Final Report - 2013-015 - Developing improved methods for stock assessment in spatially complex fisheries using Blue-eye Trevalla as a case study
The first spatial mapping of Blue-eye Trevalla stocks in Australian waters was completed between 2013 and 2016 using a variety of techniques, novel approaches, leading edge technology, a synthesis of historical data, and input from knowledgeable commercial fishers.
Each of our three primary analyses provided evidence for stock structure within the broad southern Australian distribution of Blue-eye Trevalla: spatial differences in age and growth (phenotypic variation) and otolith chemistry of the adult life stage implied there was local and regional residency by adults. Dispersal potential indicated a broader scale connectivity amongst regional populations was likely during early life. By overlaying these spatial patterns, we identified four broad Blue-eye ‘stock areas’: West, South, East and Seamounts-Lord Howe. Each of these stock areas represents an interconnected ‘meta-population’, i.e. a group of discrete adult sub-populations resident on the continental slope and seamounts without extensive migration between them. Stock areas do not reflect truly separated biological stocks because there is some exchange between them during pelagic early life history, and some of the adult sub-populations act as larger ‘sinks’ than others, i.e. benefiting more from recruitment derived from ‘upstream’ spawning areas.
These findings will help identify spatial management options for fishery managers (other than additional spatial closures). New options have the potential to enhance the effectiveness of the present stock assessment and management arrangements through an appropriately-set Total Allowable Catch (TAC). Options would need to account for the area of Blue-eye habitat already protected in each of the stock areas, and a preliminary analysis is provided. This study can be used as the basis to develop and evaluate options for other species with similar characteristics of broad distribution, unknown spatial sub-structure, and management plans that presently lack spatial and temporal dimensions.
A multidisciplinary approach increased certainty in the results by enabling a comparison and validation of conclusions from different methods. The work was only possible though a collaboration that combined the knowledge and specialist skills of a group of researchers from CSIRO, Fisheries Victoria, Geoscience Australia and the Fish Ageing Services (FAS), and with the generous contribution of time and knowledge from commercial fishers.
Principal Investigator: Alan Williams
Key Words: Blue-eye Trevalla; Hyperoglyphe antarctica; stock structure; age and growth; otolith chemistry; dispersal modelling; stock assessment; fishery spatial management
Summary: Blue-eye Trevalla is a deep water fish that commands a premium price as an iconic seafood species in Australia’s domestic fish markets. Despite this prominence, a number of important knowledge gaps have hampered its fishery management in Australian waters. Thus, although the species ranges widely across southern temperate Australia and extends to sub-tropical latitudes on both east and west coasts, it is currently managed as a single stock. The primary developmental need for the Blue-eye stock assessment is a better understanding of ‘stock structure’.
Defining Blue-eye Trevalla stock structure was the primary aim of the project. In this context, stock structure means spatial patterns that delineate natural sub-populations, which represent meaningful sub-units for managers of the Blue-eye fishery.
The project found independent lines of evidence for stock structure (‘stock areas’) within the broad southern Australian distribution of Blue-eye Trevalla. This indicates the need for some form of spatial management of the resource, and provides the basis for developing options for managers of the Blue-eye fishery. The value of the results for all stakeholders, especially the fishing industry, is to provide a greater confidence in the stock assessment leading, potentially, to a less risk-averse setting of the Blue-eye Trevalla TAC.
It is recommended that project results be considered in relation to potential spatial management options (other than additional spatial closures) for the Blue-eye Trevalla resource in Australian waters. These options include separate TACs for different stock areas, a voluntary agreement about the spread of catch across stock areas, or some other management initiative that helps prevent serial depletion (sequential over-catch in certain areas), and considers ‘upstream’ management of ‘source areas’ to help ensure that downstream ‘sink’ areas are replenished.