Ecology and conservation of the critically endangered red handfish (Thymichthys politus)

Marine ecosystems are currently facing an unprecedented level of anthropogenic pressure, experiencing threats such as climate change, pollution, fishing pressure, and habitat degradation. The accumulation of these threats in coastal ecosystems, where human-induced pressure is often most intense, can be especially detrimental to coastal benthic species with limited ranges. One group of fishes that is especially vulnerable is the family Brachionichthyidae (handfishes). Handfishes are small demersal anglerfishes endemic to south-eastern Australian waters, and represent one of the most threatened families of marine bony fishes in the world. One member of this family, the red handfish (Thymichthys politus), is currently only known from two fragmented locations in Tasmania’s southeast. The species is thought to be at high risk of extinction, with numbers that have significantly declined over the past few decades to the point that it is considered by some to be the rarest marine fish species in the world. Despite the high profile of the species for its charismatic nature and conservation concern, little is known about it. Before this PhD research project, no formal population estimates had been made, and basic population dynamics of the red handfish (such as growth, movement, and habitat preference) were also uncertain. Given that this knowledge is vital for the management of any species, an increased understanding of the species was needed so that conservation efforts can be implemented more effectively.

The primary goal of my PhD research was to investigate aspects of the ecology of the Critically Endangered red handfish to inform future conservation of the species. Specifically, my research aimed to: (1) Evaluate the detectability of red handfish using both environmental DNA and underwater visual census to assist in monitoring known populations and for locating new ones; (2) Establish the first estimates of population size for red handfish, and estimate other basic biological parameters, such as growth, movement, and habitat usage; and (3) Identify key threats to the red handfish and prioritise future conservation action.

The first analytical chapter of this thesis sought to understand the effort required to search for undiscovered populations of small, cryptic, marine species in shallow water vegetated coastal habitats, using the red handfish as a representative species. I designed a sampling strategy using a combination of environmental DNA (eDNA) and structured underwater SCUBA surveys to estimate detectability across large (hundreds of meters to kilometres) and small (< 100 m) scales. At large scales, the eDNA assays detected red handfish presence in situ, but inconsistently, and only up to ~100 m away from a known population. At smaller scales, however, data from underwater censuses of planted 3D-printed handfish by SCUBA divers revealed that diver-based detection and monitoring of red handfish is likely to be highly effective, giving confidence that population trend data collected by this method provide an adequate representation of trends in the true population size. Because the habitats covered in this study were typical of shallow coastal environments along sheltered temperate coasts, the lessons from this research may assist researchers of other rare or threatened species that are similarly small and inconspicuous.

In the second analytical chapter, I aimed to provide the first estimates of the population size of red handfish and reveal other important aspects of their life-history, such as growth and movement. Over three years, I monitored both of the currently known red handfish populations via SCUBA surveys (using the methods tested in the first chapter) and used photographic mark-recapture techniques to estimate these biological parameters. The study confirmed abundance of the species is very low, at around 100 total adult individuals, with one site having a population of only ~7 adult fish. The study also indicated that red handfish predominately utilise species of brown algae and seagrass as habitat. This is important because these habitats are currently under threat from overgrazing by increasing sea urchins in high densities at the red handfish sites. I also observed low net movement in the red handfish, indicating the importance of local management for their conservation (e.g., site protection and prevention of habitat destructive processes).

In the final research chapter, I developed a risk assessment-based approach that accounts for cascading ecological changes and indirect impacts between human and environmental pressures for threatened species. I then applied this approach to the red handfish to identify the greatest threats to the species, and to prioritise future conservation action. The study identified the unique life history strategy of handfishes, coastal warming, indirect ecological pressures caused by recreational fishing, urban development, and poaching as the greatest current threats to the persistence of the species. The mitigation options identified to have the greatest immediate reduction in extinction risk included an ex situ captive population and release program to bolster numbers in the wild, and engagement with the commercial sea urchin fishery to help reduce impact within critical habitat.

Overall, my research greatly increased our understanding of the red handfish from a practical perspective and will be used to refine conservation and management of this species to aid its recovery. The testing of approaches and development of tools I undertook to aid searches for new populations, evaluate monitoring methods, provide population estimates, and evaluate threats are all potentially applicable and of importance to other small and inconspicuous marine species that may be under threat. We are thus, now not only better positioned to inform the conservation of red handfish, but also to potentially better understand the status and address threats to other data[1]poor coastal marine species. Future directions for research include refining my eDNA assays, conducting genomic studies to understand the genetic diversity and differentiation of the two red handfish populations, and habitat preference research.