Spatio-temporal modelling of prawns in Albatross Bay, Karumba and Mornington Island

The seasonal life-history patterns of penaeid prawns are complex and there are distinct differences between species. Better knowledge and understanding of offshore migration to the fishing grounds of penaeid prawns is particularly important for the operation and management of the Northern Prawn Fishery, Australia. Knowledge of the location of prawns during the critical spawning period of the year is important, especially since recent work suggests that inshore areas are critical for effective spawning. This paper explores factors, such as rainfall, water depth and season, impacting prawn migratory patterns, and models the spatial and temporal distribution of three commercially valuable species of prawns (Penaeus merguiensis, Penaeus semisulcatus and Penaeus esculentus) in Albatross Bay, Karumba and Mornington Island in the Northern Prawn Fishery using modern statistical methodology. The methodology allows for flexible functional relationships between variables, including univariate smooth terms and smooth terms for interactions between covariates: a major advancement on the methodology used in previous analyses of spatial distribution of prawns in the Gulf of Carpentaria. The models interpolate well, but expose the problems associated with extrapolation beyond the data. The results suggest that there are spawners in the important inshore waters during the critical spawning period of the year, although there is no strong evidence for major congregations of prawns in these important inshore regions compared to more offshore regions. Rainfall was significant in many of the models, particularly those describing the distribution of Penaeus merguiensis. Increased rainfall mostly had a positive effect on prawn catch rates, possibly by stimulating prawns to move out of shallow coastal areas into the fishing area. However, in some cases, high levels of rainfall had a negative effect which may be due to storms associated with the heavy rainfall decreasing the effectiveness of the fishing fleet. The analyses showed no signs of major inshore migrations associated with spawning, but there were certainly some spawners close inshore at the critical spawning period.