Response of a floodplain fish community to river-floodplain connectivity: natural versus managed reconnection

To restore lateral connectivity in highly regulated river-floodplain systems, it has become necessary to implement localized, "managed" connection flows, made possible using floodplain irrigation infrastructure. These managed flows contrast with "natural", large-scale, overbank flood pulses. We compared the effects of a managed and a natural connection event on (i) the composition of the large-bodied fish community and (ii) the structure of an endangered catfish population of a large floodplain lake. The change in community composition following the managed connection was not greater than that exhibited between seasons or years during disconnection. By contrast, the change in fish community structure following the natural connection was much larger than that attributed to background, within-and between-year variability during disconnection. Catfish population structure only changed significantly following the natural flood. While the natural flood increased various population rates of native fishes, it also increased those of non-native carp, a pest species. To have a positive influence on native biodiversity, environmental flows may need to be delivered to floodplains in a way that simulates the properties of natural flood pulses. A challenge, however, will be managing river-floodplain connectivity to benefit native more than non-native species.

A tale of two seas: contrasting patterns of population structure in the small-spotted catshark across Europe.

Elasmobranchs represent important components of marine ecosystems, but they can be vulnerable to overexploitation. This has driven investigations into the population genetic structure of large-bodied pelagic sharks, but relatively little is known of population structure in smaller demersal taxa, which are perhaps more representative of the biodiversity of the group. This study explores spatial population genetic structure of the small-spotted catshark (Scyliorhinus canicula), across European seas. The results show significant genetic differences among most of the Mediterranean sample collections, but no significant structure among Atlantic shelf areas. The data suggest the Mediterranean populations are likely to have persisted in a stable and structured environment during Pleistocene sea-level changes. Conversely, the Northeast Atlantic populations would have experienced major changes in habitat availability during glacial cycles, driving patterns of population reduction and expansion. The data also provide evidence of male-biased dispersal and female philopatry over large spatial scales, implying complex sex-determined differences in the behaviour of elasmobranchs. On the basis of this evidence, we suggest that patterns of connectivity are determined by trends of past habitat stability that provides opportunity for local adaptation in species exhibiting philopatric behaviour, implying that resilience of populations to fisheries and other stressors may differ across the range of species.

DNA from historical and trophy samples provides insights into white shark population origins and genetic diversity

Characterizing genetic variation by retrospective genotyping of trophy or historical artifacts from endangered species is an important conservation tool. Loss of genetic diversity in top predators such as the white shark Carcharodon carcharias remains an issue, exacerbated in this species by declining, sometimes isolated philopatric populations. We successfully sequenced mitochondrial DNA (mtDNA) D-loop from osteodentine of contemporary South African white shark teeth (from 3 jaws), and from 34 to 129 yr old dried cartilage and skin samples from 1 Pacific Ocean and 5 Mediterranean sharks. Osteodentine-derived sequences from South African fish matched those derived from an individual’s finclips, but were generally of poorer quality than those from skin and cartilage of historical samples. Three haplotypes were identified from historical Mediterranean samples (n = 5); 2 individuals had unique sequences and 3 shared the contemporary Mediterranean haplotype. Placement of previously undescribed mtDNA haplotypes from historical material within both the Mediterranean and Pacific clades fits with the accepted intra-specific phylogeny derived from contemporary material, verifying our approaches. The utility of our methodology is in its provision of additional genetic resources from osteodentine (for species lacking tooth pulp) and cartilage of rare and endangered species held in often uncurated, contemporary and historical dry collections. Such material can usefully supplement estimates of connectivity, population history, and stock viability. We confirm the depauperate haplotype diversity of historical Mediterranean sharks, consistent with founding by a small number of Pacific colonizers. The consequent lack of diversity suggests serious challenges for the maintenance of this top predator and the Mediterranean ecosystem.

DNA from historical and trophy samples provides insights into white shark population origins and genetic diversity

Characterizing genetic variation by retrospective genotyping of trophy or historical artifacts from endangered species is an important conservation tool. Loss of genetic diversity in top predators such as the white shark Carcharodon carcharias remains an issue, exacerbated in this species by declining, sometimes isolated philopatric populations. We successfully sequenced mitochondrial DNA (mtDNA) D-loop from osteodentine of contemporary South African white shark teeth (from 3 jaws), and from 34 to 129 yr old dried cartilage and skin samples from 1 Pacific Ocean and 5 Mediterranean sharks. Osteodentine-derived sequences from South African fish matched those derived from an individual’s finclips, but were generally of poorer quality than those from skin and cartilage of historical samples. Three haplotypes were identified from historical Mediterranean samples (n = 5); 2 individuals had unique sequences and 3 shared the contemporary Mediterranean haplotype. Placement of previously undescribed mtDNA haplotypes from historical material within both the Mediterranean and Pacific clades fits with the accepted intra-specific phylogeny derived from contemporary material, verifying our approaches. The utility of our methodology is in its provision of additional genetic resources from osteodentine (for species lacking tooth pulp) and cartilage of rare and endangered species held in often uncurated, contemporary and historical dry collections. Such material can usefully supplement estimates of connectivity, population history, and stock viability. We confirm the depauperate haplotype diversity of historical Mediterranean sharks, consistent with founding by a small number of Pacific colonizers. The consequent lack of diversity suggests serious challenges for the maintenance of this top predator and the Mediterranean ecosystem.

Fisheries stocks from an ecological perspective: Disentangling ecological connectivity from genetic interchange

The concept of a stock of fish as a management unit has been around for well over a hundred years, and this has formed the basis for fisheries science. Methods for delimiting stocks have advanced considerably over recent years, including genetic, telemetric, tagging, geochemical and phenotypic information. In parallel with these developments, concepts in population ecology such as meta-population dynamics and connectivity have advanced. The pragmatic view of stocks has always accepted some mixing during spawning, feeding and/or larval drift. Here we consider the mismatch between ecological connectivity of a matrix of populations typically focussed on demographic measurements, and genetic connectivity of populations that focus on genetic exchange detected using modern molecular approaches. We suggest that from an ecological-connectivity perspective populations can be delimited as management units if there is limited exchange during recruitment or via migration in most years. From a genetic-connectivity perspective such limited exchange can maintain panmixia. We use case-studies of species endangered by overexploitation and/or habitat degradation to show how current methods of stock delimitation can help in managing populations and in conservation.

Fisheries stocks from an ecological perspective: Disentangling ecological connectivity from genetic interchange

The concept of a stock of fish as a management unit has been around for well over a hundred years, and this has formed the basis for fisheries science. Methods for delimiting stocks have advanced considerably over recent years, including genetic, telemetric, tagging, geochemical and phenotypic information. In parallel with these developments, concepts in population ecology such as meta-population dynamics and connectivity have advanced. The pragmatic view of stocks has always accepted some mixing during spawning, feeding and/or larval drift. Here we consider the mismatch between ecological connectivity of a matrix of populations typically focussed on demographic measurements, and genetic connectivity of populations that focus on genetic exchange detected using modern molecular approaches. We suggest that from an ecological-connectivity perspective populations can be delimited as management units if there is limited exchange during recruitment or via migration in most years. From a genetic-connectivity perspective such limited exchange can maintain panmixia. We use case-studies of species endangered by overexploitation and/or habitat degradation to show how current methods of stock delimitation can help in managing populations and in conservation.