Mitochondrial DNA mixed-stock analysis of American shad: Coastal harvests are dynamic and variable

Populations of American shad Alosa sapidissima are generally declining in s ize along the U.S. East Coast. Coastal harvest of migrating mixed-stock ass emblages has been proposed as a possible cause of the decline, but the exte nt of fishing pressure on any particular stock has not been evaluated. To a ssess origin of shad harvested in the coastal fishery, we applied genetic m ixed-stock analysis by using mitochondrial DNA (mtDNA) variation. We examin ed American shad from the coastal mixed fisheries off both Virginia and Mar yland in 1992 and off Virginia alone in 1993 (N = 250 and 270, respectively ), comparing mtDNA restriction fragment patterns for the coastal individual s with an archival database containing mtDNA genotypes of 1,734 individuals from 16 North American river stocks distributed from Florida to Canada. We used two different maximum-likelihood approaches to estimate the contribut ing-stock composition of oceanic mixtures, one yielding conditional estimat es of stock composition, given observed haplotype frequencies, and the othe r yielding unconditional estimates and adjusted haplotype frequencies. The unconditional approach performed slightly better, but both statistical appr oaches indicated that composition of the coastal harvests was dynamic and v ariable from year to year and from location to location. For stocks contrib uting substantially to the fisheries, composition estimates derived through genetic analysis were roughly concordant with estimates based on returns f rom a concurrent tagging study.