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Paul Yoxon BSc Hons PhD CBiol MIBiol
Grace M Yoxon BA Hons
Aknowledgements
We would like to thank the following people who took part in the survey and climbed boulders above and beyond the call of duty.
The Survey Team
Lorraine Gibson
Terry Gibson
Alex Hardy
Helen Hardy
Ray Morton
John Noorani
Alison Northeast
Clive Northeast
Some of the survey team
Coll lies some 50 miles from Oban, and extends to about 13 miles in length and 3 miles across. It is composed of Lewisian gneiss and is intersected by Tertiary dolerite dykes. The island is low lying with the highest point on the island being Ben Hogh (104m).
Otters were known to be present on Coll: an afternoon visit 12 years ago revealed spraint at two locations (personal observation)and they are also mentioned in the Guidebook to the Hebridean island of Coll, (Lunghi, M, 2001). However no detailed survey has ever been undertaken and the island (along with Tiree) was omitted from the Vincent Wildlife Trust surveys in 1977, 1984 and 1994. (Green, G and Green R, 1980, 1987 & 1997), when they undertook the National Otter Surveys for the UK government.
METHODOLOGY
For the purposes of the survey the Coll coastline was divided into 162 sections of 500m in length measured at the High Water Mark. Personal experience had shown that such sections are easily manageable for fieldwork and are generally long enough to observe otter activity from a set vantage point without losing a part of the section around a corner or blocked off by a hill.
The NW coastline and SE coastline were analysed separately. The NW coastline was defined as the area from Calgary Point to the west of the island to Garbh Aird Bheag to the east. Some of the freshwater lochs were also looked at with regard to otter activity.
Isle of Coll
Over the years the surveys we have carried out have examined many variables and there is a lot of data on shore type, inland vegetation and slope of the shoreline but for future surveys we will only be concentrating on otter activity and secondary signs. Therefore in this report the following factors were examined:
a) Otter activity
b) Number of major holts, minor holts and lie-ups
c) Number of freshwater pools
a) Direct observations of otters (with telescope and binoculars)
Observations were made from a vantage point in each 500m section before a search of the coastline to count spraints and look for the freshwater pools. During this observation, the coastline was scanned using 10X40 binoculars and a X30 telescope.
b) Number of freshwater pools
A freshwater pool was defined as an area of standing water with a minimum width or length of 0.5m and minimum depth of 0.1m. It was found from personal observation that otters show no signs of utilising pools with dimensions less than this. All freshwater pools, which met these criteria and had associated sprainting points were counted.
c) Numbers of holts
Otter holts are very variable and for the purpose of this study they were divided into three distinct types.
Holts: Defined as a tunnel system with signs of regular use by otters (Kruuk et al, 1989). The evidence of use consisted of tracks, flattened grass, fresh spraints and the characteristic odour in the holt. On Skye, holts consist dominantly of rock piles within 50m of the shore. If two holts were more than 10m apart they were considered separate.
Lie-ups: Otters use resting places throughout their home range and these are mostly hollows under rocks or within reed/rush beds. They have a path leading to them and a sprainting point associated with them.
For the purpose of analysis only the holts were included in the count of holt numbers.
RESULTS
A total of 73 coastal sections were examined accounting for 44% of the coastline, together with the following freshwater lochs:
Loch Urbhaig (NM 232:578)
Loch Ronard (NM 203:555)
Loch na Easgannan (NM 117:525)
Lochan a’ Chuirn (NM 119:524)
Loch Airigh Meall Bhreide (NM 215:565)
Loch a’ Gharbh-airde (NM 215:558)
Otter Sightings
A total of 9 otters were observed during the survey at the following locations:
Sunday 4 August: Bagh Craimneach (NM180:590)
Cliad Bay (NM199:607)
Monday 5 August: Sorisdale Bay (NM 275:633)
Traigh Bhousd (NM 255:642)
Wednesday 7 August: Eilean Tomulam (NM146:520) [male and female]
Traigh Bhousd (NM 255:642)
Thursday 8 August: Eilean nam Muc (NM245:574)
Eilean Tomulam (NM146:520)
The otter activity in relation to the NW and SE coastline is shown in Table 1. A Chi squared test was undertaken to test whether the differences in the otter activity on the NW and SE coastlines could be attributed to chance. There was a highly significant difference between the two coastlines (c2 = 21.8, df = 2, p<0.001), the SE coastline having the highest density of otter sightings, holts and freshwater pools.
Table 1. Otter activity in relation to the NW and SE coastlines of Coll.
|
Otter Activity per 500m section |
NW Coastline (n=24) |
SE Coastline (n=49) |
|
Otter sightings |
0.2 |
0.1 |
|
Major holts |
0.25 |
0.49 |
|
Freshwater pools |
1.2 |
2.8 |
There was a high correlation between the holts and freshwater pools on Coll (rs = 0.5, df=72 p<0.001). This compares with similar data from the Isle of Skye, Barra, Canna and Shetland.
Estimating Otter Numbers
On Shetland there was a good correlation between otter numbers and holt numbers (Moorhouse, 1988, Kruuk et al, 1989), enabling the number of holts to be used as an index for the number of otters. The same was true on Skye (Yoxon, 1999). As no detailed work over a long period has been undertaken on Coll, relationships from Shetland and Skye were used to estimate otter numbers.
The relationship on Shetland from intensive work over many years was:
0.33 females per holt
Total Otters = 1.83 X number of resident females
The relationship on Skye on four study areas over four years was:
0.52 females per holt
Total Otters = 1.64 X resident females
An estimate can be made for the total number of coastal holts on Coll: the NW coastline estimate is 17.6 holts and the SE is 44.8 holts giving a the total number of holts for Coll of 62.4. Using the two relationships above the total number of otters on Coll lies between 38 and 53.
DIET ANALYSIS USING SPRAINTS
In the coastal environment, spraints are found most frequently on rocky coasts, on well-marked sites at the mouths of rivers, on otter runs or, at certain times of the year, at the entrance to holts (Conroy and French, 1985, 1987). The droppings add nitrogen to the surrounding area making sprainting points very obvious: grassy areas tend to be very green with a prominent growth of nitrophylous grasses. Rocks may be covered by green algae, and some sprainting areas on the coast are without vegetation exposing soil as a result of the continuous addition of urine (Conroy et al, 1993).
Analysis of spraints
Spraints were collected for analysis and stored in labelled plastic bags. In the laboratory, mucus was removed using Co-op false teeth cleansing solution (Composition greatest first: sodium chloride, sodium percarbonate, trisodium phosphate, magnesium carbonate, methol, peppermint oil and sodium lauryl sulphate). The samples were soaked in the solution for 48 hours then washed and sieved using a 0.5mm sieve. This method has been used by many workers including Erlinge (1967, 1968); Watson, (1978) Beja, (1995) and Yoxon (1999).
The bulk of the prey remains consisted of fish bones and rabbit hair, with occasional bird feathers and small mammal bones. The prey was identified by comparing the fish vertebrae with a reference collection and also compared with the published keys of Watson (1978) and Conroy et al (1993).
No method of expressing the results of spraint analysis describes the diet of otters accurately (Wise et al, 1981; Kyne et al, 1989; Rowe-Rowe, 1977; Jenkins et al, 1980; Heggberget, 1993; Carss, 1996); however, the two most commonly used methods are described by Conroy et al (1993).
a) Percentage Frequency of Occurrence, where all the remains of each species in a spraint are taken to represent only one specimen and its frequency is calculated as the number of times it appears in all spraints in the sample.
It is calculated as:
Number of spraints containing a particular prey item
X 100
Total number of spraints in sample
b) Relative Frequency of Occurrence, where all the bones are identified in every spraint, and the frequency of each species in that spraint determined.
It is calculated as:
Number of occurrences of a prey in each spraint
X 100
Total number of all prey items identified in the spraint
These methods, however, have important limitations, which will be considered in the Discussion.
The results of the Coll spraint analysis are presented here by Percentage Frequency of Occurrence.
Results
Ten spraints were analysed including two from freshwater lochs yielding ten
prey types. The results are shown in Figure 2. 
Marine fish dominated the diet (91%), with the most common prey species being Viviparous Blenny (23%), Five-Bearded Rockling (23%), Saithe (10%), Flatfish (10%) and Sea Scorpion (5%). Rabbit consisted of 10% of the diet with a small amount of evidence that the otter had taken birds. The two freshwater spraints contained dominantly marine species, Saithe, Five-bearded Rockling and Flatfish, indicating that the otters using the freshwater systems are also feeding on the coastal fringe.
Comparisons with other studies
Studies of the diet of coastal otters have been undertaken in Norway (Heggberget and Moseid, 1984), Loch Broom (Mason and Macdonald, 1980), Mull (Watt, 1995), Skye (Yoxon, 1999) and Shetland (Kruuk et al 1987; and Kruuk and Moorhouse, 1990) and the data present some interesting comparisons.
A significant correlation existed between Coll and Skye (rs = 0.95, df=8 p<0.001), with similar correlations between Loch Broom, Shetland and Mull.
The important conclusion once again is the importance of the five key prey species (Viviparous Blenny, Five-Bearded Rocking, Butterfish, Sea Scorpion, and Saithe) in all of the NW Scottish Islands but on Coll Flatfish also seem to be important.
DISCUSSION
Based on the data, an estimate of 62.4 major holts can be given for the Coll coastline. The estimate for coastal otters on Coll is derived by working out a relationship between resident females and active holts and a relationship between resident females and other otters, and thus the Coll island population is estimated to be between 38 and 53. This estimate is based on the numbers present at a particular point in time plus births, minus deaths, plus immigrants, minus emigrants. Stubbs (1977) has shown that animals from permanent habitats like the vertebrates tend to show an under-compensating or exactly- compensating mortality, and this would clearly be relevant when considering the Coll population. The estimate given here is, however, likely to be an underestimate because of otters living in inland freshwater systems.
The figure of 38-53 is based on this survey of 44% of the coastline and the following assumptions:
1. 44% of the Coll coastline is a fair representation of the entire Coll coastline. The sampling of 44% is very high compared with the 35% achieved in studies on Shetland (Kruuk et al, 1989), (Jolly, 1969).
2. All otter holts were found during the survey. Some holts may have been overlooked due to the vegetation and in places difficult terrain of the area surveys and this would have underestimated holt numbers.
3. The total number of otters in the study areas was estimated correctly. We do not know if the same relationship exists between resident females to holts on Coll but we can assume this to be the case as it has already been shown in Shetland and Skye. The Skye data may be assumed to give a better figure as the Lewisian gneiss is found on Skye and Coll and therefore this would lean towards the higher figure.
REFERENCES
Beja, P.R. (1991). Diet of otters (Lutra lutra) in closely associated freshwater-brackish and marine habitats in south west Portugal. Journal of Zoological Society of London, 225, 141-152.
Carss, D.N. (1996). Errors associated with otter (Lutra lutra,) faecal analysis. Assessing general diet from spraints. Journal of the Zoological Society, London, 238, 301-3 17.
Conroy, J.W.H. and French, D.D. (1985). Monitoring otters in Shetland. Shell Oil Terminal report.
Conroy, J.W.H. and French, D.D. (1987). The use of spraints to monitor populations of otters (Lutra lutra). Symposium of the Zoological Society of London, 58, 247-262.
Conroy, J.W.H., Watt, I., Webb, J.B. and Jones, A. (1993). A guide to the identification of prey in otter spraint. Occasional publication 16, The Mammal Society, London.
Erlinge, S. (1967). Home range of the otter (Lutra lutra) in southern Sweden. Oikos, 18. 379-97.
Erlinge, S. (1968). Territoriality of the otter (Lutra lutra). Oikos, 19, 8 1-98.
Green, J and Green, R. (1980). Otter Survey of Scotland 1977-1979 Vincent Wildlife Trust. London.
Green, J and Green, R. (1987). Otter Survey of Scotland 1984-1985 Vincent Wildlife Trust. London.
Green, J and Green, R. (1997). Otter Survey of Scotland 1991-1994 Vincent Wildlife Trust. London.
Jenkins, D., Walker, J.G.K. and McCowan, D. (1979). Analysis of otter (Lutra lutra) faeces from Deeside, north east Scotland. Journal of the Zoological Society of London, 187, 235-44.
Jolly, G. M. (1969). The treatment of errors in aerial counts of wildlife populations. East African Agriculture and Forestry Journal, 34, 50-55.
Harris, S., Morris, P., Wray, S. and Yalden, D. (1995). A review of British mammals: population estimates and conservation status of British mammals other than cetaceans. JNCC Peterborough.
Heggberget, TM. (1993). Marine feeding otters (Lutra lutra) in Norway: Seasonal variation in prey and reproductive timing. Journal of the Marine Biological Association, 73, 297-312.
Kruuk, H., Moorhouse, A., Conroy, J.W.H. and Durbin, L. (1989). An estimate of numbers and habitat preferences of otters (Lutra lutra) in Shetland. Biological Conservation, 49, 241-254.
Kruuk, H. and Balharry, D. (1990). Effects of sea water on thermal insulation of the otter (Lutra lutra). Journal of the Zoological Society of London, 220, 405-4 15.
Kyne, M. J., Small, CM. and Fairley, I.S. (1989). The food of otters (Lutra lutra) in the Irish midlands and a comparison with that of mink (Mustela vison). Proceedings of the Royal Irish Academy, 89B, 33-46.
Lovett, J.L. (1994). Utilisation of freshwater pools by the Eurasian otter in a marine environment. MSc Thesis, Aberdeen University.
Lovett, J.L., Kruuk, H. and Lambin, X. (1997). Factors influencing the use of freshwater pools by otters (Lutra lutra) in a marine environment. Journal of the Zoological Society of London, 75, 825-83 1.
Lunghi, M. (2001). Eilean Colla pdf Guide to the Isle of Coll. Toad Wine Press
Moorhouse, A. (1988). Distribution of holts and their utilisation by the European otter (Lutra lutra) in a marine environment. MSc Thesis, University of Aberdeen.
Pond, C.M. and Mattacks, C.A. (1985). Anatomical organisation of mammalian adipose tissue. Fortschritte der Zoologie, 30, 445-448.
Priestley, S.H. (1996). The salinity of water pools used for bathing by the Eurasian otter (Lutra lutra). MSC thesis. DICE.
Read, P. and Watson, J. (1972). Beginning Geology. Macmillan and Co.
Rowe-Rowe, D.T. (1977). Food ecology of otters in Natal, South Africa. Oikos, 28,2 10-219.
Stubbs, M. (1977). Density dependence in the life cycles of animals and the importance in k- and r- strategies. Journal of Animal Ecology, 46, 677-688.
Tarasoff, F.J. (1974). Anatomical adaptations in the River Otter, Sea Otter and Harp Seal with reference to thermal regulation. In Functional Anatomy of Marine Mammals, 2 (ed. R.J. Harrison), 111-142. Academic, London,
Van der Zee, D. (1981). Prey of the Cape Clawless Otter (Aonyx capensis) in the Tsitsikama Coastal National Park, South Africa. Journal of the Zoological Society of London, 194, 467-483.
Watson, H. (1978). Coastal otters (Lutra lutra) in Shetland. Vincent Wildlife Trust. London.
Wise, M.H., Linn, I.J. and Kennedy, C.R. (1981). Comparisons of the feeding biology of mink (Mustela vison) and otter (Lutra lutra). Journal of the Zoological Society of London., 195, 181-213.
Yoxon, P. (1999). The effect of geology on the distribution of the Eurasian otter (Lutra lutra) on the Isle of Skye. Phd Thesis, Open University, Milton Keynes.
