Population Structure of American Shad
PhD project, Dan Hasselman
Anadromous fishes, species that spawn in rivers but spend extensive periods of time in the ocean, provide an opportunity to examine how dispersal, colonization, and homing behaviour lead to the establishment of distinct populations, and how the structure and genetic integrity of populations are maintained across geographic space and over time. Anadromous fishes are also often of particular conservation concern, because their dependence on rivers for reproduction makes them particularly vulnerable to human impacts, including pollution, dams and fishery exploitation. Because of their socioeconomic importance and fascinating biology, salmon have received the bulk of scientific study among anadromous fishes. However, the east coast of North America in general and that of eastern Canada in particular harbour a wide diversity of ecologically and economically important non-salmonid anadromous fishes. My research focuses on one of these, the American shad (Alosa sapidissima).
Population and conservation biology of shad
American shad is a wide ranging member of the herring family (Clupeidae) native to the Atlantic coast of North America, where it spawns in rivers from the St. Lawrence (Quebec) to the St. Johns (Florida). The life history of American shad changes with latitude: south of Cape Hatteras shad spawn only once before dying, whereas to the north an increasing proportion survive spawning to reproduces again in subsequent years. Regardless of where they spawn, shad undertake lengthy migrations in the ocean, including southward movements in the winter and a feeding migration that takes them through the Bay of Fundy in the summer. Tagging studies have shown that shad are philopatric, that is, they return to the same river to spawn year after year. This suggests that shad form reproductively isolated populations that correspond to the rivers in which they spawn. The results of early mitochondrial DNA (mtDNA) studies (e.g. Bentzen et al. 1989) were consistent with this prediction, but lacked the resolution to determine the exact spatial scale of population structuring in shad (i.e. regional, river by river, or tributaries within major rivers).
American shad are of increasing conservation concern across their native range. Widespread population declines have lead to the closures of once commercially important shad fisheries (Maryland 1980, Virginia 1994).
These populations declines have been attributed to the impacts of human activities (e.g. overfishing, dams with inadequate fish passage, pollution, siltation of spawning habitat), the most important of which is habitat loss or degradation. The construction of dams with inadequate fish passage blocks access for shad to historic spawning grounds, which in some cases are located hundreds of kilometers upstream. Within the Canadian portion of their range, a number of spawning runs have been lost, and among the remainder most are at a fraction of historical abundance, or of uncertain status. In fact, Canadian shad populations have received comparatively little study relative to those in the U.S.
Current research
My research uses microsatellites to examine population structuring and the pattern of genetic differentiation in shad. A primary goal of my research is to identify the number of shad populations in Atlantic Canada, that is, how many river runs constitute discrete, self-sustaining populations as opposed to migrant individuals from other rivers. Concomitantly, I aim to resolve the spatial scale at which shad population structure can be identified (basin level, drainage level, tributary level). My research also explores the impacts of shad alternative shad life histories (iteroparity vs. semelparity) and biologically relevant historic events (glaciation) on the spatial distribution of genetic variation across the species native range.
This research will be valuable for prioritizing conservation efforts within the Canadian portion of the species range (i.e. protecting important components of shad genetic diversity), and preserving the evolutionary potential of American shad.
Shad publications
Hasselman, D.J. 2010.Spatial Distribution of Neutral Genetic Variation in a Wide Ranging Anadromous Clupeid, the American Shad (Alosa sapidissima). PhD thesis, Dalhousie University. 275 p.
Hasselman, D.J., R.G. Bradford and P. Bentzen. 2010. Taking stock: Defining populations of American shad (Alosa sapidissima) in Canada using neutral genetic markers. Canadian Journal of Fisheries and Aquatic Sciences. 67:1021-1039.
Bentzen, P., G.G. Brown, and W.C. Leggett. 1989. Mitochondrial DNA polymorphism, population structure and life history variation in American shad (Alosa sapidissima). Can. J. Fish. Aquat. Sci. 46:1446-1454.
Bentzen, P., W.C. Leggett, and G.G. Brown. 1988. Length and restriction site heteroplasmy in the mitochondrial DNA of American shad (Alosa sapidissima). Genetics 118:509-518.