Use of catchability equations for population estimation of marron, Cherax tenuimanus (Smith) (Decapoda : Parastacidae)

Abstract

Marron in a 0 .01-ha hatchery pond were sampled after sunset on 14 occasions over 2 years in a standardized manner using baited drop nets. An inventory of the total population was taken after each sampling by draining the pond. Variation in catchability. p, was analysed by linear logit models of p/(1 - p) using the likelihood ratio χ2- statistic G2 as a measure of goodness of fit. A reduction in G2 of 83% was produced by inclusion of water temperature. T� (C); relative position of marron in the size distribution, S (rather than absolute size); and a between-years factor, Y. The following catchability equation was calculated: In[pI/(l-pI)] = -5.40 +0.523S-0.246S2 +0.220T+1.40Y. The catchability equation was then applied to drop-net catch numbers from stocks of marron of 2-2+ years old in 10 0.1-ha farm dams. Estimated cohort numbers were compared with 'true' values obtained by independent removal of the stocks by seining. Agreement was improved by adjusting these estimates for differences in Secchi disk values, D. between dams (because daytime sampling was pursued and illuminance markedly influences catch rates) and in coefficient of size variation, V, The latter was negatively correlated with catchability and appeared to reflect the intensity of size dominance in feeding competition. A catchability equation. including these additional factors, was calculated: In[pI/(l -pI)] = -0.081 +0.664S -0.214S2 +0,284T -0.668ln D - 19.5V. Compared with other methods of estimation in which equal catchability is assumed, this method takes into consideration variation in catchability, requires no marking of individuals, and needs only one sampling occasion to obtain an estimate of crayfish numbers. Because of dependence of catchability on size of marron, sample means over estimated mean individual body weight by 13-48%. The catchability equation was also used to correct for this bias.