A new automatic drift sampler for riverine fish

Abstract

Larval drift plays an important role in the population dynamics of riverine fish species. An accurate assessment of the larval drift depends on the availability of suitable sampling methods. The present paper reviews existing drift sampling methods and introduces new approaches. A new drift sampler was constructed to conduct an automatic around-the-clock sampling procedure. The drift sampler consists of a net basket, partitioned into 6 compartments, which rotates within a frame construction. The device samples the drift in the lowest net of the basket at prescribed time intervals. The net basket is turned 60° after each time interval, during which all nets except the lowest remain closed. The drift sampler thus allows automatic collection of 6 successive samples within one sampling unit. It can be installed by two people and the sampling procedure itself requires only one person. The first implementation of the new drift sampler at a side-channel of the Danube River yielded 363 fish, primarily cyprinids, during an 8-day sampling period in summer. The day/night ratio was about 1:16 and drift peaked between 0200 and 0400 (0.22 ind./m 3 ). The maximum total length of fish reached 30 mm, with about 80 % in the range between 6 and 20 mm. The new drift sampler provides a quantitative, time- and cost-saving sampling procedure that enables large-scale drift studies to be more efficient and informative.