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Otolith chemistry indicates age and region of settlement of immature shortspine thornyhead (Sebastolobus alascanus) in the eastern Pacific Ocean

Emmanis Dorval*, Richard D. Methot, Ian G. Taylor, Kevin R. Piner

*Corresponding author:

ABSTRACT: Shortspine thornyhead, Sebastolobus alascanus, are widely distributed along the eastern Pacific coast and are assessed as a demographically homogeneous stock off the coasts of California, Oregon and Washington. The validity of this assumption has been questioned because data suggest complex ontogenetic movements. Otolith microchemistry applied to immature fish was used to determine the age of settlement and the population structure of S. alascanus. Samples were collected in 2011-2013 from bottom trawl surveys in five areas along the coast: Washington, northern Oregon, southern Oregon and northern California, central and southern California. Ba:Ca, measured from the primordium to the margin of otoliths, corresponds to the pelagic and the benthic phases of immature fish. This ratio decreased steadily before the formation of the first annulus, and increased thereafter. Water masses along the bathymetric gradient mostly correlated with the distribution of otolith trace elements (Mg:Ca, Ba:Ca) and stable isotopes (δ18O and δ13C) on the continental shelf. These results indicated that the population of immature fish belongs to two distinct groups, north and south of Cape Mendocino. Accordingly, the performance of a k-nearest-neighbor classifier was highest when structured based on these two regions, with 94% and 69% cross-validated classification accuracy, respectively into the northern and southern group. These results confirmed that S. alascanus settled after nearly one year of pelagic life, and showed that otolith chemistry could be used to test critical hypotheses regarding the migration and population structure of this species.