Effects of day and night on swimming, grazing and social behaviours of rohu Labeo rohita (Hamilton) and common carp Cyprinus carpio (L.) in simulated ponds Mohammad Musta¢zur Rahman 1 , MarcVerdegem 1 , Md Abdul Wahab 2 , Md Yeamin Hossain 3 & Qtae Jo 4 1 Department of Animal Science, Aquaculture and Fisheries Group,Wageningen University,Wageningen,The Netherlands 2 Faculty of Fisheries, Bangladesh Agricultural University, Mymensingh, Bangladesh 3 United Graduate School of Agricultural Sciences, Kagoshima University, Kagoshima, Japan 4 Aquaculture Research Team, ESFRI, National Fisheries Research and Development Institute, Gangnung, Gangwon, Korea (Correction added after online publication 27 May 2008: The author names ‘Mohammad Abdul Wahab’and ‘Mohammad Yeamin Hossain’were corrected to‘Md AbdulWahab’and‘MdYeamin Hossain’respectively.) Correspondence: M M Rahman, Aquaculture Research Team, ESFRI, National Fisheries Research and Development Institute, Gangnung, Gangwon 210-861, Korea. E-mail: musta¢zu.rahman@yahoo.com Abstract Diel rhythmicity of grazing, swimming, resting and social interactions of rohu ( Labeo rohita ) (weight 66.5^ 68.3g) and common carp ( Cyprinus carpio ) (79.9^ 82.0 g) were observed in 1m 2 simulated ponds using video images. Fish behaviour was monitored during a full 24-h period, starting at 08:00 hours with a15-min recording, which was repeated every 3 h. Rohu spent more time grazing during the day than at night. Graz- ing activity peaked at the beginning and at the end of the day. Grazing and resting of rohu were negatively correlated. Common carp grazed day and night, show- ing no distinct grazing peak. Considering the grazing time of rohu and common carp, a feeding schedule with two feeding times at 07:30 and16:30 hours might be appropriate for better food intake and conversion in rohu^common carp polyculture system. Keywords: swimming, grazing, behaviour, rohu, common carp Introduction Together, rohu Labeo rohita (Hamilton) and common carp Cyprinus carpio (L.) contribute 26% (rohu 11%, common carp15%) of the total world freshwater aqua- culture production outside China (FAO 2005). Nowa- days, rohu^common carp polyculture is being widely practiced in South Asia. Rohu is a highly appreciated consumption ¢sh, fetching good market prices (Dey, Paraguas, Bhatta, Alam, Weimin, Piumsombun, Koeshandrajana, Dung & Sang 2005; Rahman,Wahab & Verdegem 2007), while common carp is gaining quick popularity as a bottom feeder in South Asia because it has a faster growth rate than the native bottom feeder mrigal Cirrhinus cirrhosus (Bloch) (Milstein,Wahab & Rahman 2002; Rahman 2006). The e¡ects of addition of arti¢cial feed and di¡er- ent densities of common carp on natural food avail- ability, food intake and growth and production in rohu ponds were studied (Rahman, Verdegem, Nagelkerke,Wahab, Milstein & Verreth 2006). It was observed that both arti¢cial and natural foods are im- portant for the growth of rohu and common carp. The pond study showed synergistic e¡ects on natural food availability in rohu^common carp polyculture. The question was raised as to whether the observed synergism was mainly caused by increased natural food availability or by a change in feeding behaviour. Behavioural studies in ponds are di⁄cult due to high turbidity and, consequently, low visibility.Therefore, to be able to observe the feeding behaviour, pond treatments were simulated in aquaria. The e¡ects of arti¢cial feed application and di¡erent stocking den- sities of common carp (0, 1 and 2tank  1 ) in rohu ponds on grazing, swimming and resting were re- ported (Rahman, Verdegem, Nagelkerke, Wahab & Verreth 2008). In this paper, the di¡erences in beha- viour of rohu and common carp between day and night were analysed. Aquaculture Research, 2008, 39, 1383^1392 doi: 10.1111/j.1365-2109.2008.02007.x r 2008 The Authors Journal Compilation r 2008 Blackwell Publishing Ltd 1383