Changes in the diet of a recovering Eurasian otter population between the 1970s and 2010

Abstract

ABSTRACT1. Diet is an essential element for understanding how the Eurasian otter (Lutra lutra) is re-colonizing its former range in England following a population decline observed in the 1950s to 1970s.2. Otter spraints from the River Glaven catchment (north Norfolk, eastern England) were collected seasonally between 2009 and 2010 from three habitat types (ponds, 'retenus' (small in-stream reservoirs), and stream stretches). Feeding tactics of the otter were compared between habitat types and, using previously published data, with regard to seasonal changes since the 1970s when the species was in decline.3. Otter diet composition varied greatly, with predation on aquatic invertebrates, crayfish, fish, and tetrapods. Fruit seeds were also found in the spraints. Diet generally reflected habitat, with northern pike (Esox lucius) remains found in spraints from ponds and retenus, whereas European eel (Anguilla anguilla) and gudgeon (Gobio gobio) were prominent in spraints from stream sections. Trophic diversity was particularly high in spraints collected next to ponds. Between the 1970s and the present, crayfish, brown trout (Salmo trutta), cyprinids, Eurasian perch (Perca fluviatilis), birds and mammals increased in prominence, whereas eel and threespine stickleback (Gasterosteus aculeatus) decreased, with notable seasonal variations between the study periods.4. Otter spraints revealed few prey preferences, with only that for gudgeon being ecologically relevant. The consumption of popular sport fishes, in particular common carp (Cyprinus carpio), has given rise to conflicts between recreational anglers and otter conservationists, but no demonstrated preference for carp was observed in the present study.5. Overall, otters have increased their consumption of 'less preferred' (i.e. lower energetic value) food types, and because the species normally preys on the most abundant species available, this suggests declines in 'preferred' prey types. These dietary shifts could contribute to otter dispersal and re-colonization of parts of its original European range where food availability may be higher.

Fish selection by riverine Eurasian otters in lowland England

“…It was unclear whether the results of this study reflected a high abundance of common bullheads in the environment or if otters were preferentially selecting bullheads, because the EA does not quantify their abundance. However, several other dietary studies have also found a large number of bullheads in otter diet (see Copp and Roche 2003), with some noting preferential selection (Almeida et al 2012). Cote et al (2008) also found a preference for slow-swimming marine cottids in North American river otters.…”

“…Since then, otter populations have slowly recovered; the most recent survey shows that, in England, otters are present at 56.3 % of 3327 sites surveyed (Crawford 2010). However, as otters continue to recolonize British waterways, there is growing concern amongst some sectors of the angling and fish-farming community that otter predation will impact fish populations, both in rivers and artificial stillwaters (Almeida et al 2012;Scholey 2012).…”

“…Otters are considered to be an opportunistic species, who select prey largely according to availability (see Almeida et al 2012;Reid et al 2013 and references therein). Otter diet is usually dominated by fish (Kruuk 2006).…”

“…When studying otter diet, it is important to remember that otters show considerable trophic plasticity, even within relatively focussed geographical ranges (Reid et al 2013) and that prey selection may be further influenced by season and local conditions (Almeida et al 2012). Predicting to what extent otter diet overlaps with human interests is therefore difficult.…”

Importance of small fishes and invasive crayfish in otter Lutra lutra diet in an English chalk stream

“…At freshwater aquaculture sites across Europe, conflicts with piscivorous fauna, such as otter Lutra lutra have been widely reported (e.g., Kloskowski, 2005Kloskowski, , 2011. In freshwaters with less intensive management, such as ponds and rivers used for catch and release recreational angling, anglers frequently report fish losses to otter predation as a major concern, specifically for the loss of larger bodied fishes Almeida et al, 2012). Where dietary data are available, however, they suggest that these fishes are a minor component of their diet, with species including eel Anguilla anguilla and bullhead Cottus gobio more commonly taken (Britton et al, 2006;Grant and Harrington, 2015).…”