The most
comprehensive study to ever be undertaken in
healthy and continue to expand across
environment.
The Environment
Agency Science Department carried out a series of studies into the deaths of
almost 1000 otters between 1992 and 2003, which looked at the overall health of
otters, factors affecting their survival, the levels of chemicals in otters and
the impacts these have on otter populations.
Otter populations
have begun to expand again following the decline in numbers across
that one factor behind this recovery may be decreasing levels of organochlorine chemicals (OCs), such as the insecticides dieldrin
and aldrin, in the environment.
Conservation Officer Lyn
Jenkins, who managed the
study into otter health in southern and south west
was progressively withdrawn from use from 1962 and banned by 1989.
“This was a very potent
insecticide, used extensively by farmers, as it remained active for a long time
after they applied it. But we now know it can take up to 25 years for 95% of
dieldrin in soil to disappear,” Lyn Jenkins said.
“This persistence, and
the fact that it passes from animal to animal through the food chain, was the
reason it was eventually banned. It had a devastating impact on animals. By
interfering with vitamin A levels, it caused reproductive abnormalities and
other conditions. Previous research has strongly linked its use to declines in
predators such as peregrine falcon, kestrel, sparrowhawk and heron.
“Otter numbers dropped
significantly during the late 1950s when dieldrin and aldrin first came into
use, especially in the south and east, and it seems otters in this country are
only just recovering from the effects.”
Lyn Jenkins said although post mortems revealed
a decline in levels of dieldrin in otters over time,
this was the first time a link had
been made between OC levels in otters
and the effects of these chemicals on their physiology.
“In the south west and
southern
almost 30% of otters found between 1996 and 1999 had retinal dysplasia, a
distorted retina in the eye linked to low vitamin A metabolism during
development, which can make otters almost blind. Concentrations of dieldrin
were more than three times higher in otters with this abnormality than in
otters with normal eyes,” she said.
“There were numerous
stories of ‘blind’ otters being encountered in the 1950s and 1960s, which fits
very closely with the widespread use of OCs in the countryside. And we know from the early work in this study
that otters with high OC loadings showed this eye disorder.”
The study also found that:
–
road traffic
accidents are the significant cause of otter mortality, and a major concern is
the number of breeding females killed
–
most of the
otters were in excellent condition, though a small number (particularly
orphaned or abandoned young) were suffering from infections, ill health or
starvation
–
many of the
adults had serious bite wounds, almost certainly due to fighting other otters,
and some young otters were killed by dogs
–
in a small number
of otters, some suffered from infections, including lung infections and several
tooth abscesses.
Lyn Jenkins said there were
no population estimates for otters in the
but it was unlikely the number of otters killed over the past 15 years was
having a significant impact on the population as a whole.
“All we know is that
otters lived on all rivers in the
in the 1920s, and now otter
populations have begun to grow and expand again following the decline in
numbers between the 1950s and 1980s. It’s also promising that a otter was found
on the River Thames last year – not far from Tower Bridge
– and it was the first wild otter to be seen in
central London for more than a 100 years.
“But we rely on
volunteers to help us collate this information. Without the volunteers who
report dead otters to us, we wouldn’t be able to undertake this work to
increase our knowledge about otter populations in
and
Lyn Jenkins said.
The series of
studies, which are available on the Environment
Agency’s website,
looked at the results of post
mortems carried out on 341 otters found dead in south west and southern
between 1996 and 2003, and on 609 dead otters found in
and the rest of
between 1992 and 2003. The post mortems were carried out by Vic Simpson
from The Wildlife Veterinary Investigation Centre in
If anyone finds a dead otter
it should be reported to the Environment Agency on 08708 506506.
Otter fact
file:
Otters are members of the Mustelid family
which includes badgers, polecats, martens, weasels, stoats and mink. There are 13
known species of otter in the world but the only one found in the
Length: Average 1.2m (dogs), 1m (bitches)
Weight:
Average 10.3kg (dogs), 7.4kg (bitches)
Colour:
Medium to dark brown above and lighter underneath.
Diet:
Mainly fish, but also birds, small mammals, amphibians,
crustaceans and molluscs.
Breeding:
Only once every 2 years as the cubs remain dependent on their
mother for a year.
Offspring:
1-4 cubs
Lifespan:
Average 4 years due to so many threats, although they can live
8-12 years.
Habitat:
Mainly rivers, but also canals, marshes, small streams,
ditches, ponds and lakes. They also inhabit estuaries and coastal areas.
Home
range: Depends on food supply but can be from 1km along a rich
coast to 40km along some rivers.
Resting
sites: Called holts, often tree roots but also drains, caves
and holes in rockfalls. They will also rest above ground in vegetation,
sometimes called couches.
Social
structure: As otters are very territorial they tend to live
alone, except during mating and for a period, dependent on the mother, after
the cubs are born.
Physical
features: Short legs, webbed feet and claws, long stream-lined
body, small ears and a broad muzzle, sensitive whiskers around snout to help
detect prey, 2 layers of fur – a thick waterproof outer one and a warm inner
one.
Special
abilities: The European otter has an acute sense of sight,
smell and hearing. The eyes are placed high on the head so that it can see when
the rest of the body is below water.
Did you know…? Otters can swim at
speeds of 12km/hr underwater and can travel for up to 400m before surfacing for
air. When it dives, an otter closes its nostrils and ears. Otters are the only
truly semi-aquatic members of the Mustelid family.
