Response to The University of Miami Independent System for Peer Review Review of Alaskan Harbor Seal Stock Assessment

Although we agree that additional analysis of bi-parentally inherited, nuclear DNA markers, such as microsatellites, could strengthen our understanding of population structure in Alaskan harbor seals, we fundamentally disagree with Dr. Hoelzel that inclusion of nuclear DNA is a necessary component to identifying stock structure and note a confusion between demographic isolation and reproductive isolation. We went to some length in the Administrative Report to pull language from agency guidelines on the definition of “stock” to avoid confusion on this issue. The salient sentences are “For the purposes of management under the MMPA, a stock is recognized as being a management unit that identifies a demographically isolated biological population.” And “Demographic isolation means that the population dynamics of the affected group is more a consequence of births and deaths within the group (internal dynamics) rather than immigration or emigration (external dynamics). Thus, the exchange of individuals between population stocks is not great enough to prevent the depletion of one of the populations as a result of increased mortality or lower birth rates.” And finally, “Interbreed when mature is acknowledged to include cases in which either: i) mating occurs primarily among members of the same demographically isolated group; or ii) the group migrates seasonally to a breeding ground where its members interbreed with members of the same group and with members of other demographically distinct groups that have migrated to the same breeding ground from other feeding areas (e.g., North Atlantic and central North Pacific humpback whales).” These regulatory definitions allow for stocks to be demographically isolated while not necessarily being reproductively isolated, as is the case for humpback whales. In such a case, the demographically isolated units would be identifiable through their mtDNA but not through nuclear DNA (a more detailed explanation follows in the next paragraph). Using these definitions, a positive finding of structure using mtDNA can never be negated by negative findings using nuclear DNA.