MARINE MAMMAL SCIENCE, 24(1): 183–201 ( January 2008)
C©
2007 by the Society for Marine Mammalogy
DOI: 10.1111/j.1748-7692.2007.00175.x
Determining spatial and temporal scales
for management: lessons from whaling
PHILLIP J. CLAPHAM
National Marine Mammal Laboratory,
Alaska Fisheries Science Center,
7600 Sand Point Way NE,
Seattle, Washington 98115, U.S.A.
E-mail: phillip.clapham@noaa.gov
ALEX AGUILAR
Department of Animal Biology,
Faculty of Biology,
University of Barcelona,
08071 Barcelona, Spain
LEILA T. HATCH
Gerry E. Studds Stellwagen Bank National Marine Sanctuary,
National Ocean Service,
Scituate, Massachusetts 02066, U.S.A.
ABSTRACT
Selection of the appropriate management unit is critical to the conservation of
animal populations. Defining such units depends upon knowledge of population
structure and upon the timescale being considered. Here, we examine the trajectory
of eleven subpopulations of five species of baleen whales to investigate temporal
and spatial scales in management. These subpopulations were all extirpated by
commercial whaling, and no recovery or repopulation has occurred since. In these
cases, time elapsed since commercial extinction ranges from four decades to al-
most four centuries. We propose that these subpopulations did not recover either
because cultural memory of the habitat has been lost, because widespread whal-
ing among adjacent stocks eliminated these as sources for repopulation, and/or
because segregation following exploitation produced the abandonment of certain
areas. Spatial scales associated with the extirpated subpopulations are frequently
smaller than those typically employed in management. Overall, the evidence indi-
cates that: (1) the time frame formanagement should be atmost decadal in scope (i.e.,
<100 yr) and based on both genetic and nongenetic evidence of population sub-
structure, and (2) at least some stocks should be defined on a smaller spatial scale
than they currently are.
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184 MARINE MAMMAL SCIENCE, VOL. 24, NO. 1, 2008
Key words: management units, population structure, whaling, recovery, whales.
In terms of sheer biomass, the commercial hunting of whales in the 20th cen-
tury represents one of the greatest wildlife exploitation episodes in human history.
Between 1904 and 2005, more than two million whales were killed in the South-
ern Hemisphere alone (Clapham and Baker 2002). The International Convention
for the Regulation of Whaling, signed in December 1946, established the Interna-
tionalWhaling Commission (IWC) to overseemanagement-related research onwhale
stocks and to set quotas that would not exceed sustainable levels of exploitation. This
objective, however, was clearly nominal at best: For most of the century, catch levels
were invariably excessive, and populations of several species were reduced to very low
levels. In a number of cases, local subpopulations appear to have been completely
extirpated.
The IWC quotas are set for specific “stocks,” which do not necessarily equate to
genetic subpopulations but rather refer to spatial groupings of whales that are conve-
nient for the purpose of management (Donovan 1991). Selection of the appropriate
management unit is critical to the conservation of a subpopulation. For example,
if two distinct subpopulations subject to anthropogenic removals are managed as a
single unit, and if the removals are similar in their numbers for both subpopulations
but one is smaller than the other, the risk that the former subpopulation will be lost is
substantially increased because the impact of mortality in it will be underestimated
(see Taylor 1997). A similar risk may occur if the two subpopulations are of similar
size, but the bulk of the removals occurs in only one of them. This process is identical
to the well-known case of multispecies fisheries, in which the risk of overexploiting
a scarce species is proportionally much higher than for the species that predominates
in the catch, making these operations more risky than those directed at a single
target species. Thus, in single-species fisheries, or in a whaling operation exploit-
ing a single subpopulation, catch per unit effort rapidly reflects overexploitation,
and the increased costs of obtaining a given catch discourages fishers to continue
exploitation; the resource reaches “commercial extinction” before “biological extinc-
tion” takes place. This process does not occur in a multispecies or a multistock fishery
because catches may continue indefinitely on the basis of the profits obtained from
the more abundant species or subpopulation, while negligible but sustained catches
would continue on the scarcer species or stock and eventually bring it to extinction.
An accurately defined management unit is thus the key to avoiding extirpation of
subpopulations or local forms.
If the animals inhabiting a specific area are severely reduced in number, repopula-
tion may occur from a remnant (founder) stock and/or by immigration from adjacent
areas. However, the remnant stockmay be too small to be viable, and distance or other
factors may limit or preclude recolonization from the nearest viable subpopulation.
Because of this, management should be based upon units that confer the maximum
protection and minimize the possibility of local extirpation. Despite the importance
of this principle, management has extensively disregarded it, mostly because of the
technical (and sometimes political) difficulties of establishing such discrete units.
Thus, the “management unit” will not necessarily be synonymous with a biological
subpopulation, or even a “genetic” population, a distinction that is now commonly
recognized by conservation biologists (e.g., Donovan 1991, Bossart and Prowell 1998,
Moritz and Faith 1998, Taylor and Dizon 1999, Taylor 2005).