Integrated use of otolith shape and microchemistry to assess Genidens barbus fish stock structure

Otolith composition (edge vs core: Mg/Ca, Mn/Ca, Zn/Ca, Sr/Ca, Ba/Ca) by LA-ICP-MS and Elliptic Fourier analysis were integrated to evaluate spatial segregation of adult and juvenile stages of Genidens barbus from specimens collected from five coastal areas off Brazil (Paraiba do Sul River, Guanabara Bay, Itapanhau River mouth, Paranagua Bay), Argentina and Uruguay (La Plata Estuary). Fisheries of this diadromous catfish have largely collapsed in the southwest Atlantic coastal region due to overexploitation. An understanding of population structure is now critically needed for improved management strategies for this endangered species. PERMANOVA based on otolith edge chemistry showed significant differences (p < 0.05) between all sites, except Itapanhau River and Paranagu ' a Bay. Shape, by comparison, found significant differences between all sampling sites, except Guanabara Bay and Paranagu ' a Bay, and Itapanhau River and Paranagu ' a Bay. Discriminant analysis cross-classification success based on chemistry ranged from 33.3 (Paranagu ' a Bay) to 100% (La Plata Estuary), and 66.7 (Paranagu ' a Bay) to 100% (La Plata Estuary) for otolith edges (mean = 61.3%) and cores (mean = 78.9%), respectively. For otolith shape, the jackknifed rate (mean = 45.9%) was relatively low for all sites (32.1-44.7%) except La Plata Estuary (67.6%). Although we do not find otolith shape to be particularly useful; otolith microchemistry supports the presence of different management units. The results revealed that on a small geographic scale (similar to 300 km) microchemistry might not be efficient to discriminate between some sampling sites.

Automated summary

Otolith chemistry showed significant differences between sampling sites, while otolith shape differences were also significant except for certain sites. The discriminant analysis success rate ranged from 33.3% to 100% for chemistry, and from 32.1% to 67.6% for otolith shape, indicating the potential limitations of using microchemistry on a small geographic scale.