Both the catch and range of marron have reduced over the last 25 years. A re-evaluation of the range will provide both the current extent and potential of the recreational marron fishery (RMF) and allow a re-interpretation of current production of the RMF.
Selecting two indicator sites will allow the focusing of research effort to achieve a new, useful level of detail on fecundity, recruitment and survival in indicator stocks, develop new performance measures and new models of productivity (stock recruitment and yield-per-recruit models). Although focused on indicator stocks, this will produce generic tools applicable to other marron stocks, providing a suite of powerful management indicators.
Changes in legal gears have occurred and these gears are likely to have different efficiencies and may explain a proportion of the decline in marron catches. By quantifying the relative efficiencies of the three legal gears, the historical data set can be re-evaluated to allocate a proportion of the decline in catches to changes in gears and predictions of the impact of future gear restrictions.
Identifying and ranking sources of marron mortality will provide key information on marron survival at various life-stages and allow management to focus resources on important mortality sources.
Environmental variables and management of catchments and water resources profoundly influence the extent and productivity of the entire RMF. The collection of data and development of models will provide fishery managers to identify key influences and engage with other management agencies to promote a more sustainable and productive RMF.
Final report
The distribution of marron in the southwest of Australia has seen many changes since European settlement. Reconstructions of their range from historical records suggested that marron inhabited the waters between the Harvey River and Denmark River. Due to translocation, their range has expanded as far north as the Hutt River and as far east as Esperance. Although at present marron still exist in all the original rivers within the southwest, their distribution within these rivers has contracted. Poor water quality, salinity, low rainfall and environmental degradation in the upper and lower reaches have restricted marron populations.
Historically, management decisions in the Recreational Marron Fishery have been based on fishery-dependent CPUE data collected using a logbook survey and phone survey. A critical assumption has been that the fisheries-dependent CPUE values were proportional to abundance. However, raw or nominal fisheries-dependent CPUE effort data are seldom proportional to abundance and relative abundances indices based on nominal and even standardised CPUE data are notoriously problematic and often provide little useful guidance for management. Although, the fishery-dependent programs provide high quality data on changes in the fishery, in isolation, these data provided limited information on the effects of fishing and the impact of fishery regulations on marron abundance. Standardising the fishery-dependent CPUE data for just one (introduction of snare-only areas during the 1990s) of the numerous management changes illustrated the significant bias in raw, nominal CPUE data. The use of biased fishery dependent data as measures for Recreational Marron Fishery productivity was probably one of the contributing factors limiting the success of developing predictive models using non-fishery variables (e.g. rainfall, river flow).
After a thorough review of (historical) sampling methods, a new fishery-independent annual research program using inexpensive box traps was implemented in 2006. Trapping allowed technical staff to sample several sites (2-4) simultaneously instead of just one site per night. More importantly, traps were set late afternoon and retrieved the following morning, removing the serious occupational health and safety issues associated with the historical late night (18:00- 1:00) sampling trips using drop nets and scoop nets. Furthermore, trapping removed the high level of subjectivity (e.g. operator skill level) associated with the traditional methods, especially scoop netting. Trap data appeared to be the most suitable as an index of relative abundance of marron. Interestingly, comparing trap catches with density data obtained through visual surveys using scuba revealed that at least over soft substrate in dams, trap catches can be used as both a measure of relative and absolute (#/m2) abundance.
