Vertical segregation of age-0 and age-1+ polar cod (Boreogadus saida) over the annual cycle in the Canadian Beaufort Sea
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The offshore marine ecosystem of the Canadian Beaufort Sea faces the double pressure of climate change and industrialization. Polar cod (Boreogadus saida) is a pivotal forage species in this ecosystem, accounting for 95 % of the pelagic fish assemblage. Its vertical distribution over the annual cycle remains poorly documented. Hydroacoustic records from 2006 to 2012 were analysed to test the hypothesis that age-0 polar cod segregate vertically from larger congeners. Trawls and ichthyoplankton nets validated the acoustic signal. Fish length, weight, and biomass were estimated from new regressions of target strength and weight on standard length. Polar cod were vertically segregated by size in all months, with small age-0 juveniles in the epipelagic (<100 m) layer and larger age-1+ deeper in the water column. From December to March, the biomass of age-1+ peaked in a mesopelagic layer between 200 and 400 m. With increasing irradiance from April to July, the mesopelagic layer deepened and extended to 600 m. Starting in July, age-0 polar cod formed an epipelagic scattering layer that persisted until November. From September onward, age-0 left the epipelagic layer to join small age-1+ in the upper mesopelagic layer. Low biomass in the mesopelagic layer from February to September likely resulted from large polar cod settling on the seafloor to avoid diving marine mammals. Longer ice-free seasons, warmer sea-surface temperatures, or an oil spill at the surface would likely impact epipelagic age-0, while mesopelagic age-1+ would be vulnerable to an eventual oil plume spreading over and above the seafloor.
KeywordsVertical distribution Ontogenic migrations Acoustics Target strength Diel vertical migrations
We thank the officers and crew of the CCGS Amundsen and F/V Frosti for their dedication and professionalism. The calibration of the echosounder in 2012 was conducted with the help of George Cronkite at the Pacific Biological Station (DFO). Several technicians and colleagues contributed to sample collection from 2006 to 2012. Special thanks to Jane Eert and Mike Dempsey (DFO) for technical support on board the Frosti, to Shani Rousseau and Dominique Robert (Université Laval) for preliminary analysis, and to Yvan Simard (DFO) for advice during calibration and operation of the Amundsen’s echosounder. This manuscript benefited from the constructive comments of four anonymous reviewers and two editors. The National Oceanic and Atmospheric Administration graciously lent the transducers used during the 2012 survey. This is a contribution to Québec-Océan at Université Laval, ArcticNet, and the Canada Research Chair on the Response of Arctic Marine Ecosystems to Climate Change.
Aboriginal Affairs and Northern Development Canada (Beaufort Region Environmental Assessment program), ArcticNet, and Fisheries and Oceans Canada provided financial support. Imperial Oil Resources Ventures Limited and BP Exploration Operating Company Limited partly funded ship time from 2009 to 2011. MG benefited from scholarships from the Natural Sciences and Engineering Research Council of Canada and the W. Garfield Weston foundation.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
All applicable international, national, and institutional guidelines for the care and use of animals were followed. This active acoustics research project was reviewed by the Environmental Impact Screening Committee (EISC) for the Inuvialuit Settlement Region (www.screeningcommittee.ca). Following the reviews, scientific research licences were issued by the Aurora Research Institute in accordance with the Northwest Territories Scientists Act.
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