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OTTER DIET ALONG THE UPPER LITTLE OUSE ROWENA LANGSTON & ARTHUR RIVETT Background The upper reaches of the Little Ouse River are typical of many East Anglian rivers in having a largely arable catchment, historically being straightened and over-deepened, with a very shallow gradient, and having a riparian vegetation often dominated by nettles (Fig. 1a). However, the Little Ouse headwaters are also bordered by a mosaic of valley fens, woodlands, scrub, heaths and grasslands, ranging from wet to dry and from calcareous to more Figure 1a: Upper section of the Little Ouse acidic conditions. For over twenty years, River: straight, over-deepened channel the Little Ouse Headwaters Project (LOHP, www.lohp.org.uk) and its predecessor voluntary groups have been working to reconnect and restore habitat remnants in the valley (Fig. 1b), complementing the work of the Suffolk Wildlife Trust (SWT) to safeguard and enhance these valuable wildlife habitats. In 2012, in-channel river restoration works were carried out along a 1-km section of our study area. This involved using a digger to push short sections of riverbank, on alternate sides of the river, Figure 1b: a section following in-channel into the wide channel, forming low restoration works terraces and forcing the water into a narrower, more sinuous course. This had the desired effects of increasing flow-rate and improving aeration. As water levels rise, the water spills over the terraces to fill the widened channel. In flood, the river overspills on to the surrounding fen floodplain. These works enhanced the river habitat for wildlife, although silt influx in the catchment makes the water quite turbid in places. Otter studies along the Little Ouse River The decline, and subsequent recovery, of Eurasian otters Lutra lutra across the UK has been well-documented (e.g. Cousins et al. 2011). In 1996/97, SWT carried out a survey of otters in Suffolk, on behalf of the Environment Agency, and subsequently there were at least partial annual surveys until 2008/09 (Hemphill, 2009). Otters have shown an increase in activity along the Little Ouse over this time period. In November 2015, Richard Woolnough gave the LOHP’s Annual Talk, prompting interest in carrying Trans. Suffolk Nat. Soc. 56 (2020)
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out a study of otters along the Little Ouse and the establishment of an LOHP mammal study group. Following some preliminary work, we commenced a three-year study in March 2016, with the aims of increasing our knowledge of otter activity and diet along the upper Little Ouse River. We undertook a combination of year-round spraint (faeces) collection and analysis, aiming for monthly samples, and use of video cameras set at key sprainting locations. Our study group is affiliated to the Suffolk Otter Group, an umbrella organisation for volunteers studying otters in the county (https://suffolkotters.wordpress.com). Methods Otter spraint analysis Prior to commencement of the study, the group identified contiguous stretches of river, from the source downstream to Euston, to search for prime sprainting locations which would form the main sampling points for the study (Fig. 2). Mink rafts,
Figure 2: Little Ouse River study area, showing sampling locations for otter spraint deployed by SWT and LOHP for the purpose of monitoring and trapping the invasive, non-native American mink Neovison vison throughout the catchment, were favoured sprainting locations, as were riverbanks, stones beneath bridges, riverside tree roots, and hard structures around sluices (Fig. 3). On each sampling occasion, the freshest spraints were collected; we did not remove all spraints accumulated since the previous visit. Each spraint was put in a clean pot, using a spoon, tweezers or nitrile glove and the pot labelled with the date, location, and observer initials. GPS coordinates were recorded for each sampling location. The majority of spraints were washed by one stalwart member of the group, who also maintained the data spreadsheet and sample collection. Spraint analysis is a standard methodology for studying otter diet, although caution is needed in its interpretation (Carss & Parkinson 1996, Chanin 2013). Spraint analysis depends upon the identification of prey remains evacuated in faeces and so is likely to better represent prey types which have distinctive, hard features, for example fish scales, bones or invertebrate exoskeletons, rather than entirely soft-
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Figure 3: Spraint locations. A) Fresh otter spraint on riverbank tree root, B) otter spraint on tree root, C) otter spraint on stone under bridge, D) otter sprainting point on mink raft bodied prey (although we recorded some soft-bodied items, including frogs’ spawn). Generally, we were unable to identify cyprinid fish to species, because their vertebrae and scales, which were most often recorded, are very similar across species. Rudd Scardinius erythrophthalmus and roach Rutilus rutilus are known to occur in the river. Diet cannot be fully quantified, in terms of biomass or exact percentage composition of items, but frequency should provide information on the range of prey and on the gross patterns in seasonal variation in prey composition. Most spraint analysis was carried out at group workshops (Fig. 4), in collaboration with more experienced members of the Suffolk Otter Group, thereby enabling shared learning and exchange of ideas. We recorded frequency of occurrence of prey i.e. the Figure 4: Otter spraint analysis workshop proportion of spraints containing each in action
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prey type or species. Samples were checked twice by different observers to improve consistency and accuracy of identification. Most analysis was done using digital microscopes connected to laptop computers, which have the advantage of enabling projection on to a slide screen to discuss particular features, but a binocular microscope was used to verify more challenging specimens. Photographic records were taken for some specimens, contributing to the Suffolk Otter Group’s online identification resource (Fig. 5).
Figure 5: Bullhead, part of the account from the online guide to otter spraint analysis. See link under Surveys/Diet on: https://suffolkotters.files.wordpress.com/ Once samples had been double-checked, each was stored in a small, labelled and sealed polythene bag, so we have the option to return to them at a later date, to refine the analysis. The statistical analysis presented here used Minitab 19 (©minitab.com). Video cameras Video cameras were positioned overlooking several sprainting locations, spacing them at intervals along the main study corridor between TM030787 and TM014788, in Thelnetham/ Blo’Norton/ Hinderclay, with the aim of identifying sequential movement of otters along the watercourse. In addition, one camera was deployed overlooking the river through Scarfe Meadows in Garboldisham, at TL99698067, and occasionally overlooking the sluice at TL99738083.
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Placement of video cameras along the river posed quite a few challenges. In particular, aligning cameras to capture otter behaviour, their direction of movement along the river, individual characteristics, or at least evidence of multiple animals. The flashy nature of the Little Ouse means that water level varies markedly, rising rapidly at times of flood and dropping rapidly as the pulse of floodwater passes through. We had to rescue cameras before they were inundated by flood-water in April 2018. The river was an obvious location to target our small number of cameras, but most of our observations do not account for otter activity away from the river. We had some problems with cameras ceasing operation, dates and times reverting to the factory default, and operator error! Also, we were conscious of the need for concealment of cameras to minimise the risk of interference, given that the study was conducted in areas accessible to the general public. Thankfully, we have had very few problems attributable to interference with the study. Results Spraint analysis The results presented here are drawn from a sample of 326 spraints, comprising those collected during the main study, between March 2016 and March 2019, and a preliminary sample from October 2015. The diet of otters along the upper Little Ouse consists predominantly of fish, present in over 90% of spraints (Fig. 6), of which European bullhead Cottus gobio and sticklebacks were most frequent (Fig. 7). Perch Perca fluviatilis and cyprinids were the next most frequent fish recorded, then Pike Esox lucius. Stone loach Barbatula barbatula was recorded occasionally, whilst Eels Anguilla anguilla were not recorded at all in spraints collected for this study.
Figure 6: Summary of prey types found in 326 otter spraints sampled along the upper Little Ouse River
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Figure 7: Percentage frequency of occurrence of different fish species in 326 otter spraints Few spraints were found in summer, June to August (n=33), but reasonable sample sizes were obtained for other seasons - autumn, September to November (n=83), winter, December to February (n=82), spring, March to May (n=128). Simple data plots indicate seasonal differences in prey frequency (Figs. 8 & 9). Overall, fish occurrence showed little seasonal variation in otter spraint (Fig. 8). The seasonal frequency of bullhead in spraint was more consistent than for sticklebacks (Fig. 9). The slightly greater frequency of sticklebacks in spring was perhaps a consequence of the more prominent behavioural activity by sticklebacks during courtship. Both Three-spined stickleback Gasterosteus aculeatus and Ninespined or Ten-spined stickleback Pungitius pungitius were recorded. Perch (χ23 = 15.62, P < 0.05, n = 116, i.e. perch were recorded in 116 of the 326 otter spraints) and cyprinids were found in spraint most frequently during winter (Fig. 9). Amphibians were most frequently recorded in spraint during spring (χ23 = 15.89, P < 0.05, n = 69) when they are mating and spawning (Fig. 8). Crustaceans recorded in spraint comprised mainly the invasive non-native Signal crayfish Pacifastacus leniusculus, confirmed by Riverfly sampling along the Little Ouse (ARMI), and shrimps Gammarus. Crayfish were most frequently recorded in summer, then in spring and autumn, and least recorded in winter (χ23 = 16.64, P < 0.05, n = 60). Bird remains were most frequent in summer when young birds – notably ducklings and young moorhens Gallinula chloropus – were abundant along the Little Ouse (χ23 = 11.18, P < 0.05, n = 113). Mammals were the least frequently recorded prey, found in just 15 spraints
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Figure 8: Seasonal variation in occurrence of different prey types found in 326 otter spraints
Figure 9: Seasonal variation in occurrence of different fish species found in 326 otter spraints
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mainly in autumn and winter. Most mammal remains were unidentified to species, but we did find fur in one spraint that we thought was from European water vole Arvicola amphibius. Beetles, notably Dytiscus diving beetle remains, were often recorded, but showed little seasonal variation (χ23 = 5.32, P = 0.15, n = 127). Video cameras We regularly put out three cameras along the main study stretch of the Little Ouse, and a fourth camera occasionally, ensuring periods of simultaneous operation. Sequential video clips were obtained indicating movement along the river, either upstream or downstream, as well as behavioural information – foraging, sprainting, and social interactions. Attempts at using a spreadsheet to interpret the video records proved unwieldy, so graph paper was used to Figure 10a: adult male otter showing record each date and time of day an distinctive throat markings otter was observed at each location. Most video clips gave lateral views which made individual identification difficult, as most recordings were of animals moving rapidly through the field of view. Very few videos provided sufficient detail to distinguish individuals (Fig. 10a). Estimation of the number of otters using the river was based on the observed sequences of adult female with cub(s) (Fig. 10b), adult male, and a solitary adult female, over the three years of the study. These records suggested that there were at least three to five individual otters regularly using the main study section of the upper Little Ouse during 2016 to 2019, the difference arising from an adult female with a single cub in one year
Figure 10b: adult female otter with two cubs
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(2018-19), an adult female with two cubs in another year (2016-17), and no cub in 2017-18. It is not known whether otters seen in successive years were the same individuals. Otters were observed foraging on several occasions, especially the adult female with cub(s). They used the mink rafts as launching pads and, sometimes, feeding places. Successful forays were observed and, occasionally, it was possible to identify prey items, including a perch captured during a foraging bout. Behavioural interactions between mother and cub(s) were recorded. Sequential video clips routinely showed movement between observation points along the watercourse, based on time elapsed and distance travelled. They also indicated that the mother and cubs were often followed by an adult male. One clip shows courtship and mating by adult otters soon after the 2018-19 cub reached independence. Supplementary information recorded by the cameras included regular passage of mallard Anas platyrhynchos broods, from recently-hatched ducklings to sub-adults, accompanied by adults, as well as moorhens with chicks at various stages of development. These are all potential prey for otters and are likely to have featured in the spraint, although we were rarely able to identify bird remains to species. Kingfishers Alcedo atthis sometimes used mink rafts as foraging platforms, and occasionally occurred in otter spraint. The camera on Scarfe Meadows was operational during September to November 2017, recording one probable female and a male otter on separate occasions throughout this period. There was also brief deployment of a camera under the Hopton Bridge, at TM00258005, which confirmed the presence of otters, including an adult male. Owing to the timings of observations in the main study area, and the gap of several kilometres between there and Garboldisham, we believe the otters in the two areas were different individuals, but cannot rule out overlap. Discussion It cannot be assumed that all prey remains found in our sample of otter spraint were from prey captured in the river alone. However, it is reasonable to assume that the spraint sample is representative of diet from the Little Ouse, together with associated wetlands and waterbodies in the catchment used by the otters in our study area. Our results fit within the range observed by other studies of otter diet in freshwater, notably for a slow-flowing, lowland river in its upper reaches (Cousins et al. 2011, Almeida et al. 2012, Common 2016). Otters are opportunistic and adaptable predators. Whilst fish predominate in their diet, seasonal variation in diet is influenced by prey availability and abundance. Small prey items accounted for much of the resident otters’ diet in this study, the smallest of which are likely to have been ingested incidentally with other prey, for example Gammarus. However, earlier studies of otter spraint, including from captive-reared otters, confirmed that some small prey items such as diving beetles were actively consumed, as opposed to incidentally ingested (Foster & Turner 1991, Carss & Parkinson 1996). Given the predominance of smaller prey found in spraint, there is uncertainty over the adequacy of the Little Ouse to fully meet the food requirements of the otters using
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the river. It is estimated that spraints are passed within approximately 24 hours of ingestion. It cannot be ruled out that otters using the study area also fed away from the area as camera evidence indicated that they were not present along the river all the time, but regularly moved along the section covered by cameras, with gaps of 1-3 days recorded; occasionally more. It seems likely that the river may have formed part of a larger foraging range possibly including wet fens and small water bodies in the valley. We found some spraint away from the river, but were unable to routinely check the much larger catchment during this study. The absence of Eels in the otter spraint, together with a general lack of records from the area in recent years, suggest they may be absent from the upper Little Ouse, or at least very scarce. Eels feature in the diet of otters on several other Suffolk rivers including the Blyth and its tributaries (N. Rowbottom, pers. comm.). They were found in up to 39% of spraint in autumn at the Dingle Marshes Nature Reserve (SWT/RSPB/ Natural England), north of Dunwich, as well as occurring in spraint from other coastal locations including Minsmere and the River Alde at Orfordness (N. Rowbottom, pers. comm.). As recently as 2005, commercial catch returns were registered for eels on the Little Ouse, although the locations are not specified, and no Eels were found in the EA Fisheries Monitoring Programme sampling points nearest to our study area during 2001-2005 (DEFRA 2010). However, otters have been observed eating eels along the river in Thetford (e.g. Rae, 2013) and historically they were present further upstream. There are several barriers to movement upstream along the Little Ouse for which efforts are being made to install fish passes. It is unclear whether these barriers preclude movement upstream by eels, for example during low flows, or whether the headwaters’ habitat is no longer suitable, or whether the general decline in eels accounts for their apparent absence. Due to their broad and flexible diets, otters provide a useful indicator of environmental conditions and the health of the wetlands. The fact that several otters occupy the area and breed successfully is testament to the suitability of the catchment habitat to support them, notwithstanding the comments above about the ability of the river alone to fully meet their requirements. The spraint analysis of otters has given an insight to the nature of the current fish populations on the Upper Little Ouse and provided records of several species of particular interest such as Stone Loach. The video cameras also provided records of many other mammal, and bird, species using the valley, including water vole and Eurasian water shrew Neomys fodiens (Appendix I). These records complement other surveys, notably of water voles along our study section. As indicated by our spraint analysis, water voles may be predated occasionally (e.g. Chanin 2013). There were no records of mink on the cameras during this study, although one was recorded on a camera on the upper Waveney during the same period. The cameras remain a useful means of detecting mink. We cannot be complacent about their potential return to the area, following the successful concerted effort to cull them in the catchment over ten years ago. We will continue to deploy video cameras and check for spraint, in readiness to carry out further spraint analysis in response to any perceived changes in activity
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levels or known changes to water quality or quantity. This could include estimating the size-frequency distribution of key prey items. Additionally, we aim to place cameras in new locations, away from the river. Through this study and others on different river catchments in Suffolk, the Suffolk Otter Group has maintained an overview of the status of otters in Suffolk, and is developing guidance for identifying fish bones and other prey remains that may be found in otter spraint, adding substantially to previous reference material See link under Surveys/Diet on the following page: https://suffolkotters.files.wordpress.com/ References Almeida, D., Copp, G. H., Masson, L., Miranda, R., Murai, M., & Sayer, C. D. (2012). Changes in the diet of a recovering Eurasian otter population between the 1970s and 2010. Aquatic Conservation: Marine and Freshwater Ecosystems 22 (1): 26-35. ARMI. Anglers Riverfly Monitoring Initiative. www.riverflies.org Carss, D. N. & Parkinson, S. G. (1996). Errors associated with otter Lutra lutra faecal analysis. I. Assessing general diet from spraints. J. Zool. 238: 301-317. Available from https://zslpublications.onlinelibrary.wiley.com/doi/abs/10.1111/j.14697998.1996.tb05396.x. Accessed 27/03/2020. Chanin, P. (2013). Otters. Whittet Books Ltd., Stansted. Common, J. (2016). The Eurasian Otter (Lutra lutra) is an incredibly opportunistic predator with prey diversity varying greatly across its range in the Northern Hemisphere. Short review article available at www.countrysidejobs.co.uk/articles/ otter-diet-in-freshwater-ecosystems. Accessed 24/03/2020 Cousins, L., Tansley, D. & Hepburn, L. (2011). Investigation into the dietary habits of the Eurasian Otter (Lutra lutra) in the County of Essex. IUCN/SSC Otter Specialist Group Bulletin 28 (2): 76-83. Available at https://www.iucnosgbull.org/Volume28/ Cousins_et_al_2011.html. Accessed 27/03/2020. DEFRA. (2010). Eel Management plans for the United Kingdom: Anglian River Basin District. Report available at www.defra.gov.uk or www.broads-authority.gov.uk. Foster, J. & Turner, C. (1991). Insects and otter diet. The Entomologist 110(4): 166-169. Hemphill, P. (2009). Suffolk Otter Survey. Suffolk Wildlife Trust Report. Rae, M. (2013). Otters in the Norfolk landscape. Available at mikerae.com. Accessed 06/04/2020. Suffolk Otter Group. (2020). The Wooly Guide to Otter Spraint. See link under Surveys/Diet on the following page: https://suffolkotters.files.wordpress.com/ Acknowledgements LOHP mammal study group and Suffolk Otter Group members: Ellie Beach, Alan & Angela Beeken, Peter Hughes, Reg & Rowena Langston, Sue Lawrence (chief spraintwasher), John McCormack, Richard Minter, Graham Moates, Arthur Rivett, Nicky Rowbottom, John Shrouder, Mark Smith, Richard Woolnough. Also, our thanks to Suffolk Wildlife Trust for hosting the workshops at their Redgrave & Lopham NNR visitor centre, to Rob Fuller for helpful comments on an earlier draft of the article, and to Martin Sanford for producing Fig. 2.
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Figure 11: Richard Woolnough demonstrating the use of a digital microscope for analysing otter spraint, at the LOHP AGM 25 November 2015. Rowena Langston Walnut Tree Farm, Thorpe Street, Hinderclay, Diss, Norfolk IP22 1HT rowena.langston@lohp.org.uk Arthur Rivett 7 Neve Gardens, West Row, Bury St Edmunds, Suffolk IP28 8PP arthurrivett057@btinternet.com Footnote: Sadly, Richard Woolnough, who was the driver and mentor for studies of otters in Suffolk, died in October 2019 (Fig. 11). Richard was key player in Suffolk Mammal studies for over 40 years. In 1983, he was one of the founders of the Suffolk Bat Group. At around the same time he recruited the late Margaret Grimwade and more volunteers to set up the Suffolk Badger Group, turning out regularly at meetings to inspire both old and new members. He was not only very knowledgeable about wildlife and conservation, but so enthusiastic about it and willing to share his knowledge and enthusiasm with other less experienced people. Trans. Suffolk Nat. Soc. 56 (2020)
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Appendix I: Mammal and bird species recorded on video cameras Mammals
Birds
Species name
Common name
Species name
Common name
Arvicola amphibius
European Water Vole
Alcedo atthis
Common Kingfisher
Capreolus capreolus
Roe Deer
Anas crecca
Eurasian Teal
Lepus europaeus
Brown Hare
Anas platyrhynchos
Mallard
Lutra lutra
Eurasian Otter
Buteo buteo
Common Buzzard
Meles meles
European Badger
Certhia familiaris
Eurasian Treecreeper
Muntiacus reevesi Muntjac
Columba palumbus
Common Woodpigeon
Mustela erminea
Stoat
Cuculus canorus
Common Cuckoo
Mustela nivalis
Weasel
Dendrocopos major
Great Spotted Woodpecker
Mustela putorius x Mustela putorius furo
Polecat Ferret
Egretta garzetta
Little Egret
Neomys fodiens
Eurasian Water Shrew
Gallinula chloropus
Moorhen
Rattus norvegicus
Brown Rat
Garrulus glandarius
European Jay
Sciurus carolinensis
Grey Squirrel
Mareca strepera
Gadwall
Vulpes vulpes
Red Fox
Motacilla cinerea
Grey Wagtail
Phylloscopus collybita
Common Chiffchaff
Picus viridis
European Green Woodpecker
Rallus aquaticus
Water Rail
Strix aluco
Tawny Owl
Turdus pilaris
Fieldfare
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