The potential utility of otolith microchemistry as an indicator of nursery origins in Pacific halibut (Hippoglossus stenolepis)

Abstract

Identifying the nursery origins of individual Pacific halibut (Hippoglossus stenolepis) is critical to a refined understanding of regional productivity, the effects of environmental forcing on population dynamics, and the spatial impacts of mortality and bycatch. Otolith microchemistry may provide a tool for identifying fish origins, but a precursor to using microchemistry-based assignment models is to evaluate the extent to which there exists detectable spatial structure in these natural tags. Here, we examined assignment accuracy as a function of spatial scale using canonical discriminant function analyses (DFA) applied to microchemical data from age-2 Pacific halibut in the western Gulf of Alaska (GOA) and southeastern Bering Sea (SEBS). Element:calcium ratios were assayed for fourteen trace elements using inductively coupled plasma mass spectrometry; δ13C and δ18O were determined via isotope ratio mass spectrometry. Substantial assignment success (∼75–90 %) was observed at spatial scales that are consistent with stock management. Elemental signatures were defined primarily by δ18O, δ13C, and 88Sr:48Ca, with a minor contribution from 55Mn:48Ca. Individuals were most commonly mis-assigned to adjacent locations. However, discontinuities were observed within the western GOA that suggest that elemental signatures do not vary along strictly longitudinal and latitudinal clines. These results highlight the need to exercise caution when attempting to use otolith microchemistry to assign fish to their origins when the baseline elemental data are unable to resolve missing nursery sources from the locations that are included in the discrimination models.