Anthropogenic changes, such as overfishing and marine invasions, have greatly altered the ecology of the North Atlantic Ocean. Yet setting a meaningful benchmark for restoration is difficult in the absence of historical data. Genetic methods can be useful in studying the history and effects of these alterations.; Marine Invasions. (1) The first record of the European green crab, Carcinus maenas, beyond its native range was in the eastern United States in 1817. Since then it has expanded north to Nova Scotia and Prince Edward Island. To analyze the history of this widespread invader, I sequenced a 502-bp fragment of the mitochondrial cytochrome oxidase 1 gene from 295 samples collected from Long Island to Prince Edward Island. Contrary to a priori assumptions, southern populations have lower genetic diversity than recent, northern populations, indicating that a new invasion has occurred in the Canadian Maritimes. High genetic diversity suggests that modern invasions may carry far more propagules than earlier intrusions, reducing the founder effect and increasing the chances of long-term survival of alien species. (2) A phylogeographic analysis of European populations based on 230 individuals shows slight structure between the North Sea and populations in western Europe. Populations on the Faeroe Islands and Iceland showed strong structure, with Iceland represented by a single lineage. An assignment test and pairwise FSTs indicate that the point of origin for the Nova Scotia invasion was probably the North Sea. (3) In 2002, I collected 578 juvenile green crabs from the Gulf of Maine and Scotian Shelf to examine changes in the genetic cline across two generations. No significant change was seen between 2000 and 2002, indicating that larvae had been retained close to parental populations. This research can be used as a model for larval dispersal in native species, helping to manage harvests and design marine reserves.; Marine Exploitation. Whaling has reduced populations of all species of baleen whales. Although managers rely on preexploitation population sizes to set recovery goals, these numbers are difficult to estimate. I used coalescent methods to estimate long-term population size of three exploited species; the North Atlantic probably sustained at least 240,000 humpback, 360,000 fin, and 265,000 minke whales. These populations are far larger than current population estimates and greater than those previously calculated for prewhaling populations. Genetic methods provide a new approach, independent of historical records, to enumerate natural abundances in perturbed ecosystems.
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