Carapace width-weight relationships of blue swimmer crab Portunus armatus (A. Milne-Edwards, 1861) (Crustacea: Brachyura: Portunidae) in southwestern Australia: influences of sex, decadal change, environment, and season

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 (R2 = 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.