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Spraints demonstrate small population size and reliance on fishponds for Eurasian otter (Lutra lutra) in Hong Kong


Bonebrake, Timothy Carlton et al. (2022), Spraints demonstrate small population size and reliance on fishponds for Eurasian otter (Lutra lutra) in Hong Kong, Dryad, Dataset,


Lack of data on population sizes and resource requirements are major impediments to the effective conservation of rare species globally. The conservation of the Eurasian otter (Lutra lutra) in Hong Kong reflects many of these key challenges for elusive and difficult-to-study mammals. It is a rare carnivore that has narrowly escaped extirpation, now surviving within a human-dominated environment. Using sign surveys and spraint analysis, we recorded only 40 fresh spraints from 246 otter signs locations, over four months of intensive sampling across two years. Records were restricted to the Mai Po wetlands, confirming this as the core area for Hong Kong’s otter population. Molecular analysis and microsatellite genotyping identified a minimum of seven individuals, two pairs of which were likely related. The genetic and sign data together strongly indicate a small population. Fish dominated the otter diet, highlighting the importance of fishpond habitats as a premium foraging resource. Given the rapid changes surrounding the Mai Po area (especially the new Northern Metropolis Development Strategy), maintaining quality and connected habitats, in addition to sustaining commercial fishponds will be key to otter recovery and long-term population viability in Hong Kong.


Surveys for otter spraints were undertaken during the winter (dry) season of 2018 and 2019 in Hong Kong, avoiding higher temperatures and humidity (affecting DNA quality of samples), and overgrown vegetation (affecting sample detectability). Surveyors experienced in otter survey traversed the banks of all accessible fishponds, gei wai, freshwater, and tidal watercourses across the study area (located in the north-west New Territories of Hong Kong, along the east coast of Inner Deep Bay in the Pearl River Delta). The survey was repeated at least two times each season to increase the probability of detection and thus decrease the probability of false negatives (MacKenzie et al., 2006). During the surveys, spraints between 1 and 3 days old were collected and frozen while being transported to the lab (Taastrøm and Jacobsen, 1999). Spraints were soaked in a diluted solution of alkaline detergent at room temperature prior to being rinsed and air-dried (24-48 hr) and then sieved through a 1 mm mesh. Identifiable hard prey-remains were separated, examined using a dissecting microscope, and weighed.

For the otter diet study, six main food categories were considered: fish, amphibians, birds, mammals, crustaceans, and other invertebrates, based on the following undigested remains: bones, scales, feathers, fur, exoskeleton, antennae, articulate extremities, and insect wings. Fish species were identified to the lowest taxonomic level using a personal reference collection developed, existing databases, and taxonomic keys (Conroy et al., 2005; Groves and Cornwall Mammal Group, 2020; University of Nottingham, n.d.). Where present, amphibians, and reptiles were typically identified from their characteristic bones; bird prey from feathers and beak and leg/claw remains; mammal remains from teeth and hair; undigested integuments for insects, and shells for crustaceans. No attempt was made to identify non-fish categories below class level.

A minimum of eleven taxa were recovered from the spraint samples. Fish represented the major component of the otter diet, while birds, other invertebrates, and crustaceans represented a very small proportion of the diet.

To investigate relatedness between otter individuals, faecal DNA was extracted and amplified with seven microsatellite markers from an initial set of fourteen markers (Dallas and Piertney, 1998; Huang et al., 2005). All samples were genotyped following the multiple-tubes approach to ensure reliable genotyping (Taberlet et al., 1996).

Usage Notes

This folder contains two csv files, with “Otter_Spraints_Prey_Items.csv” containing data regarding otter diet, and “Otter_microsat_data.csv” containing microsatellite data regarding otter relatedness.


Environment and Conservation Fund, Award: 2016-68

Ocean Park Conservation Foundation, Hong Kong, Award: 2016/17

Wildlife Reserves Singapore Conservation Fund