Environmental drivers of variation in southeast Australian Giant Mud Crab (Scylla serrata) harvest rates

Abstract

Catch rate data is typically used as a proxy for relative abundance in stock assessment model, under the assumption of constant catchability. However, environmental variation and operational aspects of a fishery can decouple catch rates and abundance. As such, there is a need to quantify drivers of variation in catchability to enable robust stock assessment. Here, we present an analysis of Giant Mud Crab (Scylla serrata) catch rate data, obtained from a fishery-wide observer program implemented in southeast Australia (i.e., New South Wales, NSW). Using generalized additive mixed models (GAMM), we examined the influence of ‘soak’ (or immersion) time of commercial traps, water temperature, river flow, wind speed and lunar phase while controlling for gear configurations (e.g., number of entrances/escape gaps) and fisher ‘skill’. Our modelling showed soak time has little effect on catch rates, while warmer temperatures generally enhance catch rates. To a lesser degree, river flow, wind speed and the lunar phase also influenced catch rates. Overall, our results emphasize the importance of quantifying the impact of environmental variation on catch rates to ensure they are a reliable proxy for relative abundance when incorporated into stock assessments.