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Paul Yoxon BSc Hons PhD CBiol MIBiol
Grace M Yoxon BA Hons
International Otter Survival Fund, Broadford, Isle of Skye, IV49 9AQ, Scotland
Tel: 01471 822 487. email: info@otter.org. Web: www.otter.org
Phot: Tim Corke
Acknowledgements
We would like to thank the following people who took part in the survey: Tim Corke, Liz Cornish, Alison and Phillip Collyer, Alison and Clive Northeast and Ruth Thomson.
We would also like to thank the people of Tiree for their help, interest and information on otter sightings. It seems that although the actual number of otters on the island is low they are frequently seen by local people and visitors, especially as one home range is actually outside the local Co-op store.
Tiree is the outermost of the inner Hebrides and like most of the Outer Hebrides it is composed of a rock called Lewisian gneiss. The island measures 24km in length and varies in width between 2-13km and it has some of the finest areas of machair land of all the Hebrides. The population of the island reached a peak of 4,450 in 1831, but this dropped to 2,700 in 1846 due to the potato famine. Today the population is about 600.
Otters were known to be present on Tiree but the distribution and population numbers were not known. No detailed survey had ever been undertaken and the island (together with the neighbouring island of Coll) was omitted from the National Otter Surveys carried out by the Vincent Wildlife Trust in 1977, 1984 and 1994. (Green, G and Green R, 1980, 1987 & 1997).
METHODOLOGY
In order to carry out the survey, the Tiree coastline was mapped for secondary otter evidence - sprainting points, spraints, freshwater pools, lie ups and holts (defined below). Time was also spent looking for otters with binoculars and telescopes. Some of the inland freshwater lochs were also studied for signs of otter evidence.
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 throughout the course of the survey and 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. Personal observation has shown 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, but in other areas they can also be holes in the ground, under trees, etc. 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
The entire coastline of Tiree was mapped and the coastal areas used by otters are shown in map 1, together with the following freshwater lochs: Aulaig, Raighain, Phuill and Bhasapool.
Otter Sightings
A total of five otter sightings were recorded during the survey at the following locations:
Sunday 3 August: Lon Fhadamuill (NM0756/4967) Mother and cub.
Monday 4 August: Ard Mor (NL 9949/4732) Old male
Wednesday 6 August: Rubh Dubh (NM0420/9370) One otter
Thursday 7 August: Lon Fhadamuill (NM0756/4967) Brief view one otter.
Otter activity was confined to the rocky areas around the coast with the main beaches showing little evidence of otters, although there was a little evidence of activity in the dunes. Freshwater pools were abundant throughout the coastline but the island lacked suitable holts with only a total of nine major holts being counted around the island.
Map 1. Otter activity (shown in red) along the Tiree coastline
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 Tiree, 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
The total number of holts found on Tiree was nine, and using the above relationships it gives a total adult otter population of 6-8 otters.
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 Tiree spraint analysis are presented here by Percentage Frequency of Occurrence.
Results
Twelve spraints were analysed including two from freshwater lochs at Aulaig and Fhaing, and they yielded eight prey types. The results are shown in Figure 1.
Marine fish dominated the diet (80%), with the most common prey species being Viviparous Blenny (32%). Rabbit consisted of 8% of the diet. 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.
Figure 1. Diet of otters from Spraint analysis.
DISCUSSION
Based on the data, nine major holts were identified on the Tiree coastline. The estimate for coastal otters on Tiree 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 Tiree island population is estimated to be between 6-8. 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 Tiree population.
The figure of 6-8 is based on this survey of 100% of the coastline and the following assumptions:
1. All otter holts were found during the survey. Some holts may have been overlooked due to the vegetation and this would have resulted in an underestimate of holt numbers.
2. 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 Tiree but we can assume this to be the case as it the relationship has already been shown to be similar in Shetland and Skye. The Skye data may be assumed to give a better figure as the Lewisian gneiss is found on Skye and Tiree and therefore this would lean towards the higher figure.
The Tiree Eurasian otter population is relatively low and compares with low numbers on islands like Barra. The reasons for this are likely to be the lack of suitable holt areas, where the female can bring up cubs in a coastal home range. There is a possible migration of otters between Coll and Tiree and Tiree and Coll across Gunna island which is only 1km from Tiree and less than ½ km from Coll.
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