Autonomous vehicle surveys indicate that flow reversals retain juvenile fishes in a highly advective high‐latitude ecosystem

Abstract

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.

Northward Range Expansion of Subarctic Upper Trophic Level Animals into the Pacific Arctic Region

“…Surveys during the recent period of extreme warming (2017-2020) indicate that while the distributions of the other pelagic forage fishes have not significantly changed, pollock abundance in the Pacific Arctic has substantially increased (Figure 2). In the eastern Chukchi Sea, juvenile pollock were widespread and highly abundant in 2017 and 2019 and found in comparable densities to Arctic species (Levine et al, 2021; recent work of author Levine). In the Russian sector, surveys in 2018 and 2019 found a significant increase in both juvenile and adult pollock north of the Chukotka Peninsula (Orlov et al, 2020).…”

Uncrewed surface vehicle (USV) survey of walleye pollock, Gadus chalcogrammus, in response to the cancellation of ship-based surveys

“…The USVs were equipped with a Simrad WBT mini split-beam echosounder equipped with an ES38-18/200-18CR transducer (18 • split-beam 38 kHz and 18 • single beam 200 kHz) gimbal mounted on the keel at a depth of 1.9 m (De Robertis et al, 2019;Levine et al, 2021). The echosounders continuously transmitted 0.512 ms narrowband signals every 3 sec.…”

“…Thus, the development of reliable methods to convert acoustic measurements to species and size without trawling has the potential to transform the use of echosounderequipped USVs. More widespread use of frequency response (Korneliussen, 2018), target-strength measurements (Levine et al, 2021), broadband measurements (Bassett et al, 2017), resonance scattering (Stanton et al, 2012), and application of machine learning (Brautaset et al, 2020;Proud et al, 2020), may ultimately result in better characterization of acoustic targets. Sampling methods that can be adapted to USVs such as small cameras (Fernandes et al, 2016), and environmental DNA samplers (Yamahara et al, 2019;Berger et al, 2020) may prove valuable.…”

“…USVs using wind and wave power for propulsion are promising for large-scale fisheries surveys (Greene et al, 2014;Swart et al, 2016) due to their long endurance. Initial short-term deployments (Greene et al, 2014) have been extended to >100 days (De Robertis et al, 2019;Chu et al, 2019;Levine et al, 2021), and USVs capable of conducting acoustic surveys are now commercially available (Verfuss et al, 2019). Low-power echosounders suitable for acoustic measurement of fish abundance from USVs have also been developed (Benoit-Bird et al, 2018).…”

Sea Surface Salinity Variability in the Bering Sea in 2015–2020

“…Some in situ salinity observations might not be close enough to the surface and often yield large biases when validating L-band radiometric SSS, which are representative of the first top centimeter. Recently, Saildrone, Inc. uncrewed surface vehicles (USVs) have been widely deployed to make near-surface measurements in the Bering Sea, Chukchi Sea and Arctic Ocean [37,38]. The authors in [39] analyzed saildrone data in 2019 and the simultaneous remote sensing salinity products and concluded that many of the mesoscalesubmesoscale variability were captured.…”