Otolith chemistry of juvenile walleye pollock Gadus chalcogrammus in relation to regional hydrography: evidence of spatially split cohorts Marine Ecology Progress Series

For many marine ecosystems, uncertainty about nursery locations and the spatial dynamics of juvenile fish impedes our understanding of fish production. Walleye pollock Gadus chalcogrammus occur throughout the coastal North Pacific Ocean and support some of the world’s largest fisheries. We used otolith microchemistry to answer questions about whether cohorts of young-of-the-year (age-0) walleye pollock are spatially split in the western Gulf of Alaska (GOA). Demographics indicate a possible cohort split between habitat influenced by the Alaska Coastal Current (ACC) (Semidi regions) and habitat more influenced by oceanic influxes (Kodiak region). We used a stratified-random sampling design to select 204 age-0 juveniles collected with a small-mesh trawl during late summer 2007 and 2011. Laser ablation-inductively coupled plasma mass spectrometry was then used to measure the composition of elements assimilated into their otoliths within 1 wk of capture (otolith edge) and over their life histories (otolith edge to core). Otolith edge chemistry varied by region of capture, primarily in strontium:calcium (Sr:Ca), barium:Ca (Ba:Ca), and manganese:Ca (Mn:Ca). Semidi-region otoliths were discriminated from Kodiak otoliths by lower Sr:Ca and higher Ba:Ca and Mn:Ca with 78% (2007) and 79% (2011) success. We estimated that exchange between these 2 habitats was limited for ≥3 (2007) and ≥7 (2011) weeks, sufficient to explain observed demographic differences. We hypothesize that a Semidi-Kodiak split buffers the western GOA population against losses due to density-dependent mechanisms and downstream transport.