Philopatry and migration of Pacific white sharks.

Advances in electronic tagging and genetic research are making it possible to discern population structure for pelagic marine predators once thought to be panmictic. However, reconciling migration patterns and gene flow to define the resolution of discrete population management units remains a major challenge, and a vital conservation priority for threatened species such as oceanic sharks. Many such species have been flagged for international protection, yet effective population assessments and management actions are hindered by lack of knowledge about the geographical extent and size of distinct populations. Combining satellite tagging, passive acoustic monitoring and genetics, we reveal how eastern Pacific white sharks (Carcharodon carcharias) adhere to a highly predictable migratory cycle. Individuals persistently return to the same network of coastal hotspots following distant oceanic migrations and comprise a population genetically distinct from previously identified phylogenetic clades. We hypothesize that this strong homing behaviour has maintained the separation of a northeastern Pacific population following a historical introduction from Australia/New Zealand migrants during the Late Pleistocene. Concordance between contemporary movement and genetic divergence based on mitochondrial DNA demonstrates a demographically independent management unit not previously recognized. This population's fidelity to discrete and predictable locations offers clear population assessment, monitoring and management options.

GEOGRAPHIC POPULATION STRUCTURE AND SPECIES DIFFERENCES IN MITOCHONDRIAL DNA OF MOUTHBROODING MARINE CATFISHES (ARIIDAE) AND DEMERSAL SPAWNING TOADFISHES (BATRACHOIDIDAE).

Restriction-fragment length polymorphisms in mitochondrial DNA (mtDNA) were used to evaluate population-genetic structure and matriarchal phylogeny in four species of marine fishes that lack a pelagic larval stage: the catfishes Arius felis and Bagre marinus, and the toadfishes Opsanus tau and O. beta. Thirteen informative restriction enzymes were used to assay mtDNAs from 134 specimens collected from Massachusetts to Louisiana. Considerable genotypic diversity was observed in each species. However, major mtDNA phylogenetic assemblages in catfish and toadfish (as identified in Wagner networks and UPGMA phenograms) exhibited contrasting patterns of geographic distribution: in catfish, distinct mtDNA clades were widespread, while such clades in toadfish tended to be geographically localized. By both the criteria of species' ranges and the geographic pattern of intraspecific mtDNA phylogeny, populations of marine catfish in the western Atlantic have had greater historical interconnectedness than have toadfish. Results are also compared to previously published mtDNA data in freshwater and other marine fishes. Although mtDNA differentiation among conspecific populations of continuously distributed marine fishes is usually lower than that among discontinuously distributed freshwater species inhabiting separate drainages, it is apparent that historical biogeographic factors can importantly influence genetic structure in marine as well as freshwater species.