Citation preview
Essex Otter Survey 2008 Darren Tansley February 2009
ESSEX Wildlife Trust
Surveying for otters Even when otters are using a river they are
rarely seen. Their main prey, eels and other fish, are more
sluggish and easier to catch at night so riverine otters have
adapted to a nocturnal lifestyle. They can also occupy very large
territories with males typically holding 15-25km overlapping with
several females. It has therefore been important to develop a
method of surveying that does not rely on actual sightings.
Otters communicate with each other by depositing droppings known as
spraints on prominent features such as fallen trees, boulders,
grass tussocks, shingle bars and road bridge supports. The males
travel greater distances and need to territory mark more regularly
so their spraints tend to be smaller than the females or juveniles
(Woodroffe, 2007). With training and practice spraints can be
easily located and identified. Dark and sticky when fresh, spraints
contain fish bones and scales and have a distinctive scent,
variously described as smelling like fresh hay, ‘jasmine tea’ or
fishy and musty.
Survey transects generally begin at bridges or other prominent
features and include a total of 600m of bank in the most easily
accessible direction from the starting point. When a sprainting
site is found the spraints are counted (but not removed) and the
survey is complete. Footprints and other field signs are noted but
a survey point is only identified as positive if spraints are
present or a suitable photographic evidence is produced for
verification.
The easiest time for surveying is from February to the end of May
before waterside vegetation becomes too dense. Surveys should not
be undertaken during times of flood or high water as field signs
will be washed away. Spraints generally begin to reappear a few
days after the water levels drop when otters remark their
territories. Tracks may also be more obvious after floods in
recently deposited silt. Where access is possible sites can be
searched at any time of year and some sites may be visited several
times.
Survey limitations
While a survey of field signs can reveal the presence of otters, it
tells us little or nothing about the individual or the number of
animals present in a catchment. Only detailed analysis of spraints
in a lab can confirm the identify of an individual and
unfortunately usable DNA is only present in an average of 20% of
samples. A further limitation of this technique is that spraints
must be collected within an hour of deposition to be fresh enough
for examination.
A new method of analysing spraints by their chemical scent markers
is now being developed at Cardiff University. The main advantage of
this in comparison with DNA is that all spraints contain scent
information even if they are not fresh. This method could
potentially identify individual otters and possibly even age, sex
and breeding status. In the future we hope to undertake such a
survey to calculate the number of otters on our river catchments,
combining the work of our RIVERSEARCH surveyors in the field with
laboratory research facilities at the University of Essex.
Introduction
It seems incredible to us now that a species so well loved and
easily recognised as the otter had never been systematically
surveyed in Essex until it was finally extinct.
The first attempt to produce a national distribution was published
half a century ago by the Universities Federation for Animal
Welfare but this was based on anecdotal sightings and hunt data
(Stephens, 1957). None of the records came from Essex although the
species was considered widespread in the county from the River
Stour on the Suffolk Border to the edge of London in Enfield.
During the 1960s and 1970s, hunt and survey reports showed fewer
animals in Suffolk and Norfolk, although again no data were
collated for Essex. Concern for the species culminated in a
national survey during 1977-1979, however once again Essex was
omitted and it was not until 1986 that the county was finally
surveyed.
No signs of otter were found.
The third national survey in 1991-1994 finally uncovered otter
spraints but at only three sites; two on the River Stour and one on
the River Colne. No positive records were found on the Chelmer or
Blackwater (Strachan and Jeffries, 1996).
In 1996 funding was secured by the University of Essex to conduct a
series of annual county surveys (1996-2002) after which the work
was co-ordinated by the Essex Biodiversity Project (2003-2006) and
Water for Wildlife (2007 to present).
RIVERSEARCH was launched in 2007 to provide volunteer surveyors
with identification skills and basic ecological training for a
range of species including otter, water vole and mink. In 2008
surveyors discovered more sites with positive otter signs than
during any previous survey, a testament to their diligent
efforts.
But as otters continue to expand their range our understanding of
their movements and impacts must also grow. Essex is an
increasingly busy county with transport, commerce, house building
and recreation all competing with wildlife for space. Our
generation will decide whether we can accommodate this species and
allow them the opportunity to recolonise the rivers and wetlands
which they inhabited just three short decades ago.
Darren Tansley Water for Wildlife Officer Essex Wildlife
Trust
Below: Left is a fresh spraint with a black, tarry appearance.
Right is an older spraint which is whiter and crumbly with obvious
fish bones and scales visible.
Essex Otter Survey 2008
Survey points
RIVERSEARCH aims to survey 252 points along 26 rivers/catchments
covering Essex and adjacent parts of neighbouring counties Suffolk,
London, Hertfordshire, and Cambridgeshire. These points cover 26
districts and boroughs - 13 in Essex, 8 in London, 2 in Suffolk, 2
in Hertfordshire and 1 in Cambridgeshire. The list includes the
original 234 survey points (Macdonald & Mason, 2003) as well as
extra locations that have subsequently been incorporated.
In 2008, 215 (85%) of the 252 survey points were surveyed by 36
surveyors, as opposed to only 198 (78%) in 2007. Of these, 41%
(n=88) were positive and 58% (n=126) negative with just one showing
other signs of otter activity, but no spraints. A comparison of the
last three years shows a dramatic increase in positive results for
the surveyed sites (see below).
Rivers and catchments In 2008, 17 catchments were surveyed in their
entirety and a further 8 partially. Once again the only water
course left totally unsurveyed was the Stebbing Brook, which
contains only one survey point. Many of the other smaller brooks
and rivers had very good coverage and several surveyors provided
results from extended searches of previously unsurveyed sections of
their water courses. While these additional areas are not used for
the comparative data below, they are incorporated into the species
distribution map (page 6) and provide important data for areas
missed by the systematic survey methodology.
River Stour (Box, Brett, Chad Brook, Glem) Surveyors:- Bill
Johnson, Carole Mander and William Mann
The River Stour forms the border between Essex and Suffolk and the
catchment includes a number of substantial tributaries which drain
south into the river from the Suffolk side. This was the first
‘Essex’ catchment to be re-colonised by otters during the late
1980s. By 1996, 66% of the catchment was occupied. That year the
Environment Agency reported that otters were breeding at
Flatford.
Although 10 points on the Suffolk side were unsurveyed there were
signs of otters at 15 locations, the highest number of positive
sites since 2005, a year when all 27 points were surveyed and 18
were found to be positive. Breeding continues in the river with
sightings of mothers with cubs in two different locations.
River/Catchment Total No. of survey points
No. of positive
No. of probable
No. of negative
Blackwater/Pant 29 6 0 14 9
Box 5 3 0 2 0
Brain 5 1 0 4 0
Brett 12 9 0 3 0
Cam 6 0 0 6 0
Can 2 2 0 0 0
Chad Brook 5 3 0 2 0
Chelmer 18 7 0 5 6
Colne 26 15 0 9 2
Crispey Brook 2 0 0 2 0
Crouch/Dengie 12 0 0 12 0
Glem 10 5 0 5 0
Hamford Water 5 0 0 3 2
Holland Brook 5 0 0 5 0
Lee or Lea 10 4 0 6 0
Pincey Brook 2 0 0 2 0
Roding 17 9 0 8 0
Roman River 7 2 1 1 3
Sandon Brook 2 2 0 0 0
Southeast Essex 15 0 0 12 3
Stebbing Brook 1 0 0 0 1
Stort 9 1 0 8 0
Stour 27 15 0 2 10
Tenpenny Brook 4 0 0 4 0
Ter 8 1 0 7 0
Wid 8 3 0 4 1
Total 88 1 16 7
Chart 1 – Percentage breakdown of surveyed sites 006-008 (does not
include unsurveyed sites)
Chart – Comparison of Essex Otter Survey for 006-008 including
unsurveyed sites
Table 1: Breakdown of the sites in each river/catchment that were
surveyed as positive or negative for otters in 008. (Shading
represents catchments with otters present.)
Positive 25%
Probable 0%
Negative 75%
0
20
40
60
80
100
120
140
160
7
Unfortunately there were also two road deaths in 2008 (at the A134
road crossing at Nayland and the Belchamp Brook near Bulmer). The
Nayland otter was a lactating female indicating she had dependent
cubs. Despite a search of several kilometres of river bank by boat
and on foot, surveyors from the Essex and Suffolk Water for
Wildlife Projects could not be locate the natal holt or cubs and it
must therefore be assumed they did not survive.
The Stour’s tributaries were once again well occupied. Signs on the
Brett and Glem were higher than previous years with 75% of the
Brett showing signs of occupation, a first for this river. The Chad
Brook and Box maintained their good levels of positive sites.
River Colne and Hamford Water (Roman River, Tenpenny Brook, Salary
Brook, Bourne Brook) Surveyors:- David Aldridge, Stephen Clarkson,
Sonya Lindsell, Peter Margetts, Darren Tansley and Jim & Sarah
Young
In 2008 there were 15 positive sites on the Colne, much higher than
any of the previous surveys. As recently as 2001 there were no
signs of otters on the main channel. During the 1990s signs would
appear one year and disappear the next, indicating only transient
animals. Confirmation of breeding in 2008 came in May when a mother
and cub were seen on a tiny tributary near West Bergholt. Otter
distribution now reaches as far inland as Great Yeldham and east to
tidal reaches of the river between Colchester and the coast.
Unfortunately there were another two road deaths on or near the A12
Colchester bypass, a notorious accident blackspot that claims at
least one otter every year.
The Bourne Brook, which threads south around Halstead, was heavily
sprainted this year. In places the channel is less than two metres
wide, useful territory for a mother to occupy while teaching her
cubs to fish. As this report was being compiled a late record was
received from a resident near Earls Colne. Two otters had been
observed in early 2008 playing on the watercourse, one whistling
loudly typical behaviour for a cub. This strongly suggests breeding
in a second location in the Colne catchment however there is a
small possibility that the West Bergholt and Earls Colne sightings
were of the same pair.
The Roman River was not surveyed in its entirety this year however
two points were positive, an increase from previous years where
spraints were only found at one location near Fingringhoe.
The Tendring Peninsula remains unoccupied. No signs have been found
on Hamford Water since 1998. These resulted from the release of
three animals at Horsey Island in 1996 that were occupying the
freshwater dykes on the island. Unfortunately these were killed by
traffic during forays onto the mainland.
Otters were found for the first time on Salary Brook when
footprints were spotted on a monitoring raft set to track mink
movements near Ardleigh. A further sighting of an animal fishing on
the Ardleigh Reservoir now confirms that otters are moving into the
area and could soon move across into Tendring.
Map 1 - Otter distribution 008 This map shows all the points that
were surveyed by the RIVERSEARCH volunteer survey team in 008
(positive sites in green). In addition, blue represents verified
records of otters gathered from independent sources or by
RIVERSEARCH surveyors during extended survey work outside their
official site locations.
Four sites of otter mortalities were recorded, two on the Stour and
two on the Colne. All dead otters are collected by the Wildlife
Trust or Environment Agency and sent to Cardiff University for post
mortem.
Riversearch
Blackwater/Pant (Brain & Ter) Surveyors:- Colin Butler, Adrian
Halliday, Hilary Jones, Sue Manning, Alec Martin, Martin Pugh, Adam
& Jo Taylor, Stephen Wilkinson and Peter Wilson
The Blackwater has the largest number of survey points of any of
the Essex Rivers (n=29) but positive sites continue to be elusive.
Of the 20 sites surveyed in 2008 only 6 were positive. Signs appear
far upstream at Walthams Cross and Gt Codham Hall then are absent
until the lower reaches between Kelvedon and Maldon where there are
regular sightings. This could indicate two distinct groups of
animals as the upper signs are close to the well sprainted Bourne
Brook in the Colne Catchment and the downstream cluster joins with
positive sightings on the Chelmer. There was another single
positive site on the Brain downstream from Witham, and for the
first time signs were found on the lower reach of the Ter near its
confluence with the main Blackwater channel.
Chelmer (Can, Wid, Sandon Brook and Stebbing Brook) Surveyors:-
Patricia Clegg, Graham Littlewood, Adrian Wall, Elizabeth &
George Wiley and Peter Wilson
While only 12 of a possible 18 sites were surveyed on the main
channel of the Chelmer in 2008, 7 proved positive, the highest
number since 2004. Sightings and signs are regularly recorded
around the confluence with the Blackwater and there were good signs
on the Can, Wid and Sandon Brook with another 7 positive sites
across the 12 survey points. These were better results than in
previous years and seem to indicate that otters have consolidated
their occupation of this central part of Essex.
River Roding (Crispey Brook) Surveyors:- Marcus Dain, Norman Lee,
Henriette Monteiro and Louise Wells
This river was virtually unsurveyed in 2007 but thanks to some new
RIVERSEARCH surveyors and a grant from the Thames Environment
Agency we were able to survey the whole catchment this year. Otter
presence was first noted when tracks appeared on monitoring rafts
at Roding Valley Meadows Essex Wildlife Trust Reserve in spring
2008. During the official survey positive signs were found at 9
sites throughout the mid and lower reaches as far south as Ilford
and South Wanstead. This is equal to the previous highest number of
positive sites found in 1999 after which the number dropped to 4 in
2001, 3 in 2002 and has not exceeded 2 sites since. While the 2008
survey was the best for a decade, there were no positive records
from surveys of the rural upper reaches at the Rodings or from the
Crispey Brook.
West Essex Border (Lee, Stort and Cam) Surveyors:- Kevin Radley,
Bob Reed and Adam Rochester
The Lee population proved stable in 2008 with 4 out of 10 sites
positive, identical to 2007. The Stort continued to provide just
the single positive site at Eastwick Lodge Farm however in Feb 2009
otter tracks appeared upstream near Spellbrook Lock. There was a
puzzling lack of otter signs on the Cam, a river that usually
provides between 2-4 positive sites. The surveyor did note
extensive feeding on frogs, which is typical of otter activity in
the spring, but in the absence of prints or spraint it was
impossible to verify presence.
South Essex (Mardyke, Ingrebourne, Dengie and Crouch) Surveyors:-
Norman Edney, Chris Govus, Graham Hart, Tim Love, Adam Rochester
and Peter Wilson
Sites in this part of the county continue to remain steadfastly
negative in spite of good survey coverage. There are continuing,
unconfirmed reports of otters on the Crouch, Roach and from small
streams in the area but in the absence of definite evidence these
are impossible to verify. Both seals and mink could easily be
mistaken for otters, especially when glimpsed at a distance, so
could account for some of these sightings.
River/Catchment Total No. of survey
points
Box 5 4 3 3 2 2 3
Brain 5 n/s 0 0 1 1 1
Brett 12 0 4 5 7 6 9
Cam 6 0 1 4 2 3 0
Can 2 1 0 n/s 1 1 2
Chad Brook 5 n/s n/s n/s 2 3 3
Chelmer 18 5 7 2 6 3 7
Colne 26 3 5 10 10 9 15
Crispey Brook 2 n/s n/s 0 0 0 0
Crouch/Dengie 12 0 1 1 0 0 0
Glem 10 n/s n/s n/s 2 4 5
Hamford Water 5 n/s 0 0 0 0 0
Holland Brook 5 0 0 0 0 0 0
Lee or Lea 10 3 1 n/s 2 4 4
Pincey Brook 2 0 0 0 0 0 0
Roding 17 4 2 0 1 0 9
Roman River 7 0 0 1 0 1 2
Sandon Brook 2 n/s 0 n/s 2 2 2
Southeast Essex 15 n/s 0 0 0 0 0
Stebbing Brook 1 n/s n/s n/s n/s n/s n/s
Stort 9 1 1 1 1 1 1
Stour 27 4 14 18 3 12 15
Tenpenny Brook 4 0 0 0 0 0 0
Ter 8 n/s 0 0 0 0 1
Wid 8 5 1 0 1 2 3
Total 1 8 7 88
Table : Summary of the total sites in each catchment recorded
positive for otters since 00 (n/s = not surveyed)
Map - Mink records 008 This is not the full distribution of mink in
Essex but has been compiled from records reported to the Essex
Water for Wildlife Project during 008. Mink are widespread
throughout the county and are present on most rivers.
Essex Otter Survey 2008
11
Summary of the Essex Otter Survey 2008 Positive sites have
increased dramatically since Essex University recommended that an
annual survey should be conducted at the conclusion of their 7 year
funded project (Mason & MacDonald, 2002). In 2003 there were 31
positive records, a figure that has nearly tripled in 2008 to 88
positive sites and one probable. Many rivers produced the highest,
or equal highest, number of positive records since 2003 (Colne,
Roding, Blackwater/Pant, Chelmer, Brain, Brett, Can, Chad Brook,
Glem, Lee, Roman River, Sandon Brook, Stort, Ter) while the Stour
showed exceptionally good occupation in the section that was
surveyed. Only one river, the Cam, gave cause for concern, with no
positive signs found for the first time. As only a short section of
the headwater is contained within the Essex border, it is possible
that only one animal was occupying this reach and was absent during
the survey for some reason.
More significantly there is good evidence of breeding in some of
our larger catchments. The River Stour has had a breeding
population since the survey began in 1996 but the Colne, which has
been variable in the past, is now a river with an equally robust
population. From 1999 to 2001 there were no signs of otters at all,
then only patchy records from what was thought to be a single
transient animal until 2004. From 2005-2007 positive records
steadied at 9 or 10 sites in the middle reaches but this has
improved significantly in 2008 and we now appear to have more than
one breeding female present.
The rivers in the middle and west of the county now have good
records of otter activity although this is often broken by sections
of river that appear unoccupied. The Blackwater shows good signs of
occupation, including regular sightings in and around Maldon and
Kelvedon, but there are no further positive sites upstream until
the upper headwaters. While this seems rather odd, the distribution
suggests that these animals may be linked to otter activity on the
Bourne Brook and River Colne. In fact the county can almost be
divided into two distinct meta-populations, one in the north and
one in the south west.
The River Roding was surveyed in its entirety by a surveyor from
the Essex Mammal Group who was also looking at water vole
distribution in the catchment (work funded by the Thames Region
Environment Agency). Combined with good coverage by our RIVERSEARCH
surveyors this resulted in the highest number of positive sites
since the survey began, equalling the result of 1999 when otters
were first recorded. It is still unclear whether breeding is taking
place here as the only sightings have been of a single animal. The
pattern of high numbers of signs one year and few the next was also
typical of the River Colne up to 2003, when only a single animal
was thought to be present, so it will be interesting to what
pattern emerges on the Roding over the next few years.
The survey was unable to confirm any expansion of range into the
eastern coastal margin stretching from Tendring in the north, to
Dengie and the Thames marshes in the south. Eels are a favoured
food of otters in East Anglia and they are present in many of the
marsh ditch and stream systems so the limiting factors may be
adequate holt sites or restricted access across transport networks.
There have been reports of ‘otter-like’ animals in the Crouch and
Roach, but no verifiable evidence has been provided to underpin
these sightings. Both seals and mink can be mistaken for otters
when swimming or if sighted from a distance so while these reports
appear to indicate that otters are moving into their old coastal
haunts, we still cannot confirm this to be true.
In addition to the official survey, ad hoc reports of otters and
records of dead animals have been included on the 2008 map to build
up a more complete picture of otter distribution in the county.
While incorporating records such as this does provide a better idea
of the true range, it is still important to have the comparative
data provided by the volunteer survey team to allow a year by year
comparison.
Presence of mink
Surveying North American mink Mustela vison is not fundamental to
the Essex Otter Survey but where evidence of this species was noted
by surveyors this was recorded. In addition we receive records of
mink from members of the public, and through mink control efforts
which form part of the Essex Water Vole Recovery Scheme. Map 2
(page 9) represents only those records we have been able to verify
and should not be considered as a full species distribution. No
systematic mink survey of the county has ever been
undertaken.
Currently mink signs are found almost everywhere that otters are
present which indicates a level of co-existance that may at first
appear surprising. However mink also inhabit off channel areas such
as arable land, hedgerows and woodland and can therefore avoid
direct contact with otters. While otters can kill mink they risk
bite wounds or broken teeth that can lead to infection or a reduced
ability to hunt and feed. This may eventually result in the death
of the injured otter.
Recent work in the Tendring Peninsula by the Water for Wildlife
Project revealed that mink were present throughout the coastal area
where they were previously considered absent or rare. Where mink
threaten water vole strongholds they are now being removed or
prevented from entering those areas, so mink distribution may alter
to reflect this intervention.
Otter road deaths
Otters still fall victim to traffic accidents as they attempt to
cross busy roads, especially in the winter when floods or generally
high water levels make it impossible for otters to pass beneath
road bridges. Design of river crossings is vitally important and
now in most cases is recognised as something to be incorporated at
the construction stage. Where accident blackspots occur near old
bridges it is important that these are identified and the
appropriate mitigation applied.
When otters are killed, it is essential that the location of the
body is reported to Essex Wildlife Trust or the Environment Agency
for collection. The information gained from post mortems each year
helps warn us of potential disease, parasitic or environmental
threats, not only in our county, but across England and Wales. In
2007 a fire retardant considered unable to pass through animal
tissues, was identified as present in an otter sent for post
mortem. It is only through this process that we can inform
Government bodies of these potential dangers to wildlife.
Conclusion
The Essex Otter Survey continues to provide an important record of
an expanding distribution of otters in the county and helps to
identify potential problems. There are still rivers where breeding
has not been confirmed and parts of the county where there are no
signs at all, but the number of positive sites continues to
increase year on year despite the annual death toll on our
roads.
By 2010 it is hoped that we will be able to combine the traditional
survey with a new otter spraint analysis technique, using chemical
scent markers to identify individual otters and perhaps even their
sex, breeding status and relative age. RIVERSEARCH surveyors will
be at the forefront of this ground-breaking work which will at last
answer that perennial question - how many otters are there on our
rivers?
Acknowledgements
I am extremely grateful to all those surveyors who took the time to
conduct their local surveys and submit records, Ursula Broughton
who input the survey data and Michelle Knight who assisted me in
analysing the results. I would also like to thank Rebecca O’Shea at
the Thames Environment Agency who provided extra funding for the
combined water vole and otter survey of the River Roding, Richard
Allen for his superb otter illustration, Andrew and Peter at Reed
Digital and the numerous landowners and managers who continue to
give access to their estates which enables this valuable work to
continue.
References
Crawford, A. (2003) Fourth Otter Survey of England 2000 – 2002
Environment Agency Macdonald, S. M. & Mason, C. F. (2003) The
Otter Lutra lutra in Essex 1996-2002. Essex Naturalist 20 (New
series) 159 - 176 Macdonald, S M & Mason, C. F. (2003) Essex
Otters – struggling in a hostile environment? British Wildlife 14:
153-159. Mason, C. F. & Macdonald, S. M. (2002) American Mink
Mustela vision in Essex. Essex Naturalist 19 (New series) 95-99
Stephens, M.N. (1957) The Natural History of the Otter.
Universities for Animal Welfare, London Strachan, R and Jefferies,
D.J. (1996) Otter survey of England 1991-1994. Vincent Wildlife
Trust, London Tansley, D. (2008) Essex Otter Survey 2007. Essex
Wildlife Trust / Water for Wildlife Thompson, G. (1999) A wild
future for Essex. The Essex Biodiversity Action Plan. Essex County
Council/Essex Wildlife Trust
Otter road deaths This tributary of the River Roding flows under
the M through a culvert that is potentially dangerous even during
low flow. Otters avoid swimming beneath such structures but ledges
can be fitted to allow them to walk through and avoid attempting to
cross the high speed road network.
is a unique partnership of The Wildlife Trusts, working with water
companies,
the Environment Agency and these other key partners to provide a
more consistent and targeted approach to wetland conservation
across the UK.
Essex mammal Group
If you want to know more about the RIVERSEARCH volunteer training
program or would like to take part in the annual Essex Otter Survey
please contact:- Darren Tansley, Water for Wildlife Officer, Essex
Wildlife Trust e: darrent@essexwt.org.uk t: 0161 86 Front cover
otter illustration © Richard Allen Back cover otter photo © Stuart
Hutchings Introduction otter photo © wildstock.co.uk All other
photos © Darren Tansley Printed by Reed Digital, Ipswich
The 2008 Essex Otter Survey
Introduction Surveying for otters Survey limitations 008 Survey
results Survey points Rivers and catchments Stour 6 Otter
distribution map 7 Colne & Hamford Water 8 Blackwater/Pant 8
Chelmer Roding Lee, Stort & Cam South Essex Mink records map 10
Summary 10 Presence of mink 11 Otter road deaths 11 Conclusion 11
Acknowledgements 11 References