The Use of Spraints for Surveying Otter Lutra lutra Populations: An Evaluation

1. What they did

For about ten years, otters had been surveyed in many countries by searching up to 600 m of riverbank for their signs, mostly spraints (droppings), and stopping as soon as a sign was found. Otters seem to use spraints to signal to each other and often leave them in conspicuous places, which makes them easy to find. A 1986 study of daytime-active, coastal otters on Shetland (Kruuk et al.) found no link between spraints and the time otters spent on stretches of shore, and questioned the method.

The authors set out to test the method with their own survey data, asking three questions:

• Distribution. Does the standard 600 m search reliably show where otters are? They recorded all signs in 100 m stretches over 1000 m at sites in north-east Greece, Andalucia (Spain) and Wales (92 km in total), and plotted when sites were first confirmed in a Welsh survey of 329 sites and Mediterranean surveys of 712 sites.
• Population status. Does the number of spraints per 200 m match the share of positive sites, across ten surveys in Portugal, Greece, Italy, Spain, Tunisia, Morocco, Algeria and Wales? And does it differ between regions of Greece thought to hold thriving or declining otter populations?
• Habitat. Does spraint density reflect the habitat otters use? They reviewed nine studies of spraints and habitat, three radio-telemetry studies and one experiment with a captive otter.

They also re-examined the design of the Shetland study.

2. What they found

Distribution

• Where 1000 m was searched, 73% of sites were positive within the first 600 m. The extra 400 m added only 6% more. 79% of positive sites were confirmed within the first 200 m.
• In Wales, 44% of 329 sites were positive, 74% of them within 200 m. Extending the search to 1000 m was predicted to add 11.7% more positive sites.
• In the Mediterranean, 44% of 712 sites were positive, 69% of them within 200 m. Extending to 1000 m was predicted to add 7.5%.
• Negative sites were not scattered at random. In Greece (a thriving population) they clustered in distinct sub-catchments; in Italy (otters close to extinction) the positive sites clustered. A survey of 1300 Italian sites by another researcher found a very similar picture to the authors’ 188 sites (6.2% and 8.5% positive).

Population status

• Across ten surveys, spraint density rose with the percentage of positive sites (a significant correlation). The main outlier was central Wales, surveyed in September when marking is at its lowest and straight after unseasonal heavy rain and flooding.
• In Greece, central Greece had far fewer spraints per 200 m (2.69) than Thrace (10.04) or the western Peloponnese (9.90), matching its lower share of positive sites (56.4% against 92.3% and 85.0%). Thrace and the Peloponnese did not differ.

Habitat

• All nine studies that looked for it found more spraints where there was more cover: woodland and trees along the bank (especially ash and sycamore), scrub, brambles, willow, reeds and potential holts (resting sites).
• Radio-telemetry supported this. In Perthshire, otters fed a radioactive marker left labelled spraints concentrated at their centres of activity, and used ash and sycamore roots as holts. Released otters in East Anglia spent 53% of their time in woodland. In Idaho, river otters hardly used stretches with plenty of food if there was too little cover.
• A captive otter entered and left the water mostly by bankside bushes, and moved their entry points when the bushes were moved.
• Cover is not everything: one stretch of bare grazed pasture in Wales was heavily sprainted, especially in winter. It faces the meeting of two major tributaries with the main river, and the marking may have social importance at this “crossroads”.

The Shetland study

• The Shetland otters were daytime-active, at high density on the coast, unlike nocturnal river otters at lower density along a line of river. That study did find more spraints near holts.
• Observations were irregular, and time spent watching blocks of coast varied more than fifteen-fold. No otters were seen in the three least-watched blocks, although spraints were found there. Recorded activity correlated with observation time, suggesting observers watched longest where they expected otters.
• Spraints were collected only three times (May, July, September), but spraints weather away in about four weeks on Shetland, and half of marked spraints disappeared within three weeks in Shropshire. Many spraints linked to observed activity would have been missed.

3. What they concluded

• Spraint surveys using the standard method give a reliable picture of where otters are. A whole catchment is very unlikely to be wrongly recorded as having no otters.
• Used with care, and with large enough samples, spraint density gives a broad indication of the status of an otter population. It is more valuable as part of repeated monitoring than as a single survey.
• The link between spraint density and cover is real, so spraint surveys can identify habitat features important for otter conservation.
• The Shetland study’s conclusions may reflect its design. Proving there is no link would need radio-tagged otters producing labelled spraints.
• Most surveys were a first step towards conservation, meant to find where otters are widespread and raise local awareness, not to find every otter population. Local otter groups in Iberia, Italy and Greece suggest this aim is being met.

4. How tracking was or was not used

The whole method is a sign survey: observers search riverbanks for otter spraints, and occasionally footprints, and count spraints per 200 m. Sites count as positive when any sign is found. The paper does not describe how spraints were identified or how fresh and old spraints were told apart.

5. Tracker or observer certification

Not reported. The surveys were done by the authors and others (one author examined the 329 Welsh sites); no qualification or test of the observers’ skill is described.

6. Other technologies, skills or innovations

• Cumulative detection curves (sites confirmed against distance searched) to judge how long a survey stretch needs to be.
• Radio-telemetry and spraints labelled with a radioactive marker (from a cited study) to link spraints to where otters actually spent their time.

7. Implications for conservation (from the paper)

• Spraint surveys can map otter distribution well enough to guide conservation at the scale of catchments.
• Spraint density linked with bankside cover identifies habitat features worth protecting for otters.
• Surveys straight after unseasonal floods should be avoided where possible, and the season of sprainting should be kept in mind.
• Repeated monitoring of contrasting sites with the same methods could refine estimates of population status.