KORNFIELD AND BOGDANOWICZ: MITOCHONDRIAL DNA IN ATLANTIC HERRING 



the occurrence of common composites need not 

 imply current gene exchange (Avise et al. 1984; 

 Neigel and Avise 1986); population sizes are suffi- 

 ciently large to support the co-occurrence of com- 

 mon ancestral composites and their derivatives. 



The presence of 20 composites which were 

 specific to spawning groups suggests that there 

 may be some degree of genetic isolation among 

 stocks. Given the limited number of individuals 

 sampled, it is difficult to know whether "unique" 

 composites are actually restricted to specific 

 stocks. For example, rather than increasing the 

 abundance of previously observed unique com- 

 posites, the second sample from Trinity Ledge 

 generated additional composites. There is thus 

 little indication that "unique" composites may be 

 useful in defining stocks. The great composite di- 

 versity displayed in the samples of Atlantic her- 

 ring most probably reflects the very large popula- 

 tion sizes involved. 



In the absence of gene flow among spawning 

 populations, we would expect a unique composite 

 to be found in the same population as its most 

 probable precursor. In three out of seven in- 

 stances, precursors of unique composites occurred 

 in diff'erent spawning populations (this result 

 holds for all other equally parsimonious net- 

 works). For example, composite 9, which only oc- 

 curs in the St. Lawrence sample, is the immediate 

 ancestor of composite 20 from Trinity Ledge. In 

 addition, the two unique composites which were 

 maximally divergent (12 steps) occurred in the 

 same population (Trinity Ledge 1985). These con- 

 siderations, as well as the absence of any consis- 

 tent geographic pattern of unique composites are 

 consistent with the idea of gene flow. 



Evidence for the ability of mtDNA analysis to 

 detect subtle population differentiation is com- 

 pelling (Avise et al. 1979; Lansman et al. 1981; 

 Wilson et al. 1985; Bermingham and Avise 1986). 

 However, since differentiation of mtDNA restric- 

 tion patterns is a time dependent process (Kessler 

 and Avise 1985), it is possible that there has been 

 insufficient time to accumulate population 

 specific differences in Atlantic herring (Grant 

 1984, 1985). Atlantic herring stocks, as they cur- 

 rently exist, can not predate the origin of the Gulf 

 of Maine following glacial withdrawal 18,000 

 years ago (Kellogg 1980). In addition, since the 

 effective population sizes of Atlantic herring 

 stocks are very large, they would be expected to 

 diverge only very slowly by lineage sorting 

 (Neigel and Avise 1986). 



Consistent, significant genetic differences 



among spawning groups of Atlantic herring is a 

 sufficient, but not a necessary, condition to regard 

 populations as discrete stocks. Our results do not 

 support the hypothesis that discrete Atlantic her- 

 ring stocks exist throughout the Gulf of Maine; 

 however, the absence of such differences does not 

 allow us to rigorously conclude that there is gene 

 flow among the populations in question. More 

 comprehensive sampling of mtDNA composites 

 within and among populations in the western 

 North Atlantic may better allow resolution of this 

 problem. Regardless, for the sake of preserving 

 variability, resources like the Atlantic herring 

 should be managed under the assumption that 

 every spawning group is a semi-discrete genetic 

 entity. 



ACKNOWLEDGMENTS 



We thank David Pierce (Massachusetts Fish 

 Division), Susan Safford (University of Massa- 

 chusetts), Clarence Bourque and Sandy Seagle 

 (Fisheries and Oceans, Canada), and the captain 

 and crew of the FV Barnegat (Gloucester, MA) for 

 assistance in field collections. Thomas Dowling 

 kindly provided computer programs for mtDNA 

 analysis. Robert Chapman critically commented 

 on an early draft of this manuscript; Eldredge 

 Bermingham and an anonomyous reviewer pro- 

 vided comments which greatly improved the final 

 version. Our work was supported by BRSG 507 

 RR07161 from the National Institutes of Health 

 and by funds from Sea Grant and the Migratory 

 Fish Research Institute, Orono ME. 



LITERATURE CITED 



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