Genetic structure of cetacean populations in sympatry, parapatry, and mixed assemblages: implications for conservation policy

Many cetaceans have a wide distribution in one or more oceans, and in some spe- cies, individuals migrate over an extensive range. When breeding sites are geo- graphically distant from foraging grounds, as for a number of baleen whales, ge- netic stocks may be geographically isolated during a breeding season and together in a mixed assemblage during a separate feeding season. These assemblages can be spatially and temporally dynamic and pose a special problem for managers when whales are hunted on feeding grounds (as is typical). For other species sea- sonal migrations are less pronounced, but the same effect of locally mixing genetic stocks can develop through other mechanisms. Examples are described where in- traspecific foraging specializations appear to be important in limiting gene flow between sympatric and parapatric populations. It is proposed that learning could be important in the generation and maintenance of these specializations. The ef- fective conservation of genetic diversity in these species will require further data on both spatial and temporal components of population genetic structure.