Almost a year has now past since I handed in my completed undergraduate thesis, oh how time flies! During my time at university I developed somewhat of a fixation with Otters, so much so that when the time came to decide on our dissertation projects I forsook my usual quarry of birds in favor of an investigation into the habits of these secretive wetland denizens. Given the expanding nature of Otter populations and the increased potential for human/otter conflict I eventually ended up conducting a dietary assessment of a population of otters on the River Petteril, located near the town of Penrith, Cumbria. To the best of my knowledge Otter diet had not been previously studied here thus any results would be new and somewhat interesting. Otter diet has been studied extensively elsewhere in the UK with results pointing towards Otters as generalist predators rather than the “fish specialists” they are so often portrayed. My results favored this theory and showed that this population of Otters at least were far from picky when it came to their next meal. Indeed the Otters on the Petteril consumed a broad range of species; making use of seasonal abundances in prey items and clearly adapting to an ecosystem that has suffered substantial degradation over the years. I have included below a few extracts from my thesis, absent much of the technical mumbojumbo that makes academic writing such a pain to comprehend.
Photo: Catherine Trigg
In keeping with prior assumptions fish were the most important component in otter diet throughout the year (60%) with otters consuming fish from six families; Anguillidae, Cottidae, Cyprinidae, Gasterosteidae,Percidae and Salmonidae. Few bones were found that allowed easy differentiation between the various salmonid and cyprinid species and as such data collected for these two groups were combined where necessary. Among the fish species consumed Three-Spined Stickleback (18.3%) and cyprinids (16.6%) were of most importance followed by salmonids (10.6%) and Bullhead (8.3%). The remaining species (Perch and European Eel) were barely represented (1.6%) and as such are of little importance as a food source contrasting greatly with the notion that otters “favour” Eels. Both birds (10%) and mammals (12%) occurred regularly and in significant quantities though to a lesser extent than fish and as such are of secondary importance. Among these otters consumed species from among six families; Lagomorpha, Soricomorpha, Rodentia, Anseriforme, Passeriforme and Rallidae with only one species the Rabbit occurring in significant quantities (5%). Among the other prey groups consumed by L.lutra amphibians were of little importance (5%) contrasting with data from elsewhere across Europe and invertebrates though occurring frequently (10%) were likely consumed as a by-product of fish depredation and as such were of similarly low importance.
The below graph gives a break down of the overall diet; highlighting species as well as prey groups. As you can see the otters were consuming far more than just fish!
Of the trends revealed during the course of the investigation the most obvious is the significance of avian prey during the late spring/early summer period with birds making up 38% of otter diet between March/May and 30% during June whilst failing to occur at all outside this period. This spike in predation coincides with the known avian breeding season and as such it is likely otters are making use of the inexperience of juvenile birds or the increase vulnerability of brooding adults. Rabbit predation shows a distinct trend rising from 0% during the winter/spring period to 20% during the month June before again falling to 12% during November and December. As is the case with avian prey it seems otters are making use of a seasonal glut associated with the abundance of rabbits during the warmer summer months. Few trends are present among fish species with Stickleback predation remaining high year round and cyprinid predation declining only moderately as the year progresses. In contrast with these species Salmonid predation does however show a distinct trend with predation rising considerably to 62% during the early winter. The frequency of remains and large size of the bones recovered possibly suggests that otters are making use these species as they return to their spawning grounds or as the dead adults are washed down stream after spawning. This seems likely given the low frequency of salmonid predation during Jan/Feb and March/May where the only bones recorded where that of fry. Both Atlantic salmon and Sea Trout are known to persist in the Petteril and both migrate on mass at certain times of year however bone morphology (jaw) and the size of the vertebrae in question points towards Salmo Trutta as the species in question.
The below graph shows the seasonal importance of prey items to the otters in question:
The diet of Lutra lutra on the River Petteril is consistent with evolutionary adaptation and similar in comparison with the diet of otter populations from across the Palearctic zone. Befitting prior assumptions fish formed the main component of otter diet though the species consumed were almost universally of small size and low calorific value with stickleback, bullhead and small cyprinids (Minnow) making up the bulk of the species consumed. Birds and mammals were revealed to be of secondary importance though predation rates of these species were somewhat elevated when compared with similar studies likely due to both abundance and degraded fish stocks. Amphibians occurred infrequently and were of little importance in the diet of L.lutra showing no distinct temporal trends in frequency likely due to nature of the habitat in question. This study provides clear evidence for prey abundance as the main catalyst for otter predation with prey consumed based on their frequency in the environment rather than by individual bias.
The results of this study pose a number of questions when in regards to the health of the Petteril ecosystem with numbers of both eels and salmonids substantially lower than previous estimates with the low abundance of A.anguilla in the Petteril likely corresponding with wider declines across both Britain and the Palearctic zone. Adult salmonids were insignificant in the diet of L.lutra suggesting their low abundance in the ecosystem in keeping with prior findings regarding the nature of otter predation. The low occurrence of salmonids and the lack of any distinguishable large cyprinids suggest that otters pose no threat to angling interests in the area.
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James is a nature writer, conservationist, blogger and birder; holding an MSc in Wildlife Management and working previously in the fields of ecology and practical conservation. He maintains a popular natural history blog at commonbynature.co.uk, writes regularly for Northumberland Wildlife Trust and, as its managing director, runs New Nature - the youth nature magazine.