Maugean skate (Zearaja maugeana)

Applicant: University of Tasmania

Species/Taxon: Maugean skate, Zearaja maugeana

Location: Animals will be collected from Macquarie Harbour and housed in a temporary facility in Strahan and a longer-term facility in Taroona, Tasmania

Title of research: Creating an Insurance Population for Maugean skate

Aim of project: The overarching aim of this project is to establish a Maugean skate insurance population outside of Macquarie Harbour. 

Justification: The endangered Maugean skate is only known from two isolated estuarine systems located on the west coast of Tasmania, Bathurst and Macquarie Harbours. However, a recent environmental DNA study demonstrated that the vast majority, if not all, of the remaining Maugean skate live only in Macquarie Harbour. 

There is mounting evidence that low dissolved oxygen conditions are impacting the Maugean skate population in Macquarie Harbour, including inducing mortality events. Using catch per unit effort (CPUE) during monitoring activities as a measure of relative Maugean skate abundance, IMAS demonstrated a substantial decline in CPUE between 2014 and 2021 of 47%. The scale of the overall decline and the scarcity of new recruits created significant concern for the conservation of the species and highlighted the need for immediate action.

In response to this significant concern the Maugean Skate Recovery team was co-established by the Commonwealth Department of Climate Change, Energy, Environment and Water and the Department of Natural Resources and Environment Tasmania (NRE Tas). One of the first acts of this recovery team was to commission a population viability analysis (PVA) of the Maugean skate. A PVA is a species-specific method of risk assessment frequently used in conservation biology. It is traditionally defined as the process that determines the probability that a population will go extinct within a given number of years. 

The PVA estimated a best-case scenario of an 88.9% population decline by 2041 and a worse-case decline of 99.7% including extinction probabilities of >25% by 2041. The PVA also noted that while the probability of full extinction in 2041 is relatively low, the species would be functionally extinct prior to this date without significant intervention – hence the need for an insurance population. 

It is anticipated that Maugean skates bred in captivity may be released back to Macquarie Harbour to supplement the wild population in the future. 

IMAS are experts with significant experience in elasmobranch biology and ecology, and are also collaborating with the Zoo and Aquarium Association (ZAA), various skate specialist researchers and aquarists around the world, along with a specialist elasmobranch vet.

Maximum likely numbers of individuals involved: 

• Captive population: Up to four adults (two males and two females) and 50 eggs, for long-term captivity, noting that it is estimated that approximately only 40% of eggs will be viable (i.e. 20 viable eggs). 

• Temporary holding: Up to 25 gravid females to be temporarily held in Strahan for 2 weeks to lay eggs and then released back to the wild.  

Note: Temporary holding will only occur if 50 eggs cannot be collected via diving. In this case, collection of eggs laid by captured females may be required. PVA indicates it is important to collect the eggs during the next 12 months, as collections in future yearswill have a significantly greater impact on the viability of the wild population. Initially, 5 females may be captured to trial egg collection in captivity. If the trial is determined to be successful by NRE Tas, a further 20 females may be captured and held in temporary captivity in Strahan to lay eggs and then released back to the wild. Females lay eggs in batches of 2, with months between batches. It is estimated that up to 25 gravid females will be required, because if 2 eggs are collected from each female and 40% are viable, this would result in 20 viable eggs. 

Activities undertaken and methods: 

Collection: Up to 29 adults may be captured by gillnet or baited line during routine monitoring of the Maugean skate. Fifty eggs will be collected in two ways: primarily dive collection, or if insufficient eggs are located, bringing females with eggs ready to lay into temporary captivity, with the females then released after laying their eggs. 

Housing: Four adults and 50 eggs will be transported to IMAS Taroona aquaria to initiate the captive population. Animals and eggs will be transported to the IMAS Taroona facility immediately following capture, in holding tanks containing high levels of dissolved oxygen, with 2-hourly water changes during transport, using best practices. Transport is expected to take around 7-8 hours. IMAS have successfully transported skates, sharks and eggs previously using this method. At IMAS Taroona, the adult skates will be housed in large recirculating saltwater tanks containing high levels of dissolved oxygen, in low light conditions and temperatures similar to Macquarie Harbour. Eggs and juveniles will be housed in a separate tank, with the same system as described for adults. Animals are monitored by staff and a specialist elasmobranch vet.

Up to 25 females may be temporarily housed at Strahan in tanks for up to 2 weeks while eggs are being laid. Eggs are usually laid within 2 weeks. Once the females are released, eggs are to be transported to IMAS Taroona as described above. The Strahan facility will be overseen by vets and will include an Animal Ethics Committee (AEC) inspection. Animals will be transported to the Strahan facility immediately following capture, in holding tanks containing high levels of dissolved oxygen, using best practices. Transport is expected to take approximately 45 minutes. At Strahan, the adult skates and eggs will be housed in holding tanks containing saltwater pumped from Macquarie Harbour, with controlled levels of dissolved oxygen, temperature and filtration, as well as low light conditions similar to Macquarie Harbour.

Fate of animals: 

Captive population: The length of captivity of the animals kept at IMAS Taroona will be determined based on the outcomes of breeding and husbandry success, but it is currently assumed that they will be held for the full term of their life, which will be approximately 4-6 years for the adults and 10 years for the embryos in the viable eggs, assuming a 10-year lifespan. Offspring will be retained to facilitate an ongoing captive population. It is anticipated that Maugean skates bred in captivity may be released back to Macquarie Harbour to supplement the wild population in the future.

Temporary holding: The animals kept temporarily at Strahan will be released back into Macquarie Harbour following egg collection or a maximum of two weeks, whichever occurs first. 

The mortality rates of captured adult Maugean skates is estimated to be 12.5% based on previous studies. There is little information on the mortality rate of Maugean skate eggs, so a precautionary rate of 10% has been applied.

Likely impact on species involved (including any by-catch): The Maugean skate PVA demonstrated that the likelihood for extinction, taking into account the removal of 50 eggs and four adults before the end of 2024, as proposed here, was essentially the same as the scenario where no animals were removed. The animals kept temporarily at Strahan will be released back to Macquarie Harbour, so are not being removed from the population and so are not considered to influence the overall extinction risk.

Scientific permits to collect threatened fauna species are issued for a maximum of 12 months.