The Eurasian water shrew: an unsuitable candidate species for a vertebrate bio-indicator of aquatic pollution

The Eurasian water shrew (Neomys fodiens) is a semi-aquatic predator of freshwater invertebrates. As water quality affects the diversity and abundance of aquatic invertebrates, water shrews could potentially be used as a vertebrate bio-indicator of water quality. To date, no detailed studies have empirically examined the impacts of water quality on Eurasian water shrew occurrence. Bait-tube surveys were undertaken in winter and summer over 3 years at 26 different wetland locations across Sussex, UK, which varied in water quality. Bait tubes were used to confirm water shrew presence at specific sites and derive an index of activity using frequency of occurrence of faeces within tubes. Water quality was measured using six direct physical and chemical indicators (dissolved oxygen, pH, water temperature, ammonia, nitrate and phosphate) and two derived indices of biological indicators based on aquatic invertebrate composition. We found no linear relationship between any physical, chemical or biological water quality indicators and water shrew occurrence. Generalized linear models indicate that water shrew presence and frequency of occurrence are more affected by site and season than water quality. Thus, water shrews may be more tolerant of poor water quality than previously envisaged. Overall, our study indicates that water shrews are not suitable vertebrate bio-indicators of water quality.