The commercial scallop (Pecten fumatus) fisheries in south-eastern Australia have long been characterised as boom-and-bust (Tracey & Lyle 2010). While historic overfishing has contributed to this (Young 1989), unpredictable cycles of alternating abundance and large-scale die-off characterise the species, particularly in the eastern portion of the region. For instance, there have been five sudden die-offs on the eastern side of the Tasmanian fishery (TSF) and Commonwealth fishery (BSCZSF) combined since 2005. Whereas, since most recently being fished in 2014, the scallop beds in the King Island region of the BSCZSF have been harvested each year due to predictable and constant recruitment and scallop conditioning.
The relative difference in predictability between the regions likely lie with the changing nature of the EAC on the east coast bringing warm, nutrient-poor water to the east coast and the Leeuwin current bringing cold nutrient-rich water to the west coast, with these differences likely to be further exacerbated due to climate change. A case in point is the Tasmanian fishery, which after being closed for five years due to the stocks being depleted, opened in 2021 off Babel Island (east) only to find the bed had died-off only a few months post-preseason surveying. A sudden influx of warm water was likely the cause of the die-off, with beds in the eastern portion of both the BSCZSF and the Victorian scallop fishery (OSF) simultaneously suffering a significant loss of condition but not death (Semmens unpublished). In 2022, again a major die-off has impacted the TSF, with beds at White Rock (east) found to be dying off upon opening in late June. The unpredictability of these die-offs confounds management decisions, as a lack of understanding into the drivers of die-offs means that even if beds with commercially significant biomass are surveyed and opened, they may be lost before fishing begins. There is a clear need to understand these die-offs, determine if they can be predicted and adapt management such that it can be reactive and is tailored to the region in which the bed occurs (e.g., east vs west). Fitting management strategies to the fishing region also makes sense biologically, with the east and west portions of the species’ distribution displaying different life history features (e.g., spawning and settlement times, growth rates, etc; Semmens et al. 2019) and this may be a contributing factor to die-offs.
This project will use a collaborative industry/management/research approach to investigate the factors causing mass die-off of scallop beds, characterising the impacts of stressors including fishery practices, such as the use of tumblers, survey method (e.g., dredge vs video) and environmental factors, such as location of beds, sea temperatures (considering both absolute temperature and rate of change) and food availability, and assessing them in a framework that fits management practices to the relative risk of loss of fishable stock. Developing an understanding of the factors driving mortalities will also enable evaluation of existing data capture capabilities to identify whether potentially harmful conditions can be identified before beds are lost. Where deficiencies are identified, new data collection techniques will be evaluated, including video surveying of closed regions (both permanent, e.g., MPAs, and fishery closures) to allow more flexibility in decision making around when an area should be fished. The outcome of this research will provide the evidence needed to develop a decision-making framework that will enhance the rapid response capabilities of management of scallop fisheries in the future, but also ensure that they fit the changing environment and region within which the stocks sit, improving the sustainability of this vulnerable industry.

There are areas of the Western Zone Abalone Fishery where Greenlip Abalone are depleting, with biomass levels well below carrying capacity and historical levels. Some areas may not recover quickly without intervention and recovery may be further impeded by climate change. Thus, the Western Zone wild-catch abalone industry is seeking to establish a commercial-scale stock release program to accelerate Greenlip Abalone stock recovery in South Australia using release of hatchery-reared juveniles. The Central Zone wild-catch abalone industry is seeking to establish a commercial-scale stock release program to re-build Greenlip Abalone stocks in depleted areas that will use hatchery-reared juveniles.
There are two key needs for commencing a stock recovery program using hatchery-reared juveniles. The key industry need is to test release of juvenile Greenlip Abalone in the Western and Central Zones to evaluate the long-term economic viability. To support this important industry goal, the key Government need is for data to underpin release policy. This includes knowledge of the geographic distribution of Greenlip Abalone genetic differentiation (after Miller et al. 2014, Sandoval-Castillo et al. 2017), to inform policy review.

References:
Miller et al. 2014 – Molecular genetics to inform spatial management in benthic invertebrate fisheries: a case study using the Australian Greenlip Abalone.
Sandoval-Castillo et al. 2017 – Seascape genomics reveals adaptive divergence in a connected and commercially important mollusc, the greenlip abalone (Haliotis laevigata), along a longitudinal environmental gradient.

Critical knowledge gaps identified by the cross-sectoral harvest strategy working group are encapsulated within three priority areas for mulloway in NSW:

1) Information on the spatial extent of population structure

Whilst mulloway in NSW have been shown to be part of a single genetic stock along the east coast (Barnes et al. 2015), which is managed at the jurisdictional level (Queensland, New South Wales, Victoria – Earl et al. 2021), the overall general small scales of movement and connectivity (Hughes et al. 2022), and spatial variation in otolith chemistry (Russell et al. 2021), suggest the potential for fine-scale population structuring within the broader stock. Such population structuring may occur over various time scales (e.g. evolutionary, generational or lifetime) relevant to management of the species. Identifying the spatial extent of population structure is therefore critical to inform the potential utility of spatially structured monitoring, assessment, and management of the species in NSW, including the potential need for cross-jurisdictional collaboration with Queensland and Victoria.

2) Refined and updated population life-history parameters

As described above, evidence indicates the potential for fine-scale within-generation population structure of mulloway within NSW, as has been demonstrated elsewhere in Australia (Ferguson et al. 2011). For mulloway in NSW, sub-populations may be subject to variation in environmental variables (e.g. habitat, water temperature, salinity), particularly those that vary with latitude. Such population structure may therefore manifest itself in spatial variability in demographic characteristics, such as growth, size and age composition, and mortality that affect stock productivity and subsequent resilience to exploitation. Information on mulloway reproductive biology was collected in the early 2000s and established size- and age-at-maturity (Silberschneider & Gray 2005), however information on the spatial and temporal extent of spawning is not clearly defined and the body-size fecundity relationship for mulloway in NSW is not well known. An updated examination of spatial variation in size and age structures, growth, mortality and reproductive biology are therefore urgently required to underpin length- and age-based components of future stock assessments for mulloway in NSW.

3) Assessment of gear selectivity and discard/release mortality for the main fishing methods.

Despite the majority of the commercial mulloway catch (~60%) being taken using gillnets (termed ’mesh nets’) in NSW, to date there has been very little research into selectivity, bycatch, discarding and post-release mortality of mulloway caught in this gear. Research on discard (‘release’) mortality from recreational fishing has shown that the two key predictors of mortality are deep-hooking (Butcher et al. 2007) and barotrauma (Butcher et al. 2013, Hughes et al. 2019), however, most of this work was restricted to small mulloway ( 45 cm TL) and no data are available on the fate of larger angled and released conspecifics. Research into the selectivity, rates of discarding and unaccounted fishing mortality of mulloway caught in the main gears and sectors are therefore urgently required (e.g. by defining selectivity functions and rates of discarding and post-release mortality for use in future stock assessment models).

Other knowledge gaps fall under priority areas already being addressed by existing NSW DPI-Fisheries initiatives (e.g. improved fishery data from all sectors, development of fishery-independent survey methods and updated comprehensive ERA; Figure 1).

Successfully fulfilling all knowledge gaps will generate data that will underpin a fourth priority area:

4) Development of a dynamic population model.

This is the essential tool that will be developed to reduce uncertainty in the species stock assessment, service the requirements of the harvest strategy to rebuild the stock, and guide future management to maintain the stock at a level that improves access to, and use of, the resource by all sectors. Any model(s) must also support an expandable assessment approach, capable of determining stock status with reasonable confidence from limited data available during the stock rebuilding phase, but with the ability to integrate additional data sources as they become available (once the rebuilding phase is complete) and maintaining continuity with previous assessments. The role of climatic/environmental drivers on mulloway population dynamics will also be examined within the integrated assessment model(s) that will be developed.

Without the improved knowledge encapsulated in the above priority areas for research, any reasonable assessment of the status of the resource, estimation of appropriate harvest levels, harvest strategy development, and implementation of appropriate management to rebuild the resource and maintain sustainability in future, will not be possible. This will in turn directly impact the magnitude, profitability, and social outcomes derived from the resource. Funding from the FRDC is therefore needed to address these key identified knowledge gaps, representing an urgent research priority for all harvesting sectors of the resource in NSW.