Population Structure of American Shad within and among Canadian Rivers
Daniel J. Hasselman, Rod G. Bradford, and Paul Bentzen
Knowledge of population structuring is essential to the management of diadromous fishes, within the context of fisheries that may target mixed stock assemblages, to maintain biodiversity, and in the context of species at risk. Fishery-based indices, or simple presence–absence, are not always adequate, or accurate, for resolving the population structure of highly migratory species. These considerations apply to American shad Alosa sapidissima, a widespread and seemingly loosely structured population-rich anadromous alosine of increasing conservation concern. American shad are philopatric and form reproductively discrete spawning runs that are genetically distinguishable (Bentzen et al. 1989). The largest spawning runs, within the Canadian portion of their range, are found in the Annapolis, Shubenacadie, Saint John, Miramichi, and St. Lawrence rivers (Chaput and Bradford 2003). However, shad have been reported from several additional drainages that are not known to support spawning runs (Leim 1924), and it is uncertain whether these accounts constitute genetically discrete spawning populations.
Genetic analyses can resolve population structure but may be compromised by stocking efforts. Canadian populations of American shad have not been subject to artificial stock transfers, unlike their U.S. counterparts, and present an opportunity to study population structuring in the absence of human-mediated gene flow, on a variety of spatial scales. Here, we examine broad and fine spatial scale population structuring among, and within, six drainages tributary to the Bay of Fundy using 13 polymorphic microsatellite loci.
Whole genomic DNA was extracted from tissue acquired from 50 to 100 live specimens captured above the head of tide in Nova Scotia: Annapolis River in 2004, Gaspereau River in 2004 and 2006, Kennetcook River in 2005, Shubenacadie River in 2004–2006, River Hebert in 2004–2005, and Saint John River, New Brunswick in 2003–2006. Eight sites were sampled from the Saint John River, and three sites were sampled from the Shubenacadie River between 2004 and 2006. Samples were genotyped across four dinucleotide (Aa14, Af6, Af13, Af20), four trinucleotide (Asa2, Asa4, Asa16, AsaB020), and five tetranucleotide (Asa8, Aps7C, AsaD042, AsaD429, AsaD029) microsatellite markers. Broad spatial scale analysis involved comparisons among drainages, while fine spatial scale analysis involved comparisons within drainages for the Saint John River and Shubenacadie River. Analyses were conducted using Bayesian clustering analysis (BAPS 4.1) and factorial correspondence analysis. Isolation by distance among drainages was assessed to determine whether an increasing degree of genetic differentiation with increasing geographic distance was evident. Geographic distances among sampled drainages were calculated following a counterclockwise migration route hypothesized for shad within the Bay of Fundy (Dadswell et al. 1987).