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28 Rethinking the Stock Concept eastern tropical Pacific, for example, was originally based entirely 01;1 distribution and morphology (Perrin et al. 1979), but later was supported by differences in reproductive parameters (Barlow 1984, 1985). Phenotypic Data The concept of self-sustaining stocks was formulated by Heincke through his extensive, examination of discon- tinuous geographic variation in Atlantic herring mor- phology (Sinclair 1988). Stocks have been defined by morphological differences of every type: color patterns, body size, shape, and skeletal characters are some ex- amples. (Electrophoretically detectable differences in proteins are treated as genetic characters if there is ev-' idence that such differences are inherited.) For homeo. therms, morphological differences probably represent underlying genetic differences, and analyses of DNA and morphology should provide similar evidence regarding phylogenetic structuring of other groups (Sytsma 1990). The relative morphological differences between populations can be assumed to be a record of past (or present) differences in selection pressures or genetic drift. However, there are instances where morphologi- cal and molecular evidence differ due to either meth- odological or actual biological problems (see Box 1, Sytsma 1990). For instance, differential migration be- tween sexes might produce strong mtDNA differences but little difference in chromosomal markers if males stray between populations and females do not. Some morphological patterns, possibly color patterns or size differences, may be ecophenotypic, that is, not stable to environmental variation. However, differences in mor- phological characters in mammals and birds usually yield clear information regarding population unique- ness; because the variation is assumed to be both adap- tive and heritable, it is a record of past selection pres- sures. Given adequate sampling, morphological differences between two or more populations strongly suggest limited gene flow among these populations due either to extrinsic barriers or selection. like the popu- lation response criteria, these types of phenotypic data integrate movement patterns over a much larger time scale than do abundance data. Morphological differentiation has been the predomi- nant reason for assigning stock structure to marine mammal populations. Perrin et al. (1985), for example, documented population-specific differences for spinner, spotted, and common dolphins (Delpbinus delpbis) in the eastern tropical Pacific; stock status and mortality quotas have been applied to the take of those popula- tions by U.s. tuna purse-seiners (Anonymous 1987). Roest (1976), and more recently Wilson et al. (1990), used skull morphology to separate sea otters from Alas- kan and Californian waters into separate stocks subse- quently used as management units. CoascrfttIoa Biology Volume 6, No. I, Match 1992 Dizon et aI. Genotypic Data Booke (1981) defines a genotypic stock as a random mating population maintaining Hardy-Weinberg equilib- rium; presumably, it can be defined as':fuy random mat- ing population that shows fixed genetic or temporally stable gene frequency differences when compared to other populations. Data include those from all studies that involve the analysis of some part of the genotype. The studies can be classified into analysis of proteins via isozyme electrophoresis or immunological techniques, molecular cytogenetics, and analysis of DNA by DNA- DNA hybridization, restriction site analysis, and se- quence analysis (see Hillis & Moritz 1990 for a thorough review). The techniques involved in genetic studies and their application to marine mammals have been re- viewed recently by Hoelzel and Dover ( 1989) and have been the subject of a recent International Whaling Com- mission workshop (Hoelzel 1991). The evidence obtained from genetic methods is con- sidered by resource managers as the most unequivocal for differentiating species and their intraspecific struc- ture. Using genetic information for management pur- poses presents problems, however. Clearly it could be the ideal tool if we could directly examine the genes that constitute the locally adapted genome. One cannot do this and must rely on the analysis of "neutral" genes, primarily using allozymes and mtDNA, that are assumed to be mostly independent of selective forces. If this as- sumption can be made and if complete isolation if as- sumed, the degree of genetic divergence between two populations is a measure of the relative time since they shared a common ancestor. While we believe that sig- nificant morphological differences usually represent adaptive evolution in disparate environments, mtDNA differences between two populations may simply be a measure of time of separation. Rapid evolution through drift could accumulate significant mtDNA differences in different allopatric populations that inhabit similar en- vironments, Still, none of these populations would pre- sumably harbor unique adaptive genetic variability and would perhaps not warrant separate management status. In the opposite situation, lack of significant mtDNA dif- ferentiation between two populations does not neces- sarily mean that stocks are separate. Where barriers are "leaky," mtDNA genomes can rapidly penetrate neigh- boring populations independent of the adaptive chro- mosomal genome (Ferris et al.' 1983). These "foreign" mtDNA genomes are not selectively removed from the population-they are presumably neutral. The appear- ance of these foreign genomes may argue for the pres- ence of "homogenizing" gene flow and lumping the populations as one stock. In actuality, significant varia- tion between the populations still may develop in the chromosomal genome due to differential selection pres- sures. As an extreme example, in Lake Victoria, 14 mor-