The Water Shrew Neomys fodiens is the largest of the shrews found in Britain, measuring around 170mm and weighing between 12 and 18 g, with pregnant females often reaching up to 28 g. The main identifying features include the distinctive dark/black dorsal fur and pale white/silvery underbelly, and there are usually white tufts of fur on the ears and white hairs around the eyes (Carter and Churchfield, 2006). Occasionally adult shrews with brown fur or even completely black fur all over have been recorded and pale, gingery specimens have been recorded in Northumberland (Kevin O’Hara, pers. comm., 2012).
All shrews can swim, but the Water Shrew is the only British shrew currently known to forage mainly underwater. In the wild most dives are to depths of between 30 and 200 cm (Schoelth, 1980; Churchfield, 1998). Whilst swimming, the velvety fur of the Water Shrew traps thousands of tiny air bubbles which makes the fur appear silvery whilst underwater and provides vital insulation. It has a very distinctive fringe of tiny silvery hairs under the tail and on the margins of the feet, which aid it in swimming. The main prey of Water Shrews is underwater invertebrates although they are known to eat terrestrial invertebrates such as earthworms and beetles. It has venomous saliva making it the only venomous British mammal.
Water Shrews appear to be particularly associated with fast flowing, unpolluted rivers and streams but they will use a wide range of wetland types. Their burrows in the bank sides are approximately 2 cm in diameter; the size of the burrow is important because Water Shrews use the burrows to squeeze excess water from their fur after swimming before grooming themselves. A maximum of 3.2 per hectare have been recorded in water-cress beds in southern England but this is probably an underestimate (Churchfield, 1984). They usually have a home range of between 20-30 metres² on land and 60-80 metres² in water (Illing et al, 1981).
It is a very difficult species to survey due to its secretive behaviour and discrete field signs. Its specialised habitat means that it is not encountered in small mammal trapping as frequently as the other shrew species. The local Environmental Records Information Centre has less than 20 records for the period prior to 2000. This is approximately the same as the number of 10 km squares shown for the region in The Atlas of Mammals in Britain (Arnold, 1993), though these are not all the same records. After 2000, records for this species have increased significantly with the total now standing at over 100 records across the region. This is largely down to an increased focus on mammal recording, including two surveys specifically targeted at this species: the bait tube surveys run by the Mammal Society and a joint project between Northumbria Mammal Group and Northumberland Wildlife Trust called “Researching Ratty”. The “Researching Ratty” project in particular added greatly to our knowledge of Water Shrew distribution. It found that the species was quite commonly encountered with results reflecting survey effort and volunteer distribution, which supports the idea that the relative paucity of records across much of the region is down to under-recording. In particular it found a good number of records around the Ponteland, Morpeth and Wallington waterways as can be seen from our distribution map. It also found them at higher altitudes, for example up the River Rede as far up as Byrness and Catcleugh. Another significant factor in the increase in records is the number of Water Shrews that are caught as by-catch in Great Crested Newt surveys where Water Shrews are often found in bottle traps set for trapping and then releasing newts; unfortunately more often than not the shrews die in the traps.
The distribution of records, outside of the “Researching Ratty” surveys in Northumberland, shows a particular concentration around the North Tees Marshes and surrounding area though they are also widely distributed across the Tees Valley. Other small clusters of records centre around places with a history of natural history recording such as the Wildfowl and Wetlands Trust at Washington, Joe’s Pond next to the Durham Wildlife Headquarters at Rainton Meadows and the National Trust’s Gibside site. There are relatively few records in the west of the region though this is a common trend in all small mammal records, but the fact that there is at least the occasional record from there indicates that there is no ecological barrier to them in that area. They are not necessarily dependent on running water as they are one of the species caught in small mammal traps on Lindisfarne, which has no running water but just a single Lough with ditches running from it to the sea. There is a report from Lindisfarne of Water Shrew using a garden pond, complete with fountain (Veronica Carnell, pers. comm., 2012).
The increase in records post-2000 has also resulted in more records involving multiple animals with one sighting of up to five Water Shrews. However this is still some way short of an account in British Mammals (Harrison Matthews, 2009) which claimed to report “a ‘mass migration’ of water-shrews in which some hundreds of the animals are said to have been seen swimming, packed close together, upstream in a narrow drain running through a pasture to join the river in Upper Teesdale”.
At the turn of the 20th century the distribution of Water Shrews was described as “not by any means a rare animal, but would appear to be of local distribution” (Barrett-Hamilton and Hinton, 1910-1921) and “fairly common in England and Wales, as well as in Scotland” (Millais, 1906). However, whether or not the poor number of records we have now show a population decline is difficult to determine because it has been an under-recorded species, but now more recording is being done we are starting to get a better idea of their actual distribution.
Written by Rhia McBain (last updated Nov 12)