|Country or region name
|| Myocastor coypus
|English common name
|Substantially same species (synonym)
|Year of invasion or detection
|| South America
|Situation of establishment
|| Category 4: Settled after 1951, but not distributed all over the
|Expansion of distribution area
| The nutria was introduced in Okayama Prefecture during and after
World War II to provide a source of fur. However, the nutria escaped into
surrounding fields through broken fences or after the boom in rearing this
species ended and the animals were abandoned. The escaped nutria became
established around reclaimed land near Kojima Bay because the area provides
very good conditions for this species: the habitat is warm, and there are
many creeks with abundant aquatic plants.
The escaped nutria expanded their distribution around Okayama Prefecture
during the 1970s (Miura, 1976). The escaped nutria have now expanded into
the Okayama, Hyogo, Hiroshima, Gifu, Kyoto, Aichi, and Mie Prefectures.
This expansion was estimated to represent secondary dispersal and increased
the population that became established for the first time in the original
habitat around the reclaimed land (Miura, 1994). It was reported that thus
an original habitat existed in Gifu Prefecture too, which the expansion
started (Okada et al., 1988).
Nutria suffer easily from frostbite, and amputation of the tip of the tail
and of the legs is observed with high frequency at cold latitudes. As a
result, they cannot inhabit regions where the water freezes.
Nutria live around slowly flowing rivers, in ponds, and in marshes, and
seldom moves farther than 10 m from the water. They burrow their nests
into the banks. They are usually nocturnal, and rest in their nests during
the day. The burrows of nutria are complicated, with many branching tunnels,
and sometimes reach lengths of more than 10 m. In winter, they construct
floating nests on the water by gathering aquatic plants together; these
structures are called "platforms", and the nutria often rest
on them. The platforms typically range between 30 and 100 cm in diameter.
The platform's surface is flattened, and often contains food residues and
feces; the latter, which are sausage-shaped, are commonly observed on the
platforms. The presence of platforms is a sure sign that an area is inhabited
by nutria (Miura, 2002).
| The impact of the nutria on riparian vegetation has been heavy because
they eat the roots and rhizomes of these plants as part of their staple
food. There have also been impacts on bivalve populations.
1) Simplification of the riparian flora
It is known that at least one aquatic plant, Trapa japonica, has been fed upon extensively by nutria and has vanished from certain
areas. As a result, populations of residual plants that remain after browsing,
such as the sweet flag (Acrus calamus), have increased as a result of increased room in which to grow. Such
changes have simplified the riparian flora into less-diverse communities
2) Predation on Anodonta woodiana (a bivalve)
It has been reported that nutria consume many Anodonta woodiana in agricultural reservoirs in Okayama Prefecture (Mori, 2002). Predation
by nutria on freshwater bivalves has also been reported in England (Gosling,
1991). Because A. woodiana in ponds provides a bed for oviposition by freshwater fish and performs
mass filtration of organic matter in the water, this predation may have
grave consequences for the region's aquatic ecology.
| Nutria also cause damage by eating rice plants and vegetables grown
around wetlands, and can cause safety problems around stream banks because
of their habit of burrowing into these banks, leading to occasional collapses
of stream banks.
| Reproduction is observed all year round. Nutria have no specific
reproductive season; they are polyestrous and can potentially bear offspring
two to three times in a year. Pregnant females of wild populations in Okayama
Prefecture have been caught year-round. Nutria are mammals, with copulation-induced
ovulation, and their pregnancy period is normally 127 to 132 days. The
average number of babies per litter is about five. The number of fetuses
averaged 5.87 in a wild population in Okayama Prefecture (Miura, 1994).
| Nutria are large aquatic rodents native to South America, and adults
reach weights of more than 10 kg. They have forms adapted to an aquatic
life, such as small, round ears, webbed hind legs, and female dugs arranged
somewhat towards the back to help the babies suckle while in the water.
The birth weight of nutria is ca. 225 g, and the weight of the young increases
by ca. 300 g each month, and can reach more than 5 kg after 20 months in
captivity. The period until sexual maturity is estimated at 3 to 4 months
for babies born in the summer, versus 6 to 7 months for babies born in
the autumn (Miura, 1994). Nutria are sensitive to cold, and their mortality
is highest in winter. The greatest longevity that has been observed in
captivity is more than 10 years. If we accept the method for estimated
ages based on the presence of cementum layers in the teeth, the greatest
longevities in wild populations in Okayama Prefecture are 8 years for males
and 11 years for females. Miura (1994) estimated that 60% of this wild
population was younger than 1 year old.
| Nutria can be efficiently prevented from invading agricultural and
other areas by means of wire fences erected around rivers and ponds. However,
this method is expensive and requires ongoing maintenance of the fences.
A more effective approach may be to alter their habitat by removing important
food species such as aquatic plants.
However, because nutria is an exotic species in Japan, it would be preferable
to eradicate them. Nutria were designated as a species subject to hunting
in 1963 in Japan, and their numbers have been controlled not only during
an open season but also during a closed season based on Japanese laws on
wildlife preservation and hunting. As nutria feed mostly at night, and
hunting with guns is not permitted at night, the pressure exerted on populations
of nutria by hunting is generally low.
Instead, nutria can be caught easily using box-type wire live-traps (30
x 40 x 75 cm) baited with carrots (Miura, 1992). As a result, such traps
should be used whenever it is necessary to remove nutria in a hurry. Poisonous
baits such as zinc phosphide are frequently used to control nutria in the
West, where poisoned carrots are set on the floating platforms. For example,
the eradication of nutria is conducted using this method in England (Miura,
|Writer's name and affiliation
| ©Mori, I. Okayama Pref. Nature Conservation Center (Written in January