Environmental drivers of commercial blue swimmer crab (Portunus armatus) catch rates in Western Australian fisheries

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.

Automated summary

This study investigated the impact of environmental variables on commercial CPUE of blue swimmer crab in five Western Australian fisheries, finding that water temperature had a significant positive effect on CPUE universally. Wind speed and tide height also showed significant positive effects in three out of five fisheries. However, lunar illumination and freshwater flow had a smaller, negative impact on CPUE in only a few fisheries. Standardizing CPUE for these environmental effects highlighted clear variations from raw CPUE, emphasizing the importance of accounting for environmental factors in fishery assessments.