Overview of methods (pros and cons)

PlatReintro05releaseThere 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 thermal stress peaks) as compared to mid-to-late autumn (Serena and Williams 2012).
  • 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 accordingly always needed when comparing the number of platypus detected in relatively wet versus relatively dry periods.

Advantageous and limiting features of each of the three approaches are outlined below.

Photo: APC

Visual methods

Positive features:

platypus-maffra-2014-sep-gwyther-e1562732440196.jpg

  • 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).
  • Don’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:

  • Don’t provide reliable information about platypus sex, age or condition.
  • Experience has shown that roughly 5% of platypus sightings are in error or are untrustworthy due to the nature of the sighting (very brief view, the animal was seen in poor light, etc.) – records must be supported by photographic evidence or independently vetted for accuracy to ensure reliability.
  • Require humans to be active near a water body.
  • Require 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:

  • Trap KParr check nets IDDF9250Currently the only option for describing population attributes such as sex ratio and reproductive success and gaining accurate information about size and condition.
  • Allows animals to be permanently marked and subsequently identified (normally by means of miniature implanted pet-type transponders)
  • Enables tissue samples to be obtained for genetic studies, animals to be fitted with radio-tags or acoustic transmitters for tracking studies, etc.
  • Provides quantitative data for longer-term population monitoring (though can be difficult to obtain enough captures for 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 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 exercising 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:

  • a-nr-trailstrack-crossing-look-u.s..jpgDoesn’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 better suited to mapping distribution than monitoring changes in activity at a given location).
  • For best results, water samples must be collected with care and transported promptly to a specialist lab.
  • Though generally less expensive than live-trapping surveys, robust sampling and analysis entails 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 a variety of seasonal and environmental factors.

Most useful application: This can be a good approach to adopt for broad-scale mapping of distribution (especially in districts where the potential for sightings to be recorded is limited) as long as negative results are interpreted with appropriate caution.

Photo: APC