MITOCHONDRIAL-DNA DIVERGENCE AMONG POPULATIONS OF AMERICAN SHAD (ALOSA-SAPIDISSIMA) - HOW MUCH VARIATION IS ENOUGH FOR MIXED-STOCK ANALYSIS

We investigated the geographic distribution of the mitochondrial DNA ( mtDNA) diversity of American shad from 15 North American rivers in 199 2 with the intent of assessing sampling efficiency for future mixed-st ock analysis. We observed 116 haplotypes among the 988 individuals ass ayed. Because no single or group of haplotypes completely discriminate d river stocks or regional complexes, we investigated haplotype freque ncies as stock descriptors. Analysis of four unique indices of haploty pe divergence indicated that including rather than suppressing restric tion site heteroplasmy increased resolution; however, the final result s were not overwhelmingly dependent on this choice. A redundancy of va riation among restriction enzymes diminished information returns rapid ly after considering the six best enzymes, caused by physical linkage of restriction sites on the mtDNA molecule. Stock discriminatory power was tested by computing allocation efficiencies of mtDNA characters. When each individual was temporarily removed from the data set and rea llocated to the various candidate populations on the basis of haplotyp ic similarity, 28% of the reallocations were correct, a fourfold incre ase over random success. We demonstrate that although the specific sto ck identity of individuals cannot be confidently established, the hapl otypic arrays from baseline stocks can support stock identification an d mixed-stock analysis for shad because rivers support stock-specific haplotype frequencies.