OTTER SURVEY OF PABAY

Paul Yoxon BSc Hons PhD
Grace M Yoxon BA Hons

An Otter Cub

Before you read this very detailed survey here are some brief comments made by Dr.Yoxon in a recent Email:

From work elsewhere the female otters have the territory and if the habitat is OK they occupy about 3 to 4 km of coastal zone with males wondering some 30 to 40km. Since you have about 4 to 6 km of coast the population of 3 to 5 would probably be about normal. with one or two females, some juveniles and males.

I noted about 8 years ago when I was on Pabay that the droppings contained much evidence of rabbit hairs and it would seem to be the same today. We have also seen otters from the Arnish Peninsula swimming across to Pabay, so during dispersion they are obviously using this route. All in all Pabay has a healthy viable otter population probably doing quite well.

Now here is the full survey. Why not download it and read it a your leisure.

Pabay Otter Survey 23 August 2000

This report deals with a survey carried out on the isle of Pabay in August 2000; it is aimed at obtaining data on distribution and population density, to look at what factors influence the distribution of the otter population around the coastline. The Isle of Pabay is situated on the northwest seaboard of Scotland, lying in Broadford Bay. It is about 2km long by 1.5km wide, it is composed of Lower Jurassic sedimentary rocks.

METHODOLOGY

 For the purposes of my research the Pabay coastline was divided into 9 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 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. Two sections on the watercourses inland where also look at giving a total of 11, 500 metre sections. 100 % of the total Pabay coastline was looked at and approximately 42% of the inland.

 In this report the following factors were examined

 a) Width of tidal zone

b) Height of point 25m from High Water Mark

c) Coastal type

 d) Inland vegetation

 e) Number of Sprainting Points

f) Otter Activity

g) Number of holts

h) Number of freshwater pools

 

a) Width of Tidal Zone

The width of the tidal zone was measured using the maximum width over the 500m sections. It was measured using the Ordnance Survey 1:25,000 map. Since the maximum width is used in all sections it will give a standard benchmark for the width of the intertidal zone.

b) Height of point 25m from High Water Mark

The height of the point 25m from the High Water Mark was used as an indicator of the general slope of the shoreline and measured from the 1:25,000 maps. The reading was taken from the centre of the 500m sections at the relevant contour line nearest this 25m mark.

c) Intertidal Make up

The intertidal coastal zone was divided into six shore types shown below. Excluding the rock outcrop all are related to grain size as shown in Table 2.3. Examples of different shore types are shown in Plate 2.1. The shore type was estimated with the eye and by looking at samples on the shore. The shore type chosen for the 500m section was that which occupied the largest part of it (usually >50%). The maximum diameter of the boulder was measured using a pocket tape measure.

The table shows of different intertidal coastlines used in the study. These are standard definitions based on Gore (1997), Read (1972), and Blitt et al (1972).

Category Grain Size (Minimum) Grain Size (Maximum)
Muddy None 0.0625mm
Sandy 0.0625mm 2mm
Shingle 2mm 64mm
Boulder<20cm 64mm 200mm
Boulder>20cm 200mm None

 

Muddy

I used the definition of mud given by Gore (1997), which describes it as a mix of silt and clay and defines "mud" as a substrate containing 15% sand, 45% silt and 40% clay. Clay particles are defined as any particle less than 0.0625mm in diameter.

Muddy shores require sheltered conditions to form, and in places vegetation is dominated by the red algae Phyllophora crispa, and in the sublittoral zone by Laminaria saccharina and Chorda filum.

Sandy:

Sand grains are by definition between 0.0625mm and 2mm in diameter (Read, 1972). The definition of sand usually means quartz sand, but other sands are present on Skye following the breakdown of basalt lavas. Feldspars, olivine, pyroxene, mica and shell fragments can also be mixed with quartz to form a sand.

Sandy coasts have poor vegetation communities and are too mobile to support algae. Isolated patches of Laminaria saccharina were present with beds of Zostera marina in the littoral zone.

Shingle:

Pebble, gravel or shingle deposits range in grain size from 2-64 mm (Blatt et al, 1972). Shingle is a mixture of material accumulated on beaches or offshore bars. The shingle shores make up one of the poorest maritime habitats. Thick beds of Phymatolithon calcareum and Lithothamnium corallioides occurred on some shores and in places Laminaria saccharina was present in the sublittoral zone.

Boulder:

Boulder shorelines were classified as those with individual particle sizes ranging from 64 mm upwards and consisted of eroded country rocks and glacial material.

 

They were divided into two size groups as follows:

i. > 20 cm

ii. < 20 cm

The littoral region was dominated by Ascophyllum nodosum and Fucus serratus on the sheltered coasts and Fucus vesiculosus on the more exposed areas. In the sub littoral zone the encrusting algae Lithothamnium corallioides became common.

Rock Outcrop:

When the dominant coastal type was bare rock, this category was used to define the intertidal zone. Rock outcrop is gently sloping wave-cut platform as shown by many of the Mesozoic sedimentary rocks.

The communities on the rocky coastlines consisted of Ascophyllum nodosum and Fucus serratus in the littoral region with the sublittoral region dominated by Laminaria saccharina and Laminaria digitata on more sheltered coasts replaced by Laminaria hyperborea and Fucus vesiculosus on exposed coasts.

One of the above categories was allocated for each 500m coastal section. When more than one category was present in the zone, the dominant category in that 500m coastal zone was allocated. The "boulder" category was further divided into boulders greater or less than 20cm. A random sample of boulders were measured and again the dominant boulder size was recorded.

 

d) Inland Vegetation

Inland vegetation was recorded 20m from High Water Mark and the categories used are shown below.

Heather:

The heather moors are dominated by Ling (Calluna vulgaris) although this is not the only heather species present: Bell Heather (Erica cinerea) and Cross-Leaved Heath (Erica tetralix) occur frequently with Ling. Heather grows on well-drained soils, but hollows occur with grasses, sedges and rushes in these waterlogged areas.

Scrub:

Scrub habitat consisted of a mixture of plants including Willow (Salix sp), stunted Birch (Betula pendula), Bramble (Rubus sp) and Bracken (Pteridium aquilinum).

Grassland:

These are areas that are extensively grazed by sheep and cattle. The pastures have been modified by cultivation and draining and the grasslands have communities of tall herbs associated with the production of hay.

Native Wood:

Native woods are scarce on Barra, and are restricted to gorges and small coastal areas out of the reach of grazing animals and fire. The woods are dominated by Birch (Betula pubescens), and Hazel (Corylus avellana), with some Rowan (Sorbus aucuparia), Holly (Ilex aquifolium) and Oak (Quercus petraea). Many of the Skye woodlands have a remarkable bryophyte and fern flora, which is of international importance.

e) Numbers of Sprainting Points

Sprainting points were defined as places where one or more spraints or piles of spraints were found. 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.

f) Direct observations of otters (with telescope and binoculars)

Observations were made from a vantage point for 30 minutes 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.

g) 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, were counted and measured and it became evident from the survey that most of these pools had sprainting points adjacent to them but pools without sprainting points were also counted and measured.

h) 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, I found holts consisted 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

Environmental variables

The volunteers carried out a survey of 9 x 500m coastal sections and two inland sections. 89% of the adjacent inland vegetation was grassland with 11% being heather moorland. The dominant intertidal make up was Rock outcrop (56%) , with boulder accounting for 33% and shingle 11%.

Intertidal widths where large on Pabay ranging from 1.1km on the west side of the island to 200m on the east side. Heights also ranged from 2m on the west to 15m on the east.

Otter Activity

The otter activity in relation to the east and west did vary considerably with more pools and sprainting points occurring to the west of the island, with large amounts of activity to the north of the steading. No holts where found on the coastal fringe but two where located inland on the water system. Difficulty did occur here with regard to access because of the height of the vegetation.

 

Estimating Otter Numbers

Otters where not seen on Pabay but they have been observed regularly to the area to the north of the steadings.

In 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). Because no detailed work over a long period has been undertaken on Pabay, Shetland and Skye relationships where used to estimate otter numbers.

The relationship on Shetland on 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 4 study areas over 4 years was:

0.52 females per holt

Total Otters = 1.64 X resident females

On Pabay we counted 2 holts inland from 2 500m square sections out of a potential 5. We would therefore have an estimated total holt number of 5.

Using this data for all section we get a Pabay otter population of 3 based on the Shetland relationship and 4.2 based on the Skye relationship.

 

 

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

Only fresh spraints were collected for analysis and stored in labelled plastic canisters. 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) and Beja, 1995.

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.

I have presented the results of spraint analysis by Percentage Frequency of Occurrence.

Remains and is the only free-swimming species as the other four are small benthic fish. Small mammals played a small part (5%) and seabirds (1.5%).).

 

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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.

Blat, H., Middleton, G. and Murray, R. (1972). The Origin of Sedimentary Rocks. Prentice Hall.

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.

Gore, P. (1997). Sedimentary Geology Lecture. World Wide Web.

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 wildftfe populations. East African Agriculture and Forestry Journal, 34, 50-5 5.

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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.

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Kyne, M. J., Small, CM. and Fairley, iS. (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.

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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.

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