Dolphins, Whales and Porpoises: 2002-2010 Conservation - IUCN
Dolphins, Whales and Porpoises: 2002-2010 Conservation - IUCN
Dolphins, Whales and Porpoises: 2002-2010 Conservation - IUCN
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Chapter 2<br />
Threats Faced by Cetaceans<br />
As pointed out in the Introduction, the threats facing cetaceans<br />
have changed through time. While overkill from hunting<br />
was the most obvious <strong>and</strong> immediate threat to some<br />
species <strong>and</strong> populations during much of the twentieth century,<br />
the relative importance of other threats, particularly<br />
bycatch in fisheries, has increased dramatically during the<br />
last few decades. It is often impossible to distinguish between<br />
perception <strong>and</strong> reality, particularly where pernicious<br />
threats such as pollution <strong>and</strong> climate change are concerned.<br />
In addition, it can be all but impossible to distinguish the<br />
effects of one threat from those of another when, as is<br />
usually true, multiple threats are acting simultaneously.<br />
In this section, we identify <strong>and</strong> discuss some of the threats<br />
facing cetacean populations (Simmonds <strong>and</strong> Hutchinson<br />
1996; Twiss <strong>and</strong> Reeves 1999; Whitehead et al. 2000; Evans<br />
<strong>and</strong> Raga 2001; Perrin et al. <strong>2002</strong>; Reeves <strong>and</strong> Reijnders<br />
<strong>2002</strong>). There are undoubtedly more threats than we know<br />
about today, <strong>and</strong> even the most basic information on cetacean<br />
mortality caused by human actions is lacking for many<br />
regions. Moreover, the total impact of the various threats<br />
cannot be predicted by simply summing their effects as<br />
though they were independent. It may be difficult to describe<br />
<strong>and</strong> quantify the role of synergy among threats in<br />
causing population declines, but it cannot be neglected. For<br />
example, the immunosuppressive effects of environmental<br />
contaminants (Lahvis et al. 1995), in combination with<br />
range shifts of pathogens caused by global warming <strong>and</strong><br />
ship ballast transport (Harvell et al. 1999), could increase<br />
the susceptibility of cetaceans to emergent diseases. Wild<br />
populations are subject to pressures from both human activities<br />
<strong>and</strong> ecological variability, <strong>and</strong> there is nothing static<br />
about the task of trying to identify, track, <strong>and</strong> address the<br />
threats to a group of organisms as diverse <strong>and</strong> widespread as<br />
the cetaceans.<br />
2.1 Unsustainable use (including<br />
incidental mortality)<br />
Direct exploitation<br />
Direct exploitation is usually driven by the dem<strong>and</strong> for<br />
products, whether this means food to be consumed or exchanged<br />
at the local, household level (“subsistence”), or<br />
meat, blubber, oil, <strong>and</strong> other commodities to be sold in<br />
national <strong>and</strong> international markets (“commercial”). Without<br />
controls of some sort, the growing dem<strong>and</strong> for products can<br />
lead to overexploitation. In the history of commercial whaling,<br />
there are many examples in which direct exploitation<br />
13<br />
caused cetacean populations to decline. The great whales<br />
were sequentially over-exploited, beginning with the easiest<br />
to catch <strong>and</strong> most profitable species (right, bowhead, sperm,<br />
humpback, <strong>and</strong> gray whales), followed by the elusive but<br />
valuable blue, fin, <strong>and</strong> sei whales that could only be taken<br />
regularly once steam-powered vessels <strong>and</strong> harpoon cannons<br />
had become widely available. In some instances, populations<br />
were reduced to such an extent that their recovery may<br />
now be hindered by demographic <strong>and</strong> genetic factors (e.g.,<br />
Northern Hemisphere right whales, western Pacific gray<br />
whales, <strong>and</strong> Antarctic blue whales). Moreover, there are<br />
signs that the massive reduction in populations of baleen<br />
whales has resulted in changes at the community or ecosystem<br />
level, shifting the equilibrium conditions <strong>and</strong> making<br />
full “recovery” of some populations unlikely if not<br />
impossible (e.g., Kawamura 1994; Clapham <strong>and</strong> Brownell<br />
1996).<br />
The small <strong>and</strong> medium-sized cetaceans have been taken<br />
for hundreds of years (Figure 2), <strong>and</strong> they continue to be<br />
taken in many areas for food, oil, leather, bait, <strong>and</strong> other<br />
uses. In Japan, for example, the drive fishery for small<br />
cetaceans led to a dramatic decline in the abundance of<br />
striped dolphins (Stenella coeruleoalba) by the early 1980s<br />
(Kasuya 1999c). This decline prompted fishermen to<br />
change their target species to killer whales (Orcinus orca)<br />
<strong>and</strong> bottlenose, pantropical spotted, <strong>and</strong> Risso’s dolphins<br />
(Tursiops spp., Stenella attenuata, <strong>and</strong> Grampus griseus,<br />
respectively) to supply the profitable Japanese market for<br />
small cetacean meat (Kishiro <strong>and</strong> Kasuya 1993). In the<br />
Arctic, monodontids were over-exploited historically by<br />
Figure 2. Harbor porpoises were killed in large numbers<br />
(up to 3000 in a single year) in a Danish drive <strong>and</strong> net<br />
fishery from the sixteenth century until the mid-twentieth<br />
century. This photograph was taken near Middelfart, inner<br />
Danish waters between the Baltic <strong>and</strong> North seas.<br />
Photo: Middelfart Museum courtesy of Carl C. Kinze.