Sand Flathead account for well over half of the total catch (by numbers) taken by marine recreational fishers in Tasmania and represent the mainstay of Tasmania's recreational fishery. Furthermore, since the recreational catch of sand flathead is more than 20 times the commercial catch, trends in commercial catch and catch rates are of little value in inferring changes in stock status. This has meant that fishery independent or novel assessment methods are required. To date, IMAS has implemented a research program focused on sand flathead in the south-east of the state that provides a spatially restricted, perspective and semi-quantitative evaluation of stock condition. Given the significance of the species and a status of 'depleting' in the latest stock assessment report, there is a need to implement a more comprehensive stock monitoring approach throughout the state that can support the development of a spatially explicit quantitative stock assessment model. There is also a need to determine the appropriate spatial resolution to apply to the stock assessment model. As such, there is a need to understand the extent of adult movement and ontogenetic connectivity of regional sub-populations of sand flathead throughout Tasmania. In addition, by collation of biological data sets from historical studies and surveys conducted around Tasmania the extent and direction of potential changes in population size structures, and life history characteristics will be investigated. Where possible collated biological data will be used to assess spatial and temporal changes in life history characteristics to assess the implications of selective excessive fishing pressure and/or past and future climate change effects for this species.

Size selectivity is an important aspect of southern rock lobster stock assessment models and has been estimated for different fleets, pot types, and areas. Temporal changes in size selectivity can lead to biased model results and can bias parameter estimates, such as pre-recruit indices (PRIs). These biases can manifest as trends that are unrelated to changes in the population characteristic they are designed to monitor. Several mechanisms for changes to size selectivity have created particular concern due to their potential to influence management decisions.

Firstly, in some areas southern rock lobster stocks are increasing significantly and are expected to continue to do so. One way in which these elevated densities may affect selectivity is if large lobsters deter small lobsters from entering pots, thus lowering the estimated PRI (an index used in TACC setting in Victoria)

Secondly, increased price differential between lobster size classes coupled with high CPUE is creating substantial incentives for high grading, both through discards and through changing fishing practices targeting different sized lobsters.

Lastly, seasonal changes in size selectivity have been observed in South Australia and may occur elsewhere. These have not been quantified and may occur in other regions.

Understanding these changes in size selectivity and mitigating the impact on the ongoing stock assessment modelling and harvest strategies will ensure robust assessments and avoid future management bias.