Otters – Surveying

See the results of the otter survey from April 2014 here.

Dorset Otter Group surveying was focussed on collecting evidence of otter presence or absence four times a year (once per season). Such evidence was requested both from a fixed set of sites and by way of casual, or one-off, records. This painted a picture of recovery across the county and suggested that, by 2011, otters could be found on virtually any river or stream in the county at one time or another in the year. Although surveyors were requested to estimate the age of any spraint found, this information was not analysed. Indeed, given that surveyors could visit their sites on any day in a two-month period, analysis of spraint age and distribution would not have been meaningful. For this reason, the emphasis now has changed to everyone surveying on the same day.

The idea of blitzing an area on a single day is not new; it was used as a way of bringing a large number of observers into an area where resident surveyors were thin on the ground – on the Fleet for example. It has also been used in Somerset for many years. The difficulty is in recruiting a sufficient number of committed volunteers such that each person has a manageable quota of sites and the whole area is covered. In Dorset, this started in 2004 on the River Axe in the west of the county (a joint effort with surveyors from Somerset and Devon as the Axe catchment is shared between all three counties) and has continued twice yearly. By concentrating on the identification of fresh spraint and plotting its occurrence on a map, it is possible to make an estimate of the number of separate otters (or family groups) present.

In April 2013, we started same-day surveying across all of Dorset. Thanks to the efforts of our current team of surveyors, we followed this up in October 2013 and the map below shows the location of fresh otter signs found on that occasion. A more detailed account of the latest survey can be found in the newsletter.

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Contact us if you would like to join the Mammal Group and take part. We can train you how to survey for spraint, tracks and other signs. Although we hope soon (probably in April2016) to change to surveying on two successive days (like Somerset) – which enables certainty in identifying spraint which is new on the second day – there will always be a need to improve understanding of how spraint ages under various conditions.  Ken Hutchinson’s photo guide can be seen here.

Whilst surveying for otters we also keep an eye open for signs of mink and water vole. The former may be declining – due perhaps in part to renewed pressure from the expanding otter population? – whilst the latter is making a slow recovery after being badly affected by both mink predation and habitat loss.

It is unusual to see an otter in England. Although they are being seen more frequently in the day, it is still a noteworthy occurrence. Except perhaps at Blandford where, during 2011, locals had to get used to regular invasions of visitors keen to see one or more of the several animals regularly using the Stour in the vicinity of the town. They seemed oblivious to human watchers and were featured on various TV programmes. For the most part, however, an otter sighting is a chance affair and something to be treasured. However, the next best thing is to record one with a camera trap and many are now deployed in Dorset with excellent results. Records from these, together with actual sightings, and the sad collection of roadside corpses, tends to make us think that we may be under-estimating numbers when we rely on spraint alone. There is, in other words, much more to discover.

In addition to local surveying, there is a national survey (England only) which has taken place every seven or eight years since the late seventies. It is a broad brush approach which masks local variations but it has illustrated the recolonisation very well. The three pictures show the situation in 1979, 1994 and 2010 (left to right).

 

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Click to enlarge maps

 

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