Normal (Gaussian) response of juvenile marron, Cherax tenuimanus (Smith) (Decapoda: Parastacidae), to capture by baited sampling units

Abstract

The possibility that a population of freshwater crayfish contains members with permanent zero "vulnerability" to capture by baited nets has been investigated.

After five capture-mark-release opportunities a pond year-class cohort of juvenile marron, Cherax tenuimanus (Smith), was completely examined, as suggested by Cormack (1968). The observed frequencies of marron in the six capture classes, i.e. including the uncaptured marron, were shown to approximate to a truncated normal (Gaussian) distribution using a statistical procedure given by Thompson (1951). Thus, the uncaptured marron only differed quantitatively in their capture response in comparison to the other marron, and therefore, did not contain members with permanent zero "vulnerability".

Of the variability in probability of capture, or "catchability", 26.5% was accounted for by a positive correlation with individual size. Sex was also influential amongst the larger members of the year-class.

Since equations used to estimate population numbers are based upon a basic assumption of equal "catchability" of individuals, the results presented have considerable practical importance with respect to the validity of these estimates for marron and, at least, related species.