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.

Sound fisheries management requires knowledge of how the reproductive biology of targeted species varies over space and time and responds to external drivers. We determined the size at maturity and fecundity of the blue swimmer crab Portunus armatus (A. Milne-Edwards, 1861) (Brachyura: Portunidael; along the west coast of Australia at monthly, annual and interdecadal (1990s versus 2010s) time scales. The size at maturity of females determined by abdominal morphology (carapace width (CW) at 50% maturity (CW50) was highest in sub-tropical Shark Bay at 26 degrees S (104 mm), and declined latitudinally to temperate Geographe Bay at 34 degrees S (93 mm). A notable exception was in a marine embayment at 32 degrees S (Cockburn Sound), where CW50 was significantly lower than all other locations (83 mm). In contrast, batch fecundity was lower in sub-tropical (830,000 eggs at 127 mm CW) than temperate waters (950,000-990,000 eggs). Temporally, CW50 was significantly higher during January-June (typical growth period, encompassing the warmest months of the year) than during July-December (breeding period), and also differed between months, years, and decades. Positive correlations between water temperature, monthly and annual estimates of CW50, and functional maturity (size at first ovigery), were detected in most locations. Annual CW50 also increased with population density, particularly in locations where abundances and biomass were lowest. Results highlight plasticity in the reproductive biology of this portunid in response to environmental drivers such as temperature. Regular review of such parameters is important for ensuring robust fisheries management and stock assessments.

The relationship between carapace width (CW) and weight (W) of the blue swimmer crab Portunus armatus (A. Milne-Edwards, 1861) was determined for southwestern Australian populations using measurements collected over two decades from a range of nearshore marine and estuarine environments. A strong and significant positive linear relationship occurred between the natural logarithm (ln) of CW and ln(W) for both sexes overall and individually for males and females (R-2 = 0.985-0.987; P < 0.001). The relationships were: overall ln(W) = -10.2262 + 3.1708 ln(CW); males ln(W) = -10.4641 + 3.234 ln(CW), and females ln(W) = -10.0595 + 3.1246 ln(CW). Significant differences in the relationship were detected between sexes, decadal sampling periods, environments, and seasons (P < 0.001), with sex and decadal period being most influential. Males were heavier than females at any given CW, which was most apparent at larger (>100 mm CW) sizes. Crabs were also generally heavier at CW in a recent sampling period (2014-2019) than historically (1995-2000), with the greatest decadal differences occurring in the estuarine environment. These relationships have direct applicability for use in stock assessments of this valuable fishery species. Moreover, observed changes in the CW-W relationships over time highlight the importance of regularly reviewing biological parameters used in fisheries assessments, particularly in response to climate change as in the case of southwestern Australia.

Day-night shifts in the nearshore fish fauna of a temperate microtidal estuary were assessed using a holistic suite of structural and functional community attributes. Mean fish species richness and diversity (taxonomic distinctness) were higher at night across all regions of the estuary and seasons, concurring with the findings of numerous comparable studies reviewed worldwide, while the diel period in which mean abundance was higher varied among seasons. Likewise, species and functional guild compositions (the latter based on feeding modes and habitat use) both differed significantly between day and night, with the extent of the diel shift again varying seasonally. Daytime fish communities were characterized by higher abundances of Atherinidae, Sillaginidae and Mugilidae, while Gobiidae were far more abundant at night. Marked shifts in size composition were also evident, with smaller fishes (<100 mm total length, LT ) being more prevalent during the day and larger fishes (≥200 mm LT ) proportionally more abundant at night. The above diel shifts were feasibly related to a range of predator-prey interactions and feeding-related movements, namely a nocturnal decrease in top-order avian piscivory coupled with an increase in invertebrate prey availability, resulting in changes in the presence and catchability of certain fish species in shallow estuarine waters.

Fish communities in the shallow (<1.5m) nearshore waters of the Walpole-Nornalup Estuary, on the south-coast of Western Australia, were sampled seasonally over a twelve month period using beach seine nets. Sampling was undertaken at a comprehensive range of sites throughout five regions of the system, during both day and night. Thirty-eight fish species from 24 families have been recorded, with the most abundant taxa being the atherinids Leptatherina presbytoides and L. wallacei, and the gobiid Favonigobius lateralis. The characteristics of the ichthyofauna differed substantially between regions of the estuary, seasons and between day and night. The lower regions of the estuary were dominated by marine spawning species (e.g. Arripis georgianus, Sillaginoides punctata and L. presbytoides), while the upper reaches contain a high proportion of solely estuarine species (e.g. Acanthopagrus butcheri, Afurcagobius suppositus and L. wallacei). Seasonal variation is most likely attributed to cyclic variation in physico-chemical water quality parameters, particularly salinity and temperature. Notably, abundances of the four gobiids Arenigobius bifrenatus, Pseudogobius olorum, A. suppositus and F. lateralis were far higher in summer than any other season.