Evaluation of the recreational marron fishery against environmental change and human interaction
de Graaf, M., Beatty, S. and Molony, B. (2010) Evaluation of the recreational marron fishery against environmental change and human interaction. Western Australia. Dept. of Fisheries, Western Australia.
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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 fisherydependent 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.
There were large differences in the fishing mortality and rates of exploitation of the two key indicator stocks, Wellington Dam and the Warren River. Fishing mortality and subsequent rates of exploitation were much greater in the habitat poor dam stock relative to the relatively pristine river stock. This was attributable to the differences in habitat and food availability between these systems. Marron are also susceptible to teleost fish predation during the juvenile part of their life history. For example, small (<30 mm Orbital-Carapace Length, OCL), juvenile marron were preyed upon by feral/introduced fish like redfin perch and trout and native predators such as, freshwater catfish, aquatic birds, longneck turtles and water rats. However, the current study did not find native predators to have a significant impact on marron recruitment to the fishery. Among the feral/introduced predatory fish, redfin perch (>20 cm Standard Length, SL) consumed by far the most marron throughout the year. Small trout feed predominantly on insects with larger individuals (>30cm SL) shifting towards fish and crayfish, including marron. The highest marron densities were found in water bodies with 2 Fisheries Research Report [Western Australia] No. 211, 2010 complex hide habitat despite large numbers of introduced predatory fish like redfin perch, rainbow and brown trout; highlighting the importance of complex habitat in reducing intra and interspecific predation and maintaining recruitment to the fishery. As such, no evidence was found to support the hypothesis that marron stocks are significantly (recruitment) limited due to predation in systems with complex habitat.
The Recreational Marron Fishery has targeted (adult) marron since the introduction of a minimum legal size of 57 mm OCL in 1952. The reduction of the fishing season from 135 days (~500,000 marron) in the 1970s and 1980s, to 16 days (~50,000 marron) in the early 2000s, has significantly reduced the impact of fishing mortality.
Marron densities and population structure were strongly influenced by habitat type. Both trapping and visual surveys using scuba, clearly demonstrated that complex habitat sustained higher densities of marron over a wide size range, while fewer, but mainly larger marron, occurred over soft, flat substrate.
The key indicator populations studied in the Warren River and Wellington Dam had the highest densities of marron of all the surveyed water bodies but the population dynamics in each dam were strikingly different. In both water bodies, juveniles initially grow at roughly the same rate but the population in Wellington Dam is stunted, very few animals larger than 60 mm OCL. While in the Warren River animals between 60-90 mm OCL were common and continued to grow, tagging data showed that in Wellington Dam adult marron would moult but not increase in size. The study demonstrated that the Wellington Dam stock was effectively fully exploited and had very low productivity. By contrast, the Warren River stock had relatively low fishing exploitation and a high productivity driven by higher growth rates of trappable individuals coupled with higher densities. The differences in densities and growth of larger individuals were attributed to differences in habitat quality and food resources. The differences in exploitation rates were attributed to the greater fisher accessibility in the dam relative to the river. These differences are likely to be similar in other dam and river stocks (with the exception of Harvey Dam) and thus have direct implications for the management of the two sectors of the fishery.
A key finding of the study is that water quality (salinity) and the quality and availability of complex habitat (intact riparian vegetation providing woody debris in rivers and a lack of complex habitat in dams) are the critical bottlenecks restricting marron distribution and abundance in the southwest of WA.
Little temporal and spatial variation was observed in both ovarian (potential) and pleopodal (effective) fecundity among a dozen marron populations. Size-at-maturity showed little temporal but considerable spatial variation ranging from 30-70 mm OCL. Preliminary results indicated that at present the minimum legal size in the Margaret, Blackwood and Moore Rivers might be insufficient to protect the female breeding stock. Furthermore, it appeared that in some populations (e.g. Shannon River), females were not reproductively active every year and may trade-off growth for reproduction. Alternating between growth and reproduction has been demonstrated for Tasmanian and Victorian freshwater crayfish populations where low water temperatures restrict the growing/breeding season.
Limited information is available on the early phases of juvenile marron in the field. Visual surveys demonstrated that in dams juvenile abundance is highly correlated with the presence of complex, hard substrate. Several designs of ‘habitat traps’ were successfully tested in Wellington Dam and Warren River to capture juvenile marron. However, the design of the ‘habitat trap’ will need some further adjustment and testing, before this method can be used as a reliable recruitment abundance index.
Tagging juvenile marron with small (<1mm), internal coded micro wire tags was highly successful and proved to be a potentially powerful tool to study juvenile biology in the field. Laboratory experiments showed that tagging had no effect on mortality or growth of small juvenile marron and tag retention was high.
The degradation (salinisation, reduced surface and groundwater inflow) of the rivers in the southwest is a threat to both the recreational marron fisheries and the marron populations upon which it is based and has resulted in reduction of the inland range of the species. The exclusion of recreational fishing from public dams (e.g. Stirling Dam and possible Logue Brook Dam and Wellington Dam in the near future) reduces the size of the marron fishery but simultaneously creates unofficial protected areas for marron and other native aquatic fauna.
The major threat to the ongoing sustainability of the fishery is the decline in the health of the freshwater systems in which it currently occurs. As such, a key recommendation is to promote inter-agency cooperation in addressing the decline of river health; particularly with regard to water quality and riparian vegetation.
The future of the recreational marron fishery lies in also promoting it as a skilful, exciting, inexpensive and fun outdoor activity that can be enjoyed by the whole family.
As such, creation of a snare-only, ‘Trophy’ marron fishery with conservative bag and possession limits throughout the whole fishery would greatly simplify research, compliance and management. More importantly, such an approach would allow for a considerable increase in season length that will allow more people to participate or organise multiple trips, enhancing visitation to the southwest region and further contributing to regional economies.
|Murdoch Affiliation:||Centre for Fish and Fisheries Research|
|Series Name:||Final report to Fisheries Research and Development Corporation on Project No. 2003/027. Fisheries Research Report No. 211|
|Publisher:||Western Australia. Dept. of Fisheries|
|Copyright:||2010 Western Australia. Dept. of Fisheries|
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