Natal Source Contributions of Pacific Cod (Gadus macrocephalus) Recruits in the Southeastern Bering Sea

Population structure of Pacific cod (Gadus macrocephalus) in the southeastern Bering Sea (EBS) remains unresolved although genetic data indicate isolation-by-distance. Chemical analysis of otoliths can provide relevant information on mixing among groups of fish on ecologically-relevant time scales and at finer spatial scales than genetic analyses. We examined otolith composition of larval and juvenile cod collected in 2006 and 2008 to: (1) determine the number of chemically distinct larval sources; (2) evaluate the potential to detect larval exchange between the Gulf of Alaska (GOA) and the EBS; and (3) quantify the natal sources of juvenile recruits in the EBS. In both years, otolith elemental signatures (Mg:Ca, Mn:Ca, Zn:Ca, Sr:Ca, and Ba:Ca) differed between larvae collected in the EBS and GOA, and 80% and 70% of the larvae were correctly classified to source in 2006 and 2008, respectively. Furthermore, we examined the chemical signature of the otolith edge, which represents the last 10 d of life, for juveniles collected at six locations throughout the EBS. Seventy-nine and 71% of the juveniles were correctly classified to their collection location in 2006 and 2008, respectively. These results demonstrate that there is sufficient spatial variation in otolith composition to examine spatial structure in Pacific cod and potentially identify GOA larval contribution to the EBS population. However, otolith increment analysis indicated that sampled larvae hatched later in the season (April 28 to May 6) than juveniles (Feb 21 to Mar 18), precluding a direct comparison of natal signatures between larvae and juveniles. Therefore, natal signatures of juveniles collected throughout the EBS were classified into chemically distinct groups (sources) using hierarchical cluster analysis. In 2006, we identified two dominant sources based on natal otolith chemistry, representing 56% and 30% of sampled juveniles. In 2008, we identified three sources that were evenly represented in juvenile samples (31-36%). Most collection sites contained a mixture of larval sources although one or two sites per year were comprised of a single source. These sources potentially represent known spawning areas, which include the Alaskan Peninsula, Aleutian Islands, and Pribilof Islands. Our results highlight the potential of otolith chemical analyses to identify source contributions of Pacific cod in the EBS but larval collections throughout the spawning season are needed to provide additional detail on the spatial distribution of those sources.