ECOS 40(4): Undergraduate winner: What are our otters eating?

Over the last 40 years populations of the elusive Eurasian Otter (Fig.1) have been increasing throughout the UK as a result of conscious efforts to clean up our river systems. However, a recent study investigating the diet of otters inhabiting the Essex countryside has uncovered a surprising find: microplastics. Otters are ingesting small pieces of plastic, alongside their usual prey, which poses an uncertain threat to these charismatic creatures and the wider ecosystem.

Otters in the UK

Within the British Isles otters are an apex predator, occupying a diverse variety of aquatic habitats whilst also exploiting terrestrial environments. Otters require clean waterways and abundant prey stocks, and so are a good indicator of the ecological health of the systems they inhabit. Throughout the UK otters were widespread until the early 1960s when populations rapidly declined due to the industrial use of PCBs (polychlorinated biphenyls) and agricultural use of organochlorine pesticides, such as DDT (1). By the 1970s the species was almost extinct in the UK, reduced to a few refugial groups in Southwest England, East Anglia and Northern England (2). Over the last four decades otter numbers have been slowly rising and healthy otter populations can be found in many places in the UK. This recovery has been attributed to several factors including the establishment of legal protection of otters (1981); the banning of the organochloride pesticides (1984); the removal of lead from petrol (2000); and the extensive conservation efforts implemented by Wildlife Trusts to improve the health of river systems (3). The successful recovery of otter populations in many places throughout the country stands as a positive example of conservation work, however the battle between wildlife management and habitat degradation is ongoing. 

Figure 1: Eurasian Otter
Photo: Danny Green

Analysing otter droppings in Essex

Several locations within Essex have well established otter populations which have been carefully monitored by the Essex Wildlife Trust over the last decade. In northeast Essex otter signs including footprints, droppings (or ‘spraint’), and meal remains are commonly found, particularly along the River Stour and River Colne. Analysing the contents of otter spraint enables an improved understanding of the prey species that support the recovery of the otter, which is an important tool for otter conservation. In collaboration with the Essex Wildlife Trust I undertook my undergraduate dissertation research in the catchments of the Stour and Colne, investigating what otters in these two rivers were eating through the analysis of 63 spraint samples. Samples were collected during summer and autumn months to allow for seasonal comparison.

Ingested microplastics

The results from the dietary investigation identified fish as the dominant prey group and invasive signal crayfish as the dominant prey item in both rivers. The diversity of prey species varied between the two rivers and between summer and autumn samples. Aside from the expected prey remains (fish, amphibians, crustaceans, birds, invertebrates and mammals) plastic was also identified in the spraint samples (Fig.2).

Figure 2: Location map of the spraint sites where microplastics were found on the River Stour and Colne. The locations where microplastics were identified are indicated by red stars. Map was produced using ArcMap 10.3.

Seven microplastic beads were found in four spraint samples, one of which came from the Stour and three from the Colne (Fig.3). As otters are located at the top of the food chain it is likely that the ingested microplastics were transferred into the otters through predation of an already contaminated prey.

Figure 3: Microplastics found in spraint samples. Picture taken with a ROTEk Digital Microscope. The background grid is 1mm squares for scale.

The microplastic beads were sent to Kings College London and analysed by Dr. Stephanie Wright using Raman microscopy. All seven items were identified as primary microplastics composed of styrene-butadiene copolymers (synthetic rubbers), with wide-ranging applications such as tyres and latex, as well as belting, flooring, wire and cable insulation, and footwear.

Microplastics in otters

The global abundance and ubiquity of plastic debris is a growing environmental concern that has gained attention within both public discourse and academic discussion. The occurrence of plastic fragments in freshwater environments has been known for some time (4). However, the majority of academic research into plastic contamination of aquatic environments has predominantly focused on marine settings (5). In the last five years, the relationship between microplastic pollution and freshwater systems has begun to receive attention (6).

Due to the prevalence of microplastics in aquatic environments and the documented ingestion by other aquatic mammals e.g. seals (7), it is not surprising that otters are also ingesting microplastics. However, the impact that microplastic ingestion is having on the health of otters is currently unknown. Some initial investigations into the effect of microplastics on aquatic organisms have provided evidence of physical impacts such as exhibiting signs or stress and toxic effects to internal organs (8). The impacts experienced by aquatic organisms may be related not only with the chemicals that make up the plastic material (incorporated in production), but also the external chemical pollutants absorbed by plastic from the environment.

The presence of microplastics within ecosystems and food chains is a concern for conservation managers. Findings from previous studies imply that plastics could pose a potential threat to our otter populations and a more certain threat to the prey species that otters rely on. Further research into understanding where the microplastics have come from, the contaminants they have absorbed, how they are being transported and what impacts they may be having on our wildlife, is both important and necessary.

References

  1. ‘Concentrations of some organochlorines in otters (Lutra lutra L.) in Scotland – implications for populations’, Environmental Pollutions 92: 165-171. https://doi.org/10.1016/0269-7491(95)00099-2
  2. Jefferies, D. (1989). ‘The changing otter population of Britain 1700-1989’, Biological Journal of the Linnean Society 38: 61-69. https://doi.org/10.1111/j.1095-8312.1989.tb01563.x
  3. Crawford, A. (2010). Fifth otter survey of England 2009 – 2010. Technical report. Environment Agency. Almondsbury, Bristol.
  4. Williams, A. & Simmons, S. (1997). ‘Movement patterns of riverine litter’, Water Air Soil Pollution 98: 119–139. https://doi.org/10.1007/BF02128653
  5. Li, W. (2018). The Occurrence, Fate, and Effect of Microplastics in the Marine Environment. In Zeng, E.Y. (ed.), Microplastic contamination in aquatic environments. Amsterdam: Elsevier.
  6. Vaughan, R., Turner, S. & Rose, N. (2017). Microplastics in the sediments of a UK urban lake. Environmental Pollution , 229 pp. 10-18. 10.1016/j.envpol.2017.05.057
  7. Eriksson, C. & Burton, H. (2003). ‘Origins and Biological Accumulation of Small Plastic Particles in Fur Seals from Macquarie Island’, AMBIO: Journal of the Human Environment 32: 380-385. 10.1579/0044-7447-32.6.380
  8. Rochman, C., Hoh, E., Kurobe, T. & Teh, S. (2013). ‘Ingested plastic transfers hazardous chemicals to fish and induces hepatic stress’, Scientific Reports 3: 3263. 10.1038/srep03263

Cite:

Tetteh-Wright, Teki “ECOS 40(4): Undergraduate winner: What are our otters eating?” ECOS vol. 40(4), 2019, British Association of Nature Conservationists, www.ecos.org.uk/what-are-our-otters-eating/.

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