OTTERS !! Takes abit of reading

[joffmiester]

AbstractThis study investigates the importance of avian species in the diet of Shapwick Heath's otter population. The frequency occurrence of prey items was recorded from spraints collected between July 2006 and January 2007. Bird feathers extracted from otter spraints were investigated under a microscope. Where possible, feathers were identified to species level. The presence of fish scales in spraints was recorded, but not quantified. Bird feathers were recorded in 41% of all spraints, compared with 4.6% on Slapton Ley in 1981 and 4.7% on the Somerset Levels in 1975. The highest proportion of bird feathers was recorded in July 2006, at 61%. Ralliformes were more frequently recorded than any other family of birds. Coot Fulica atra L. were more frequently recorded then any other species; 15 spraints were found to contain coot feathers. Mallard Anas platyrhynchos feathers were recorded in six spraints. Cormorant Phalacrocorax carbo feathers were found in three spraints and Water rail Rallus aquaticus feathers were found in two spraints. Pintail Anas acuta, teal Anas creca, moorhen Gallinula chloropus and little grebe Tachybaptus ruficollis feathers were found in one spraint each. Two spraints contained feathers from both mallard and coot, respectively. The high proportion of birds in the otter's diet, in comparison with past studies suggests that otters are substituting birds for part of their traditional diet of fish. It is recommended that studies into the nature of the fish stock and the development of the aquatic ecosystem on Shapwick Heath be undertaken.

Key words
Key words
Lutra lutraShapwick Heathdietralliformesspraintfeather
Previous SectionNext SectionIntroductionFeeding strategy
Previous studies1, 2 have suggested that otters predate on small, immobile fish species with a high calorific value and a low handling time in preference to large or highly agile prey. While cubs are learning to become proficient hunters, it has been suggested by Kruuk2 that cubs favour prey that is readily available, with the emphasis on ease of capture, rather than the calorific value or shortest handling time. Perhaps the most important factor affecting rates of predation is availability of food. Otters are commonly thought of as a species that predates solely on fish, but their diet includes a wide range of species. The otter is therefore a generalist carnivore that is able to capture prey as chance allows, rather than being a specialist piscivore as was commonly perceived. Chanin3 describes the differences in otter diet on Slapton Ley in Devon, 86 miles from the study site, being attributed to seasonal changes in the activity of the prey. The increased predation on birds, in the case of Slapton Ley in the spring, may be due to vulnerable fledglings and adult birds sitting on eggs, which are easier to catch than the fish.

The feeding habits of the Eurasian otter Lutra lutra L. have been extensively researched.1–8 These studies have often overlooked or ignored the importance of individual bird species as a component in the otter's diet. Chown9 suggested that the population of certain bird species, i.e. coot Fulica atra L. and dabchick Tachybaptus ruficollis Pallas, on Shapwick Heath National Nature Reserve (NNR) have not reached the numbers that ecologists would normally have expected in an area that is of national importance for wildfowl and wading birds. Chown9 implies that the poor reproductive success of bird species, in particular some Ralliformes, may be the result of learned behaviour, i.e. predation on birds, by the otter population.

The purpose of this study is to investigate the claim that the otters on Shapwick Heath predate on the resident wildfowl population to a higher degree than previously reported. It is intended to establish if certain species of birds are favoured for predation by otters and if any seasonal variation in the predation rates can be established.

Previous SectionNext SectionMethodsFor the purpose of this study, diet was assessed using the contents of otter faeces. The use of spraint analysis to determine otter diet is considered a viable method and has been used frequently before.2, 7, 8, 10–12 The methods for spraint identification have normally been used to identify the species of fish in otters' diet; the identification of bird feathers has previously only been undertaken to family level. Identification of bird remains has in the past been carried out on the stomach content of stoats13 and this has been adapted and expanded on to enable identification of birds to species level from otter spraints.

The study site
Shapwick Heath was created from flooded peat workings that have become a habitat well suited to wetland birds. Shapwick Heath is designated a Site of Special Scientific Interest (SSSI) and a Special Protection Area (SPA). It is part of the Somerset Levels Wetlands Ramsar Site and a National Nature Reserve. The site is owned and managed by Natural England. Shapwick Heath has 394 acres in area, consisting of a mixture of eutrophic open water habitat, reedbed, fenland and meadows.

Spraint collection
Otter spraints were collected on Shapwick Heath National Nature Reserve between July 2006 and January 2007. Spraints were collected from various sites along the main path that runs from east to west of Shapwick Heath Nature Reserve OS ST 449, 397 and OS ST 423, 411. The path allows year round access to the sprainting sites. Spraints were identified by their distinctive appearance, often containing visible feather and fish scales.

In addition to visual signs, spraints were identifiable by their characteristic smell.14 Spraints were only collected from areas showing evidence of use by otters, such as runs in the vegetation on the drain banks, which displayed otter footprints. Before collection began, all spraints were removed from the site so that a clear time-scale could be given as to when the faeces were produced.

Gloves were worn at all times when handling spraints to prevent infection. Spraints were stored individually in freezer bags that were labelled with: a record of the location of where the spraint was found, the date and comments on the condition of the spraint, i.e. its freshness. While collecting the spraints, the presence of wildfowl species on the reserve and those flying over it were recorded.

Spraint contents identification
After air-drying the spraints, they were placed in a solution of water and detergent (5:1) for a minimum of 2 h. Tweezers were used to help break up the spraints and feathers were removed from the detergent solution and washed in fresh detergent and water solution. The feathers were rinsed in water and then alcohol, before being left overnight to dry out; dry feathers being easier to prepare than wet ones.

Preparation of the feathers under the microscope was adopted from Day.13 A single barb was cut from the rachis using a scalpel, mounted on a slide and held with a drop of alcohol. To prevent the barbules from closing together and remain fanned out; a cover slip was aligned with the barbule from the proximal end to the distal end. Identification from a single barbule was not always possible. If this occurred, a second barb was removed and the process repeated, until identification could be made.

The barbules were examined using a binocular Nikon Labphoto 2 light microscope, and findings were cross-referenced with information from Day13 and from the Bird Remains Identification System (BRIS).15

Previous SectionNext SectionResultsA total of 87 spraints were analysed from Shapwick Heath from July 2006 through January 2007. They were collected and analysed using procedures outlined in the Methods. As the number of spraints found present per month varied, so the number analysed varied. The majority were collected from the bridge at British Ordnance survey grid reference ST 449, 397.

Due to the structure of the bridge, the spraints remained out of the water all year round and the bridge structure prevented them from being washed away by rain. The bridge at OS ST 423, 411 at Ashcott Corner was less useful as the water level inundated the ledges under the bridge, making it impassable for otters at times of peak flow. During low flows, waterfowl often rested on the ledge under the bridge and contaminated the otter spraints by defecating on them. For these reasons, the bridge at Ashcott Corner was often unsuitable for collecting spraints. A spatial analysis of the site was attempted. However, due to the nature of the collection site, an unbiased result for a spatial analysis of the site was not possible. The majority of the spraints were collected under bridges. This is likely to represent the amount of time that a spraint survives, rather than demonstrate a difference in the levels of predation in different areas of the reserve.

Figure 1 shows the monthly trend in prey items found in otter spraints. The remains of fish have been found more frequently than any other prey remains. In July 2006, the percentage of fish remains as same as bird feathers found in the otter spraints, being 60% and 61%, respectively. In August, the amount of spraints containing fish remains was consistent with July, amounting to 61%, but bird remains were found in only 19% of the spraints. The percentage of spraints with fish remains fell during August and September from 60% to 50% and the number of spraints containing bird feathers increased to 38%. The spraints collected in October were recorded as having the third largest percentage of fish remains in them, being 65% and the percentage of spraints that had feathers in them rose to 50%. November saw the percentage of spraints with fish rising again to 85%. The amount of bird feather remains in November declined, with only 32% of the spraints collected and analysed containing bird feathers. December saw the lowest recorded percentage of fish during the monitoring period, only 20% of the analysed spraints had evidence of fish scales, the remains of birds feathers was recorded as 50%. From the spraints collected in January 2007, there was evidence of fish scales in 67% of the sampled spraints, and 50% of the spraints had bird remains.


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Figure 1.
The percentage change of spraints content through time using data from 87 spraints, which were collected from July 2006 to January 2007 on Shapwick Heath. The number of spraints collected each month is shown in brackets. Percentage distribution is used to allow for the variation in the number of spraints collected throughout the study period.

Table 1 shows observed behavioural responses of wildfowl species in relation to otters. Black-headed gulls were observed displaying mobbing behaviour in the presence of otters. They were sighted hovering over a bed of Phragmites, following the progress of an otter that emerged from the reedbed. The mobbing behaviour continued after the otter had entered the water. Moorhens were observed in the backwater of the nature reserve in smaller areas of water that was well vegetated. Moorhens were also observed foraging on dry land on the reserve. Coots were observed foraging in large groups, often in 20 or more, in areas of open water in the Roughet and West Meare Heath. They were observed resting on the water edge in West Meare Heath. Cormorants were observed at the end of the day resting on protrusions from the water such as dead trees at the Roughet. Juvenile birds were often seen closer to the water than adult birds. Mute swans and herons were often observed within a few metres of an otter in the water at West Meare Heath.

[MrV]

OTTERS !! Takes abit of reading

You weren't joking