year class) and later in that year age 3 pollock (the 2008
year class) were present. In 2012--2016 the 2008 year class was
prominent in the catches with 2015 showing the first signs of the 2012
year-class as three year-olds in the catch (@fig-catage;
\cref{tab:fshage}). However, by 2017 the 2013 year-class began to be
also evident and surpassed the 2012 year-class in dominance and persist
through to 2021. The unusual pattern of switching adjacent year-classes
was examined in 2021 to see if there was a pattern of spatial
differences. There was a distinct spatial distribution of the different
year-classes. Having adjacent strong year-classes appears to be a new
characteristic of the stock. In 2020, an unusual presence of age-2
pollock appeared in the catch, along with some from the 2014 year-class
while the 2012 year-class was a smaller part of the catch (@fig-catage).
By 2021 and 2022, the predominance of 3- and 4-year olds in the catch confirms
the abundance year-class from 2018. We note that the center of locations
of the 2018 year-class, as plotted based on the locales of samples from that
cohort, appears to be more oriented to the south east (by age) when compared to
another abundant year-class (the 2008; @fig-fsh_cohort_locales).
The sampling effort for age determinations, weight-length measurements,
and length frequencies is shown in \cref{tab:fshmeas},
\cref{tab:lwmeas}, and \cref{tab:fshn}. Sampling for pollock lengths and
ages by area has been shown to be relatively proportional to catches.
The precision of total pollock catch biomass is considered high with
estimated CVs to be on the order of 1% (@miller2005).
Scientific research catches are reported to fulfill requirements of the
Magnuson-Stevens Fisheries Conservation and Management Act. The annual
estimated research catches (1963--2022) from NMFS surveys in the Bering
Sea and Aleutian Islands Region are given in (\cref{tab:rescatch}).
Since these values represent extremely small
fractions of the total removals (about 0.02%) they are ignored for
assessment purposes.
## Surveys
### Bottom trawl survey (BTS)
Trawl surveys have been conducted annually by the AFSC to assess the
abundance of crab and groundfish in the Eastern Bering Sea since 1979
and since 1982 using standardized gear and methods. For pollock, this
survey has been instrumental in providing an abundance index and
information on the population age structure. This survey is complemented
by the acoustic trawl (AT) surveys that sample mid-water components of
the pollock stock. Between 1991 and 2023 the BTS biomass estimates
ranged from 2.28 to 8.39 million t (\cref{tab:btsbiom}) for the
design-based estimates). The values used for the assessment (VAST index,
see @sec-vast for details) are shown in @fig-bts_biom. In the
mid-1980s and early 1990s several years resulted in above-average
biomass estimates. The stock appeared to be at lower levels during
1996--1999 then increased moderately until about 2003 and since then has
averaged just over 4 million t (from the standard EBS region using
design-based estimators).
These surveys also provide consistent
measurements of environmental conditions, such as the sea surface and
bottom temperatures. Large-scale zoogeographic shifts in the EBS shelf
documented during a warming trend in the early 2000s were attributed to
temperature changes (e.g., @mueter2008). However, after the
period of relatively warm conditions ended in 2005, the next eight years
were mainly below average, indicating that the zoogeographic responses
may be less temperature-dependent than they initially appeared (@kotwicki2013).
Bottom temperatures increased in 2011 to about average
from the low value in 2010 but declined again in 2012--2013. In
the period 2014--2016, bottom temperatures increased and reached a new
high in 2016. In 2018 bottom temperatures were nearly as warm (after
2017 was slightly above average) but was highly unusual due to the
complete lack of "cold pool" (i.e., a defined area where water near
bottom was less than zero degrees. In 2019, the mean bottom temperature
was the warmest during the period the survey has occurred (since 1982;
@fig-bts_temp). In 2022 and 2023, the bottom temperatures have declined
but remain above average.
The AFSC has expanded the area covered by the bottom trawl survey over
time. In 1987 the "standard survey area" comprising 6 main strata was
increased farther to the northwest and covered in all subsequent years.
These two northern strata have varied in estimated pollock abundance. In
2023 about 10% of the pollock biomass was found in these strata compared
to a long term average of 5% (\cref{tab:btsbiom}). Importantly, this
region is contiguous with the Russian border and the NBS region, and
treatment of the extent stock shifts between regions continues (e.g.,
@oleary2021).
After the increase in 2022, the 2023 survey estimate is similar to the 2021 value and is about
72% of the long term mean. The 2023 pollock density by station appeared to be lower overall
with some slight increases on the outer shelf area to the northwest (@fig-bts_3d).
The VAST model provides density-weighted population shifts in distribution. This
can be expressed in north-south and east-west trends over time. A representation of
such center of gravity estimates indicate that the stock has moved steadily north since the mid 2000s,
but last year shifted south. This year it has shifted back north to some degree (@fig-cog).
The stock center of gravity also moved east from 2010 to about 2017, then shifted west and seems about at it's long term mean.
The BTS abundance-at-age estimates show variability in year-class
strengths with substantial consistency over time (@fig-bts_age).
The abundance of 5-year old pollock (the 2018 year-class) dropped from
2022, but still represents the most abundant year class. The abundance of age-1 pollock in
2023 appears to be about average.
Pollock above 40 cm in length generally appear to be fully selected and in some
years, many 1-year olds occur on or near the bottom (with modal lengths
around 10--19 cm). Generally speaking, age 2 or 3 pollock (lengths
around 20--29 cm and 30--39 cm, respectively) are relatively rare in
this survey because they tend to be more pelagic as juveniles. Compared
to recent years, the size compositions were consistent with the
mid-range categories and consistent with the age data
(@fig-bts_lenfreq).
Observed fluctuations in survey estimates may be attributed to a variety
of sources including unaccounted-for variability in natural mortality,
survey catchability, and **horizontal** migrations and **vertical
availability** (@monnahan2021; @oleary2022). As an example, some strong
year classes appear in the surveys over several ages (e.g., the 1989
year class) while others appear only at older ages (e.g., the 1992 and
2008 year class). Sometimes, initially strong year classes appear to wane
in successive assessments (e.g., the 1996 year class estimate (at age 1)
dropped from 43 billion fish in 2003 to 32 billion in 2007
(@ianelli2007)). Retrospective analyses (e.g., @parma1993) have also
highlighted these patterns, as presented in Ianelli et al. (2006, 2011).
@kotwicki2013 also found that the catchability of either the BTS or AT
survey for pollock is variable in space and time because it depends on
environmental variables, and is density-dependent in the case of the BTS
survey.
The 2023 survey age compositions were developed from age-structures
collected during the survey (June-July) and processed at the AFSC labs
within a few weeks after the survey was completed. The level of sampling
for lengths and ages in the BTS is shown in \cref{tab:btsn}. The
estimated numbers-at- age from the BTS for strata 1--9 (except for
1982--84 and 1986, when only strata 6 were surveyed) are presented in
\cref{tab:btsage} (based on the method in @kotwicki2014 and then using
VAST--see @sec-vast for those details). Compared to the previous
design-based age composition estimates, those derived from the
spatio-temporal model were generally very similar (@fig-bts_vast_age).
In the previous assessments, the BTS mean body mass-at-ages was computed
based on the sex-specific mean length-at-age in each year and converted
to weight using sex-specific length-weight parameters that were
estimated from data prior to 1999. In reconsidering this approach, data
on weight-at-age from intervening years have become available and some
new methods applied including those corrected by spatio-temporal
modeling (@indivero2023). This work was adopted in 2022
and values used are shown in \cref{tab:wtbts}. The
time series of BTS survey indices is shown in \cref{tab:btsabund}.
The NBS survey area was sampled in 2010, 2017, 2018 (limited to 49
stations), 2019, and 2021-2023. Given that the pollock
abundance was quite high in 2017 and 2018, a method for incorporating
this information as part of the standard survey was desired. One
approach for constructing a full time series that includes the NBS area
is to use observed spatial and temporal correlations. We used the
vector-autoregressive spatial temporal (VAST) model of @thorson2018b
together with the density-dependent corrected CPUE values from each
station (including stations where pollock were absent;
\cref{tab:btsabund}). Please refer to the @sec-vast for further details
on the implementation. The appendix also includes results that indicate the
VAST model diagnostics are reasonable and provide consistent
interpretations relative to the observations. Notably, results indicate
increased uncertainty in years and areas when stations were missing. As
noted in past assessments, application of this index within the stock
assessment model required accounting for the time-series covariance
estimate.
To date, given other commitments, work on comparing the age-and-growth
from NBS samples has stalled. We hope to evaluate these data when they
become available in the near future to look at maturity and growth
conditions from this region.
### Acoustic trawl surveys
Acoustic trawl surveys are typically conducted every other year and are
designed to estimate the off- bottom component of the pollock stock
(compared to the BTS which are conducted annually and provide an
abundance index of the near-bottom pollock). Estimated pollock biomass
for the EBS shelf has averaged over 3.2 million t since the time-series
was revised to include the water column to 0.5 m (from the historical
midwater pollock index to 3 m off bottom) starting in 1994
(\cref{tab:btsabund}). The early 2000s (a relatively 'warm' period) were
characterized by low pollock recruitment, which was subsequently
reflected in lower pollock biomass estimates between 2006 and 2012 (a
'cold' period; @honkalehto2015). In 2014 and 2016 (another 'warm'
period) with the growth of the strong 2012 year class, AT biomass
estimates increased to over 4 million t (\cref{tab:btsabund}). The
number of trawl hauls, lengths, and ages sampled from the AT survey are
presented in \cref{tab:atsn}. These surveys have also provided insight
on the relative abundance of pollock in areas considered critical to
Steller sea lions (the "SCA"; \cref{tab:atsbiom}).