title: Spatio-temporal genetic structure and the effects of long-term fishing in two partially sympatric offshore demersal fishes.
abstract: Environmental gradients have been shown to disrupt gene flow in marine species, yet their influence in structuring populations at depth remains poorly understood. The Cape hakes (Merluccius paradoxus and M. capensis) are demersal species co-occurring in the Benguela Current system, where decades of intense fishing resulted in severely depleted stocks in the past. Previous studies identified conflicting mtDNA genetic substructuring patterns and thus contrasting evolutionary trajectories for both species. Using 10 microsatellite loci, the control region of mtDNA and employing a seascape genetics approach, we investigated genetic connectivity and the impact of prolonged exploitation in the two species, which are characterized by different patterns of fishing pressure. Three consecutive years were sampled covering the entire distribution (N = 2100 fishes). Despite large estimated population sizes, both species exhibited low levels of contemporary genetic diversity (0.581 < HE< 0.692), implying that fishing has had a significant impact on their genetic composition and evolutionary trajectories. Further, for M. paradoxus, significant temporal, but not spatial, divergence points to the presence of genetic chaotic patchiness. In contrast, M. capensis exhibited a clear latitudinal cline in genetic differentiation between Namibia and South Africa (FST= 0.063, P < 0.05), with low (0.2% per generation) estimates of contemporary gene flow. Seascape analyses reveal an association with bathymetry and upwelling events, suggesting that adaptation to local environmental conditions may drive genetic differentiation in M. capensis. Importantly, our results highlight the need for temporal sampling in disentangling the complex factors that impact population divergence in marine fishes.
authors.affiliation: Evolutionary Genomics Group, Department of Botany and Zoology, Stellenbosch University, Private Bag X1, Stellenbosch, 7602, South Africa.
authors.forename: Romina
authors.initials: R
authors.lastname: Henriques
authors.affiliation: Evolutionary Genomics Group, Department of Botany and Zoology, Stellenbosch University, Private Bag X1, Stellenbosch, 7602, South Africa.
authors.forename: Sophie
authors.initials: S
authors.lastname: von der Heyden
authors.affiliation: Department of Ichthyology and Fisheries Sciences, Rhodes University, P.O. Box 94, Grahamstown, 6140, South Africa.
authors.forename: Marek R
authors.initials: MR
authors.lastname: Lipinski
authors.affiliation: Evolutionary Genomics Group, Department of Botany and Zoology, Stellenbosch University, Private Bag X1, Stellenbosch, 7602, South Africa.
authors.forename: Nina
authors.initials: N
authors.lastname: du Toit
authors.affiliation: National Marine Information and Research Centre, Private Bag 912, Swakopmund, Namibia.
authors.forename: Paulus
authors.initials: P
authors.lastname: Kainge
authors.affiliation: Molecular Ecology and Evolution Programme, Department of Genetics, University of Pretoria, Private Bag X20, Hatfield, Pretoria, 0028, South Africa.
authors.forename: Paulette
authors.initials: P
authors.lastname: Bloomer
authors.affiliation: Evolutionary Genomics Group, Department of Botany and Zoology, Stellenbosch University, Private Bag X1, Stellenbosch, 7602, South Africa.
authors.forename: Conrad A
authors.initials: CA
authors.lastname: Matthee
mesh_terms: D000818:Animals; D004272:DNA, Mitochondrial / Q000235:genetics; D005398:Fisheries; D048388:Gadiformes / Q000235:genetics*; D051456:Gene Flow; D014644:Genetic Variation; D005828:Genetics, Population; D018895:Microsatellite Repeats; D009276:Namibia; D013019:South Africa; D062211:Spatio-Temporal Analysis
chemical_list: D004272:DNA, Mitochondrial
keywords: effective population size; fishing pressure; genetic chaotic patchiness; genetic variability; microsatellites; seascape genetics