Acoustic-trawl (AT) surveys are widely used to map and quantify pelagic animals and help manage several of the planet's largest fisheries. However, quantifying their uncertainty is challenging. AT surveys sample patchy, non-Gaussian animal densities using systematic, random, and targeted samples at different spatial resolutions, so classical geostatistics cannot provide confidence intervals. Additionally, converting backscatter to biomass requires multiple scaling conversions, each contributing uncertainty. We present a semi-parametric resampling approach to overcome these challenges. This novel method simulates non-Gaussian spatial fields conditioned on observations to account for spatial sampling error, and applies a spatially weighted bootstrap to account for the uncertainty in allocation of backscatter among species and sizes based on trawl samples. We applied it to surveys of walleye pollock (Gadus chalcogrammus) in the eastern Bering Sea, estimating errors of 4%-13% for biomass and 4%-34% for numerical abundance. These were higher than errors from traditional geostatistical methods, but lower than currently assumed in the stock assessment. Spatial sampling was the largest individual source of uncertainty in this survey, followed by acoustic target strength and echosounder calibration. We discuss the advantages and disadvantages of this approach, and possible future directions for uncertainty quantification in acoustic surveys.

Despite the potential benefits for species identification, broadband acoustic data collection has yet to be widely implemented in fisheries surveys. In large parts, this is because it remains unclear whether broadband echo integration produces similar abundance estimates as traditional narrowband data. This work compares the integration of broadband and narrowband data from EK80 transceivers operating at nominal frequencies of 38, 70, 120, and 200 kHz. We sequentially transmitted broadband frequency modulated (FM) and narrowband continuous wave (CW) pings to investigate the relationship between the volume backscattering coefficients measured using the two signal types, ${S_{V,CW}}$ and ${S_{V,FM}}$. ${S_{V,FM}}$ was calculated using two approaches. The first approach calculated the pulse-compressed volume backscatter in the time domain, ${S_V}( t )$, using three different methods for estimating aggregate terms in place of frequency-dependent terms. The second approach calculated the mean volume backscatter in the frequency domain, ${S_V}( f )$. While time-domain estimates provide reasonable first approximations of ${S_{V,FM}}$, calculation of volume backscatter using the mean of ${S_V}( f )$ produces backscatter estimates that are statistically equivalent to those calculated from ${S_{V,CW}}$. These comparisons indicate that broadband signals processed in the frequency domain can be echo integrated for fisheries surveys, maintaining continuity of long-term indices of abundance and biomass.

Survey time series are used to track species- and ecosystem-level trends over time to support ecosystem-based fishery management. However, these recurring survey efforts are subject to unpredictable cancellations and reductions in effort. This occurred in 2020 when the COVID-19 pandemic forced the cancellation of the research vessel-based 2020 eastern Bering Sea (EBS) acoustic-trawl survey, which has provided an estimate of euphausiid abundance and distribution since 2004. As a partial replacement for this lost effort, three uncrewed surface vehicles (USVs) were used to collect acoustic data. In contrast to the standard vessel-based survey, which provides 4-frequency acoustic data, the USVs collected acoustic data at only two frequencies. This presented a challenge given that four frequencies are currently used to identify euphausiids in this time series. Here, we first evaluated two methods to provide comparable euphausiid abundance estimates using fewer acoustic frequencies. We found that a random forest classifier was able to produce abundance estimates comparable to those obtained in the vessel-based time series. This method was then used to estimate euphausiid abundance and distribution from the 2020 USV survey. We additionally estimated the increase in survey uncertainty due to the use of the random forest classifier and changes in the acoustic instruments. Together, this allowed for the EBS euphausiid abundance time series to be extended with fewer acoustic frequencies.

Uncrewed surface vehicles (USVs) equipped with echosounders have the potential to replace or enhance acoustic observations from conventional research vessels (RVs), increase spatial and temporal coverage, and reduce cost and carbon emission. We discuss the objectives, system requirements, infrastructure, and regulations for using USVs with echosounders to conduct ecological experiments, acoustic-trawl surveys, and long-term monitoring. We present four example applications of USVs with lengths <8 m, and highlight some advantages and disadvantages relative to RV-based data acquisitions. Sail-driven USVs operate continuously for months and are more mature than motorized USVs, but they are slower. To maintain the pace of an RV, multiple sail-powered USVs sample in coordination. In comparison, motorized USVs can travel as fast as RVs and therefore may facilitate a combined survey, interleaving USV and RV transects, with RV-based biological sampling. Important considerations for all USVs include platform design, noise and transducer motion mitigation, communications and operations infrastructure, onboard data processing, biological sampling approach, and legal requirements. This technology is evolving and applied in multiple disciplines, but further development and institutional commitment are needed to allow USVs equipped with echosounders to become ubiquitous and useful components of a worldwide network of autonomous ocean observation platforms.

Fish behavior during the capture process affects the overall efficiency of survey vessels and gear, influencing selectivity and catchability, which in turn influences stock assessments. The effects of behavior on capture are typically investigated through experiments that use video or acoustic technologies to observe behavioral responses of fish to vessels and gear directly. While these approaches provide valuable information about fish behavior, their use is limited to observing the movement of fish inside a small observed area. In this proof-ofconcept study, we demonstrate a novel application of bio-logging Pop-up Satellite Archival Tags (PSATs) equipped with accelerometer sensors to observe behavioral responses of free-swimming demersal fish to approaching bottom trawl fishing vessels and gear. By combining PSAT and fishing vessel geolocation data from the Vessel Monitoring System, we characterized diving and herding responses of three individual Pacific cod (Gadus macrocephalus) at-liberty for 21-106 days to trawling fishing vessels. We found that fish descended as trawlers approached, presumably in response to noise generated by the fishing vessel and gear, which supports the previously untested hypothesis that Pacific cod dive in response to approaching fishing vessels and gear, similar to closely related gadids. Additionally, one 88 cm fish swam ahead of a trawl towed at 1.25 m & sdot;s- 1 (i.e., 1.42 body lengths per second) for 55.6 minutes, covering a distance of 4.17 km before capture, which challenges previous assertions that Pacific cod are unlikely to outswim bottom trawl gear towed at typical towing speeds. This study demonstrates that opportunistically collected archival tag data can be used to improve knowledge of behavioral interactions between fish and fishing gear.

In the Pacific Arctic, the Chukchi Sea has been warming for decades, and exhibited an exceptionally warm period from 2015 to 2021. We examined changes in seabird distribution and abundance in the Chukchi Sea, and their relationships to environmental and prey conditions between 2 contrasting periods. We sampled systematically placed stations in late summer during 2 years before (2012, 2013) and 2 years during the warm period (2017, 2019; characterized by multiple marine heatwaves). Ship-based bird counts were used to model at-sea density of 5 seabird foraging guilds relative to oceanographic (water temperature, salinity, chlorophyll) and prey (large copepods, euphausiids, 3 forage fish taxa) variables. Relative to cool years, heatwave years were characterized by warmer, saltier waters, low abundance of large copepods and euphausiids, and elevated fish abundance, including an unprecedented abundance of age-0 walleye pollock Gadus chalcogrammus . Seabird species richness was higher during heatwave years but diversity was lower, driven by an influx of shearwaters. The best models for surface feeding and diving piscivores and diving planktivores included oceanographic and prey variables, plus a heatwave interaction term, indicating that responses to variables differed between cool and heatwave periods, with greatest disparity exhibited by diving planktivores. Models for surface planktivores were inconclusive, whereas shearwater distribution was associated with geographic variables (latitude, distance offshore), with relationships differing during cool and heatwave periods. We propose a conceptual model of how a prolonged period of marine heatwaves may affect the offshore seabird community via changes in prey species composition and distribution.

The degree to which walleye pollock (Gadus chalcogrammus, hereafter pollock) move between the US and Russian zones of the Bering Sea is a key source of uncertainty for fisheries management. To study transboundary migrations across the US-Russia maritime boundary and explore how climate variability might influence these migrations, four seafloor-mounted echosounder moorings were deployed from July 2019 to August 2020 in the northwestern Bering Sea. The observations indicated that a substantial amount of pollock moves between the US and Russia seasonally, with a period of southeast movement into the US as winter as sea ice forms and northwest movement into Russia in early summer as waters warm. Over the deployment period, 2.3-times more pollock backscatter moved into the US zone in fall and winter than exited the subsequent spring and summer. We hypothesize that the difference in the net movement between regions was driven by pollock moving farther into Russia during the historically warm conditions at the start of deployment period and reduced northwest return migration the following summer when temperatures were relatively cooler. This supports the hypothesis that temperature affects pollock distribution, and that continued warming will lead to a larger proportion of the stock in Russian waters.

Identifying sound-scattering organisms is a perennial challenge in fisheries acoustics. Most practitioners classify backscatter based on direct sampling, frequency-difference thresholds, and expert judgement, then echo-integrate at a single frequency. However, this approach struggles with species mixtures, and discards multi-frequency information when integrating. Inversion methods do not have these limitations, but are not widely used because species identifications are often ambiguous and the algorithms are complicated to implement. We address these shortcomings using a probabilistic, Bayesian inversion method. Like other inversion methods, it handles species mixtures, uses all available frequencies, and extends naturally to broadband signals. Unlike previous approaches, it leverages Bayesian priors to rigorously incorporate information from direct sampling and biological knowledge, constraining the inversion and reducing ambiguity in species identification. Because it is probabilistic, a well-specified model should not produce solutions that are both wrong and confident. The model is based on physical scattering processes, so its output is fully interpretable, unlike some machine learning methods. Finally, the approach can be implemented using existing Bayesian libraries and is easily parallelized for large datasets. We present examples using simulations and field data from the Gulf of Alaska, and discuss possible applications and extensions of the method.

Acoustic-trawl surveys are widely used to measure the abundance and distribution of pelagic fishes. The echo integration method used in these surveys requires estimates of the target strength (TS, dB re 1 m2) of individuals to estimate abundance. In situ TS measurements are attractive to establish length-to-TS relationships because they are obtained under realistic conditions utilizing the same tools as those during surveys. However, these measurements are usually made in restricted circumstances, are limited in number, and data processing is often subjective. Here, we present a novel method to estimate TS from a large volume of previously collected survey data recorded at trawl sites. By applying a series of published filtering methods to TS data based on frequency response, packing density, and degree of co-location in overlapping beams, single fish echoes can be reliably isolated from existing survey data. The automated approach produced measurements representative of survey conditions (e.g., collected at survey speeds on free-swimming fish) from existing surveys. We applied this method to 30 surveys of walleye pollock (Gadus chalcogrammus) in Alaska and estimated a new length-to-TS relationship. The results were largely consistent with the relationship currently used for walleye pollock; depth and geographic area were significant covariates.

Baseline surveys of offshore pelagic fishes in the eastern Chukchi Sea in 2012 and 2013 found that age-0 Arctic cod (Boreogadus saida) dominated the pelagic fish community in summer, with relatively few adults present in the region. Since this time, drastic changes in the ocean-atmosphere-ice feedback loop have led to continued warming, further reducing ice cover, and increased northward transport has led to an increase in Pacific-origin waters on the Chukchi shelf in summer. To examine potential bottom-up effects of these environmental changes on pelagic fishes in this rapidly changing environment, we extended a time series of large-scale acoustic-trawl surveys with additional surveys in 2017 and 2019. Age-0 Arctic cod were the most abundant pelagic fish in all four survey years, comprising 68–93% of fish abundance. However, age-0 walleye pollock (Gadus chalcogrammus), which were scarce (<0.1% of fishes) and confined to the southern Chukchi in 2012 and 2013, were present in high abundance (>21% of fish abundance) throughout the Chukchi shelf in 2017 and 2019. Age-0 Arctic cod were substantially more abundant in 2017 than in other years, possibly due to increased survivorship of larvae under warm conditions. Unlike in 2017, Arctic cod and pollock were spatially separated in 2019 due to enhanced transport, with Arctic cod primarily present in the northeastern portion of the survey area, which was characterized by cool surface and bottom temperatures. The substantial increase in abundance of age-0 pollock in recent years suggests that environmental conditions now allow this species to extend its northern range into the southern and central Chukchi Sea, at least on a seasonal basis. The changes in abundance and species composition of pelagic fishes in the 2012–2019 time series are tightly coupled to recent changes in sea ice, temperature, and the increasing transport of Bering Sea waters through Bering Strait into the Chukchi Sea. Given that the environment is expected to experience further warming and increased transport, these northward shifts in species distribution are likely to persist in the future.

Recent summer surveys of the northeastern Chukchi Sea found pelagic fishes were dominated by large numbers of age-0 Arctic cod (Boreogadus saida, Gadidae) and walleye pollock (Gadus chalcogrammus, Gadidae), while adult fishes were comparatively scarce. The source and fate of these young fishes remain unclear, as sampling in this region is impeded by seasonal ice cover much of the year. Seafloor-mounted echosounders were deployed at three locations in the northeastern Chukchi Sea from 2017 to 2019 to determine the movement and seasonal variability of these age-0 gadids. These observations indicated that the abundance of pelagic fishes and community composition on the Chukchi Sea shelf were highly variable on seasonal time scales, with few fish present in winter. Tracking indicated that fish movements were strongly correlated with local currents. Fishes were primarily displaced to the northeast in summer and fall, with periodic reversals towards the southwest driven by changes in regional wind patterns. The flux of fishes past the moorings indicated that the prevailing northward currents transport a large proportion of the age-0 pelagic fishes present on the Chukchi shelf in summer to the northeast by fall, leading to relatively low abundances of age-1+fishes in this environment.

Pelagic trawls are one of the primary methods of sampling midwater fishes. However, these trawls are species- and size-selective, and small fish can escape through trawl meshes. This can introduce uncertainty and bias into survey abundance estimates if not accounted for. The small, abundant pelagic fishes of the Alaska Arctic are challenging to sample with trawls as they are sufficiently motile to avoid small fine-mesh trawls but are also small enough to escape through the meshes of trawls designed to capture larger fishes. A pelagic herring trawl equipped with a fine-mesh codend liner was used to quantify the size and species composition of pelagic fishes during a baseline acoustic-trawl survey of the Chukchi Shelf. Subsequent experiments with recapture nets attached to the outside of the trawl netting suggested that escapement of small fishes was substantial, particularly in the aft net section. Thus, the trawl was further modified by reducing the taper in the aft net section and adding a small-mesh section in front of the codend to potentially reduce escapement. Further use of recapture nets during two subsequent acoustic-trawl surveys confirmed that this trawl modification substantially increased retention of small fishes and resulted in less size selectivity. These improvements will reduce biases in estimates of abundance, size, and species composition of pelagic Arctic fishes. This work highlights the importance of quantifying escapement from survey trawls and demonstrates that escapement estimates can guide successful trawl modifications.

We used genetic techniques to identify gadids (cods) to species in the Pacific Arctic during a time of substantial physical change in the marine ecosystem between 2012 and 2019. The dominant fish species in the Chukchi Sea is Arctic Cod (Boreogadus saida); however, other gadids such as Saffron Cod (Eleginus gracilis), Pacific Cod (Gadus macrocephalus) and Walleye Pollock (Gadus chalcogrammus) have been observed. Two aims in this study were to evaluate the accuracy of at sea morphological identification (which can be difficult for juveniles) with genetic species identification and to document potential variation in species composition and distribution of gadids in the Pacific Arctic in response to changing environmental conditions. Microsatellite and mtDNA genetic results revealed that most B. saida collected in the Chukchi Sea in 2012 and 2013 were correctly identified at sea. Conversely, genetic results from samples collected in 2017 and 2019 revealed a large number of G. chalcogrammus and some G. macrocephalus and E. gracilis that were initially identified at sea as B. saida. The majority of misidentification occurred between B. saida and G. chalcogrammus. This study indicates a northward shift of G. chalcogrammus and B. saida during warmer conditions. In addition, juvenile Polar Cod (A. glacialis), which is not typically found in the Chukchi Sea and was not identified at sea, was genetically detected on 3 hauls on the northern Chukchi Shelf, outside of its documented distribution. Accurate species identification, especially during a time of changing marine landscapes, is not only important for survey abundance estimates but for downstream analyses as well. This emphasizes the value of implementing strategies for correct identification of the gadid species to better capture and monitor responses to varying and likely changing conditions. Our results provide strong evidence of distributional shifts and range expansions of gadid species in the Arctic, which may be the result of changing climactic conditions.

Sand lance or sand eels ( Ammodytes spp. ) are small planktivorous forage fishes that play an integral role in pelagic ecosystems in the Northern Hemisphere. Arctic sand lance ( Ammodytes hexapterus ) is prevalent in the North Pacific in the Sea of Okhotsk, northern Bering Sea, Chukchi Sea, and Beaufort Sea. Few studies have focused on this species despite its critical position in energy transfer and trophic food webs. Recent surveys in the Chukchi Sea and Beaufort Sea have noted an increase in the prevalence of this species in concert with reduced ice extent. Sand lance are unique among forage species in having close associations with specific water column conditions as well as a reliance on specific sand substrates for burrowing. Life history and habitat dependency is influenced by both dynamic oceanographic conditions and static benthic substrata. This taxa, therefore, provides a unique opportunity to examine the potential expansion of a boreal species into Arctic marine habitats. We use comprehensive surveys conducted throughout the Chukchi Sea shelf over multiple years to evaluate spatial distribution and abundance relative to oceanographic variables in the water column and sediment composition on the seafloor. We applied logistic regression and generalized additive models to investigate presence and relative abundance of Arctic sand lance and to evaluate spatial distribution, as a function of oceanographic and benthic environmental variables. Spatial distribution shifted considerably between years in response to environmental conditions. Arctic sand lance presence was influenced by surface water mass and positively associated with Alaskan Coastal Water. Relative abundance was positively associated with high surface temperature, low surface salinity, and coarser substrates. Evidence is mounting that the distributions of many boreal species are expanding on the margins of the Arctic. Our research reports increased aggregations of Arctic sand lance in the Chukchi Sea and at the highest latitude on record for this species, at the shelf break of the Arctic Basin.

In 2020, the developing COVID-19 pandemic disrupted fisheries surveys to an unprecedented extent. Many surveys were cancelled, including those for walleye pollock (Gadus chalcogrammus) in the eastern Bering Sea (EBS), the largest fishery in the United States. To partially mitigate the loss of survey information, we deployed three uncrewed surface vehicles (USVs) equipped with echosounders to extend the ship-based acoustic-trawl time series of pollock abundance. Trawling was not possible from USVs, so an empirical relationship between pollock backscatter and biomass established from previous surveys was developed to convert USV backscatter observations into pollock abundance. The EBS is well suited for this approach since pollock dominate midwater fishes in the survey area. Acoustic data from the USVs were combined with historical surveys to provide a consistent fishery-independent index in 2020. This application demonstrates the unique capabilities of USVs and how they could be rapidly deployed to collect information on pollock abundance and distribution when a ship-based survey was not feasible. We note the limitations of this approach (e.g. higher uncertainty relative to previous ship-based surveys), but found the USV survey to be useful in informing the stock assessment in a situation where ship-based surveys were not possible.

Fin Balaenoptera physalus and humpback Megaptera novaeangliae whales share foraging areas and may compete for the same prey, but little is known about the extent to which they partition prey resources. Visual cetacean surveys and simultaneous acoustic-trawl surveys of prey were conducted around 2 submarine canyons off Kodiak Island, Alaska, in 2004 and 2006. Statistical models were used to examine the associations between sightings of fin and humpback whales and measures of their potential prey and environment. Observations and models indicate that fin whales were disproportionately abundant in areas with the highest observed euphausiid concentrations, while humpback whales were abundant at lower euphausiid concentrations and in areas where juvenile walleye pollock were abundant. Fin whales were abundant in the areas where euphausiid biomass was deepest and in the deepest areas surveyed (>150 m depth). In contrast, humpback whales primarily occurred in shallower areas and near more shallowly distributed euphausiids. The different depth and prey affinities of fin and humpback whales suggest niche and habitat partitioning between these 2 co-occurring species. Abundance models built using acoustic estimates of prey density are a useful tool to further understanding of the abundance, distribution, and behavior of these animals.

Summer surveys of the Chukchi Sea indicate that high densities of age‐0 gadid fishes, historically Arctic cod (Boreogadus saida) but recently also walleye pollock (Gadus chalcogrammus), dominate the pelagic fish community. Adults are comparatively scarce, suggesting that either overwinter survivorship of age‐0 gadids is low, or that they emigrate to other areas of the Pacific Arctic. To examine population movement, we conducted repeat acoustic surveys with saildrone autonomous surface vehicles equipped with echosounders throughout summer 2018. The saildrones' range and endurance enabled two large‐scale surveys of the U.S. Chukchi shelf. Acoustic backscatter, a proxy for fish density, was highest in regions with sea surface temperatures of 6–8°C, and lowest in areas influenced by recent ice melt. A subarea of the central Chukchi was surveyed a total of four times; backscatter in this subarea increased by > 85% from late‐July to mid‐September. As summer progressed, fish developed more extensive diel vertical migrations and backscatter from individuals doubled. Both changes suggest increases in backscatter were driven primarily by increasing body size. Particle tracking simulations indicated age‐0 gadids were likely retained over the Chukchi shelf by extended periods of wind‐driven southward flow during the survey period before strong northward flow in late fall transported them to the north. These findings suggest that in summer 2018, age‐0 gadids were advected northward to the Chukchi shelf from the northern Bering Sea, where they were retained during a period of growth until late fall before being advected farther north toward the Chukchi and Beaufort shelf breaks.

Identifying biological scatterers is a perennial challenge in fisheries acoustics. Most practitioners classify backscatter based on direct sampling and frequency-difference thresholds, then integrate at a single frequency. However, this approach struggles with species mixtures, and discards multi-frequency information when integrating. Inversion methods do not have these limitations, but are seldom used, because their species identifications are often ambiguous and their algorithms complicated to implement. We address these shortcomings with a probabilistic, Bayesian inversion method. Like other inversion methods, it handles species mixtures, uses all available frequencies, and extends naturally to broadband signals. Unlike prior approaches, it leverages Bayesian priors to rigorously incorporate information from direct sampling and biological knowledge, constraining the inversion and reducing ambiguity in species identification. Because it is probabilistic, it can be trusted to run automatically: it should not produce solutions that are both wrong and confident. Unlike some data-driven machine learning models, it is based on acoustical scattering processes, so its inferences are physically interpretable. Finally, the approach is straightforward to implement using existing Bayesian libraries, and is easily parallelized for large datasets. We present examples using simulations and field data from the Gulf of Alaska, and discuss possible extensions and applications of the method.