An improved electrofishing methodology for the assessment of habitat use by young-of-the-year fishes

Abstract

A new electrosampling device was developed and evaluated for the assessment of densities of and microhabitat utilization by young-of-the-year (YOY) fishes in the River Ourthe (Southern Belgium). This device consists in electrofishing frames powered by a direct current generator. A steel plate anode is placed in the centre of the frame whose perimeter is delimited by a closed cathode. The cathode acts as an electric barrier and allows a more precise delimitation of the sampled area. The frame is installed in 5 min by two operators (camouflage of the anode in the substratum). Delays for recolonization of the sampling area by YOY averaged 6.5 min (N = 25) and never exceeded 15 min. The straying and mortality rates of YOY were dependent on voltage used, size of anode and distance anode-cathode (logistic equations ; p <0.05 ; 14 DF). The best compromise was obtained when using a 0.04 m 2 anode within a 2 m 2 frame energetized with 240 V (straying and mortality <5 %). This configuration was tested on 16 sites within a single station. 1,241 YOY (21-66 mm FL) were captured belonging to eight cyprinid species, the most represented species being L. leuciscus (51.04 % of captured YOY). Densities as high as 133 YOY/m 2 were attended in shallow and weakly-sloped nurseries and were significantly (R 2 = 0.972, 14 DF) correlated with water temperature in the sampling site relatively to the thermal gradients in the station. Microhabitat use by YOY is interpreted within the contexts of predation and habitat availability in reduced flows. The consistent results obtained with this new methodology indicate that its precision is relevant for the assessment of microhabitat use by juvenile fishes. Further applications and improvements of the methodology are discussed.