Methods of sampling aquatic beetles in the transitional habitats at water margins

Populations of water-beetles (Hydrophiloidea) were studied during the ice-free seasons of 2 years at a lake shore near Stockholm, Sweden. They were sampled by means of an absolute quantitative method (twenty-four bottom cores/sample), extracted by Berlese-Tullgren funnels, and a relative searching method (time-catch or catch per unit effort). The sampling efficiency was tested by charging empty samples with known numbers of beetles. ‘Smaller’ species were less efficiently extracted than ‘larger’ ones. Absolute and relative population estimates of eight species were compared by means of regression analysis, the relative estimate being considered as the dependent variable. Only three statistically significant correlations were found, but a correlation always seemed to exist, and the relative estimates are considered as crude estimates. To obtain better ones when studying several species in one habitat much work is required, and then the advantages of the simple relative method disappears. The different sizes of the species were evaluated as sources of errors affecting the relative estimates. The estimates of the 'smaller’ species were more affected than the ‘larger’ ones. It is considered that relative population estimates would gain much if compared with independent absolute estimates. This seems to have been done rarely with invertebrates in aquatic habitats. Further, it is thought that behavioural extraction methods might be applied to several aquatic animal groups in eulittorals and littorals, saving much time compared with mechanical methods or hand sorting.