There are currently three major ways to approach platypus survey and monitoring: by recording sightings, by setting nets to capture animals, and by detecting traces of platypus DNA (referred to as environmental DNA or eDNA).
All three approaches have the following features in common:
- Results for all methods will vary with platypus activity – how far animals travel (and how long they are active) in a given day. Greater activity means that animals are more likely to encounter nets, be seen by observers, and leave widespread traces of DNA in the water. Activity in turn can be affected by numerous factors, including an animal’s reproductive status and energy requirements. For example, based on monthly variation in platypus fyke-netting results in Victoria, the likelihood of capturing an adult platypus is nearly three times higher in mid-winter (just before the start of the breeding season and when water is coldest) as compared to mid-to-late autumn (Serena and Williams 2012). The monthly frequency of platypus sightings near Melbourne follows a very similar pattern (Easton et al. 2008).
- Along with seasonal variation, results for all methods are also expected to vary with the strength of stream or river flow. Flow affects how well nets can be set and the concentration and distribution of eDNA in the water. Flow can also influence platypus activity. For example, by monitoring the movements of animals implanted with acoustic tags, Bino et al. (2018) found that daily travel was reduced when water levels in a river were low. Caution is therefore needed when comparing the number of platypus detected in relatively wet versus relatively dry periods.
Positive and limiting features of each of the three approaches are outlined below.
Photo: APC
Recording sightings
Positive features:
- Can provide abundant data suitable for reliable monitoring (if based on standardised protocols).
- Less constrained by inclement weather/high flows than netting or eDNA sampling.
- Far more cost-effective than netting or eDNA sampling (especially if information is routinely collected by citizen scientists).
- Doesn’t entail significant risk or disturbance to platypus or other wildlife (especially if carried out in daylight hours).
- Intensity of monitoring effort isn’t compromised by trap-shyness.
- Most effectively applied in relatively substantial water bodies (ponds, lakes, rivers and large streams) that are particularly likely to be important platypus habitats.
Limitations:
- Doesn’t provide reliable information about platypus sex, age or condition.
- Experience has shown that roughly 5% of platypus sightings reported to the APC are either untrustworthy or clearly in error due to the nature or circumstances of the sighting (the animal was only glimpsed far away in poor light, the observed behaviour doesn’t conform to normal platypus behaviour but is like that of a water-rat, etc.) – sighting records should therefore be supported by appropriate photographic evidence or otherwise vetted, including asking for more supporting details in the case of questionable sightings.
- Depends on people being active on or near a water body.
- Requires the water surface to be visible from vantage points along the bank (unless sightings are recorded from a boat or raft).
Most useful application: This is the technique of choice for cost-effective platypus mapping and monitoring (especially in the case of reasonably large water bodies that are regularly visited by people).
Photo courtesy of J. Gwyther
Use of live-trapping nets
Positive features:
Currently the only option for describing population attributes such as sex ratio and reproductive success and gaining accurate information about animal size and condition.- Allows animals to be permanently marked and subsequently identified (normally by means of miniature implanted pet-type transponders).
- Enables animals to be fitted with radio-tags or acoustic transmitters for tracking studies, tissue samples to be collected for genetic studies, etc.
- Provides quantitative data for longer-term population monitoring (though it can be challenging to obtain enough captures for really reliable findings).
- Best-practice techniques are based on knowledge gained over several decades by researchers working in a wide range of water bodies.
Limitations:
- Normally can only be conducted by qualified biologists working in accordance with research permits issued by state wildlife and/or fisheries agencies.
- Requires specialised equipment and is logistically demanding (hence expensive).
- Nets can only be set effectively in a limited range of water depths and flow (so their use is restricted to a subset of platypus habitats and is also weather-dependent).
- Animals can become trap-shy if nets are set repeatedly in a given area.
- Entails some costs to animal welfare (both to platypus and non-target species) though these can be reduced by taking appropriate care when setting and checking nets.
Most useful application: This is the technique of choice if detailed biological information is needed, either to evaluate a population’s conservation status or as part of a scientific study.
Photo courtesy of Ken Mival
Use of environmental DNA (eDNA)
Positive features:
Doesn’t entail significant risk or disturbance to platypus or other wildlife.- Intensity of sampling effort isn’t compromised by trap-shyness.
- Very well suited for use in relatively small streams, including those where platypus may be difficult to observe.
- Can be used in remote and inaccessible locations where humans rarely visit.
Limitations:
- Findings are inferential and currently limited to platypus presence/absence, so eDNA is much better suited to mapping distribution than monitoring how that distribution may change over time. In practice, the amount of eDNA sampling needed to confirm a change in usage is inversely related to how often animals are detected: change is easier to detect in places where platypus are abundant than where they are rare. For example, it’s been estimated that nearly 400 sites would have to be resampled in the Yarra River catchment (average platypus detection frequency = 53% of site-visits) to enable a 20% change in site occupancy to be confirmed with 95% confidence, and more than 2000 sites would have to be resampled in the Werribee River catchment (average platypus detection frequency = 8% of site-visits) to achieve the same outcome (Griffiths et al. 2019).
- Water samples must be collected with care and sent to a specialist lab for processing.
- Though generally less expensive than live-trapping surveys, robust sampling and analysis entail a significant cost.
- Reflecting the fact that this approach has only recently been developed, a great deal remains to be learned about how to optimise eDNA sampling and the extent to which findings are affected by seasonal variation in platypus behaviour and environmental factors such as water temperature and turbidity.
Most useful application: This is best suited for mapping where platypus occur (especially over broad geographic areas or where the potential for sightings to be reported is limited) as long as negative results are interpreted with appropriate care (for example, by ensuring that circumstances expected to limit platypus usage – for example, seasonally ephemeral or intermittent flow – are properly considered).
Photo: APC
Literature cited
Bino, G., Kingsford, R. T., Grant, T., Taylor, M. D., and Vogelnest, L. (2018). Use of implanted acoustic tags to assess platypus movement behaviour across spatial and temporal scales. Scientific Reports 8, 5117.
Easton, L., Williams, G., and Serena, M. (2008). Monthly variation in observed activity of the platypus Ornithorhynchus anatinus. The Victorian Naturalist 125, 104-109.
Griffiths, J., Maino, J., Tingley, R., and Weeks, A. (2019). Distribution and relative abundance of platypuses in the greater Melbourne area: survey results 2018/19. Report prepared for Melbourne Water by cesar, Parkville.
Serena, M., and Williams, G. A. (2012). Effect of sex and age on temporal variation in the frequency and direction of platypus (Ornithorhychus anatinus) captures in fyke nets. Australian Mammalogy 34, 75-82.