i

Characterizing Movement and Spatial Occupancy of 

Northern Bering Sea Pacific Halibut

Principal Investigator: Andrew Seitz1

Collaborators: Austin Flanigan1, Julie Nielsen2, Tim Loher3,

1 College of Fisheries and Ocean Sciences, University of Alaska Fairbanks

2 Kingfisher Marine Research

3 Martingale Marine Ecological Research

May 2024

Final Report

OCS Study BOEM 2024-026
ii

Contact Information: 

uaf-cmi@alaska.edu 

https://www.uaf.edu/cfos/research/cmi

 

This study was funded by the University of Alaska Coastal Marine Institute and the U.S. 

Department of the Interior, Bureau of Ocean Energy Management Alaska OCS Region 

(cooperative agreement M21AC00013). This report, OCS Study BOEM 2024-026, is available 

electronically from https://www.boem.gov/akpubs

The views and conclusions contained in this document are those of the authors and should not be 

interpreted as representing the opinions or policies of the U.S. Government. Mention of trade 

names or commercial products does not constitute endorsement by the U.S. Government. 

Citation: Seitz, A.C., Flanigan, A.J., Nielsen, J.K., Loher, T. 2024. Characterizing movement 

and spatial occupancy of northern Bering Sea Pacific halibut. Coastal Marine Institute Final 

Report for OCS Study BOEM 2024-026 
iii

Table of Contents

List of Figures................................................................................................................................ iv

List of Tables .................................................................................................................................. v

Abstract: ........................................................................................................................................ vi

Chapter 1:........................................................................................................................................ 1

Introduction:................................................................................................................................ 1

Methods: ..................................................................................................................................... 3

Fish Capture ............................................................................................................................ 3

Tagging and Release ............................................................................................................... 3

PSAT Tag Specifications and Settings................................................................................... 3

Data analysis:.............................................................................................................................. 4

Results:........................................................................................................................................ 5

Discussion:................................................................................................................................ 11

Chapter 2:...................................................................................................................................... 14

Introduction:.............................................................................................................................. 14

Methods: ................................................................................................................................... 15

Data:...................................................................................................................................... 15

Data Analysis:....................................................................................................................... 15

Results:...................................................................................................................................... 17

Discussion:................................................................................................................................ 19

Acknowledgments: ....................................................................................................................... 21

Study Products:............................................................................................................................. 22

References:.................................................................................................................................... 23

Appendix:...................................................................................................................................... 28
iv

List of Figures

Figure 1.1: Bering Sea Outer Continental Shelf lease planning areas and Pacific halibut (Hippoglossus 

stenolepis) tagging locations, represented by the pink (Nome, AK) and yellow (Savoonga, AK) circles. ..2

Figure 1.2: Pop-up locations for all satellite tags attached to Pacific halibut (Hippoglossus stenolepis) in 

the northern Bering Sea that remained affixed until at least January 1st of the following year and 

successfully transmitted a location (n=42). Pop-up locations are color-coded by the month the tag 

detached from the fish. Symbol shape corresponds to tagging location, with octagons representing tags 

attached to Pacific halibut near Nome, AK and squares near Savoonga, AK. Outer Continental Shelf lease 

planning areas are overlaid. ..........................................................................................................................7

Figure 1.3: Monthly occupied depths (A) and temperatures (B) of Pacific halibut (Hippoglossus 

stenolepis) satellite tagged in the northern Bering Sea whose tags transmitted time-series data (n=61). 

Boxes represent the first and third quartiles, while the horizontal line is the median; whiskers extend out 

to the minimum and maximum values, except for any values that lie farther than 1.5 times the 

interquartile distance from the box, which are represented as points. ..........................................................8

Figure 1.4: Most likely movement paths for Pacific halibut (Hippoglossus stenolepis) satellite tagged 

near Nome, AK (Left, n=14) and Savoonga, AK (Right, n=20). Circles, color coded by month, are 

estimated daily locations generated from the Hidden Markov Model probability surface using a Viterbi 

algorithm. The black triangles represent Pacific halibut tag and release locations, while the purple 

squares are tag pop-up locations. Outer Continental Shelf lease planning areas are overlaid......................9

Figure 1.5: Depth time-series data (A) from a recovered satellite tag that was attached to a Pacific halibut 

(Hippoglossus stenolepis) near Nome, AK, that exhibited spawning rises. Data are at a 5-second 

resolution and plotted as a continuous line. The two rises occurred while the halibut was >200 m, with 

the first rise (B) occurring on 20 January and the second (C) on 25 January, lasting approximately 7 and 3 

minutes, respectively...................................................................................................................................10

Figure 2.1: Distribution of mean daily temperatures occupied by individual Pacific halibut (Hippoglossus 

stenolepis) in the northern Bering Sea. Boxplot summaries represent the first and third quartiles, the 

center bar is the mean and whiskers extend to the 95% level. All observations outside of this range are 

outliers and represented as points. ..............................................................................................................17

Figure 2.2: Extent of potentially suitable thermal habitat of Pacific halibut (Hippoglossus stenolepis) 

based on monthly mean bottom water temperatures from the pan-arctic Regional Ocean Modeling System 

output for the Bering and Chukchi seas in 2012. Polygons represent thermal conditions within the 50% 

core (red) and 95% (orange) thermal occupation range of Pacific halibut, as well as conditions below the 

lower end of the 95% thermal range and above the thermal minima threshold (blue). ..............................19

Figure 2.3: Extent of potentially suitable thermal habitat of Pacific halibut (Hippoglossus stenolepis) 

based on monthly mean bottom water temperatures from the pan-arctic Regional Ocean Modeling System 

output for the Bering and Chukchi seas in 2019. Polygons represent thermal conditions within the 50% 

core (red) and 95% (orange) thermal occupation range of Pacific halibut, as well as conditions below the 

lower end of the 95% thermal range and above the thermal minima threshold (blue). ..............................20
v

List of Tables

Table 1.1: Summary of the number of satellite tags deployed on Pacific halibut (Hippoglossus stenolepis) 

in the northern Bering Sea by tagging year, location, and pop-up schedule (when the tag is programmed to 

float to the surface to transmit data to satellites). For each group of tags, the number of tags used in end 

location, depth and temperature time-series, and Hidden Markov Model (HMM) analyses is also 

provided. .......................................................................................................................................................6

Table 2.1: Extent of suitable thermal habitat (km2

) for Pacific halibut (Hippoglossus stenolepis) in the 

Chukchi Sea by month in the cold year of 2012. The total habitat represents the sum of the 95% summer 

and the thermally tolerable waters. .............................................................................................................18

Table 2.2: Extent of suitable thermal habitat (km2

) for Pacific halibut (Hippoglossus stenolepis) in the 

Chukchi Sea by month in the warm year of 2019. The total habitat represents the sum of the 95% 

summer and the thermally tolerable waters. The increase multiplier is the magnitude of increase in 

available habitat between the 95% summer occupation ranges of 2012 and 2019.....................................18

Table A1: Deployment information for pop-up satellite archival tags attached to 84 Pacific halibut 

(Hippoglossus stenolepis) in the northern Bering Sea. For tags that successfully transmitted, the date the 

tag began transmitting data, and the number of messages sent to satellites, is provided. Additionally, 

when the tag provided a useful end location, the horizontal displacement (km) of the tag from its tagging 

location is provided.....................................................................................................................................28
vi

Abstract:

Since the 1970s, the fish assemblage has been changing in the northern Bering Sea (NBS), where 

rising water temperatures have correlated with increases in abundance of sub-arctic fish species. One 

such species is the Pacific halibut (Hippoglossus stenolepis), an important flatfish that is a valuable 

resource in the NBS and may expand into the southern Chukchi Sea. To characterize the possible 

interactions between Pacific halibut and potential future human activities in the Bering and Chukchi seas, 

such as oil and gas exploration, marine mineral extraction, and renewable energy development, this 

species’ movements and spatial dynamics must be understood. This information is currently scarce, and 

as such, the possible interactions between human activities and Pacific halibut are unknown. This study 

examined spatial dynamics of large female Pacific halibut in the NBS using pop-up satellite telemetry 

tags, from which reconstructed movement tracks were generated using Hidden Markov modeling 

(Chapter 1). Additionally, to evaluate potential halibut occupation of the Chukchi Sea, recorded thermal 

conditions from tagging efforts in the NBS were compared to modeled water temperatures in the Chukchi 

Sea to determine the spatial and temporal extent of suitable conditions within the region (Chapter 2). 

Tagged individuals remained within the NBS and central Bering Sea and crossed multiple Outer 

Continental Shelf (OCS) lease planning area boundaries, as well as the Russian maritime border. During 

the winter season, Pacific halibut made long migrations to the continental shelf edge for inferred

spawning purposes, with fish occupying St. George Basin and Navarin Basin planning areas, as well as 

coastal Russian waters along Kamchatka. Individuals returned to their tagging location the following 

year, which indicated inter-annual site fidelity to summer foraging areas within Norton Basin in the NBS. 

No Pacific halibut were tracked entering the Hope Basin planning area in the southern Chukchi Sea, 

despite the presence of thermal conditions that are similar to those currently occupied in the NBS. Pacific 

halibut primarily occupied waters from 6.0 to 8.2°C during the summer months, but were regularly found 

in conditions from 3.8 to 9.8°C during the summer and conditions around 0.9°C during the winter. The 

availability of these conditions in the Chukchi Sea was relatively limited during a recent cold year, 

constrained to eastern coastal waters near Alaska with maximal availability during August and 

September, but were 2.5 to 7.8 times more prevalent during a warm year, comprising a large portion of 

the Chukchi Sea from July to October. 

Results indicate Pacific halibut occupy multiple OCS planning areas throughout the year, with 

seasonal variation in occupied areas. Pacific halibut may interact with potential resource exploration and 

developments across these planning areas in the Bering Sea, but at their current extent are much less 

likely to do so in the Chukchi Sea. However, the potential for these interactions remains, where suitable 

thermal habitat for Pacific halibut is currently available in the Chukchi Sea, although alternative factors 

such as food availability appear to be discouraging these movements.
1

Chapter 1: 

Introduction: 

The Bering Sea is a productive high latitude, sub-arctic sea that is traditionally characterized by 

rich benthic infauna populations and large concentrations of winter sea ice cover (Cui et al., 2012). Large 

sea ice extents traditionally generate a deep-water cold pool, when cold surface water made more saline 

by freezing and brine formation sinks to the ocean floor and persists throughout the warm summer 

months (Mueter and Litzow, 2008). These waters range between -2 and 2°C and may encompass a large 

portion of the eastern Bering Sea shelf, with cold years seeing these conditions extend from Bristol Bay to 

the Russian coast (Figure 1.1) (Kinney et al., 2022). This cold pool creates a thermal barrier, limiting the 

northward migration of more temperate species in the Bering Sea; however, in some recent years the cold 

pool has not formed, and as a result, bottom water temperatures have increased by several degrees Celsius 

(Sheffield, 2019). This rapid change has resulted in a regime shift, with the distribution of numerous 

species expanding northward into the northern Bering Sea (NBS) (Grebmeier et al., 2006). In particular, 

sub-arctic fish species traditionally found further south have been observed moving northward to exploit 

warming sea temperatures, expanding into these newly habitable waters (Stevenson and Lauth, 2019). 

Groundfishes including Pacific Cod (Gadus microcephalus) and Walleye Pollock (Gadus 

chalcogrammus) are among the species exhibiting these northward movements into the NBS (AFSC, 

2019). Another species exhibiting these movements, and a consequential increase in abundance, is the 

Pacific halibut (Hippoglossus stenolepis) (Conners et al., 2002). 

The Pacific halibut is a large, long-lived, demersal flatfish species that is distributed across the 

North Pacific Ocean. The Pacific halibut (hereafter referred to as “halibut”) is a highly migratory species 

that exhibits large-scale movements when they move between summer foraging areas in shallow 

continental shelf waters and deep winter spawning areas on the continental shelf edge and slope (Loher 

and Seitz, 2008). The species is commercially valuable and has been heavily targeted throughout its 

range, where in Alaska, the species also supports important recreational and subsistence fisheries 

throughout the Bering Sea, Aleutian Islands, and Gulf of Alaska (Figure 1.1). This is especially true in 

the NBS, a remote area where rural residents rely heavily on subsistence and commercial harvest 

opportunities. Therefore, the northward expansion and increasing abundance of halibut may have 

profound impacts on the food security and livelihoods of local individuals by providing new harvest 

opportunities in the region (Thoman Jr et al., 2020). 

As their distribution expands, halibut may also interact with other human activities, such as 

potential resource exploration, extraction, and development in the NBS and Chukchi Sea. This area is 

divided into a number of Outer Continental Shelf (OCS) lease planning areas, including Hope Basin in 

the Chukchi Sea and Norton Basin, Navarin Basin, St. George Basin, and St. Matthew-Hall in the Bering 

Sea (hereafter referred to by lease planning area name). Many of these OCS lease planning areas may be 

explored for oil, minerals, and renewable energy suitability in the future, and as such, it is important to 

examine the spatial dynamics and occupied environmental conditions of halibut in these regions to 

provide valuable benchmark information that will aid in understanding possible interactions between this 

important food source and exploration activities. While halibut have been studied and comprehensively 

surveyed throughout much of their range, minimal research has occurred in the NBS. Currently, 

monitoring during an annual survey is primarily restricted to the outer shelf domain of the southern and 
2

central Bering Sea, with limited coverage across the central shelf and no stations in the NBS (IPHC, 

2023). As such, the recent expansion of halibut into the NBS has resulted in an understudied region, 

leaving an information gap in halibut movements and spatial dynamics. This study sought to use satellite 

tagging methods to assess movements and spatial occupancy of halibut in the NBS, including 1) 

identifying Outer Continental Shelf (OCS) planning areas in the Northern Bering Sea and potentially in 

the Southern Chukchi Sea that Pacific halibut currently occupy throughout the course of a year, 2) 

examining the timing and pathways of Pacific halibut movements among OCS planning areas, as well as 

the duration of occupation within a given planning area, 3) characterizing Pacific halibut spawning events 

to identify spawning grounds and the OCS planning areas in which they occur, and 4) characterizing the 

depths occupied by Pacific halibut to determine the potential exposure risk to disturbance events such as 

an oil spill, under the pretext that Pacific halibut are not exclusively benthic. By addressing these key 

knowledge gaps, this study sought to identify potential regions, and timing, where halibut may interact 

with potential future hydrocarbon activities. 

Figure 1.1: Bering Sea Outer Continental Shelf lease planning areas and Pacific halibut (Hippoglossus 

stenolepis) tagging locations, represented by the pink (Nome, AK) and yellow (Savoonga, AK) circles. 
3

Methods: 

Fish Capture 

All halibut were captured in the northern Bering Sea in waters north of Saint Lawrence Island, 

Alaska, within Norton Basin (Figure 1.1), during the commercial fishing season of July, August, and 

September. These efforts were based out of Nome and Savoonga, Alaska (Figure 1.1). Nome (64.351 N, 

166.440 W) is located on the north shore of Norton Sound and has a small commercial longline fleet. 

Savoonga (63.782 N, 170.406 W) lies on the north shore of Saint Lawrence Island. Savoonga is an 

Alaska Native village, supporting a small handline fleet that captures halibut for both commercial and 

subsistence purposes. During the summers of 2019, 2021, and 2022, fishing vessels were chartered out of 

both locations to conduct tagging efforts; in summer 2020, fishing was only conducted near Nome due to 

COVID-19 travel restrictions to Savoonga. Fishing gear varied across tagging vessels, but all fishing 

efforts were conducted using either fixed- or snap-hook longlining gear with baited (pink salmon 

Oncorhynchus gorbuscha or Pacific herring Clupea pallasii) circle hooks soaked for 3 to 12 hours. All 

halibut were brought on board by net or hand, precluding the use of a gaff. 

Tagging and Release 

All captured halibut were evaluated for tagging suitability once on the boat deck. Only fish that 

were in good health were considered for tagging to minimize post-release mortality, and any fish that had 

significant injuries, poor body condition, or large numbers of parasites were excluded from consideration. 

For healthy fish, fork length (FL) was measured and fish >100 cm FL were selected for tagging as halibut 

above this threshold are likely to be sexually mature females (Loher and Stevens, 2011). Due to 

numerous COVID-19 complications in 2020, tagging efforts were less selective and some tags were 

placed on fish smaller than this size threshold. 

Satellite tags were attached following previously described procedures (Seitz et al., 2003; Seitz et 

al., 2011). Tags were affixed to fish with a tether system that consisted of the tag, a leader, and a dart. 

The leader was a 13-cm length of 1.8-mm-diameter nylon monofilament and the dart a 62-mm long 

custom-fabricated stainless steel halibut dart. The leader was secured to the dart and the tag using 2.38-

mm stainless steel crimped swage sleeves wrapped at both ends in 3:1 adhesive-lined polyolefin (4.76 

mm). To insert the dart in the fish, a pilot hole was made in the dorsal surface on the eye side of the 

halibut, just behind the widest part of the fish. Using a dart applicator, the dart was inserted through the 

pilot hole and past the pterygiophores, or fin ray supports, at which point the applicator was retracted and 

the dart affixed under the pterygiophores. After ensuring the tag and tether were properly secured to the 

fish, the individual was released head first. 

PSAT Tag Specifications and Settings 

Satellite tags were Wildlife Computers miniPATs, which weigh 60 g in air with total dimensions 

of 124 mm in length by 38 mm in width. These tags have a memory of up to 64 megabytes and an 

operating life up to two years. Each tag contains light, temperature, pressure, and wet/dry sensors. Over 

the course of a deployment, each tag measures and records depth (resolution 0.5 m, range 0–1700 m), 

temperature (resolution 0.05°C, range -20–50°C), and ambient light (10-2 to 10-10 W/cm2 at 440 nm) 

readings at 5 second intervals. These tags remain attached to the fish collecting these data until a prescheduled pop-up date, at which time the tag releases from the fish and floats to the surface of the ocean

to transmit summarized archived data to satellites, during which time an end location is calculated.

Scheduled pop-up dates used in this study varied across tagging years, but in general tags were 

set to release during two time periods: winter and summer. Winter pop-ups served to identify spawning 
4

grounds and the magnitude of annual migratory movements for NBS halibut, while summer pop-ups 

provided an indication of the degree of site fidelity to feeding grounds in the NBS. Winter tags were set 

to release between January 15th and April 1st of the year following tagging, and summer tags released 

approximately 1 year after deployment, from mid-July to mid-September. If a tag came free from the 

halibut prior to the scheduled pop-up date, the tags were set to begin transmission after 1 day at the 

surface, determined via the tags’ pressure sensors. When transmitting, the tags’ internal programming 

summarized archived data to minimize the number of outgoing messages. Ambient light data were

condensed into sunrise and sunset light curves for each day, while depth and temperature recordings were

sampled at a regular interval, providing equally spaced point readings for each day at liberty. During 

2019 and 2020 these were sub-sampled at 10-minute intervals, and in the subsequent years of 2021 and 

2022 at five-minute intervals. To reduce the number of messages needed for the increased temporal 

resolution in 2021 and 2022, temperature data were only transmitted for every third day at liberty. For 

tags that are physically recovered, the full data time-series can be recovered, providing occupied depth 

and temperature readings every 5 seconds during the tags’ time at liberty. 

Data analysis: 

To understand the horizontal movement of tagged halibut, end locations were assigned as the 

location of first transmission of each tag with an Argos location class of 2 or 3, corresponding to an 

accuracy of <500 m and <250m, respectively. Horizontal displacement was calculated as the great circle 

distance between tagging and end locations. For tags that reported before their scheduled pop-up dates, 

only end locations from those that reported after 1 January of the year following tagging were included in 

horizontal displacement analyses. 

Habitat occupation of halibut was characterized by determining occupied mean daily depths and 

temperatures for each individual. For all days during which a tag was attached to a halibut and provided 

time-series data, a daily mean depth and temperature was calculated from all available subsampled 

readings. These individual daily means were then aggregated across all individuals for each day that data 

were available, providing a distribution of occupied daily conditions that were summarized in monthly 

boxplots, providing the median, quartile, and 95% range of occupied conditions of all tagged halibut. For 

subsequent movement modeling, movement states were assigned for each halibut by qualitatively 

examining the mean daily depth data. Periods when daily means changed steadily, indicating onshore or 

offshore movement across the Bering Sea shelf, were assigned as a migratory state. Conversely, times 

when these depths did not change appreciably, indicating localized movements, were assigned as a 

residency state. Occupied depth and temperature conditions were also summarized (mean ± SD) for the 

periods designated as the residency state, as these time periods correspond to the summer foraging and 

winter spawning seasons: Here, the daily depth and temperature means were aggregated across all 

individuals across all days in the specified time period. 

For all tagged halibut for which end locations and depth time-series data were available, most 

likely movement paths were reconstructed using a Hidden Markov Model (HMM). HMMs are highly 

flexible non-parametric state space models (Pedersen, 2008) that use a movement model to predict where 

an individual can move within the study area and a likelihood model to identify where in the study area 

conditions best align with observed tag data (Nielsen and Sibert, 2007). An HMM was adapted for use on 

demersal fishes in the Bering Sea (Nielsen et al., 2019; Nielsen et al., 2023), in which the model 

framework discretized the Bering Sea into grid cells (3-km2 resolution) on a daily time scale. The 

movement model executes a random walk to simulate fish movement using a symmetrical two-
5

dimensional diffusion kernel that can vary in size. Herein, the kernel varied based on movement state, 

with 20 km2 and 100 km2 being selected for the residency and migratory states, respectively: After 

testing multiple kernel sizes, these values were chosen as they allowed model convergence 

across individuals, and were consistent with values chosen to represent both residential (Nielsen 

et al., 2014) and migratory (J.K. Nielsen, unpublished) groundfish movements within the region. 

The likelihood model relies on depth and light data, which are used to generate maximum occupied depth 

and longitude estimates daily. Maximum daily depth was utilized following the assumption that halibut

occupy the seafloor at some time each day, and therefore the maximum occupied depth for each day 

corresponds to ocean depth. Longitude estimates were generated via Wildlife Computer’s proprietary 

software, Global Position Estimator 2 (GPE2), and qualitatively filtered due to previous studies 

documenting a high degree of variability in generated estimates (Seitz et al., 2006): Only series of 

consistent estimates that did not result in a daily change of >2° in longitude were retained (Teo et al., 

2004). The model executes a forward filter to iteratively estimate the probability of a fish’s presence 

across days based on the movement and likelihood models, subsequently followed by a backwards 

smoothing that adjusts estimated probabilities based on the end location, yielding a probability density 

surface for each day the fish was at liberty. Movement tracks were generated from the final smoothed 

probability using a Viterbi algorithm, which estimates the most likely locations for each day (Nielsen et 

al., 2023). The lengths of these tracks were determined for individuals and summarized (mean ± SD) for 

each tagging location, and one-sided independent t-tests with unequal variances were used to test for 

differences in track lengths for both winter and summer pop-ups between the two deployment locations. 

Reconstructed movement tracks and final HMM probability density surfaces were also used to 

assess occupation and potential cross-boundary movements of halibut among OCS leasing areas. Each 

individual movement track was qualitatively examined and the proportion of halibut that emigrated into

regulatory areas outside that in which they were tagged was determined for each tagging location. 

For any tags that were physically recovered, depth time-series data were assessed for spawning 

activity. Active spawning behavior was identified as serial rises in the water column separated by 

approximately 65 hours (Seitz et al., 2005), when halibut make rapid ascents of 75 m or more, starting at 

depths >200 m, to release their gametes, followed by a steady return to depth (Loher and Seitz, 2008).

For all identified spawning rises, a location was assigned from the corresponding day of the reconstructed 

movement track for the individual. All analyses were conducted using R version 4.3.1 (R Core Team, 

2023).

Results:

A total of 84 satellite tags were deployed in the NBS (Table A1) on halibut ranging in size from 

89 to 175 cm in fork length (112.98 ± 16.77 cm). In total, 56 tags were deployed out of Nome and 28 out 

of Savoonga, AK with the deployment durations split between winter (n=52) and summer (n=32) pop-ups

(Table 1.1). Of the deployed tags, 64 (76.2%) successfully communicated with satellites and transmitted 

data, with received messages ranging from 3 to 8,575 (2,401 ± 2,167) for individual tags. Three of these 

tags were recovered after transmitting, providing the complete data time-series from the deployment. Not 

all tags provided sufficient data for each analysis, and of these 64 tags, 42 were utilized in horizontal 

displacement, 61 in occupied habitat, and 34 in HMM modeling analyses. Tags that had insufficient data 

for analyses experienced premature releases (n=10), extended delays between popping up and data 

transmission resulting in unusable end locations (n=7), and either low numbers of or corrupted messages 
6

that yielded extremely sparse time-series data (n=13). Additionally, 20 (23.8%) tags failed to 

communicate with satellites and did not provide any data. 

Table 1.1: Summary of the number of satellite tags deployed on Pacific halibut (Hippoglossus stenolepis) 

in the northern Bering Sea by tagging year, location, and pop-up schedule (when the tag is programmed to 

float to the surface to transmit data to satellites). For each group of tags, the number of tags used in end 

location, depth and temperature time-series, and Hidden Markov Model (HMM) analyses is also 

provided. 

# OF 

TAGS

TAGGING 

YEAR

TAGGING 

LOCATION

POP-UP 

SCHEDULE

END 

LOCATION

TIME 

SERIES

HMM

5 2019 Nome Winter 0 2 0

5 2019 Nome Summer 0 0 0

3 2019 Savoonga Winter 2 3 2

3 2019 Savoonga Summer 2 3 2

15 2020 Nome Winter 7 13 7

10 2020 Nome Summer 4 8 4

5 2021 Nome Winter 2 3 2

3 2021 Nome Summer 3 3 2

6 2021 Savoonga Winter 5 5 2

4 2021 Savoonga Summer 3 3 3

9 2022 Nome Winter 5 7 4

4 2022 Nome Summer 1 1 1

8 2022 Savoonga Winter 6 5 3

4 2022 Savoonga Summer 2 3 2

Of the 42 tags with useful end locations (Nome n=22, Savoonga n=20), all were located in the 

northern and central Bering Sea (Figure 1.2). End location displacements ranged between 305 and 1022

km (699.3 ± 170.7 km) for tags that popped-up during the winter period and 14 and 675 km (324.8 ± 

245.9 km) for those that popped-up during the summer. Winter end locations were located at or near the 

continental shelf edge, distributed from the Pribilof Islands to the Russian coast. Of the 28 tags that 

provided winter end locations, 8 (28%) were located in Russian waters, 14 (50%) were along the central 

shelf of Navarin Basin, and 6 (22%) were proximate to the Pribilof Islands in St. George Basin. Summer 

end locations were distributed across the continental shelf, from waters proximate to offshore winter 

locations to near the original release locations. Of the 14 tags that provided summer end locations, 4 

(28%) were in Russian waters, 5 (36%) were in distributed across St. Matthew-Hall, and 5 (36%) were 

proximate to initial tagging locations in Norton Basin. 

Temperatures occupied by halibut ranged from -1.9 to 13.0°C (Figure 1.3B), while occupied 

depths ranged from 0.5 to 696.5 m (Figure 1.3A). While halibut predominantly occupied waters near the 

seafloor, they were seen occupying shallow waters (Figure 1.3A) and making ascents into the water 

column (Figure 1.5A), when they often approached or reached the ocean’s surface. Tagged fish remained 

in shallow waters throughout the months of August, September, and October, when occupied depths (30.8 

± 12.3 m) and temperatures (6.1 ± 2.2°C) were relatively consistent across individuals. All individuals 

then made an offshore migration towards the continental shelf edge and slope during the months of 
7

November, December, and January. The halibut remained in deeper waters for the months of February, 

March, and April when occupied depths (196.5 ± 148.8 m) were relatively deep (maximum 696.5 m) and 

thermal conditions displayed little variability (2.2 ± 1.3°C). Following the winter deep-water occupation, 

halibut returned to shallow waters during the months of May, June, and July. 

Figure 1.2: Pop-up locations for all satellite tags attached to Pacific halibut (Hippoglossus stenolepis) in 

the northern Bering Sea that remained affixed until at least January 1st of the following year and 

successfully transmitted a location (n=42). Pop-up locations are color-coded by the month the tag 

detached from the fish. Symbol shape corresponds to tagging location, with octagons representing tags 

attached to Pacific halibut near Nome, AK and squares near Savoonga, AK. Outer Continental Shelf lease 

planning areas are overlaid.
8

Figure 1.3: Monthly occupied depths (A) and temperatures (B) of Pacific halibut (Hippoglossus 

stenolepis) satellite tagged in the northern Bering Sea whose tags transmitted time-series data (n=61). 

Boxes represent the first and third quartiles, while the horizontal line is the median; whiskers extend out 

to the minimum and maximum values, except for any values that lie farther than 1.5 times the 

interquartile distance from the box, which are represented as points.
9

Reconstructed movement tracks (Figure 1.4) were generated for fish tagged near both Nome 

(n=20) and Savoonga (n=14). For fish tagged near Nome with winter pop-ups (n=13), winter track 

lengths (range 961 to 1354 m, mean ± SD 1163.8 ± 121.5 km) were significantly longer than for halibut 

(n=10) tagged near Savoonga (range 571 to 1356 km, mean ± SD 911.7 ± 241.9 km; t(12) = 3.015, p = 

0.005). This was not the case for halibut with summer pop-ups, as fish tagged near Nome (n=7) had 

summer track lengths (range 1285 to 2430 km, mean ± SD 1938.4 ± 377.6 km) that were not significantly 

longer than for halibut (n=4) tagged near Savoonga (range 1199 to 2111 km, mean ± SD 1586.8 ± 384.1

km; t(6) = 1.470, p = 0.096). 

Figure 1.4: Most likely movement paths for Pacific halibut (Hippoglossus stenolepis) satellite tagged 

near Nome, AK (Left, n=14) and Savoonga, AK (Right, n=20). Circles, color coded by month, are 

estimated daily locations generated from the Hidden Markov Model probability surface using a Viterbi 

algorithm. The black triangles represent Pacific halibut tag and release locations, while the purple 

squares are tag pop-up locations. Outer Continental Shelf lease planning areas are overlaid.

Pacific halibut moved across multiple OCS leasing area boundaries during their tag deployments

(Figure 1.4). All reconstructed movement tracks crossed OCS leasing area boundaries, with many 

individuals tagged near Nome crossing into St. Matthew-Hall on their way to the continental shelf edge, 

with 6 (30%) then entering St. George Basin, 10 (50%) entering Navarin Basin, and 4 (20%) crossing into 

Russian waters. Fish tagged near Savoonga exhibited similar movements among planning areas, with 2 

(14%) entering St. George Basin, 4 (29%) moving into Navarin Basin, and 8 (57%) leaving US waters 

and residing across the Russian maritime border. For both tagging regions, the occupation of Norton 

Basin was predominantly during the late-summer and early-fall months (August, September, October) 

prior to offshore movements across St. Matthew Hall and into Navarin Basin, St. George Basin, and 
10

Russian waters during the late-fall and early-winter (November, December, January). The occupation of 

the offshore planning areas (Navarin Basin and St. George Basin) and Russian waters persisted through 

the late-winter and early-spring (February, March, April) before fish again crossed St. Matthew-Hall 

during the late-spring and early-summer (May, June, July) and returned to Norton Basin.

Figure 1.5: Depth time-series data (A) from a recovered satellite tag that was attached to a Pacific halibut 

(Hippoglossus stenolepis) near Nome, AK, that exhibited spawning rises. Data are at a 5-second 

resolution and plotted as a continuous line. The two rises occurred while the halibut was >200 m, with 

the first rise (B) occurring on 20 January and the second (C) on 25 January, lasting approximately 7 and 3 

minutes, respectively. 
11

Spawning rises were identified in the depth time-series of one of the recovered tags. This halibut 

(ID 202305) occupied waters >200 m from 18 January to 31 January, during which time it conducted two 

spawning rises, both in St. George Basin. The first was ~150 m in magnitude and occurred on 20 

January, while the second was ~75 m in magnitude and occurred on 25 January (Figure 1.5). Spawning 

rises were not evident in the depth time-series of the other two recovered tags. 

Discussion:

Halibut in the NBS appear to exhibit a similar annual migratory cycle as individuals from farther 

south in the species' range. Occupation of shallow continental shelf waters in Norton Basin is likely for 

foraging purposes, where these fish are predominantly present in the region during the warm months 

(August, September, and October) when halibut are thought to be primarily foraging (Best, 1979). 

Various prey species of halibut, such as Pacific Cod and Walleye Pollock, have also been observed 

inhabiting this region during this time (Mecklenburg et al., 2018; Stevenson and Lauth, 2019). Following 

the summer feeding period, halibut made large-scale offshore movements, migrating hundreds of 

kilometers. Unlike in other regions, where some halibut are observed occupying shallow coastal habitat

throughout the year (Loher and Seitz, 2006; Nielsen and Seitz, 2017), all fish tagged in the present study 

undertook this migration. This is likely a result of thermal conditions in the NBS being inhospitable to 

halibut over long periods of time, when the onset of sea ice results in bottom waters being colder (<0°C) 

than deeper and more southerly waters in the Bering Sea (Mueter and Litzow, 2008). Halibut were seen 

moving to both the west and south, presumably to reach these more favorable thermal conditions. It is 

also possible that winter emigration from the NBS continental shelf is due to the disappearance of 

available prey, as species such as the Pacific cod have been observed making similar southward 

movements (Nielsen et al., unpublished). 

In addition to escaping the unfavorable NBS winter conditions, the fall migratory movements of 

halibut are also likely for reproductive purposes. The tagged individuals traveled hundreds of kilometers 

offshore to deep continental slope waters for the winter months, conditions that correspond to previously 

inferred halibut spawning depths (Seitz et al., 2005) and spawning season (Loher and Seitz, 2008). Past

studies have documented halibut moving large distances to reach winter spawning grounds that ranged 

from the Gulf of Alaska (Loher and Seitz, 2008) to Pribilof and Middle canyons along the central Bering 

Sea continental shelf edge (Seitz et al., 2011). In the present study, halibut were observed migrating as 

far south as Pribilof Canyon in St. George Basin, and as far north as Navarin Basin and the Russian coast. 

One of the individuals occupying St. George Basin conducted two inferred spawning rises (Figure 1.5), 

further evidence that these movements and occupation of this region are for spawning purposes. These 

observations corroborate previous inference on the existence of spawning grounds in the Central Bering 

Sea that are also thought to be utilized by halibut tagged further south in the Bering Sea (Seitz et al., 

2011). Additionally, these observations provide the first evidence of inferred spawning activity in 

Russian waters by halibut that are commercially targeted within US waters. 

Following their winter occupation of offshore habitat, some halibut returned to shallow inshore 

waters proximate to their initial tagging location. This is an indication of site fidelity to foraging habitat 

in Norton Basin, where individuals were seen within 14 km of their tagging location after migrating as far 

as 2400 km during their 1-year at liberty. Fidelity to foraging habitat has been observed in the past in 

halibut tagged in the Gulf of Alaska (Loher, 2008) and Glacier Bay, Alaska (Nielsen and Seitz, 2017). In 

contrast, many individuals tagged in the NBS were still hundreds of kilometers away when their 

scheduled pop-off date was reached, but many of these release locations appear to have occurred while 
12

the fish were still in transit to their summer feeding locations. This is likely due to interannual variability 

of tagging procedure and ocean conditions in the NBS. When this project was initiated, the NBS was 

extremely warm (Stabeno and Bell, 2019) and halibut were captured and tagged in July, resulting in their 

tags being set to release in the following July. In contrast, the following years saw colder conditions, with 

satellite tag pop-up locations occurring in shelf waters south of the NBS and the NBS fleet reporting 

limited catches up until mid- to late-August. This suggests that halibut migration occurred later than in 

warm conditions, with their arrival to the region being delayed due to colder waters. As such, tagging 

efforts and summer pop-off schedules were instead set to occur in August in the final year of tagging. 

This yielded summer displacements of 14, 78, and 371 km from initial release locations, supporting the 

presence of fidelity to NBS foraging habitat.

Despite being primarily a demersal fish species, tagged halibut were also seen utilizing the water 

column throughout the year. Regardless of where potential human activities and development occur, 

there is potential for interaction with halibut throughout the entire water column. However, occupation of 

surface waters was short-term and irregular, predominantly occurring outside of the halibuts’ spawning 

season. As such, the probability of interactions between halibut and human activities in surface waters is 

greatly reduced relative to pelagic species that regularly occupy surface waters, such as the Dolly Varden 

charr (Salvelinus malma)(Seitz et al., 2014), and surface-based human activities are not likely to impact 

halibut spawning behavior. 

While satellite tagging experiments on halibut in the Bering Sea had high success rates in past 

studies (Seitz et al., 2011; Loher, 2022), the lower success rate seen in this study is likely due to several

factors. Poor data transmission occurred due to batteries dying prematurely, resulting in no (n= 20) or a 

low number of transmissions (n=13). This appears to be a result of faulty batteries from the manufacturer

and from tags being deployed at below optimum voltage levels due to long term storage resulting from 

tagging complications in previous tagging seasons (D. Short, Wildlife Computers Redmond, USA, 

personal communication). In addition, there were many tags that succeeded in transmitting archived data

but did not reach their full deployment length. The anchor for some tags appears to have become 

dislodged from the fish after a short period at sea as the tag reported prematurely with the release pin 

intact (n=17); two of these individuals appeared to have been consumed by marine mammals, as several 

days prior to transmission the tags reported temperatures reflective of mammal viscera (~ 36°C) and 

repeated rises to the surface, presumably for the animal to breathe (sensu Seitz et al., 2019). Furthermore, 

the programming used to direct the tag to transmit upon reaching the surface (i.e., “premature release 

detection”) failed for seven of these tags, resulting in them floating until their pre-scheduled pop-off date, 

at which point they began data transmission. The reasons behind this are undetermined. 

This study provides the first look at the spatial dynamics of halibut at the northern extreme of 

their range, providing valuable benchmark information for future assessment of potential interactions with 

hydrocarbon exploration activities. Halibut were seen seasonally occupying four OCS lease planning 

areas, which comprise most of the eastern Bering Sea shelf, but were not observed entering Hope Basin in 

the Chukchi Sea. This suggests that halibut may interact with oil and gas exploration while foraging, 

spawning, and migrating in the Bering Sea, but not in the Chukchi Sea. While it does not appear as 

though halibut currently occupy the Chukchi Sea at this juncture, the potential for further northward 

expansion of halibut into this region in the future remains (Chapter 2). Additionally, predominantly adult 

females were evaluated herein, and as such this study mainly represents findings from sexually mature 

individuals from a small portion of the Bering Sea sub-population. With satellite tagging studies having

focused on adult individuals, the movements of juvenile halibut remain largely unstudied, and have only 
13

been theorized in the NBS based on larval drift analyses (Sadorus et al., 2020). Additional tagging and 

monitoring efforts may enable some of these questions to be answered. 
14

Chapter 2: 

Introduction: 

Climate change, and associated increases in ocean temperatures, have large-scale impacts on the 

spatial dynamics of fish species across the globe. As conditions warm, the ranges of fishes expand and 

contract, resulting in shifts in the spatial extent of fish assemblages (Mueter and Litzow, 2008). This 

phenomenon is particularly drastic at northern latitudes, where it is resulting in a process known as 

borealization, in which Arctic specialists are replaced by more boreal, sub-arctic species as they exhibit

northward range expansions (von Biela et al., 2022). This has been observed in continental shelf waters 

throughout the northern hemisphere, including the Barents Sea in the northern Atlantic Ocean and the 

Bering and Chukchi seas in the northern Pacific Ocean (Mueter et al., 2021). Changes have been 

particularly drastic in the Pacific Ocean, where the recent disappearance of the Bering Sea deep-water 

cold pool (Stabeno and Bell, 2019), and transport of warm Pacific water into the Chukchi Sea (Yang and 

Bai, 2020), have allowed the abundance and range of sub-arctic fish species to increase dramatically 

(Thoman Jr et al., 2020). 

Recent studies have observed gadids exhibiting these northwards shifts, particularly Pacific Cod 

(Gadus macrocephalus) and Walleye Pollock (Gadus chalcogrammus). Pacific Cod have displayed a 

significant increase in abundance in the northern Bering Sea (NBS) (Stevenson and Lauth, 2019), and 

were observed in the southern Chukchi Sea (SCS) for the first time in 2018 (Cooper et al., 2023). 

Walleye Pollock juveniles and adults have been observed in the NBS and SCS in insignificant numbers 

for decades (Mecklenburg et al., 2018), but adults have recently been documented in numbers that may be 

capable of supporting a commercial fishery (Orlov et al., 2021). The increase of these species has 

coincided with a decrease in abundance and distribution in the NBS and SCS of Arctic gadids such as 

Polar (Boreogadus saida) and Saffron cods (Eleginus gracilis) (Baker, 2021; Kuznetsov et al., 2023). 

Shifts in available species, and their relative abundance, may have profound impacts on the food security 

and livelihoods of residents in this region, as they rely heavily on both subsistence and commercial fish 

harvests. While the ability to harvest traditional Arctic species may decrease, the increasing abundance of 

sub-arctic species provides new opportunities for residents in the region (Thoman Jr et al., 2020). 

Additionally, range expansion of sub-arctic fish species may expose them to potential future human 

activities, such as hydrocarbon exploration, marine resource extraction, and renewable energy 

development. As such, characterizing and predicting these ongoing shifts in species distributions is 

critical for both maximizing harvest opportunities and understanding possible interactions with potential 

future human activities. With continued environmental change, additional fish species may exhibit 

similar northward movements, such as the Pacific halibut (Hippoglossus stenolepis). 

The Pacific halibut is a wide ranging and commercially valuable boreal demersal fish species 

found in Gulf of Alaska and Bering Sea waters (NOAA, 2019). During the winter months, Pacific halibut 

are known to inhabit deep offshore waters along the continental slope, where they are presumed to spawn 

(Loher and Seitz, 2008). During the summer months, Pacific halibut migrate into shallow continental 

shelf waters to forage (Flanigan et al.,Chapter 1; Loher, 2008). In the Bering Sea, these foraging areas 

were thought to occur primarily in the southern portion and the outer shelf domain, but as conditions have 

warmed, Pacific halibut have expanded into the NBS (Conners et al., 2002), where they are currently 

targeted by commercial and subsistence fisheries based in Northwestern Alaska, including Nome and 

Savoonga. Despite this, Pacific halibut still do not occur in sufficient abundance to be targeted in fisheries 
15

north of the Bering Strait in the SCS, although local knowledge suggests that these fish occasionally 

move this far north. However, Pacific halibut co-occur with and prey on Pacific Cod and Walleye 

Pollock, both of which have an increasing in presence in the SCS (Stevenson and Lauth, 2019). As water 

temperatures continue to rise, the spatial and temporal extent of thermally suitable habitat in the SCS will 

increase. This may allow Pacific halibut to utilize the SCS for summer foraging opportunities in the 

coming years, providing a potentially valuable resource for local residents while concurrently exposing 

this species to potential future human activities such as hydrocarbon exploration and development. To 

assess the potential for Pacific halibut to expand into the SCS, this study sought to characterize the 

thermal environment currently occupied by Pacific halibut in the NBS and compare it to existing bottom 

temperatures in the SCS to infer potential for range expansion northward. 

Methods:

Data: 

This study compared thermal occupancy data of Pacific halibut collected by electronic tags 

(Flanigan et al., Chapter 1) to model-generated bottom water temperatures of the Chukchi Sea (Danielson 

et al., 2020). Thermal time series data were available from 53 pop-up satellite tags attached to Pacific 

halibut in the northern Bering Sea during 2019–2022. Tags were deployed near Nome and Savoonga, 

Alaska, during July, August, and September. Wildlife Computers miniPATs (resolution 0.05°C, range -

20–50°C) recorded temperature readings at 5-second intervals while affixed to the fish. These data were 

binned and averaged at 10-minute intervals for every other or every third day of the deployment (Flanigan 

et al., unpublished) and transmitted to satellites after the tags released from the fish and floated to the 

ocean’s surface.

Chukchi Sea thermal conditions were modeled using the three-dimensional physical Regional 

Ocean Modeling System (ROMS), a free-surface, hydrostatic primitive equation and finite volume 

(Arakawa C-grid) ocean circulation model (Shchepetkin and McWilliams, 2005). In particular, the PanArtic ROMS (PAROMS) model was selected (Danielson et al., 2020), which was developed for use in 

northern latitudes by coupling ROMS with a sea-ice model (Hunke and Dukowicz, 1997; Hunke, 2001; 

Mellor and Kantha, 1989). The model uses a telescoping grid that extends from south of the Aleutian 

Islands (50°N) to Iceland (65°N), covering the entire Bering Sea, Arctic Ocean, and Canadian 

Archipelago. Model resolution varies spatially but is highest for the Chukchi and Beaufort Seas, where it 

varies from 5-6.5 km (Vestfals et al., 2021). Resolution also varies with depth, with this model 

containing 50 vertical depth levels that are terrain following, leading to increased resolution in shallower 

waters and in the depth layers near the ocean bottom and surface. Given the benthic nature of Pacific 

halibut, temperature output from the bottom layer was selected for this study. Specifically, the mean 

bottom-water temperature was obtained for each month from June to November, corresponding to the 

Pacific halibut NBS foraging season and the immediately adjacent months. 

Data Analysis:

Thermal Occupation:

Thermal occupancy of all tagged Pacific halibut was assessed by aggregating and summarizing 

transmitted temperature data. For all individuals, the average daily occupied temperature was determined 

for each day at liberty by averaging across all 10-minute intervals for which temperature recordings were 
16

received. These were then pooled across individuals for each day of the year when data were available, 

providing a distribution of average daily occupied temperatures for Pacific halibut in the NBS. 

To characterize Pacific halibut cold-water tolerance, and hence maximum potential extent of 

geographic range, the pooled temperature time series were evaluated during May and June when they 

were occupying continental shelf waters < 200 m, as this corresponds to the time of year and conditions

when these fish occupy thermal minimum conditions (Flanigan et al., unpublished). From this period, the 

50th percentile of pooled average daily temperatures was taken as the cold-water tolerance threshold; this 

was assumed to provide a conservative functional lower limit for the thermal conditions that Pacific 

halibut can occupy for an extended period of time, or what is thermally tolerable. 

To characterize Pacific halibut thermal conditions, and hence potential geographic range under 

typical conditions, the thermal time series data were evaluated during August and September, as this 

corresponds to the peak Pacific halibut summer foraging period in the NBS and the period they are most 

likely to expand into the SCS. Two thermal ranges were identified during this time: the 50% core range 

(hereafter referred to as core range) of summer foraging temperatures, which was found by identifying the 

25th and 75th percentiles of occupied thermal conditions, and the 95% range of summer foraging 

temperatures, which was similarly found by identifying the 2.5 and 97.5 percentiles of occupied thermal 

conditions. 

Ocean Conditions: 

The spatial extent of suitable thermal habitat available to Pacific halibut in the SCS during the 

summer foraging period was inferred in two ways. First, the greatest hypothetical extent of thermal 

habitat for Pacific halibut was determined by finding all waters that exceeded the cold-water tolerance 

threshold. Second, the extent of thermal habitat within the core and 95% range of summer foraging 

conditions in the NBS was determined. These analyses generated three polygons for each month 

depicting: 1) bottom waters with temperatures within the core range of summer occupancy 2) bottom 

waters with temperatures within the 95% range of summer occupancy and 3) bottom waters with 

temperatures below those of summer occupancy, but greater than the cold-water thermal tolerance 

threshold. To quantify the spatial extent of these conditions, the area (km2

) of each polygon north of the 

Bering Strait was calculated.

To understand the impacts of climatic variability on available thermal habitat for Pacific halibut 

in the SCS, the extent of potential habitat during a recent cold (2012) and warm-water (2019) year were 

compared. Conditions are known to vary significantly from year to year in this region, with cold-years 

being characterized by extensive sea ice formation and an expansive deep-water cold pool, while warmyears consist of limited sea ice and reduced cold pool extent (Mueter and Litzow, 2008). This variability 

in temperature across years in the SCS could have large impacts on the ability, or lack thereof, of Pacific 

halibut to occupy the region. As such, the impact of thermal variation in the Chukchi sea on the extent of 

suitable thermal habitat was quantified by comparing the relative change in area of the generated 

polygons between years, specifically the magnitude of increase in the area of waters within the 95% 

summer occupancy range between 2012 and 2019. 
17

Results: 

The Pacific halibut cold-water thermal tolerance threshold was 0.95°C, while the 50% core 

summer foraging temperatures ranged from 6.00 to 8.2°C and the 95% summer foraging temperature 

ranged from 3.79 to 9.77°C (Figure 2.1). 

Figure 2.1: Distribution of mean daily temperatures occupied by individual Pacific halibut 

(Hippoglossus stenolepis) in the northern Bering Sea. Boxplot summaries represent the first and third 

quartiles, the center bar is the mean and whiskers extend to the 95% level. All observations outside of 

this range are outliers and represented as points.

In the cold year of 2012, the temporal and spatial extent of summer foraging conditions in the 

Chukchi Sea was relatively limited (Table 2.1). Suitable thermal foraging conditions were not widely 

available until August and were completely absent by November. During the period that these conditions 

were available, they were almost exclusively near the coast of Alaska in the eastern Chukchi Sea (Figure 

2.2), with the 95% range covering 70,909 km2 at their greatest extent during the month of September, 

with a core ranges of 30,122 km2

. Waters above the thermal tolerance threshold added an additional 

89,940 km2 of habitat in the central Chukchi Sea during this time, providing a total of 160,849 km2 of 

available habitat. 
18

Table 2.1: Extent of suitable thermal habitat (km2) for Pacific halibut (Hippoglossus stenolepis) in the 

Chukchi Sea by month in the cold year of 2012. The total habitat represents the sum of the 95% summer 

and the thermally tolerable waters.

Month Thermally 

Tolerable 

95% Summer 

Foraging 

Core Summer

Foraging 

Total Habitat

June 4,483 3,064 1,250 7,547

July 26,041 11,360 3,182 37,401

August 67,885 56,953 19,259 124,838

September 89,940 70,909 30,122 160,849

October 128,879 32,533 0 161,412

November 119,937 0 0 119,937

Table 2.2: Extent of suitable thermal habitat (km2

) for Pacific halibut (Hippoglossus stenolepis) in the 

Chukchi Sea by month in the warm year of 2019. The total habitat represents the sum of the 95% 

summer and the thermally tolerable waters. The increase multiplier is the magnitude of increase in 

available habitat between the 95% summer occupation ranges of 2012 and 2019. 

Month Thermally 

Tolerable 

95% Summer 

Foraging 

Core Summer 

Foraging 

Total 

Habitat 

Increase 

Multiplier

June 77,578 14,720 1,900 92,298 4.8

July 101,060 86,320 27,706 187,380 7.6

August 98,383 140,265 38,285 238,648 2.5

September 70,493 200,393 43,079 270,886 2.8

October 81,678 254,934 98,665 336,612 7.8

November 253,145 35,348 0 288,493 NA

In the warm year of 2019, the spatial and temporal availability of suitable thermal habitat was far 

greater than in the cold year, with suitable foraging temperatures in the Chukchi Sea lasting from June to 

November (Table 2.2). These conditions extended to the western Chukchi Sea, and even excluded the 

coastal eastern Chukchi Sea during August and September due to conditions being warmer than those 

currently occupied by Pacific halibut in the NBS (Figure 2.3). Much of the central shelf in the Chukchi 

Sea warmed to suitable foraging conditions, with September and October providing the greatest spatial 

extent of suitable habitat, with 200,393 and 254,934 km2

 falling within the 95% range and 43,079 and 

98,665 km2 within the core range, respectively. For months when suitable foraging conditions were 

available in both years, the warm year provided an average of 5.1 times (range 2.5 to 7.8 times) more 

thermal habitat within the 95% range of summer occupation than the cold year. Additionally, when 

combined with thermally tolerable waters during October, the month with the greatest extent of available 

thermal habitat, a total of 336,612 km2 was available to Pacific halibut in the Chukchi Sea. 
19

Figure 2.2: Extent of potentially suitable thermal habitat of Pacific halibut (Hippoglossus stenolepis)

based on monthly mean bottom water temperatures from the pan-arctic Regional Ocean Modeling System 

output for the Bering and Chukchi seas in 2012. Polygons represent thermal conditions within the 50% 

core (red) and 95% (orange) thermal occupation range of Pacific halibut, as well as conditions below the 

lower end of the 95% thermal range and above the thermal minima threshold (blue). 

Discussion:

A comparison of occupied thermal conditions of Pacific halibut in the NBS with available 

temperatures in the SCS revealed that the Chukchi Sea portion of the Arctic Ocean contains suitable 

thermal habitat for this fish species in the summer months, suggesting the potential for this species to 

move into the region in the coming years. Despite the current availability of suitable thermal habitat and 

the apparent potential for Pacific halibut to move into the SCS, these fish are not currently encountered in 

survey efforts, or targeted in directed fisheries, within the region due to their extremely low abundance. 

As such, other factors may be limiting their expansion into this habitat. One potential explanation for the 

apparent lack of Pacific halibut occupation in the Chukchi Sea is substrate type. Pacific halibut are 

known to prefer bottoms composed of gravel and sand, enabling them to settle into the substrate and 

ambush prey (Bishop et al., 1993; Carlson et al., 2005). The minority of the Chukchi Sea bottom has a 

high prevalence of mud and clay substrates (Mecklenburg et al., 2007), which may limit the amount of 

suitable habitat for Pacific halibut. However, the majority of the bottom of the SCS is predominantly 

gravelly and sandy substrates (Feder et al, 2004), and as the most proximate region to the currently 

occupied NBS, would be the area Pacific halibut expand into first. As such, it is likely that another factor 

is responsible for their lack of occupation, such as food availability. Pacific halibut movements into the 
20

NBS are thought to be associated with foraging (Flanigan et al., Chapter 1), where the region apparently 

supports sufficient opportunities to merit individuals migrating hundreds of kilometers from their 

overwintering and spawning habitats along the Bering Sea shelf edge. While the abundances of prey fish 

species have been increasing in the SCS (Baker et al., 2023), the prevalence of these fishes may not yet be 

sufficient to motivate additional movements into SCS waters, with the energetic cost of reaching the SCS 

outweighing any potential additional foraging success. This may change in coming years if waters 

continue to warm and the availability of these prey species continues to increase, similar to what was 

observed in the Gulf of Alaska in the late 20th century (Anderson and Piatt, 1999). Such changes in 

potential foraging success will increase the likelihood that Pacific halibut expand into this habitat. 

Figure 2.3: Extent of potentially suitable thermal habitat of Pacific halibut (Hippoglossus stenolepis) 

based on monthly mean bottom water temperatures from the pan-arctic Regional Ocean Modeling System 

output for the Bering and Chukchi seas in 2019. Polygons represent thermal conditions within the 50% 

core (red) and 95% (orange) thermal occupation range of Pacific halibut, as well as conditions below the 

lower end of the 95% thermal range and above the thermal minima threshold (blue). 

If Pacific halibut expand into the SCS, there is a potential for the species to become a valuable 

harvest opportunity for local residents and for possible interactions with potential future human activities 

such as hydrocarbon exploration. However, the extent of potential occupation of Pacific halibut in the 

Chukchi Sea in the future is likely highly dependent on whether it is a warm- or cold-water year, as the 

availability of suitable habitat is correlated with the Pacific Decadal Oscillation and atmospheric blocking 

in the Bering Sea, both of which have significant impacts on sea ice formation and ocean temperatures 

(Kodaira et al., 2020). As a result, the distribution and abundance of Pacific halibut may differ greatly 

across years. Consequently, if Pacific halibut are to be targeted in the SCS in the future, this potential 
21

fishery may be subject to large-scale fluctuations from year to year and may not provide consistent 

harvest opportunity for local residents. Further, these annual fluctuations will result in varied exposure to 

hydrocarbon exploration activities among years. This suggests that while Pacific halibut may provide a 

valuable harvest opportunity when present, and potentially interact with oil and gas explorations, 

fluctuations in the availability of suitable thermal habitat may change the degree to which these occur 

from year to year. 

Future efforts are needed to monitor these ongoing changes within this region. Currently, models 

such as PAROMS are suitable for constructing past ocean conditions, but have reduced efficacy 

projecting these conditions into the future (Haidvogel et al., 2008). As such, diligent survey efforts are 

needed to quantify ongoing shifts in stock presence and abundance, as well as in ocean conditions. With 

waters hypothesized to continue warming (Hermann et al., 2021), the number and degree of shifts in fish 

stocks will likely increase as fish assemblages reorganize. The method presented herein provides a tool to 

assess the possibility of these shifts, where it can be applied to many sub-arctic fish species with the 

potential for future range expansion. 

Acknowledgments:

Dawn Wehde from Norton Sound Econmic Development Corporation (NSEDC) provided 

essential logistical support throughout this project, as well as aided in tagging efforts. NSEDC employees 

Wes Jones and Renae Ivanoff also provided administrative support throughout the project. Dr. Remi 

Pages provided PAROMs data and support for analyses. Tagging efforts were conducted on a number of 

fishing vessels, with captains Phil Pryzmont and Richmond Toolie providing key assistance across

multiple years. Funding for fieldwork and associated travel was provided by NSEDC, which purchased all 

satellite tags and supported tagging efforts. Phil Pryzmont also aided in funding tagging efforts, 

providing vessel time that was needed to complete tagging work. Funding to support graduate student 

tuition and stipend was provided by the Rasmuson Fisheries Research Center fellowship, the Bureau of 

Ocean Energy Management, and the University of Alaska College of Fisheries and Ocean Sciences. 

Additional support for conference travel was provided by the North Pacific Research Board Graduate 

Student Research Award. 
22

Study Products:

Flanigan, A.F., Wehde, D., Loher, T., Seitz, A.C. 2022. Understanding Pacific Halibut (Hippoglossus 

stenolepis) Spatial Dynamics in the Northern Bering Sea. Alaska Marine Science Symposium. Virtual. 

January 2022. (Poster) 

Flanigan, A.F., Wehde, D., Loher, T., Seitz, A.C. 2022. Understanding Pacific Halibut (Hippoglossus 

stenolepis) Spatial Dynamics in the Northern Bering Sea. American Fisheries Society Alaska 

Chapter. Virtual. March 2022. (Oral) 

Flanigan, A.F., Wehde, D., Loher, T., Seitz, A.C. 2022. Understanding Pacific Halibut (Hippoglossus 

stenolepis) Spatial Dynamics in the Northern Bering Sea. American Fisheries Society University of 

Alaska Fairbanks Student Sub-unit Symposium. Fairbanks, AK. March 2022. (Oral) 

Flanigan, A.F., Wehde, D., Loher, T., Seitz, A.C. 2022. Understanding Pacific Halibut (Hippoglossus 

stenolepis) Spatial Dynamics in the Northern Bering Sea. Ecosystem Studies of Subarctic and Arctic 

Seas. Seatlle, WA. June 2022. (Oral)

Flanigan, A.F., Wehde, D., Loher, T., Seitz, A.C. 2022. Understanding Pacific Halibut (Hippoglossus 

stenolepis) Spatial Dynamics in the Northern Bering Sea. PICES. Busan, South Korea. September 2022. 

(Oral)

Flanigan, A.J., Wehde, D., Loher, T., Seitz, A.C. 2023. Assessing spawning behavior at the northern 

extreme of Pacific halibut. Alaska Marine Science Symposium. Anchorage, AK, January 2023. (Oral) 

Flanigan, A.J., Wehde, D., Loher, T., Seitz, A.C. 2023. Assessing spawning behavior at the northern 

extreme of Pacific halibut. American Fisheries Society. Fairbanks, AK, March 2023. (Oral) 

Flanigan, A.J., Wehde, D., Loher, T., Seitz, A.C. 2023. Assessing spawning behavior at the northern 

extreme of Pacific halibut. American Fisheries Society Alaska Chapter Student Sub-Unit 

Symposium. Fairbanks, AK, March 2023. (Oral) 

Flanigan, A.J., Wehde, D., Loher, T., Seitz, A.C. 2023. Assessing spawning behavior at the northern 

extreme of Pacific halibut. Western Groundfish Conference. Juneau, AK, April 2023. (Oral) 

Flanigan, A.J., Wehde, D., Loher, T., Seitz, A.C. 2023. Assessing spawning behavior at the northern 

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Flanigan, A.J., Wehde, D., Loher, T., Seitz, A.C. 2023. Characterizing the scale of Pacific halibut 

distribution in the Bering Sea. ICES Annual Meeting. Bilbao, Spain. September 2023. (Poster) 
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Appendix: 

Table A1: Deployment information for pop-up satellite archival tags attached to 84 Pacific halibut 

(Hippoglossus stenolepis) in the northern Bering Sea. For tags that successfully transmitted, the date the 

tag began transmitting data, and the number of messages sent to satellites, is provided. Additionally, 

when the tag provided a useful end location, the horizontal displacement (km) of the tag from its tagging 

location is provided. 

TAG ID FORK 

LENGTH 

RELEASE 

DATE

TAGGING 

REGION

DEPLOYMENT 

OUTCOME A 

POP-UP 

DATE

HORIZONTAL 

DISPLACEMENT 

MESSAGES

182704 102 7/19/19 Nome No Report

182705 103 7/17/19 Nome No Report

182706 130 7/19/19 Nome No Report

182707 132 7/17/19 Nome Floater 3/20/20 313

182708 141 7/19/19 Nome No Report

182709 118 7/19/19 Nome Premature 8/15/19 6042

182710 113 7/17/19 Nome No Report

182711 104 7/17/19 Nome No Report

182712 124 7/17/19 Nome No Report

182713 120 7/17/19 Nome No Report

182715 89 9/9/19 Savoonga Good 1/15/20 437 2846

182716 108 9/9/19 Savoonga Good 1/15/20 401 1235

182718 108 9/9/19 Savoonga Premature 9/9/19 3301

182720 126 9/8/19 Savoonga Good 3/5/20 644 2185

182721 112 9/8/19 Savoonga Floater 9/8/20 4174

182722 97 9/9/19 Savoonga Good 9/8/20 41 5127

202294 95 7/30/20 Nome Premature 9/24/20 4945

202295 112 7/30/20 Nome Floater 4/1/21 2220

202296 94 7/30/20 Nome Good 2/18/21 927 2261

202297 100 7/30/20 Nome Good 4/2/21 677 1793

202298 90 7/30/20 Nome Floater 9/29/20 3662

202299 113 7/30/20 Nome Good 4/2/21 916 2237

202300 92 7/30/20 Nome Good 7/21/21 596 3708

202301 98 7/30/20 Nome Good 4/18/21 836 1826

202302 94 7/30/20 Nome Good 7/21/21 365 8575

202303 90 7/30/20 Nome Floater 12/29/20 4334

202304 91 7/30/20 Nome Premature 10/5/20 5080

202305 115 7/30/20 Nome Good 7/22/21 40 1264

202318 147 7/30/20 Nome Premature 10/13/20 7720

202319 101 7/30/20 Nome Premature 9/17/20 1741

202320 121 7/30/20 Nome No Report

202321 91 7/30/20 Nome Good 3/2/21 871 2118

202322 89 7/30/20 Nome Floater 7/20/21 5232

202323 95 7/30/20 Nome Premature 9/10/20 7386

202324 131 7/31/20 Nome Good 7/21/21 54 2663

202325 96 7/31/20 Nome Premature 9/3/20 7575

202339 116 7/30/20 Nome No Report

202340 160 7/30/20 Nome No Report

202341 103 7/30/20 Nome Good 3/16/21 845 2154

202342 92 7/30/20 Nome Good 3/16/21 815 2145

202343 120 7/30/20 Nome Floater 3/15/21 3443

202336 112 8/18/21 Nome Good 7/5/22 660 5490

202337 136 8/1/21 Nome Good 7/16/22 225 2971
29

TAG ID FORK 

LENGTH 

RELEASE 

DATE

TAGGING 

REGION

DEPLOYMENT 

OUTCOME A 

POP-UP 

DATE

HORIZONTAL 

DISPLACEMENT 

MESSAGES

202338 106 8/3/21 Nome Poor Data 7/16/22 469 29

220912 129 9/9/21 Nome Good 3/20/22 791 3298

220915 100 8/18/21 Nome No Report

220918 116 8/1/21 Nome Premature 11/3/21 1608

220922 116 8/4/21 Nome No Report

220923 110 8/3/21 Nome Good 3/20/22 799 1923

202306 108 7/23/21 Savoonga Good 7/16/22 515 2067

202307 130 8/31/21 Savoonga Good 7/16/22 444 6224

202311 121 7/23/21 Savoonga Good 5/13/22 675 1179

202313 100 8/31/21 Savoonga No Report

202314 102 8/31/21 Savoonga Good 3/16/22 565 2171

202316 105 8/31/21 Savoonga Poor Data 3/16/22 636 609

202326 128 7/19/21 Savoonga Poor Data 3/22/22 530 15

202331 103 8/31/21 Savoonga Premature 10/19/21 2245

202333 116 8/31/21 Savoonga Poor Data 4/19/22 717 19

202334 101 8/31/21 Savoonga Good 3/17/22 620 1120

220913 118 8/18/22 Nome Good 3/16/23 765 402

220914 107 8/19/22 Nome Good 3/16/23 1022 1484

220916 111 8/19/22 Nome Poor Data 3/22/23 18

220917 107 8/19/22 Nome No Data 3/26/23 3

220919 129 8/19/22 Nome Good 3/16/23 695 585

220920 120 8/19/22 Nome Poor Data 3/26/23 52

220921 128 8/18/22 Nome Good 3/19/23 696 848

220924 108 8/19/22 Nome Poor Data 3/16/23 36

220925 105 8/19/22 Nome Poor Data 3/16/23 816 145

220926 108 8/19/22 Nome No Report

220927 120 8/19/22 Nome No Report

220928 114 8/19/22 Nome No Report

220929 106 8/19/22 Nome Good 8/16/23 14 2380

202308 103 8/26/22 Savoonga Poor Data 6/7/23 147

202309 111 8/26/22 Savoonga No Report

202310 101 8/23/22 Savoonga Good 8/16/23 371 2333

202312 139 8/26/22 Savoonga Good 8/16/23 78 585

202315 175 8/26/22 Savoonga No Report

202317 107 8/26/22 Savoonga Good 3/16/23 547 1350

202327 116 8/23/22 Savoonga Poor Data 3/16/23 639 1064

202328 154 8/26/22 Savoonga Good 3/16/23 804 966

202329 113 8/23/22 Savoonga Poor Data 3/26/23 305 21

202330 107 8/26/22 Savoonga Good 3/16/23 569 927

202332 127 8/24/22 Savoonga No Report

202335 144 8/24/22 Savoonga Poor Data 3/22/23 390 45 A “Good Data” refers to tags that released from fish after 1 January and transmitted both robust time-series data and a viable end 

location. “Poor Data” refers to tags that released from fish after 1 January and either failed to provide a viable end location or 

only transmitted time-series data for a small number of days. “No Data” refers to tags that communicated with satellites but failed 

to transmit any usable data. “Premature” refers to tags that reached the surface and began data transmission prior to 1 January. 

“Floater” refers to tags detached from the fish prematurely but did not begin transmission until the scheduled pop-up date. 

“Missing” refers to tags that failed to transmit to satellites and were unaccounted for.
30

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