While many global fisheries rely on commercial catch and effort statistics to inform stock assessments and management, alternative approaches are required for small-scale recreational fisheries. In this study, a citizen science program was implemented to collect recreational catch and effort and fishery-independent data using three Western Australian blue swimmer crab (Portunus armatus) fisheries as case studies. Between 2013 and 2019, an opt-in Research Angler Program (RAP) was supported by 102 diarists submitting data from 1,705 fishing events, with direct and regular contact proving important for maintaining participation. Representing the only current recreational crabbing information for the three fisheries, the RAP data proved valuable in assessing management options (e.g. increases in size limits, bag limit reductions, seasonal closures) during a recent review of the Western Australian P. armatus resource. Two follow-up surveys to canvass diarist attitudes towards the program identified a desire to contribute to research and management, and concerns about fish stocks, as key motivations for involvement. In addition, 26 diarists assisted with fishery-independent breeding stock surveys (FIS) to collect valuable stock assessment data. The RAP and FIS proved complementary, as the first provided useful spatial/temporal coverage of catch rates and catch composition, while the second gave a targeted assessment of breeding stock abundance. We demonstrate how the data can generate annual fishery performance indices, such as standardised recreational catch rates and a fishery-independent egg production index, to support a potential harvest strategy. This study highlights a cost-effective and highly valuable citizen science approach to obtain key information for the assessment and management of small scale recreational fisheries, with suggested recommendations for researchers considering a similar approach elsewhere.

Sustainable fisheries management requires accurate stock assessment and an understanding of how targeted populations vary over space and time and respond to external stressors. Commercial fishery catch rate data (i.e. catch per unit of effort; CPUE) are widely used to assess fishery performance and can provide a measure of stock abundance. However, changes in environmental conditions can influence the two determinants of CPUE, i.e. catchability (q) and abundance (N). We explore the influence of environmental variables (water temperature, wind speed, rainfall, freshwater flow, wave height, lunar illumination, tide height and depth) on commercial blue swimmer crab Portunus armatus CPUE in five Western Australian fisheries. Generalised additive mixed models (GAMMs) were used to identify which variables had the greatest influence on CPUE, and enabled predictions of how these can vary with environmental change. Water temperature had a significant positive effect on CPUE in all fisheries, although some fisheries exhibited a CPUE decline above an optimal temperature range. Wind speed and tide height had significant and generally positive (but lower magnitude) effects in three of the five fisheries investigated, while lunar illumination and freshwater flow caused a decline in CPUE in two or less fisheries. When annual CPUE were standardised for these significant environmental effects, they displayed clear variation from nominal (raw) CPUE. These results highlight the importance of measuring and accounting for environmental effects (e.g. through catch rate standardisation) when using CPUE for fishery assessments.

Stock assessments to support sustainable management in data-limited fisheries present a challenge to fisheries scientists and managers. This is the case with the Shark Bay Crab Fishery, which has expanded rapidly in the past 10 years, to become Australia’s highest-producing blue swimmer crab fishery. The resource is harvested commercially by two sectors, the Shark Bay crab trap and trawl fisheries (combined catch of ~800 t), as well as supporting a small but important recreational fishery. Commercial catch and effort data have been collected for the fishery since the early 1980s, commercial trap-monitoring data since 1999, and fishery-independent trawl-survey data since 2001. There is conflicting evidence on the impact that significant increases in catch and effort over the past decade has made on this fishery, such as legal catch rates remaining relatively constant, but declines occurring in the abundance of large crabs. There has also been concern over the level of latent effort in the fishery, with the trap sector currently operating at 70–80% of its potential effort and the capacity for further increases in crab landings by the trawl fleet. Since July 2011, the relative abundance of all size classes of crabs declined significantly. The reasons for this unexpected decline are yet to be understood, but are likely to be linked to adverse environmental extremes (flooding and very warm water temperatures) during the summer of 2010–2011, associated with a very strong La Niňa event. Preliminary assessment indicated that the spawning stock that led to the low recruitment was within historic ranges. The current challenge for the research and management of this fishery is to clarify the causes for this recent decline, and establish suitable biological indicators that will determine the appropriate level of catch and harvest strategy to ensure the future sustainability of the stock.

Commercial blue swimmer crab (Portunus pelagicus) catches in Cockburn Sound, Western Australia, have declined significantly since 2000, due to low stock abundance resulting in closure of the fishery in December 2006. The fishery has remained closed for 3 years with predicted catches, based on juvenile recruitment indices, indicating that recovery has been slow. Like many other blue swimmer crab fisheries, management relied on a minimum legal size set well above the size at sexual maturity to allow crabs to spawn at least once before entering the fishery to presumably provide adequate protection to the breeding stock. However, a combination of biological, environmental and fishery-dependent factors contributed to a collapse and include: (1) vulnerability to environmental fluctuations as this species is at the southern extreme of its temperature tolerance, (2) a life cycle contained within an embayment and is self-recruiting, (3) a change in fishing method from gill nets to traps which increased fishing pressure on pre-spawning females in winter and reduced egg production to one age class, (4) four consecutive years of cooler water temperatures resulting in poor recruitment and (5) continued high fishing pressure during years of low recruitment resulting in low breeding stock. The strength of recruitment (juvenile 0+) from the previous season's spawn (September to January), and the residual stock (1+) near the completion of the current fishing season, have been incorporated in an abundance index that indicates the size of the next season's breeding stock and predicts the commercial catch. This catch prediction has been used in the development of a draft decision-rule framework for the future management of this fishery to determine the appropriate amount of fishing effort. The application of this approach to research and management may assist the sustainability of other blue swimmer crab fisheries at the extreme of their natural distributions.

The geomorphology of Cockburn Sound is unique to the lower west coast of Australia due to the relative paucity of sheltered nearshore marine embayments along this coastline. Numerous studies on many of the commercially and/or recreationally important species that inhabit Cockburn Sound have demonstrated that for many of them this marine embayment constitutes an integral part of their life history. Since the commencement of industrial and urban development in the mid 1950s, in the waters and along the shores of Cockburn Sound, the marine fauna utilising this area have been exposed to ongoing changes in environmental quality. These detrimental, anthropogenic influences have impacted on many attributes that affect the faunal composition, including, for example, habitat alteration and/or loss. Recent data has also revealed that blue swimmer crab stocks in Cockburn Sound are more vulnerable than previously thought with this fishery collapsing in 2005/06. The broader impacts of the declining numbers of crabs on the ecology, i.e. predator-prey relationships, are unknown. Likewise, adult populations of snapper in the lower west coast region have been identified as being low/depleted. Recent studies on the biology of snapper have identified the nearshore marine embayments of Owen Anchorage, Cockburn and Warnbro Sounds as important sources of recruitment for the adult population. Until recently, very little research had been done on the structure of the faunal community in these embayments to which blue swimmer crab, snapper and other commercially and/or recreationally important species belong. A broader understanding of the diversity of fauna that resides in this marine embayment will provide insights into their ecological interactions and thus aid in their sustainable management.