Threats to Dolphins
- Types of Threats
- Directed Takes
- Incidental Takes
- Habitat Degradation
- Specific Cases
- Eastern Tropical Pacific Tuna Purse-Seine Fishery
- The Root of the Problem
- Dolphin Behavior in the Nets
- History of the Fishery
- Attempts to Reduce Kills
- New Concerns
- Black Sea Fishery
- Chilean Crab Bait Fishery
As with a lot of other types of animals, the advancement of human
civilization has started to threaten the survival of many types of
small cetaceans. Two threats that profoundly affect small cetacean
populations are direct taking, where the target of the hunt is the dolphin
being caught, and incidental taking, where dolphins are caught or killed as a
side effect of the method being used to catch other species. In addition,
many species are affected by habitat degradation, pollution, and disturbance.
River dolphins are especially vulnerable to these threats; the most endangered
species of small cetacean is the Yangtze river dolphin or Baiji.
Types of Threats
In recent years, there has been a general increase in the directed taking
of small cetaceans in lesser developed countries. These new and expanding
fisheries require careful, but tactful monitoring. Many fisheries are for
cetaceans in general rather than for specific types, so managing them on the
basis of individual species is difficult. In Japanese waters, there is
extensive directed take, mainly for human consumption. Some fisheries are for
certain species of small cetacean, while others are generic. In South
America, some of the resident species are hunted for crab bait, and many of
those species are already vulnerable to incidental take. Sri Lanka has many
generic small cetacean fisheries which might threaten some local
Incidental takes most often occur when specimens get caught in large nets
set to catch fish. Unable to escape or reach the surface, they drown and
perish. Certain types of fishing gear, such as set nets, are particularly
dangerous to small cetaceans. This problem may represent a greater threat to
cetaceans than directed takes. For several populations, including the Baiji
of the Yangtze River, the hump-backed and bottlenose dolphins off eastern
South Africa, and the striped dolphins of the Mediterranean Sea, the mortality
rate due to incidental taking is greater than the maximum amount that the
population can sustain without significant decline. The effects of this type
of threat on the dusky dolphins of the eastern South Pacific and the northern
right whale dolphins of the central North Pacific is also a source of
A major concern is large-scale monofilament pelagic driftnetting. It
causes overexploitation of the fish population, and marine mammals and
seabirds often get entangled in them. Also, discarded nets, sometimes called
"ghost nets," remain active for several years, during which they continue to
entrap cetaceans. In a resolution made on 22 December 1989, the United
Nations General Assembly agreed to end pelagic driftnetting by the end of
June, 1992 unless and until procedures to reduce the problems of this type of
fishery could be implemented. Some countries took steps to forbid any use of
pelagic driftnets within 360 kilometers of their coasts.
The encroachment of human civilization on natural habitats also represents
a serious threat to the survival of some stocks. Coastal development may
destroy fish nursuries, eliminating the food supply of the local cetaceans.
Coastal mangrove swamps, found in tropical regions, are a very important
habitat for some species of cetecean, especially the hump-backed dolphins of
South Asia and West Africa. These areas are both fish nursuries and shelters
for adult fish. Continued urban development and industrialization threatens
these regions. The damming of rivers on such systems as the Ganges and the
Amazon has a nasty effect on riverine populations. In addition to the
ecological effect resulting from the change in the flow of water, riverine
populations may be permanently separated. Individual stocks may not be viable
in the long term and seasonal migrations may be disrupted.
Pollutants enter the food chain near its bottom and build up in top
predators, such as cetaceans. In addition, cetaceans may injest the wastes
and debris from discarded fishing gear. Mineral exploration and exploitation
are also a concern, but unless cetaceans are caught in survey explosions or
oil spills, the danger is probably minimal. The effects of such toxins on
cetaceans are not entirely clear. The death of cetaceans due to pollution is
difficult to recognize, especially when it occurs in large numbers. In other
mammals polychlorinated biphenyls (PCBs) adversely affect reproduction, and
the same may hold true for cetaceans.
Ironically, an important method of improving public awareness about
cetaceans and promoting conservation measures may also be a serious threat to
these creatures. In addition to normal ship traffic causing death due to
collisions and propeller injuries, the increase in visitors that come for
whale watching expeditions may disturb these animals. Studies show that if
the visitors are careful and treat the resident cetaceans with propriety, the
animals will have no difficulty acclimating. Commercial whale watching
outfits are careful to ensure that visitors are tactful, but private
individuals may not understand all of the necessary provisions.
Eastern Tropical Pacific Tuna Purse-Seine Fishery
Every year between 1959 and 1972, the tuna fisherman of the eastern tropical
Pacific killed hundreds of thousands of dolphins, mostly spinners and spotted
dolphins. In a report by the National Marine Fisheries Service's Southwest
Fisheries Center made in 1979, the estimated total number of dolphins killed
between 1959 and 1972 was 3,796,658. That figure is probably an
underestimate, since it does not include individuals that were injured in the
nets and died later. The level of incidental take is sufficient to pose a
serious threat to the survival of some stocks.
The Root of the Problem
The basis of the problem is that the two main species involved associate in
large numbers with yellowfin tuna. The nature of the association between the
dolphins and the tuna is still a mystery, although there are some theories
that try to explain it. It is possible that the relationship is symbiotic, as
the tuna are most active during the day and the dolphins are most active
during the night, so one group can rest while the other group feeds. A
similar relationship is seen between spinners and spotters.
Regardless of the reason, the fact that the three species school together
allows the tuna fishermen to find schools of tuna very easily. In 1959, they
discovered this relationship and began to exploit it. Fishermen look for a
commotion on the horizon, indicating feeding seabirds, leaping dolphins, or
both, and could indicate the presence of tuna Since spinners and
spotters aggregate in the thousands, schools are not difficult to see even at
a distance. When the tuna boat is in range, speedboats known as pongas are
lowered and herd the school together. When the animals are compactly
herded, the seiner lowers the net and surrounds the herd, trying to trap both
dolphins and tuna. At first, the purse seine is about 1.6 km long and open,
forming a wall. It is also open at the bottom, but herded individuals do not
go under it to escape, probably because the water at a depth of 200 meters is
too cold. The pongas continue to roar around the perimeter, making another
barrier. When the dolphin school is completely surrounded, it is pursed, that
is, it is closed at the bottom using a cable that passes through a series of
rings. Every creature in the school is trapped. The net is then drawn onto
the boat through a power block, the one invention that made this kind of
fishing possible. Captured individuals are stacked methodically on the deck
as the enclosure shrinks.
Dolphin Behavior in the Nets
Further compounding the problem is the fact that spinners and spotters tend
to be timid, easily frightened, and fearful of objects. They depend on the
presence of other dolphins and do not react well to new situations. As a
result, they tend to react to the purse seines by panicing rather than by
attempting to escape as they easily could given their acrobatic abilities.
The passive behavior of these animals is described as a form of "capture
myopathy" and often leads to death. Although normally a factor that aids
their survival, in the case of purse seines, their timid nature is deadly.
When not herded or chased, spinners and spotters swim slowly and spread
over a long distance, often with between 20 and 30 body lengths between
different individuals. When they are chased, they crowd together, leaving
only 2 to 3 body lengths between them. They also move more quickly, making
low leaps from the water, as it is the most efficient method of swimming.
As the school is surrounded by the nets, various behavior patterns emerge.
In a practice called milling, a large percentage of the school stays in
one area, swimming and diving. Early in the set, there is some leaping, but
that quickly subsides. In addition, some individuals start rafting,
where a group hangs vertically in the water, head up. Individuals align
themselves in layers, discrete bands oriented horizontally. Groups of
four to five individuals make up each layer, and there may be up to four
layers stacked vertically. After surfacing to breath, always return to their
original position. Often some dolphins just passively sink slowly to the
bottom of the nets, probably as a reaction to the stress from the boats, the
noise, and the wake. This type of behavior is understandable considering that
spinners and spotters are used to the open ocean where they have no barriers
or confinement. Although spinners usually separate from the spotters in a
net, they do exhibit similar behavior. However, spinners tend to move around
more in the nets, and are located more in the periphery.
History of the Fishery
Before the fishermen discovered the dolphin-tuna aggregations, they used a
method of giving the tuna bait until they were in a feeding frenzy, at which
point they would bite anything, including unbaited hooks. By 1960, however,
the new purse seine method had largely replaced the old method. In 1966, 62%
of the tuna caught in the Pacific were in association with dolphins.
In the early years of the purse seine fishery, everything in the net was
hauled aboard. The valuable tuna was kept, while the dolphins and other fish
were tossed overboard. The tuna fishermen did not publicize this slaughter,
although the information did eventually leak. The problem was noticed by
William F. Perrin, who was working on a Ph.D. dissertation on Stenella
at UCLA. Scientists quickly began a study of the dolphins from this fishery.
Much information was collected, and the species involved went from little
known to some of the most intensively studied.
Based on an extrapolation form the early irregular data supplied by
fishermen, some estimates were made on the annual kill rate. In 1959, before
the entire fleet had switched to using purse-seine, up to 100,000 dolphins were
killed. By 1960, when the entire fleet had switched, an estimated 500,000
individuals were killed. Before the examination started, no records were kept
on the proportions of species involved. Later studies showed that the species
most involved was the spotter, then the spinner, and then the common dolphin.
Several additional species were involved in small quantities.
Attempts to Reduce Kills
As a by-product of the tuna-dolphin problem, the Dedicated Vessel Program
was started. Under this program, a tuna boat performs its regular duties with
a team of biologists on board. These scientists monitor the dolphin kills and
the behavior of the dolphins in the nets. As a result of these observations,
much more care is taken by the tuna fishermen. On one tuna boat in 1977, only
11 dolphins died in a total of 20 sets. On the Queen Mary in 1978, out of 17
sets, only three dolphins perished. In addition, it was seen that some
dolphins were already familiar with the nets. They stopped swimming and knew
how to escape the net using the methods developed by the tuna fishermen to
lower the casualty rate.
Very soon after the purse-seine fishery developed, various techniques were
put into use to protect the dolphins. These methods were used not necessarily
to help the dolphins, but to reduce the labor of removing the dead dolphins
from the deck, as well as to avoid depleting the entire dolphin population,
lest there be no more dolphins to use. These strategies mostly revolved
around releasing the dolphins from the nets before they were killed. In one
method, known as "backing down," when one half of the net had been hauled in,
the vessel was put in reverse, and the net was drawn into a finger-shaped
configuration. At the far end of the finger, the water pressure against the
webbing cased the net to sink. The dolphins literally have the net pulled out
from under them. Although this technique worked to some degree, thousands
still died every year. In addition, tuna sometimes escaped during this
maneuver, and some captains were more interested in preserving the tuna then
in preventing the slaughter of the dolphins. Fine meshed panels like the
Medina panel and the super-apron were also developed to prevent entanglement
and facilitate the escape of the dolphins.
By the 1970s, the United States government was under a great deal of
pressure from humane and conservation groups and began monitoring the
situation. When the American public noticed the problem, their outcry helped
to cause the passage of the Marine Mammal Protection Act of 1972. The tuna
industry was given a two year grace period to reduce dolphin kills. They did
not abide by this, and only after a long drawn-out legal battle was any
action taken. The techniques developed to release the trapped dolphins were
probably not strictly enforced throughout the international tuna fleet. In
addition, some of the reduction in the reported kill may have been because
ships previously under United States registration changed their flag to avoid
strict US regulations.
Despite the problems in enforcing the legislation, the kill rate was
reduced significantly during the 1970s. Whereas in 1973, a total of 114,087
dolphins were killed, by 1978, only 21,805 were killed. The most important
factor was the pressure on the fishermen and the techniques to release trapped
dolphins. In October of 1980, a quota of 20,500 dolphins was set for all
species by the United States government. The previous year's kills had not
been that high, and it appeared that the problem had vanished.
New concerns have cropped up, though. The Porpoise Stocks Workship in La
Jolla analyzed the earlier data and found that the estimates on population
were seriously flawed. The actual number of spotters in the eastern tropical
Pacific was much lower than previously assumed. Earlier estimates were based
on the estimates of school size given by the observers and crew members on
tuna boats, while estimates in 1979 were based on aerial observations. The
1979 report gave an estimated offshore population of 34-55% of the 1959 level.
This is below the optimal sustainable pouplation, where the population level
leads to maximum net productivity. The same data analyzed in 1973 gave a
population of 92-95% of the 1959 level, which is in the OSP range. The
population in 1959 was probably more like three million than the four million
originally assumed, and as more than four million dolphins were killed between
1959 and 1978. Although some did reproduce, the population was probably
at a critical stage.
The government could declare the spotters depleted, if the population has
declined much over several years, and if the decline continues it will be
eligable for the Endagered Species Conservation Act of 1969, or if the
population is below the optimum carrying capacity for the species or stock
within the environment. If it does so, no taking will be allowed at all.
Tuna fishermen complain that they will be shut down entirely if this happens.
They state that they have cooperated with environmentalists and reduced kills
significantly. They threaten to change the flags of their ships to countries
without provisions if the government declares the species depleted. Doing
this would place the tuna fishermen outside of the jurisdiction of the laws of
the United States and would free them to kill as many dolphins as they wish.
As a result, conservationists are sometimes on the side of the tuna
Black Sea Fishery
Between 1870 and 1983, in the Black Sea, there existed a fishery for three
species of cetacean: the bottlenose dolphin, the harbor porpoise, and the
common dolphin. The method of catch was similar to that of the eastern
tropical Pacific tuna purse seine fishery, but the dolphins are the prime
target instead of the tuna. When herding the dolphins, fishermen clang
together cobblestones, causing the dolphins to panic and run into the nets.
When the fishery started operation, harpoons were used, but that method
was later replaced by purse seining. The main products of the fishery were
food and oil.
Four countries were involved in this fishery: Turkey, Romania, Bulgaria,
and the USSR. An estimated total of 200,000 dolphins were taken each year,
a shocking 20% of the population. The effects of depletion became evident
after the Second World War, when the USSR could not reach the same rates of
catch as in the 1930s. Between 1931 and 1941, the USSR took 110,000-130,000
dolphins per year, but in later years, it only averaged 75,000 per year. The
Turkish fishery's destruction of the breeding stock and the young during
breeding season was blamed for this. In 1966, the fishery operations in all
countries except Turkey stopped, due to a collapse of the population. The
Turkish fishery is not well documented, since many individuals were killed from
small boats using rifles and shot guns, leading to a high loss rate.
The Turkish fishery was banned in 1983, however there is much concern that
it might reopen when a reassessment of the stock is made. The fishing
industry is putting forth much pressure to end the ban. Although the European
Community bans the import of cetacean products of commerical uses, Japan is an
open market for the products of this fishery, and it is already a destination
of many of the products of current Turkish fisheries. The Berne Convention
includes an exemption clause, allowing species listed under Appendix II to be
taken to prevent serious damage to fisheries, as long as the survival of the
species is not threatened. The fishery might reopen under the provisions of
Chilean Crab Bait Fishery
The waters of the Strait of Magellan are an important source of crab meat,
and the fishery there is expanding. Unfortunately, the fishermen who need
bait to catch these crabs have turned to the local dolphin population. Under
Chilean law, the slaughter of dolphins for crab bait is illegal, but because
the region is so remote, the laws are difficult to enforce. As of the early
1990s, only four of the 26 companies operating in the Magellans provided crab
bait to the fishermen, and in most cases, the amount supplied wasn't nearly
adequate. The Commerson's and black dolphins are known to be involved in this
operation, as well as seals, sealions, penguins, guancos, and other wildlife.
There are very few estimates of the level of take, but the dolphins killed may
number in the thousands. It is feared that the dolphins are being killed too
rapidly to be replenished naturally.
Ellis, Richard. Dolphins and Porpoises. New York: Alfred & Knopf, Inc., 1982.
Klinowska, Margaret. Dolphins, Porpoises, and Whales of the World: The IUCN Red Data Book. Gland, Switzerland: World Conservation Union, 1991.