Otolith shape analysis as a tool for species identification and management of cryptic congeners in the northern Benguela ocean warming hotspot

Adaptive management is critical to maintaining resilient fisheries in our rapidly changing ocean environments. However, establishing appropriate and cost-effective monitoring programmes that adequately capture the responses of fisheries to climate change have been elusive. The coastal waters of central and northern Namibia are a global hotspot of ocean warming, with several responses by fish impacting its coastal fisheries. One documented change is a poleward distributional shift of the west coast dusky kob, Argyrosomus coronus into Namibia, where it has begun to hybridize with the congeneric A. inodorus. With considerably different life histories, it is critical that managers can differentiate between these species and the hybrids to appropriately manage the most important recreational and commercial linefish species (bycatch and biomass) in Namibia. In this paper, we used otolith shape analysis, Linear Discriminant Analyses (LDA) to show that otolith shape can be used to distinguish between species and their putative hybrids, identified based on mitochondrial DNA and nuclear microsatellite data. A total of 342 individuals from northern and central Namibia (northern Benguela) between 18.5 and 22.7°S were genetically identified, with a subsample of 217 paired with otolith shape analysis. Otolith shape analyses with LDA and leave-one-out cross validation showed successful species identification at 96.3 % accuracy, and improved accuracy of hybrid identification compared to using body morphology by 50 %. The Fourier descriptors provided excellent classification accuracy for separating A. coronus from A. inodorus (and putative hybrids, but poorer classification accuracy for separating the hybrids from A. inodorus (4/8). We thus recommend using otolith shape analysis for distinguishing the two species but suggest that genetic identification is still required to monitor hybridization. Monitoring for adaptive management should therefore include routine otolith collection and species classification to improve stock assessment and promote the development of appropriate management strategies for this important fishery in the northern Benguela.