Complex population structure
In migratory fishes, the resolution of populations (and corresponding management units) can be confounded by two factors:
1 Migratory overlap, in which populations mingle in feeding habitats or during migrations. Examples of such overlap can be found in the anadromous Sockeye Salmon (Oncorhynchus nerka; Grant et al. 1980) and Striped Bass (Morone saxatilis; Wirgin et al. 1997), as well as marine species such as the Bluefin Tuna (Thunnus thynnus; Carlsson et al. 2007) and possibly
cod (Gadus morhua; Svedäng et al. 2007). When independent breeding populations overlap at shared feeding habitats, a critical question is whether genetic exchange occurs. If fish are not breeding during the period of overlap, those populations could be isolated management units.
2 Sex-biased dispersal, in which gene flow between populations is accomplished primarily by one gender. For many mammals and birds, males disperse prior to reproduction, while females remain in natal areas (Greenwood 1980).
Both population overlap and sex-biased dispersal are common in migratory marine fishes. Female site fi delity can be countered by opportunistic mating by males, so that each gender yields a different population genetic signal. This is known as complex population structure (Bowen et al. 2005), and the most common outcome is that female-inherited mtDNA shows population structure while biparentally inherited nDNA surveys show no structure (Goudet et al. 2002). This pattern is apparent in the Brook Charr (Salvelinus fontinalis; Fraser et al. 2004), Patagonian Toothfish (Dissostichus eleginoides; Shaw et al. 2004), and Shortfin Mako Shark (Isurus oxyrinchus; Schrey & Heist 2003). In a survey of White Sharks (Carcharodon carcharias) in the Indian Ocean, the mtDNA sequences reveal significant population structure (Fst =0.81 between South Africa and Australia), while a microsatellite survey indicated a single population (Pardini et al. 2001). For these cases, dispersal by males can readily explain the lower population structure registered in nDNA relative to mtDNA.