The physio-chemical conditions in Macquarie Harbour have changed markedly since European settlement and the general decline in DO since 2009, which occurred at the same time as the rapid expansion of marine farming operations, is likely to have had a significant impact on many resident species, including the endangered Maugean Skate. Given the species poor ability to cope with low DO, any reductions in bottom DO concentrations are expected to directly influence the distribution of the skate, highlighting the need to better understand the oxygen demands of the species. Current generation acoustic tags capable of measuring DO and activity in the field, which when combined with laboratory studies, represent methods to examine tolerance and responses to varying levels of DO. Bottom DO also has implications for the development and survival of skate eggs. Limited information suggests that eggs are deposited at depths of > 20 m where they are likely to be increasingly exposed to low DO concentrations. An understanding of the relationships between environmental conditions and the development and survival of embryos, coupled with the depths in which eggs are deposited, has particular relevance to assessing the implications of changing environmental conditions on future recruitment and productivity of the Maugean Skate population.
Furthermore, the ability to monitor the status of this population, especially given the rate and extent of recent environmental changes in Macquarie Harbour is critical to assess its future viability. Tracking size composition, including data from previous studies, has the potential to provide a baseline against which reduced recruitment (if linked to the changed environmental conditions) could be detected.
The maintenance of best environmental practices by the aquaculture industry supported by effective monitoring and environmental management policies represent essential requirements if industry and Maugean Skate populations are to coexist.
The primary aim of this study is to establish the mechanistic links between the environmental conditions experienced by the Maugean Skate across multiple life stages and the implications of the changing environment in Macquarie Harbour. Specific objectives include:
- Determine the distribution of Maugean Skate eggs within Macquarie Harbour, with particular reference to depth and bottom DO.
- Examine the relationship between environmental conditions and survival and viability of Maugean Skate eggs.
- Determine what DO levels Maugean Skate experience in Macquarie Harbour, particularly when in deeper waters.
- Determine the physiological costs of the DO levels Maugean Skate are exposed to in Macquarie Harbour.
- Assess the potential of using the size composition of Maugean Skate catches as an indicator of population change, in particular recruitment variability
- Assess the implications of declining DO concentrations in Macquarie Harbour on the future viability of the Maugean Skate population.
The study comprised five main components: (1) systematic field surveys of skate egg distribution, (2) laboratory study of embryo development; (3) field assessment of skate behaviour in relation to environmental conditions; (4) laboratory assessment of physiological responses to conditions experienced in the wild; and (5) fishery-independent surveys to examine the potential to inform on changes to population status.
A combination of beam trawl and dive surveys was employed to sample for Maugean Skate egg cases in areas of known high skate abundance. Egg capsules collected in the field were reared in the laboratory to describe embryonic development and gestation period. Acoustic telemetry was used to examine links between environmental conditions (i.e. dissolved oxygen and temperature) and the distribution, movement and habitat use of the Maugean Skate within Macquarie Harbour. An acoustic array comprising 52 acoustic receivers was deployed in the Table Head/Liberty Point region of Macquarie Harbour and 25 adult Maugean Skate were externally tagged with multi-sensor acoustic tags capable of continuously transmitting information on the depth, temperature and dissolved oxygen concentrations experienced by the individuals over a 12-month period. Physiological trials were conducted in a purpose-designed field laboratory to examine physiological responses to low DO and low salinity exposure, replicating environmental conditions experienced by the skate in Macquarie Harbour. Gillnet surveys were undertaken to extend the time series of Maugean Skate size composition data and evaluate its potential as an indicator of changing population status.
Maugean Skate eggs were found across a wide range of depths (2.5 - 33 m) but appear to be most abundant in relatively shallow sites (< 10 m). Based on an analysis of the condition of egg-capsules, hatching success was estimated to be about 40%, which is comparable to success rates reported for other temperate skate species. An egg reared under laboratory conditions hatched 31 weeks (~ seven months) after oviposition. Respiratory channels opened after 15 weeks, exposing the embryo to external environmental conditions for over half of the gestation period. These findings indicate that, depending on the depths at which eggs are deposited in the wild, embryos will experience a range of dissolved oxygen, salinity and temperature conditions, reflective of localised fluctuations in environmental conditions. The implications of these varying conditions for development and survival are unknown.
This study represents the first use of animal-borne acoustic sensors to monitor long-term dissolved oxygen conditions experienced by a coastal elasmobranch and the links with behaviour. The results showed that skate are subject to wide ranging fluctuations in water chemistry. For instance oxygen levels experienced ranged from normoxic (50-100%), hypoxic (20-50%) and to near anoxic (~0-20%), with individuals often experiencing substantial variation (as high as > 90%) in the range of DO levels within the same day.
This study provides a greater understanding of the ecology and life history of the Maugean Skate and describes for the first time some of the behavioural and physiological adaptations that have enabled the species to survive in such a unique and challenging environment as Macquarie Harbour. Our results suggest that the species can mitigate some environment variability by a combination of behavioural (movement) responses and physiological capability. However, the vulnerability of early life stages to the changing environmental conditions, long-term changes in the size structure of the population, and the mortality of some tagged individuals following significant environmental events collectively highlight the vulnerability of the Maugean Skate in Macquarie Harbour and the need to consider further conservation action to support the persistence of this unique micro-endemic skate.