Plasticity in the reproductive biology of the smooth marron Cherax cainii (Decapoda : Parastacidae): A large freshwater crayfish of south-western Australia

Considerable intra-specific variability in biological traits can exist in freshwater crayfishes including those of most importance to fisheries managers. Smooth Marron (Cherax cainii) is the third largest freshwater crayfish in the world, is endemic to south-western Australia, and supports an iconic recreational fishery that has been under pressure due to environmental change (principally secondary salinisation and reduced rainfall). The fishery consists of numerous discrete populations fished from both dams and rivers and the potential variability in key reproductive biology traits among them had not been previously quantified. The current study aimed to quantify the size at maturity and fecundity of C. cainii among stocks/populations to help facilitate sustainable management of the fishery. Eleven rivers and four dams (that represented the great majority of the overall annual recreational catch) were surveyed across ~800 km of latitudinal range prior to and during the spring breeding period. Additionally, historical data were sourced from previous studies conducted in 1968, 1978, and 1999 for three of the populations. Overall size at maturity OCL50 (orbital carapace length at which 50% of the female population were mature) among all populations was 48.8 (95% CI: 47.0-50.8) mm but varied greatly among populations, ranging from 30.4-66.7 mm. There was no evidence that female size at maturity had changed over time in two populations where historical data was available. Mean ovarian and pleopodal fecundity was 583.8 (±33.1) and 389.1 (±18.9) eggs, respectively. Ovarian and pleopodal fecundity differed significantly among populations and ovarian, but not pleopodal fecundity, also differed between those populations from rivers versus those from dams. Some temporal change in ovarian and pleopodal fecundity was detected but this change was not consistent between populations. The variability in traits revealed here was most likely due to combinations of biotic and abiotic factors, with temperature found to be significantly positively related to size at maturity of female C. cainii in rivers, likely due to its effects on growth rates of C. cainii. However, several other potential factors including productivity of the systems, genetic differences among populations, and the disparate physical characteristics between rivers and dams, are also likely to contribute to the overall variability of the traits. The study has implications for the management of the fishery; for example, setting minimum legal sizes to ensure at least some of each population can reproduce before becoming exposed to fishing. A better understanding of the major factors driving this biological plasticity is required to enable predictions of how this may vary under future environmental changes.