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  • Environmental drivers of biological variation in Blue Swimmer Crab
    (2024) Hanamseth, Roshan
    Thesis
    Portunid crabs such as the Blue Swimmer Crab (BSC, Portunus armatus) are a high-value species in NSW, but catches are highly variable. Such variation in catch of portunid crabs is not unique, and other jurisdictions across Australia have sought to understand this variation by identifying links between environmental variability, recruitment, and catch for BSC. This thesis has four main chapters which investigated a novel research trap design for use in fisheries independent surveys of BSC populations; the spatial and temporal variation in BSC populations; their reproduction; and a novel approach to estimating fecundity in BSC. The retained catch from a novel small-mesh trap were compared with samples obtained using beam trawls and standard commercial round traps. I determined that the traps are more efficient at catching smaller crabs, and equally effective at catching larger size classes of crabs. In Wallis Lake, water temperature and conductivity had significant effects on abundance but there was no evidence that pulses of freshwater flow had a significant influence. Males were consistently located slightly further into the estuary compared to females, but there was no strong evidence of a spawning migration in response to low salinity. In Port Stephens, temperature and conductivity had significant effects on abundance, but the distribution of crabs within the estuary did not appear to be influenced by temperature, conductivity, or flow. The reproductive status of crabs in each estuary was highest in late winter, early spring and late summer. The highest proportion of egg-carrying (berried) females was in early spring to early summer periods in all estuaries. In Wallis Lake, the peaks in numbers of berried females were much shorter compared to Lake Macquarie and Port Stephens. I investigated a novel approach to streamline the egg count method whereby images of egg samples were digitised using a waterproofed flatbed scanner, followed by automated analysis of each image using customised computer macros. Overall, my study revealed nuanced effects of temperature and conductivity on fisheries production and on inter-annual and inter-site variation. Each estuary’s catchment and orientation to the ocean influenced their distribution and reproduction, contributing to the high variability in catch of this iconic species.