The health of many of the
world’s dolphin populations is threatened by bycatch, pollution, habitat
destruction, over-fishing and climate change. Other threats include activities
that may frighten, displace or harm these species such as underwater noise
pollution from sources such as shipping traffic, wind farms, seismic surveys
and military sonars.
Fisheries and bycatch
Global fisheries are increasing in intensity and range. The introduction of
more sustainable fishing techniques can reduce this pressure. However, the use
of destructive fishing methods and the growth of many modern commercial
fisheries continue to impact many dolphin populations around the world. The
impacts can be both direct through bycatch and indirect through loss of prey
species.
Dolphins are known to become entangled in many gear types, including
long-lines, drift nets, trap lines and mid-water trawls, but the largest
problem remains with coastal gill nets, drift nets and purse-seine nets. The
continued use of gill nets is endangering a number of coastal species of
dolphin and porpoise.
Some dolphin populations may also be threatened by the sheer scale of modern
fisheries. As fisheries compete with one another for fish, less and less prey
is available for dolphins and other wildlife to eat.
Chemical pollution
There are many different sources of chemical pollution, including domestic
sewage, industrial discharges, seepage from waste sites, atmospheric fallout,
domestic run-off, accidents and spills at sea, operational discharges from oil
rigs, mining discharges and agricultural run-off. Many rivers, estuaries and
coastal waters near large human population centres show signs of
eutrophication and heavy metal contamination. Toxic algal blooms are
increasingly common around estuaries and bays.
The impacts of chemical pollution on dolphin range from direct physical
poisoning to degradation of important habitats. The chemicals that are
probably of most concern for dolphins are the persistent organic pollutants (POPs)
including pesticides, such as DDT, and industrial chemicals; most famously the
PCBs. These substances enter marine food chains and accumulate along the chain
to the marine top predators.
Damage to the reproductive and immune systems of marine mammals (and other
species) are the likely consequences of their extraordinary pollution
“burdens’. Many dolphin populations are known to be carrying heavy
contaminant burdens which may contribute to increased mortality.
There has been a worldwide increase in reports of viral and bacterial diseases
affecting marine species as well as an apparent increase in toxic algal
blooms. Habitat degradation, in particular increased chemical contamination,
is thought to have facilitated disease outbreaks and the immunotoxic affects
of some substances has been associated with marine mammal mass mortalities.
The increasing and cumulative pressures on dolphins and the current trends of
climate change may make dolphins more susceptible to disease. The transport of
pathogens around the world, through the movement of products and ballast
water, may increase exposure to disease and environmental contaminants may be
facilitating the emergence of new diseases. In addition, exposure to chemical
substances that have immunotoxic effects may lower dolphin immune responses
and algal bloom outbreaks may further increase the toll of weakened
populations by reducing their food supply as fish die.
Ship strikes, noise, disturbance and harassment
Hearing is the most important sense for dolphins, and the ability to hear well
is vital in all key aspects of their lives including finding food, navigating
and social interactions. Any reduction in hearing ability – whether by
physical damage or masking by other sound – may seriously compromise the
viability of individuals and, therefore, populations.
Human-created noise in the marine environment contributes to an already
significant natural biological and ambient level of sound. Introduced noise
pollution comes from shipping and other vessels, military activities,
fisheries anti-predation devices, ocean research, and the air-guns used in
seismic testing to find oil and gas deposits. An emerging threat to dolphins
are the potential impacts of marine wind farms.
Whilst many of the sources of introduced noise are localized, some recent
military technologies have utilized powerful detection mechanisms that may
radiate over thousands of kilometres of the ocean.
Potential impacts of human-created noise on dolphins range from physical
damage to these animals (especially to those in close proximity to the noise
source) to altering behaviour, increasing stress and displacement from
important habitats.
In addition, the extent of harassment, whether intentional or incidental, may
be an increasing and little understood problem in coastal waters.
The impact of greatest consequence associated with noise pollution, harassment
and ship strikes may be the cumulative and long-term impact that we are
currently unable to assess and evaluate.
Habitat loss and degradation
It is important to both the individual and the survival of the dolphins
population (or species) that habitats continue to be suitable to support them.
Habitat loss is especially critical for dolphins with limited range, such as
river dolphins. In many areas habitat loss is caused by dams, fishing
structures and withdrawal of water for human use. In some parts of the world
water management, flood control and major river modification, including the
removal of surface water, has led to population decline. Dams prevent
migration and create barriers which fragment populations. Prey species may be
reduced, while sedimentation, nutrient over-enrichment and salinity, and in
turn eutrophication, increase.
Habitat loss is also a concern for coastal and offshore species. Changes in
the atmosphere, weather patterns and marine ecosystems are currently being
observed. Predictions include sea surface changes and sea level rise. Changes
in the ice-caps may affect rainfall and salinity, and temperate changes may
impact on coastal upwelling regions causing a possible reduction in nutrient
concentrations and 'productivity' which in turn can impact whole food chains.
The modification of habitats may cause shifts in dolphin food sources (through
change in upwelling patterns and prey aggregation). Species that have evolved
to find food in a highly patchy environment may have difficulties securing
prey.
The implications of climate change for dolphins are compounded by the apparent
rate of change (some 3 to 4 degrees celsius in higher latitudes in only 50
years) which is thought to be much faster than anything that dolphins have
been exposed to in the past. When considered in the context of cumulative
impacts, the ability of dolphin populations to adapt to this rapid change may
be compromised.
Deliberate hunts
Some coastal communities have exploited dolphins for centuries. There are
other documented dolphin harvests in South Asia, East Asia, South East Asia,
and parts of Africa and South America. In some cases, dolphin bycatch has
turned to directed nets or harpoon hunts by artisanal fishers. The impact of
these new directed nets hunts is not known as very little data is available on
the targeted populations or the number of animals being caught. It is likely
that the hunts are not sustainable. The belief that dolphins compete with
fisheries or damage fishing nets has prompted culls in some regions.
