Abstract

Time of day may affect the availability, distribution and behaviour of many fishes, at least in the depths that the light penetrates. Changes in the activity and position of the demersal fish as a response to the changing light levels during a diel (24 h) cycle may affect their catchability or vulnerability to the bottom trawl. Diel variability in the bottom-trawl catch rates of five sparid fish species, namely Boops boops, Diplodus annularis, Diplodus vulgaris, Pagellus acarne and Pagellus erythrinus, were investigated during seven experimental bottom-trawl surveys conducted within the same locality in İzmir Bay. All trawl haulings were performed using the same operation and sampling procedure and carried out for each seasonal survey at different periods day, including the morning, noon, afternoon, dusk, early-night, midnight, late-night and dawn. Percentage contribution by weight of the sparid fishes to the total catch varied according to the sampling season between 23-79%. Catch rates of the sparid fishes usually did not show significant diel periodicity except D. vulgaris. Daylight catch rates of D. vulgaris were significantly higher than twilight and night periods (Daylight>Twilight>Night). Information on diel variations obtained in this study could be fundamental in understanding the distribution dynamics of the five sparid fishes and developing future management approaches.