This is an ongoing monitoring project of high altitude populations (above 700m) of Elseya irwini. The turtles have been subjected to several taxonomic name changes over the past few years, traveling from Elseya dentata to Elseya strilingi to arrive at Elseya irwini. Little was known about the E. irwini population on the Atherton Tablelands and this project aims to determine the current threats to the population in the hope that this will inform future management plans. E. irwini’s conservation status was recently listed as “high priority”, thus making the importance of having baseline data all the more significant. In other words, the more we know about how they operate now the better we can recognise changes and deal with the effects.
Surveys:
Surveys have been conducted throughout the Atherton Tablelands in search for the presence of E. irwini. This involved snorkeling through rivers in the catchment area catching turtles by hand whilst recording time; allowing us to calculate CPUE (catch per unit effort). Any turtles caught were examined and released within a few minutes. The CPUE measurement gives us an idea on how dense the populations of E. irwini are in an area. If a lot of turtles are caught in a short period of the time we might conclude the population to be dense and vice versa. Surveys give us an idea as to the areas in which the species occur.
Monitoring:
For these monitoring sessions we have 4 locations, ideally visited twice a year between September and November (dependant on river conditions). Permission from landowners is required to access the riverside and most landowners have developed an active interest in the project. There are two individuals searching, in most cases, whilst timed to determine CPUE. We have one hour in which to catch as many turtles as able. Part of the monitoring process is making use of mark and recapture: before the E. irwini turtles are returned to the water they are marked and tagged so they may be identified if recaptured. This method allows us to estimate the size of the animal’s population.
Once caught there are several aspects of the turtles we are interested in:
Measurements:
Measured to the nearest millimeter using vernier calipers:
- Straight carapace length and width.
- Straight plastron length and width.
- Tail to carapace edge.
Measured to the nearest gram using a digital balance:
Straight carapace length of a female
at the Allwood's monitoring site:
Tail to carapace edge of a male
at the Allwood's monitoring site:
Weight:
Damage:
The turtles were examined for damage upon capture. Individual turtles were divided into the three categories seen in Table 1 as per the damage assessment. Most turtles suffered little damage, the worst in 2012 being a missing foot (however whether the turtle had been born thusly or had somehow lost it was unclear). The damage assessment is yet one other factor which helps determine the health of a population.
Small female from Fanning's
monitoring site with a missing foot:
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| Table 1. Definitions of broad damage categories allocated to each turtle (Freeman 2011 p.7) |
Sexing:
Turtles were identified as either female or male by comparing the ratio of tail to carapace edge length. If the tail to carapace length was too small to measure the turtle was classed as a juvenile. Males have a significantly longer tail to carapace edge length than females (Fig 1).
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| Figure 1. Ratio of Straight Carapace Length (SCL) to Tail Length (TL) for male, female and juvenile Johnstone River snappers captured during the monitoring seasons 2008, 2009 and 2011 and including incidental records from 2008 and 2010, excluding recaptures (n=205). (Freeman 2011 p.6) |
Juvenile:
Female:
Male:
A note regarding Elseya latisternum:
There is a second species of turtle found on the Atherton Tablelands (and through much of Queensland) Elseya latisternum – the saw-shelled turtle. As the turtles are caught by hand it is difficult to avoid catching the saw-shells when under water thus data is being collected on them on the side. We take the same measurements but when caught E. latisternum are merely marked and not tagged as they are a secondary interest and tags are expensive.
Why do we want this data?
These measurements help us compare:
- The populations between monitoring sites
- The differences between genders and ages.
- Marking the turtles helps us follow population trends: births, deaths, immigration and emigration.
The findings thus far:
Despite habitat modification the population is healthy:
- There is a wide range of age cohorts. Populations that are under stress or in decline tend to have a skewed age distribution (either positive or negative) i.e. a larger number of younger or older turtles. However, this means we must continue to monitor the turtle populations as areas of their habitat continues to be developed. (Freeman 2011 p. 13).
- Also, There has been little sign of damage to the animals captured.
The sex ratio has differed over the monitoring years. In other words the number of males to females has not remained constant it has changed from year to year. The study has yet to clarify the status of the species in the Atherton tablelands as it is still in progress.
Potential threats:
Feral animals are one of the biggest threats to nesting sites. In 2010 nests at the ‘Allwood's’ monitoring site were recorded. Predation occurred 3-6 days after oviposition; all known nests were predated and all known eggs destroyed (Freeman 2011). At this time A.Freeman (2011) set up camera traps and snapped a picture of a long-nosed bandicoot and giant white-tailed rat interfering with a turtles nest on the bank of the Johnstone River (Fig 2). Both quite capable of excavating a nest and been recorded eating the eggs of E. irwini (O’Malley, 2007).
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| Fig 2 Two burglars caught interfering with a turtles nest by a camera trap at the Allwood's monitoring site. |
Removal of riparian vegetation might affect the turtles adversely and may already be doing so. Vegetable matter (fruits, leaves and so on) falling from riparian vegetation is a significant source of food for these web-footed herbivores. An interesting further study would be to determine what effect its removal may have on the turtles. Also, is run off affecting the populations?
It is hoped that the monitoring will continue into the medium-term future but of course it all depends on funding and whether those higher up on the ladder see the projects importance.
With issues such as predation, land development and potential
climate change; what is the future for Elseya irwini?
References:
Freeman, A.B. and Curran, T. 2009. Johnstone
River Snapper (Elseya stirlingi) on the Atherton Tablelands, summary report for
the 2008 monitoring season.Unpublished report to Department of Environment
and Resource Management, Atherton. 16p.
Freeman, A.B. and Curran,T. 2010. Johnstone
River snapper (Elseya stirlingi) on the Atherton Tablelands, summary report for
the 2009 monitoring season.Unpublished report prepared for Department of
Environment and Resource Management, Atherton. 12pp.
Freeman, A.B. 2011. Johnstone River snapper
(Elseya irwini) on the Atherton Tablelands, summary report for the 2010
monitoring season.Unpublished report prepared for Department of Environment
and Resource Management, Atherton. 17pp.
O’Malley, A.J. 2007. The ecology of Elseya stirlingi from the
Johnstone River system
of North Queensland. Unpublished Thesis, James Cook University, Townsville. 127pp.
