Summer 2018 repeat autonomous vehicle surveys indicate age-0 Arctic cod are largely retained over the Chukchi Sea shelf

Robert Levine1, Alex De Robertis2, Daniel Grunbaum1, Rebecca A Woodgate3, Calvin Mordy4, Edward D Cokelet5, Noah Lawrence-Slavas6 and Heather M. Tabisola7, (1)University of Washington, School of Oceanography, Seattle, WA, United States, (2)NOAA Fisheries, Alaska Fisheries Science Center, Midwater Assessment and Conservation Engineering, Seattle, WA, United States, (3)University of Washington, Seattle, WA, United States, (4)University of Washington, Cooperative Institute for Climate, Ocean, and Ecosystem Studies, Seattle, United States, (5)Pacific Marine Environmental Laboratory, Seattle, WA, United States, (6)NOAA Pacific Marine Environmental Laboratory, Seattle, WA, United States, (7)University of Washington, Cooperative Institute for Climate, Ocean, and Ecosystem Studies / NOAA PMEL, Seattle, United States
Abstract:
Recent summer surveys of the Chukchi Sea indicate that 38 kHz acoustic backscatter is dominated by high densities of age-0 Arctic cod (Boreogadus saida) while adults are comparatively scarce. Thus, either overwinter survivorship of age-0 Arctic cod is low or juveniles emigrate to other areas. To examine the displacement of this population over the 2018 summer growing season, we conducted repeat acoustic surveys with Saildrone unmanned surface vehicles. Saildrones’ endurance and range enabled us to complete two large-scale surveys; mid-July to mid-August and late-August to mid-September. Backscatter increased from mid-July to mid-September and was highest in regions with sea surface temperatures of 6-8°C. Backscatter was lowest in cold, fresh areas influenced by recent ice melt. A subsection was surveyed two additional times for greater temporal coverage; backscatter in this subsection increased >65% between the first and last survey. As summer progressed, Arctic cod exhibited more extensive vertical migrations, becoming distributed deeper in the water column during daytime. This depth increase, concurrent with a 140% increase in the acoustic target strength of individuals over this period, is typically associated with growth. These increases in diel vertical migration and target strength suggest that changes in backscatter were driven by an increase in the size of the fish in the survey region. Tracking particle trajectories in a regional numerical model indicated extended periods of southward flow during the survey period. This likely acted to retain cod populations over the Chukchi Sea shelf before strong northward flow in late fall transported them to the north. Together with the survey observations, these findings suggest that in summer 2018, age-0 Arctic cod were passively retained in this region during a period of growth before being advected farther north towards the Chukchi and Beaufort shelf breaks.
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