Spatial and temporal variation in otolith elemental signatures of age-0 Pacific cod (Gadus macrocephalus) in the Gulf of Alaska

Abstract

Shallow coastal waters of the Gulf of Alaska (GOA) serve as nursery habitats for young-of-year Pacific cod (Gadus macrocephalus). However, little is known regarding the relative contribution of these areas to the adult offshore stock. Trace elements incorporated into the otolith matrix can reflect the environmental conditions to which a fish has been exposed during its lifetime. When analyzed together, a suite of elements can serve as a natural marker, characteristic of a particular environment. We evaluated the potential of otolith elemental signatures to identify nursery habitats of age-0 Pacific cod, focusing on spatial patterns within and across the eastern and western GOA. Fish were collected from shallow nearshore areas within five large embayments in summer and fall, and element:calcium ratios were measured from two different time stanzas within the otolith. Elemental ratios were found to change over short (2 month) time periods and were related to seasonal changes in temperature and salinity. Fish were classified to nursery habitats using quadratic discriminant analysis based on their otolith elemental signatures; classification accuracy to individual bays ranged from 30% to 95%, with an overall success rate of 59%. Classification accuracy improved to 78% at greater spatial scales (eastern versus western GOA). Our results point out the limitations of this application to Gulf of Alaska Pacific cod and other widely distributed species residing in coastal embayments. While some nursery habitats impart unique chemical signatures to otoliths, it may not be possible to distinguish among specific areas without considering additional factors. However, our work demonstrates that otolith microchemistry may be a useful tool for understanding source contributions to the Pacific cod population at larger regional scales within the GOA.