Seasonal Marking in an Otter Population

1. What they did

Otters had declined across much of Europe, and because they live at low density and are secretive and active at night, most otter surveys rely on finding their signs rather than seeing the animals. Yet little was known about what the different signs mean, or how otters use them to communicate.

From May 1981 to May 1984, the authors walked nine 1 km stretches of the upper River Severn in Wales every month, along one bank. At each visit they counted every sign:

• Spraints: otter droppings containing food remains.
• “Smears”: tar-like secretions with no solid food remains.
• “Jellies”: jelly-like secretions of various colours with the typical otter smell.
• Other signs: places where otters had scratched the ground or rolled, and “sign heaps” (grass or moss gathered into a small mound and often marked).

They noted which sprainting sites were new, measured all otter footprints to pick out young cubs, and noted possible holts (dens) and resting sites. They also searched much of the wider catchment for otter signs.

2. What they found

Where the otters lived

• Otters were resident on the main Severn above Welshpool. Below Welshpool, where the river is embanked and has little bankside cover, signs were found only now and then, mostly in February. Signs were rare at the top of the river among conifer plantations, and none were found above the Clywedog Reservoir.

A strong seasonal pattern

• Signs and sprainting sites rose every winter and early spring and fell to their lowest from June to October, in all three years and on every part of the river.
• Spraints were the most common sign all year round. Jellies rose and fell with spraints and made up more of the signs in winter and spring. Smears were most common in spring and summer, with a sudden peak in April.
• Scratching (55 records), rolling (34 records, 79% from November to January) and sign heaps (9 records, 7 from November to January) were concentrated in early winter.
• Each winter the otters made 11 “signal sites”: patches of grass about 1 by 2 m, scratched and marked with spraints and jelly. They were used heavily from November to February and then mostly abandoned. The same places were used in all three winters, all were on treeless banks, and 9 were at least 1 m above normal water level. Track sizes in snow showed that more than one otter visited them.

Faithful to the same spots

• Otters marked the same traditional places again and again. On one stretch, two hollows low in the bank were each marked on 29 of 36 monthly checks, an oak root on 25 and a fallen willow on 24. All four went unmarked only 3 times, during severe, long floods.
• Spraints appeared at new sites 51 times, 65% of them between November and February.

Cubs and population change

• Young cubs were recorded 9 times from their track size, between January and August, never from September to December. Four of the records came from stretch 3, which had the most possible holts and the highest share of signs.
• Winter and spring signs rose by 54% between the first and third years, with increases on 7 of the 9 stretches. On a 1 km stretch of the River Wye, January and February signs rose by 236% over four years.

3. What they concluded

• Otter marking on the upper Severn is clearly seasonal, peaking in winter and early spring, and the type of sign also changes with the season.
• The authors considered and doubted other explanations. Signs were not simply easier to find in winter, because floods made winter and spring the hardest time to survey. Cub records did not line up with the rise in signs. They thought winter food shortage was unlikely to be the cause, because salmon, trout and coarse fish are probably easier for otters to catch in winter.
• They suggest the winter and spring rise in marking is linked to building or reinforcing dominance relationships, at a time when young otters leave their mothers and look for home ranges of their own.
• The meaning of signal sites and jellies remains unclear; the authors note that radio tracking would be needed to understand signal sites.
• Judging by the rising number of signs, the otter population may be increasing. They suggest restrictions on organochlorine pesticides and the 1978 ban on otter hunting may have helped.

4. How tracking was or was not used

The whole study rests on tracks and signs. The authors recorded and counted spraints, smears, jellies, scratching, rolling, sign heaps and signal sites on monthly walks. They measured every otter footprint: prints less than 45 mm across the five toes were taken as young cubs, and prints larger than this but under 50 mm as possibly juveniles. Track sizes in snow showed that more than one otter used the signal sites. The authors note that finding prints depends on sand, mud or snow, so young otters were very likely missed at times.

5. Tracker or observer certification

Not reported. The surveys appear to have been carried out by the authors themselves; no tracking qualification or test of their sign identification is described.

6. Other technologies, skills or innovations

• Separating marking signs into types (spraints, smears, jellies, scratching, rolling, sign heaps, signal sites) rather than counting spraints alone. The authors note that earlier studies did not make these distinctions.
• Recording whether each sprainting site was traditional or new.
• Using footprint width to pick out young cubs.

7. Implications for conservation (from the paper)

• Most otter surveys, on which conservation decisions may be based, rely on finding signs, so it matters how sign numbers change with the season.
• Otters on Welsh rivers seem to need bankside cover and the roots of mature riverside trees for resting. Lack of cover may limit their use of embanked stretches. At one stretch, bankside vegetation and a known holt were destroyed during the study, and signs there declined.
• Repeating field surveys is difficult: surveyors differ in skill, and weather conditions vary.