The influence of sampling intensity on the 'catchability' of marron, Cherax tenuimanus (Smith) (Decapoda: Parastacidae)

Abstract

Sampling intensity is considered as one of the three factors contributing to the 'catchability' of animals. Theoretically, variation in 'catchability' between individuals, due to differences in 'vulnerability' ('inherent catchability'), may be magnified when sampling units are set 'non-competitively', since all the animals over the sampled area will not be equally exposed to capture; less active animals, i.e. with a lower 'vulnerability' to capture, may also be less exposed to capture because of, for example, habitat heterogeneity with respect to shelter where units cannot be placed.

A model was proposed to describe the operation of a baited net, or trap, used to catch bottom dwelling crayfish or lobsters. This model was extended to describe the different relationships between catch number per sampling unit and density of units for non-competitive, partially competitive, and competitive degrees of sampling intensity.

The results of an experiment using a known population of marron, Cherax tenuimanus (Smith), were used to quantify the above relationship for the range of net densities possible in a semi-artificial pond. After eliminating bias due to a number of causes of variability in 'catchability', it was shown that the competitive degree of sampling intensity, as described by the extended model, had been achieved.

Total catch numbers derived from the use of different net densities covering this degree of sampling intensity should not be expressed 'per net', as is done for non-competitive sampling, for comparison of relative numbers of animals. Instead catch number should be expressed in terms of the area sampled, since catch number is independent of the number of nets used on the area.

Mean 'catchability' tended to be positively correlated with seasonal water temperature. Marked differences in 'vulnerability' to capture, due to size and moult stage, could be inferred to exist between individuals. Previous capture also influenced 'catchability'.