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

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 from 2011-2013 from bottom trawl surveys in 5 areas along the coasts of (1) Washington, (2) northern Oregon, (3) southern Oregon and northern California, (4) central California and (5) southern California. The Ba:Ca ratio, measured from the primordium to the margin of otoliths, corresponds to the pelagic and 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 element (Mg:Ca, Ba:Ca) and stable isotope (delta 18O and delta 13C) ratios on the continental shelf. These results indicate that the population of immature fish belongs to 2 distinct groups, distributed north and south of Cape Mendocino. Accordingly, the performance of a k-nearest-neighbor classifier was highest when structured based on these 2 regions, with 94 and 69% cross-validated classification accuracy, respectively into the northern and southern groups. These results confirm that S. alascanus settle after nearly 1 yr of pelagic life and show that otolith chemistry can be used to test critical hypotheses regarding the migration and population structure of this species.