Pacific halibut support high-value commercial and sport fisheries in the north Pacific Ocean, making survival of Pacific halibut bycatch in trawl fisheries an important management concern. We present a method for characterizing activity and inferring survival of Pacific halibut based on accelerometer data from Pop-up Satellite Archival Tags (PSATs). A PSAT attached to a fish with a dart and tether floats freely above the fish in a vertical orientation when the fish is stationary, but switches to a more horizontal orientation when towed behind an active fish. We hypothesized that characteristic changes in PSAT orientation associated with activity could be detected by accelerometers and summarized for transmission by PSATs to provide valuable information on fish activity. We developed procedures for inferring survival of Pacific halibut with accelerometer PSATs based on a progression of analysis steps that linked PSAT orientation, PSAT acceleration data, and Pacific halibut activity patterns. Relationships between PSAT orientation and Pacific halibut activity were confirmed by observations of PSAT orientation on Pacific halibut swimming in the laboratory and comparisons with depth data from tags on free-ranging Pacific halibut. We developed two metrics summarizing 1 Hz acceleration data for transmission to the Argos satellite network. The “knockdown” metric records abrupt changes in vertical acceleration, indicating both initiation of swimming bouts and sustained swimming behavior, and is robust to the effects of tidal currents. The “%tilt” metric records the amount of time the tag tilts past a vertical orientation threshold and captures the proportion of each time bin that the fish is active. These PSAT metrics revealed diel behavior and three activity modes present in free-ranging Pacific halibut that allowed inference of survival when compared to PSAT data from fish carcasses and weighted tags. Accelerometer PSAT metrics developed in this study revealed Pacific halibut activity patterns, and thus survival, and may be extended to other fish species. Economical accelerometer PSATs can allow larger sample sizes that enhance bycatch survival studies while detecting fish activity in flat study areas. PSAT advantages over other survival estimation methods include providing outcomes for all specimens during exposure to natural conditions.

Background: Fish geolocation methods are most effective when they are customized to account for species behavior and study area characteristics. Here, we provide an example of customizing a hidden Markov model (HMM) for reconstructing movement pathways of a high-latitude demersal fish species in a remote island chain using Pop-up Satellite Archival Tag (PSAT) data. Adult Pacific cod were tagged with PSATs while occupying winter spawning grounds in the Aleutian Islands in February 2019. We adapted a demersal fish application of the HMM to (1) add light-based longitude to the data likelihood model, (2) account for possible off-bottom behavior of demersal fishes in the maximum daily depth likelihood, and (3) modify the model framework to accommodate convoluted island topography in the study area. A simulation study was conducted to explore the two primary modifications to the model framework, reflecting boundary for the movement kernel and the Viterbi method of pathway reconstruction, under known conditions. Results: Geolocation was performed on satellite-transmitted and detailed archival data sets from 6 adult Pacific cod at liberty for 21–277 days. Migration from winter spawning to summer foraging areas (range 60–395 km) was detected for the 4 tagged fish that were at liberty for more than 90 days. Light-based longitude was the primary geolocation variable for detecting migrations with precision (root mean square error) estimates of 0.56 degrees during winter and 1.3 degrees during the summer. Simulation studies confirmed the effectiveness of model framework modifications and generated guidelines for use in specific applications. Conclusions: This study demonstrates that post-spawning migrations of Pacific cod in the Aleutian Islands can be detected and characterized using PSAT data. Initial insights into migrations, summer foraging areas, and associated development of appropriate analysis tools will support future Pacific cod movement studies in the Aleutian Islands as well as other regions of Alaska. The adaptations to the HMM presented here will benefit current and future research on demersal fish in other regions as well as fish species that occupy areas with convoluted shorelines or island chain topography.

Whether male Tanner crabs, Chionoecetes bairdi, undergo a terminal molt associated with a change in claw allometry has long been debated. We measured

Pacific halibut is a large-bodied demersal fish species known to undertake large-scale winter spawning migrations. We characterized annual movement patterns of Pacific halibut relative to a Marine Protected Area (MPA) in Glacier Bay National Park, Alaska using electronic tags. In the summer of 2013, we deployed 25 Pop-up Satellite Archival Tags (PSATs) on halibut to determine seasonal movement patterns, where residency within the MPA was inferred by comparing PSAT depth and temperature records to stationary archival tag data and oceanographic survey data. To characterize within-summer movement patterns, 15 halibut tagged with PSATs were double-tagged with acoustic transmitters and actively tracked during the summers of 2013 and 2014. Home range behaviour and interannual site fidelity at spatial scales <5 km were observed for the majority of tagged fish. A small proportion (6/21 fish with PSAT data) departed the MPA on winter migrations during December 2013. A majority (4/6) of migratory fish returned to the MPA after an average of 57 d (s.d. 22 d) spent outside of the MPA. Migration timing generally coincided with existing winter commercial fishery closures. The annual movement patterns of tagged halibut relative to MPA boundaries and winter commercial fisheries closures suggest that the Glacier Bay MPA could serve as a year-round refuge from commercial harvest for both residential and migratory halibut. If halibut behaviour is similar in other areas, protected areas may provide some utility for management despite the migratory nature of halibut.