Date Awarded

2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Virginia Institute of Marine Science

Advisor

John E. Graves

Abstract

Squaloids (Order Squaliformes) are a highle diverse group of mostly deepwater habitats (> 200 m). Many species are regularly caught in commercial fisheries worldwide but their low productivity and correspondingly low intrinsic rebound potentials make them particularly vulnerable to population depletion and overexploitation. of special concern to fisheries management and conservation efforts are the spiny dogfish Squalus acanthias, the leafscale gulper shark Centrophorus squamosus and the Portuguese dogfish Centroscymnus coelolepis. These taxa have recently been declared overfished in several parts of each species' distribution but management efforts have been made to recover the "stocks" without a good understanding of the species' population structure and the level of connectivity among populations. as such, the goal of this dissertation is to elucidate the intraspecific patterns of population structure of each species, for future inclusion in fisheries management efforts, and to infer the patterns of historical dispersal of the three species of squaloid sharks. to this end, I have developed a suite of highly polymorphic molecular markers (including nuclear microsatellites and nucleotide sequences of mitochondrial DNA gene regions) and have collected tissues samples from throughout each species geographic range. The genetic population structure of S. acanthias was characterized by high genetic divergence across the equatorial Pacific, and by comparatively higher genetic homogeneity among the sample collections from the South Pacific and Atlantic oceans. Nevertheless, small but significant genetic differentiation was detected by both nuclear and mitochondrial markers among spiny dogfish collections from either side of the equatorial Atlantic. Genetic differentiation in the spiny dogfish occurred across low latitude regions characterized by warm-temperate and tropical waters, suggesting that such regions may act as effective barriers to gene flow among populations. Regarding C. coelolepis and C. squamosus, the pattern of genetic population structure uncovered for the eastern Atlantic was similar between species. Within this region, no evidence of genetic differentiation was found among sample collections ranging from off Ireland to South Africa, and including the Azores, consistent with the existence of a single genetic stock for each species within the sampled region. Furthermore, evidence for inter-oceanic dispersal between Atlantic and New Zealand populations was also found for the leafscale gulper. These results strongly suggest that both C. coelolepis and C. squamosus have high dispersal potential and no major barriers to gene flow within the deep eastern Atlantic. Overall, high genetic homogeneity was observed over large geographic areas (i.e. in the order of thousands of miles) in all three target species consistent with long-distance dispersal with gene flow. However, there were differences between the coastal and the deepwater squaloids regarding the regions of genetic discontinuity, which may be associated with adaptations to their respective habitats. In the coastal spiny dogfish, environmental factors such as unsuitable water temperatures appear to exert a strong influence in the species' distribution and in its genetic population structure. In the deepwater squaloids, stable environmental conditions but limited food supply may result in widely distributed populations as a strategy to maximizing resource partitioning while minimizing resource competition among individuals.

DOI

https://dx.doi.org/doi:10.25773/v5-z41e-2z41

Rights

© The Author

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