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30 Reth1nkh1g the Stock Concept on one population relative to another. Measures on this axis mostly tepresent the culmination of selection op- erating on many generations. The proxy for gene flow, distribution difference, is the other axis of the matrix. It is considered separately to emphasize that available information usually only in- dicates poteljltial gene flow. Actual flow of genes re- quires the prPduction of fit offspring. The advantage of looking at populations in this way is that a matrix offour population relationships can be operationally de-, scribed. It is, a hierarchical organization because with each step there is an incre:JSingprobability of the pop- ulation in question being an ESU (Phylogeographic cat- egories I through W; Fig. 2). Note that a stock is always characterized as a "population" in relation to another reference population The proposed system works as follows: Category I Populations The e:JSiest situation to deal with is that of allopatric populations demonstrating significant genetic differ- 'ences; there would be little argument that these should be managed as separate units. Category I is character- ized by a discontinuous genetic divergence pattern where locally adapted and closely related genome as. semblages are separated from others geographically and by significant genetic distances-great genetic diver- gence/stronggeographic partitioning This could have been caused by long-term, extrinsic barriers (zoogeo. graphic) or by extinction of intermediate assemblages in cases with limited gene flow. Category I situations are characterized by the pres- ence of actual geographical separation by physical bar- riers such as land masses, or oceanographic or topo- graphical barriers such as temperature clines, etc., which effectively create a margin around a population, Genetic and other differences from populations else- where are implied but are not necessarily proven The population is effectively isolated and probably is never confused with another in management programs. 1,1'1' :i~'1 31 . II [ ~: : Category II Populations Category II is characterized by a discontinuous genetic diversity pattern between groups of closely related ge- nome assemblages existing sympatrically or parapatri- cally-that is, great genetic divergence accompanied by weak geographic partitioning Avise et al. (1987) spec- ulate that this may have arisen through allopatric diver- gence and secondary contact or through some intrinsic (reproductive) barriers. lbis requires that two (or more) populations/putative stocks coexist with total sympatry or with extensive geographical overlap or be weakly defined in parapatry. Geographically there would appear to be no reason not Coasctvadoa Blo1ojjy Volume 6. No.1, Much 1992 Dizon el aI, to manage them together, but critical differences in be- havior, morphology, genetics, or some combination of these indicate reproductive isolation to some degree, lbis type of stock is perhaps the m<?~t critical and diffi- cult to manage. Avise et al. (1987) and Avise (1989) find no good examples of this category and report that it is rare to find mtDNA differences greater than 1 to 2% between individuals collected from the same locality. Using mtDNA divergence measurements, Hoelzel (1991) re- ports considerable genetic isolation between at least partially sympatric populations of killer whales (Orca orcinus) off Vancouver Island; they suggest that these groups represent a category II population structure. They are not category I (or III) populations because of their at least partially sympatric distribution and poten- tial for genetic interchange. Category III Populations The remaining two categories are characterized by a pattern of continuous genetic divergence. Category III parallels category Ii hO\v.ever, here the geographically separated assemblages are characterized by little ge- netic differentiation, for instance, less than 1 % for mtDNA diversity.'Still, the populations are clearly sepa- rate, either because of true allopatry or strict parapatry, and there is a high degree of reproductive isolation, as evidenced by various combinations of demographic and morphological differences, although there are no barri- ers to intermingling at the margins and interbreeding is feasible. There mayor may not be various combinations of demographic, morphological, and genetic differences. In any case, geographically concordant unique genome assemblages have developed within local habitats in re- sponse to local selection pressures. Category IV Populations Category W populations have extensive gene inter- change and no subdivision by geographic barriers. Pop- ulations in this category appear panmictic and occupy a broad range that blends with that of neighboring popu- lation( s) rather than abutlng on them. There are usually minimal, if any, differences in morphology, genetics, and demographic parameters between them, and the main distinguishing feature indicating that this is a separate population is that the center of abundance may be some distance from the center of neighbors. There is little or no reproductive isolation, and there is considerable in- termingling on the breeding grounds. This category typ- ifies the most nebulous classification and may show poor evidence for, any stock differentiation. Depending on the management issue, certain geographical regions occupied by such populations may still be treated as separate stocks for the sake of conservatism.