Abstract

Initially farmers can produce large freshwater crayfish (Cherax albidus Clark, commonly referred to as yabbies) in farm dams that have not been trapped previously. However, with continued trapping, the average size of the yabbies trapped decreases until the majority are below market size. We tested the hypothesis that harvesting yabbies by trapping would decrease the proportion of more catchable larger males and reduce the size of individuals in the subsequent breeding population. This hypothesis was tested by recording the sex ratio and size of yabbies trapped from research ponds and by comparing the sex ratio and size distribution of yabbies from commercially-harvested farm dams to those from a wild population. In research ponds, trapping was sex biased for males (P <0.001) but not females (P = 0.34). It removed 73 ± 4.9% (mean ± se, n = 6) of males but only 52 ± 3.3% of females. The mean weight of yabbies caught in traps (40 ± 1.4 g) was significantly larger (P <0.05) than the remaining crayfish collected afterwards by draining the ponds (35 ± 1.2 g). The sex ratio and size distribution of yabbies from 27 farm dams that have been commercially-harvested was approximately equal in the lower size grades (<20 g) but, in larger size grades, there were more females than males, with an average sex ratio of 1 male to 1.2 females. In contrast, a wild population that had not been trapped previously by commercial harvestors had more females in the lower size grades (<30 g), while animals in larger size grades (30 - >50 g) were exclusively male. Across all size grades there were more males than females (1 male to 0.82 females). A population model derived from research data shows how the current harvesting strategy used by industry can lead to skewed sex ratios and increased densities in crayfish ponds. The results from our experiments indicate that using trapping to harvest yabbies in commercial farm dams changes the sex ratio and may affect the size of yabbies in subsequent generations.

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