Modelling adult abundance and habitat distribution of Maugean skate

Declines in the Maugean skate population have coincided with the degradation of environmental conditions across its sole remaining habitat: Macquarie Harbour on Tasmania’s west coast.

Existing population estimates for the skate are highly uncertain, with no robust means of assessing change over time, particularly for juveniles. The harbour presents challenging environmental conditions and individual skates are difficult to find in a reliable way due to their scarcity, behaviour and mimetism (ability to blend into their environment).

The Maugean Skate Recovery Team and the 2023 Australian Government Conservation Advice for the Maugean skate identified the investigation of close-kin mark-recapture analysis as a priority action for determining historical abundance of the species in Macquarie Harbour, and potentially as a complimentary monitoring tool.

The first component of this hub project is investigating the use of close-kin mark-recapture analysis to estimate abundance and other demographic parameters (population trend, survival rates, connectivity) for the Maugean skate. If successful, this technique will build upon and complement existing population monitoring activities.

Maugean skate are thought to be adversely affected by low dissolved oxygen in Macquarie harbour. This is influenced by combinations of external factors (industrial usage, manipulation of flow regimes from riverine inputs) and climatic/weather events. Remediation trials are testing whether low dissolved oxygen conditions can be ameliorated through mechanical oxygenation.

The trial process requires linking biophysical models of the harbour (which predict dissolved oxygen, temperature and salinity) to observations of the habitat conditions observed to be selected by the skate.

The second component of this project is combining these two data sources in habitat selection models. Subsequent modelling will be used to generate maps of suitable skate habitat and predict whether remediation, climate or other factors are likely to affect habitat availability.

Approach

Close-kin mark-recapture

Close-kin mark-recapture has been developed and applied by CSIRO for various finfish and elasmobranch population analyses (including in NESP projects). Genetic sequencing is used to determine the prevalence of closely related individuals (parent-offspring/half-siblings) within a set of sampled animals. This information is used in population models to estimate the abundance of the breeding adults, adult survival rates, and, where possible, population trend.

CSIRO is leading this component of the project in collaboration with the Institute for Marine and Antarctic Studies (IMAS) at the University of Tasmania. IMAS has provided Maugean skate DNA wild-caught skate tissue samples collected under various research programs.

The detection of kin-pairs may be challenging given that the Maugean skate population is likely to be highly related (see Project 3.16). Nevertheless, the method is likely to will provide useful information on population status and breeding dynamics. Kin relationships among animals captured for the ex-situ breeding program at IMAS may also be determined to assist with the analysis.

Habitat modelling

The habitat modelling is drawing on biophysical predictions from CSIRO modelling of Macquarie Harbour, and IMAS telemetry data detailing the skates’ selected temperature, depth and oxygen conditions. Initial maps of potential habitat will be generated for a hindcast period (2017–2018) when the CSIRO Macquarie Harbour model was demonstrated to accurately reproduce observed water quality.

Subsequent models developed in a parallel project funded by the Department of Natural Resources and Environment Tasmania will simulate recent and evolving water quality in near real time. Maps of suitable skate habitat will be generated as the modelling results become available. The simulated habitat maps will be assessed against the known collective knowledge of skate habitat.

Expected outcomes

The close-kin mark-recapture analysis and habitat modelling produced by this project will contribute to conservation and management decisions for the Maugean skate, at a national, state and local level.

If successful, the close-kin mark-recapture analysis will provide independent estimates of the size of the Maugean skate spawning population in Macquarie Harbour, and assist in determining future monitoring options.

The habitat mapping has the potential to help identify critical habitat areas in Macquarie Harbour and future sampling strategies. It may also enable management scenario model simulations to assess the potential impact of future environmental management interventions (such as a change in river flow conditions) on the footprint of skate habitat.

Project location