Between 1995 and 2003, otter surveys were conducted on 11 islands off the north west coast of Scotland. These located holts in a 100-metre-wide strip along the coasts of these islands. The information was then used to calculate the total number of holts for each island, and from that the adult otter population for each island was calculated using the models based on studies in Skye and Shetland. The diet of marine-foraging otters was assessed from five of these islands using spraint analysis, and was compared with the diet of otters from other coastal areas in Scotland. With the exception of the free-swimming saithe, the diet was dominated by small benthic fish, with the five key prey species being viviparous blenny, five-bearded rockling, butterfish, sea scorpion, and saithe.
Since 1995 the International Otter Survival Fund has undertaken surveys of Eurasian
otters (
Islands surveyed from 1995 to 2003.
In Shetland, Moorhouse [
Similar significant correlations were found in extensive and intensive studies carried
out on Skye over a two-year period [
For the surveys, each island was divided into 500-metre lengths measured at the high watermark. Such distances are easily manageable for fieldwork, while being generally long enough
to observe otter activity from a vantage point. The following factors were examined:
otter observations, number of holts and lie-ups, number of freshwater pools.
Observations, using
Sites used by otters as holts are very variable and for the purpose of this study are defined as tunnel systems with signs of regular use by otters [
Otters use resting places throughout their range. These are mostly hollows under rocks or within reed/rush beds and usually have a path leading to them and a sprainting point nearby.
A freshwater pool was defined as an area of standing water with a minimum width or length of 0.5 metres and minimum depth of 0.1 metres. Personal observations indicated that otters do not use smaller pools. All such freshwater pools were counted and measured and it became evident that most, but not all, had sprainting points adjacent to them.
Table
Islands surveyed for otters (1995–2003).
Island | Year surveyed | Number of 500 m | Percentage of |
---|---|---|---|
sections surveyed | total coastline | ||
Skye | 1995 | 622 | 55% |
Raasay and other islands off Skye | 1995 | 75 | 19% |
Canna | 1996 | 55 | 79% |
South Uist | 1999 | 152 | 45% |
Pabay | 2000 | 11 | 100% |
Barra | 2000 | 72 | 51% |
Eigg | 2001 | 39 | 78% |
Castle, off Eigg | 2001 | 6 | 100% |
Muck | 2001 | 22 | 100% |
Coll | 2002 | 73 | 44% |
Tiree | 2003 | 138 | 100% |
In his study, Moorhouse [
On Skye, in four study areas it was possible over a 24-month period to identify individuals, and relate this to the number of holts in use at a particular time. These could be recognised by characteristics such as pink patterns on the nose, “battle scars” (nicks out of ears, scars in fur, etc.) and females with cubs could be identified.
Counts were made of otter holts in use in a number of coastal areas on Skye (Figure
Map showing the four areas used in the intensive study.
Four areas were intensively studied (Figure Bogha an t-Sasunnaich to Ard Dorch: 18 kilometres of Torridonian and Mesozoic coastlines; Camas Malag to Boreraig: 8.5 kilometres of mostly Mesozoic coastline; Drumfearn: 4 kilometres of Torridonian coastline; Loch na Dal: 3.5 kilometres of Torridonian coastline.
Each coastal section was visited for a minimum of one hour every month throughout the 24-month
period. From April to October, the four areas were visited on a weekly basis for a minimum of two
hours. In view of the length of time spent in each section, we can be confident that all the
resident otters were counted correctly.
Certain assumptions are made in this estimate. All holts were counted. All otters in each area were identified correctly. The sections sampled were representative of the Skye coastline. The ratio for the number of otters and number of holts was constant for all geological coastal zones. The ratio of numbers of females, males, non-residents, and cubs was constant.
These assumptions and their implications will be discussed in Acknowledgments.
Figure
The relationship between holts per resident female otter range in the 100-metre coastal strip in the four study areas of Torridonian and Mesozoic coastlines compared with the number of resident adult females there.
A total of 21 holts were found, with, on average, 45% in use at any one time. There is a
strong positive correlation between the two parameters over the four study areas over the two-year
period (
Mean numbers of otters and holts in the study areas.
Area | Coastal length | Resident females | Cubs | Males | Other unknown* | Holts in use |
---|---|---|---|---|---|---|
1 | 18 km | |||||
2 | 4.3 km | 2 | 1 | |||
3 | 4 km | |||||
4 | 3.5 km | 1 | ||||
Total | 29.8 km | 8.9 ± 0. 27 | 5.7 ± 0.86 | 3.5 ± 0.63 | 2.1 ± 0.76 | 17.1 ± 0.98 |
*Other unknown = mix of males and females whose sex could not be identified.
The mean estimate for the total number of adult otters in all four areas is
Where possible, areas of a particular geological composition were compared with similar areas on Skye.
The relationship on Shetland was found to be 0.33 female per holt and
The relationship on Skye on four study areas over two years was found to be 0.52 female
per holt and
Using these relationships, an estimate can therefore be made for the otter populations
on the islands surveyed (Table
Estimates of numbers of holts and adult otter numbers on the north west Scottish islands.
No. | Island | Number | Estimate of total | Estimated otter | Estimated otter |
---|---|---|---|---|---|
of holts | holts for each | population based | population based | ||
counted | island | on Yoxon [ | on Kruuk [ | ||
1 | Skye | 117 | 192.2 | 164 | 116 |
2 | Raasay and | 11 | 102.9 | 88 | 62 |
other islands | |||||
off Skye | |||||
3 | Canna | 3 | 3.8 | 3 | 2 |
4 | South Uist | 34 | 122 | 104 | 74 |
5 | Pabay | 5 | 5* | 4 | 3 |
6 | Barra | 10 | 27.2 | 23 | 16 |
7 | Eigg | 17 | 22 | 19 | 13 |
8 | Castle | 2 | 2* | 2 | 1 |
9 | Muck | 5 | 5* | 4 | 3 |
10 | Coll | 30 | 62.4 | 53 | 38 |
11 | Tiree | 9 | 9* | 8 | 6 |
*These are actual holt numbers.
Notes on islands Skye: a relationship existed between the geology of the coastline and
the number of otter holts [ Raasay and other islands off Skye: as 1. Canna: composed of Tertiary basalt lavas. South Uist: composed of Lewisian gneiss. Pabay: the whole island was surveyed and no estimate was made. Barra: composed of Lewisian gneiss. Eigg: again it varies in geology with Jurassic sediments and Tertiary lavas. Castle Island: all holts counted. Muck: all holts counted. Coll: composed of Lewisian gneiss. Tiree: composed of Lewisian gneiss.
The Isle of Skye was studied for many years and a strong relationship was found between the geology of the coastal zone
and otter activity [
The island can be divided into seven geological zones as follows.
Lewisian/Moine: old pre-Cambrian rocks to the south of the island. Torridonian: pre-Cambrian sediments (mainly sandstones) in south Skye. Cambrian: limestones and sandstones. Mesozoics: sediments mainly like limestones, sandstones, and siltstones. Tertiary lavas: lava flows to the north west of the island. Teriary intrusives: granites and gabbros in the centre of the island. Landslips: to the north and west of Skye.
By comparing the variables in the seven geological coastal zones, certain predictable patterns
emerged with more activity, holt numbers, sprainting points, spraint numbers, and
freshwater pools being found on the Torridonian coastal zone compared with all others. Both the
Tertiary lavas and Tertiary intrusives showed low numbers of each (Table
Number of 500-metre sections surveyed in each geological coastal zone and the percentage of the total coastal area covered by that particular geological type (total geology). (Activity is the number of times otters were observed in 30 minutes of observation in each 500-metre coastal zone. Sprainting points, spraint numbers, holts, and pools were found in a 100-metre-wide strip along each section.) Results show the mean numbers per 500-metre coastal section with the standard error.
Geology | Sections | % Total | Otters | Holts | Spraint | Spraints | Pools |
---|---|---|---|---|---|---|---|
surveyed | geology | points | |||||
Lewisian | 71 | 74 | |||||
Torridonian | 84 | 62 | |||||
Cambrian | 17 | 100 | 0 | ||||
Mesozoic | 99 | 100 | |||||
Tert lava | 206 | 37 | 0 | ||||
Intrusive | 104 | 98 | |||||
Landslip | 41 | 21 |
Visual observations of otter activity were recorded as otters seen per hour in each 500-metre coastal section.
Table
Chi-squared tests for the otter variables in relation to geology (
Variable | Df | ||
---|---|---|---|
Otter activity | 29.1 | 6 | *** |
Holts | 32.4 | 6 | *** |
Sprainting points | 65.0 | 6 | *** |
Spraint numbers | 83.4 | 6 | *** |
Freshwater pools | 59.7 | 6 | *** |
The greatest number of otters seen was in the Torridonian coastal zone (2.2 otters per hour observation) compared with none in the Tertiary lava and intrusive coastal zones. Holt numbers were also greater in the Torridonian zone, amounting to 1.3 holts per 500-metre section compared with no holts in the sections surveyed in the Tertiary intrusive and lava coastal zones.
Sprainting points were present in all zones from one per 500-metre section in the Tertiary intrusive zone to six in the Torridonian zone. Total spraint numbers were greatest in the Landslip and Torridonian zones and lowest in the Tertiary intrusive.
The final variable was the number of freshwater pools available to the otters; the greatest number available was again found in the Torridonian zone (4.4 per 500-metre coastal section) compared with none found in the Tertiary intrusive zone.
Overall, therefore, the Torridonian coastal zone had the greatest number of all otter variables and is the most favoured habitat for otters in this study area.
In the coastal environment, spraints are found most frequently on rocky shorelines, on well-marked sites at the mouths
of rivers, on otter runs, or, at certain times of the year, at the entrance to holts [
Spraints were collected for analysis and stored in labelled plastic bags. In the laboratory, mucus was removed by
soaking for 48 hours in Co-op false teeth cleansing solution, and the
sample was then washed and passed through a 0.5 millimetre sieve. This method has been used by
many workers, including Erlinge [
The bulk of the prey remains consisted of fish bones, with occasional bird feathers, rabbit hair, and small mammal bones. The prey was identified by comparing the fish vertebrae
with a reference collection and/or published keys [
No method of expressing the results of spraint analysis describes the diet of
otters accurately [
(a)
(b)
These methods, however, have important limitations, which will be considered in the discussion.
In this paper, the results of spraint analysis are presented by percentage frequency of occurrence and are shown in Table
Occurrence of prey items in otter spraints collected, expressed as percentage frequency of occurrence.
Prey items | Skye | Pabay | Barra | Eigg | Tiree | Coll |
---|---|---|---|---|---|---|
N | 1,480 | 15 | 20 | 139 | 12 | 20 |
Blenny-Viviparous | 29.2 | 22.6 | 20.0 | 5.0 | 32.0 | 23.0 |
( | ||||||
Rockling | 18.1 | 19.2 | 23.2 | 12.0 | 15.3 | 23.0 |
( | ||||||
Butterfish | 10.0 | 9.1 | 8.5 | 12.0 | 14.5 | 10.2 |
( | ||||||
Saithe | 10.8 | 10.9 | 12.3 | 2.0 | 5.0 | 10.0 |
( | ||||||
Sea Scorpion | 5.5 | 3.6 | 2.4 | 11.0 | 0.0 | 5.2 |
( | ||||||
Flatfish | 5.0 | 6.4 | 12.0 | 15.0 | 8.2 | 10.4 |
( | ||||||
Common Eel | 3.6 | 4.3 | 7.5 | 5.0 | 13.1 | 3.2 |
( | ||||||
Shore Crab | 3.6 | 2.7 | 1.0 | 9.0 | 0.0 | 2.0 |
( | ||||||
Conger Eel | 3.3 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 |
( | ||||||
Sand Eel | 2.2 | 0.0 | 0.0 | 0.0 | 0.0 | 1.0 |
( | ||||||
Sea Snail | 1.3 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 |
( | ||||||
Frog | 1.3 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 |
( | ||||||
Birds | 1.1 | 3.5 | 1.6 | 18.0 | 0.0 | 3.4 |
Sea Stickleback | 1.1 | 0.0 | 0.0 | 1.0 | 0.0 | 0.0 |
( | ||||||
Angler | 0.8 | 0.0 | 5.0 | 0.0 | 4.2 | 0.0 |
( | ||||||
Gobies | 0.5 | 0.0 | 0.0 | 1.0 | 0.0 | 0.0 |
( | ||||||
Small mammals | 0.9 | 2.4 | 6.4 | 0.0 | 0.0 | 0.0 |
( | ||||||
Sea Trout | 0.7 | 0.0 | 0.1 | 1.0 | 0.0 | 0.0 |
( | ||||||
Lumpsucker | 0.5 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 |
( | ||||||
Rabbit | 0.0 | 6.5 | 0.0 | 8.0 | 7.7 | 9.5 |
( | ||||||
Unknown | 0.5 | 8.8 | 0.0 | 0.0 | 0.0 | 0.0 |
Studies of the diet of coastal otters have been undertaken in Shetland [
Comparing principal prey taxa consumed by otters in four areas of Scotland and Norway. Data from Skye and Mull are from spraint analysis using percentage frequency of occurrence. Data
from Norway and Loch Broom were analysed using relative frequency of occurrence. Shetland data were from direct
observation.
Species | Skye | Mull | Loch Broom | Shetland | Pabay | Barra | Eigg | Coll | Norway |
---|---|---|---|---|---|---|---|---|---|
N Occurrences | 1,480 (4,588) | 958 | 50 | 2028 | 15 | 20 | 139 | 20 | 1074 |
30.5 | 0 | 8.6 | 33.8 | 22.6 | 20 | 5 | 23 | 7.9 | |
0 | 5.1 | 10.7 | 0.1 | 0 | 0 | 0 | 0 | 0 | |
Gadidae | 28.4 | 17.1 | 15.7 | 25.1 | 30.1 | 35.5 | 14 | 33 | 17.5 |
10.8 | 25.8 | 11.4 | 9.9 | 9.1 | 8.5 | 12 | 12 | 12.4 | |
Cottids | 5.9 | 11.8 | 2.1 | 17.5 | 3.6 | 2.4 | 11 | 5.2 | 12.8 |
Flatfish | 4.4 | 3.1 | 7.9 | 1.9 | 6.4 | 12.0 | 15 | 10.4 | 11.4 |
3.5 | 7.3 | 8.6 | 0.2 | 4.3 | 7.5 | 5 | 3.2 | 1.6 | |
Crab | 3.5 | 7.3 | 17.9 | 0.2 | 5.8 | 1 | 9 | 2 | 1.6 |
3.3 | 0.2 | 0 | 0 | 2.4 | 0 | 0 | 0 | 0 | |
0.8 | 0 | 2.1 | 0.2 | 1.7 | 0 | 1 | 0 | 7.7 | |
0.7 | 2.6 | 2.1 | 0 | 0 | 0 | 1 | 0 | 4.9 | |
Salmonidae | 0.5 | 0.1 | 2.1 | 0 | 0 | 0 | 0 | 0 | 2.0 |
Other | 7.7 | 19.6 | 10.8 | 11.1 | 14.0 | 13.1 | 27 | 11.2 | 20.2 |
Spearman rank correlation coefficients (rs) comparing data from Skye, Mull, Loch Broom, Shetland, and Norway.
Mull | Loch Broom | Shetland | Pabay | Barra | Eigg | Coll | Norway | |
---|---|---|---|---|---|---|---|---|
Skye | Ns | ** | *** | *** | *** | ** | *** | * |
Mull | Ns | Ns | Ns | Ns | * | Ns | Ns | |
L. Broom | * | ** | * | * | ** | Ns | ||
Shetland | *** | *** | * | *** | ** | |||
Pabay | *** | ** | *** | * | ||||
Barra | ** | *** | * | |||||
Eigg | ** | * | ||||||
Coll | * |
ns = not significant (
Spearman rank correlations were undertaken between these different
areas and the results are shown in Table
Significant correlations exist between most areas. Small differences did occur with more
rabbits (
The study identified the importance of five key prey species (viviparous blenny (
Significant correlations can be seen between the data from Skye, Shetland and Norway and from Shetland and Norway. There was also a significant correlation between Mull and Loch Broom.
Several differences have emerged in the composition of spraints between Skye and Mull, with
viviparous blenny dominating the diet on Skye, but being absent from the diet on Mull [
Based on the data, the estimated otter population for these islands has been achieved by working out a relationship between resident females and active holts and a relationship between resident
females and other otters. The estimate is however likely to be an underestimate because of otters living in inland freshwater
systems. From personal observations, I have seen otters in Loch
Suardal, Loch Coruisk, and other inland freshwater lochs on Skye.
However, work undertaken on freshwater
systems on Skye showed that all spraints from freshwater lochs had evidence of marine fish, indicating that otters using the freshwater systems are also utilising
the coast [
The results can be compared with those of Kruuk et al. [
Despite the difficulties of estimating otter numbers and the errors due to the above assumptions,
I am confident that these results give a reasonable estimate of otter numbers on these islands. The
figure is lower than that reported by Harris et al. [
Until now, little was known about the relationship between otter utilisation of the coast and geology.
On Shetland, Milner [
On Orkney, in sharp contrast to Shetland, there are relatively few otters (Kruuk, 1995). This
difference could possibly be attributed to distribution of prey species but it seems likely that
Orkney shores are just as productive as those of Shetland. J. Green and R. Green [
The study was carried out over eight years from 1995 to 2003, in 11 Scottish islands. The small size of the mesh of the sieve used for the preparation of the spraint for analysis ensured that all hard remains from commonly eaten prey were identified.
The problems of interpreting data from spraint analysis have been widely discussed in the
literature, and it appears that no single method provides a true picture of the importance of the
different constituents of the diet [
Percentage frequency of occurrence of prey remains is the most easily applied spraint
analysis method, and it has been used in numerous studies Mason and Macdonald [
The percentage frequency of occurrence relies on the presence of undigested hard parts;
therefore, both soft-bodied prey and large animals, where only the flesh is eaten, will leave no
remains in the spraints [
Rowe-Rowe [
Kruuk et al. [
The assumption that each occurrence of a prey item in different spraints represents a single
individual animal is not necessarily true; Carss [
In Shetland, Kruuk and Moorhouse [
Given these difficulties, percentage frequency analysis has the advantage of being relatively
quick compared to more complex and not necessarily more accurate methods of
analysis. (Counting all vertebrae and volumetric analysis have not been found to be more
accurate [
Studies of the diet of coastal otters have been undertaken in Shetland [
The author would like to thank the following people who took part in the surveys over the years and climbed boulders above and beyond the call of duty: Sara Calhim, Dick Collin, the late Alison Collyer, Peter Collyer, Tim Corke, Liz Cornish, Vera Cranmer, Sherry Fuller, Lorraine Gibson, Terry Gibson, Karen Goold, Alex Hardy, Helen Hardy, Margaret Hunter, the late Margaret Hutchinson, Sara Jupp, Tracey King, Dawn Mannerings, Justine Millard, Ruth Moffatt, Ray Morton, John Noorani, Alison Northeast, Clive Northeast, Andy Rothwell, Sylvia Sadler, Mary Taylor, Ruth Thomson, Lisbeth Tuckey, Barbara Wild, and Grace Yoxon.