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and the Victorian scallop fishery (OSF) simultaneously suffering a significant loss of condition but not death (Semmens unpublished).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.