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