Essex Otter Survey 2009-2010 Darren Tansley ESSEX March 2011 Wildlife Trust
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Essex Otter Survey 2009-2010 Darren Tansley ESSEX March 2011 Wildlife Trust Protecting Wildlife for the Future and for the People of Essex The easiest time for surveying is from February to the end of Introduction Surveying for otters May before waterside vegetation becomes too dense. Surveys should not be undertaken during times of flood or high water Even when otters are using a river they are rarely seen. Their It seems incredible to us now that a species so well loved and easily recognised as field signs will be washed away. Spraints generally begin to main prey, eels and other fish, are more sluggish and easier to as the otter had never been systematically surveyed in Essex until it was finally reappear a few days after the water levels drop as otters remark catch at night so riverine otters have adapted to a nocturnal extinct. their territories. Tracks may also be more obvious after floods lifestyle. They can also occupy very large territories, with males in recently deposited silt. Where access is possible sites can be typically holding 15-25km overlapping with several females. It searched at any time of year and some may be visited several has therefore been important to develop a method of surveying This report celebrates 15 years of the annual Essex Otter Survey; however times. otters have been part of the fossil record of the British Isles for a staggering that does not rely on actual sightings. half a million years (Harris and Yalden, 2008), illustrating just how resilient Otters communicate with each other by depositing droppings, Survey limitations a feature they have been as part of our native fauna. known as spraints, on prominent features such as fallen trees, boulders, grass tussocks, shingle bars and road bridge supports. While a survey of field signs can reveal the presence of otters, The males travel greater distances and need to territory-mark it tells us little or nothing about the individual or the number It is therefore even more startling that after enduring 500 years of deliberate more regularly so their spraints tend to be smaller than the of animals present in a catchment. Only detailed analysis of persecution, they were finally driven to near extinction in East Anglia not females or juveniles (Woodroffe, 2007). With training and spraints in a lab can confirm the identity of an individual and by hunting, but as an incidental consequence of our twentieth century practice spraints can be easily located and identified. Dark and unfortunately usable DNA is only present in an average of sticky when fresh, spraints contain fish bones and scales and 20% of samples. A further limitation of this technique is that lifestyle. The accumulation of pollutants in our waterways, ingested via the have a distinctive scent, variously described as smelling like spraints must be collected within an hour of deposition to be fish on which they fed, led to infertility in adults and debilitating deformities fresh hay, ‘jasmine tea’ or fishy and musty. fresh enough for examination. in their cubs. Legal protection in England and Wales in 1978 (1982 in Survey transects generally begin at bridges or other prominent A new method of analysing spraints by their chemical scent Scotland) may have protected them from deliberate harm, but it was the features and include a total of 600m of bank in the most easily markers is still under development at Cardiff University. The banning of harmful, persistent PCBs, organo-chlorines and the removal of accessible direction from the starting point. When a sprainting main advantage of this in comparison with DNA is that all lead from petrol that allowed the species to begin the long road to recovery. site is found the spraints are counted (but not removed) and the spraints contain scent information even if they are not fresh. survey is complete. Footprints and other field signs are noted This method could potentially identify individual otters and but a survey point is only identified as positive if spraints are possibly even age, sex and breeding status. In the future we The return of the otter in Essex over the past 25 years is especially remarkable present or if suitable photographic evidence is produced to hope to undertake such a survey to calculate the number of as it is almost entirely due to natural re-colonisation. During the 1991- enable verification. otters on our river catchments. 1994 National Otter Survey only two sites on the Stour and one on the Colne were positive but between 1996 and 2003 otters started to establish breeding territories and were already moving south onto the tributaries of the Blackwater and Chelmer. The significance of this natural movement of animals is clear; without good habitat, relatively unpolluted water and an adequate population of fish in our watercourses this expansion of range would have been impossible. For an animal that holds territories many miles long, otters are never particularly numerous on rivers (the River Colne for example might support as few as 8-12 resident animals), but the number of Essex survey sites with clear otter signs finally exceeded 100 in 2009 for the first time since the survey began. Although this dropped back in 2010, the population still expanded its range. The last few years have seen a big increase in the number of positive sites and I would like to thank the dedicated team of RIVERSEARCH volunteer Above: Otter spraints (droppings) surveyors across Essex that continue to make this long term study possible. A fresh spraint with a black, tarry appearance and (inset) an Without their skill, enthusiasm and willingness to investigate their own old spraint showing bleached fish bones and scales. Spraints rivers and streams we would have no idea how well this iconic riverside are often deposited in obvious places such as logs, tree roots, mammal was doing. in-channel debris, road bridge supports and silt bars. Photos: Darren Tansley & Geoff Empson Darren Tansley Right: A typical sprainting site Water for Wildlife Officer Otters often spraint at the base of a riverside tree. Note the bare Essex Wildlife Trust patch in the grass next to the trunk where continued urination has killed off the vegetation. Photo: Darren Tansley 2 3 Essex Otter Survey 2009-2010 Darren Tansley - Water for Wildlife Table 1: Breakdown of the sites in each river/catchment that were surveyed as positive, negative or probable for 2009-2010 survey results otters in 2009 and 2010. (Shaded catchments were positive for otters during at least one survey.) Survey points River/Catchment Total No. of No. of positive No. of probable No. of negative No. of points not RIVERSEARCH now aims to survey 255 points along 27 latter are both small watercourses with only 2 survey points survey points surveyed rivers/catchments covering Essex and adjacent parts of each so this was not considered to be a significant omission. Year (2009) 2010 2009 2010 2009 2010 2009 2010 2009 2010 neighbouring counties Suffolk, London, Hertfordshire, and Blackwater/Pant 29 11 5 0 1 15 7 3 16 Cambridgeshire. These points cover 26 districts and boroughs In fact the total number of unsurveyed sites dropped for the Box (5) 6 2 4 0 0 3 2 0 0 - 13 in Essex, 8 in London, 2 in Suffolk, 2 in Hertfordshire and fourth successive year thanks to the efforts of a number of Brain 5 2 1 0 0 1 2 2 2 1 in Cambridgeshire. The list includes the original 234 survey surveyors who returned to points that had not been visited in points (Macdonald & Mason, 2003) as well as extra locations the spring. Once again several surveyors provided results from Brett 12 9 7 0 0 2 5 1 0 that have subsequently been incorporated. extended searches of their water courses outside the official Cam 6 3 2 0 0 3 0 0 4 survey points. While these additional areas are not used for the Can 4 4 2 0 0 0 0 0 2 In 2009, 225 (88.5%) of the 255 survey points were surveyed comparative data below, they are incorporated into the species Chad Brook 5 5 2 0 0 0 0 0 3 but this dropped to 192 (75.3%) in 2010 due to extreme distribution map and helped inform the Environment Agency’s Chelmer 18 10 9 0 0 6 0 2 9 weather during the final November/December checks and National Otter Survey 2009-2010. Colne 26 10 11 1 0 8 7 7 8 the loss of a key surveyor on the Blackwater catchment. Of Cripsey Brook 2 0 0 0 0 0 2 2 0 these, 49% (n=110) were positive and 51% (n=114) negative in In 2010, 14 catchments were surveyed in their entirety and the Crouch/Dengie 12 0 0 0 0 12 12 0 0 2009 compared with 47.5% (n=91) positive and 50.5% (n=97) remaining 13 partially. The number of unsurveyed sites increased negative in 2010. The remaining 1 site in 2009 and 4 (2%) in substantially for the first time in five years. This was due to the Glem 10 6 2 0 1 4 1 0 6 2010 showed other signs of otter activity but no spraints. A severe snowfalls and subsequent high water in November and Hamford Water 5 0 2 0 0 5 1 0 2 comparison of the last eight years shows an upward trend in December (see photos opposite), the months when unsurveyed Holland Brook 5 0 2 0 0 5 3 0 0 positive results for the surveyed sites and a downward trend in sites are usually reallocated to infill areas not completed earlier Lee or Lea 10 1 0 0 0 5 5 4 5 negative results (see chart 1 below).