Aquaculture International 3, 186-195 (1995) Demand-feeding behaviour of sea bass kept in ponds: die1 and seasonal patterns, and influences of environmental factors Marie-Laure Begout Anras Centre de Recherche en Ecologic Marine et Aquaculture, B. P.5, 17137 L ‘Houmeau, France Feedingactivity was investigated from June to October in a group of 60 sea bass,Dicentrarchus labrax. The fish were kept in an earthen pond in a maritime wetland subject to the fluctuations of the natural environment. The fish fed by operating a demand feeder connected to a data logger which recorded eachfeedingact with a time reference.Meteorologicaland hydrological data were also recorded. Diurnal rhythms in feeding activity changedas the months progressed.Fish fed during daylight, preferentially at dawn in June,but in the middle of the day from Augustto October. Most feeding took place during subdued light. Feedingwas closely and positively correlated to both daylength and water temperature;daylength being the most related factor associatedwith rainfall and East-Westwind direction. KEYWORDS: Behaviour,Demandfeeding,Environmental factors,Seabass(Dicentrarchus labrax) INTRODUCTION The development of finfish farming in coastal wetlands will be dependent upon the enterprise becoming more economically attractive and ecologically acceptable. In order to achieve this, both growth performance of the fish and reduction of effluent concentrations have to be improved. The most effective way of reducing water pollution from fish farms is to minimize feed loss, and feed wastage can be reduced by presenting food when the fish are most motivated to feed (Spieler, 1977;Parker, 1984;Seymour and Bergheim, 1991;Poxton, 1991; Handy and Poxton, 1993). In European Atlantic wetlands, farming of two species is receiving particular attention: the sea bass, Dicentrarchuslabrax (L.), and the sea bream, .Sparus aurata (L.). Wetlands are subject to marked meteorological and hydrological fluctuations, so it is necessary to assess the relationships between feeding behaviour and environmental fluctuations in order to design rational feeding strategies. The investigation of feeding behaviour, and especially of demand feeding, has been used as a research tool to understand feeding activity with most work being performed on salmonids in aquaria or under intensive culture conditions (e.g. Alan;ir;i, 1992a, b; Boujard and Leatherland, 1992a, b; Boujard et a/., 1992; Alan&-a and Br?hnGs, 1993; Br&uGis and Alan;ir& 1993). However, investigations on feeding behaviour in fish seldom examine the many natural environmental fluctuations to which outdoor farmed fish are subject (Hirata, 1957, 1973; Smith et al., 1993). The aim of this study was to investigate feeding activity of sea bass kept in semi-extensive culture conditions in relation to the natural fluctuations of different environmental factors occurring in wetlands and salt0967-6120 0 1995 Chapman & Hall Demand-feeding behaviour of sea bass 187 marsh areas. To this end, seasonal variations in feeding activity of a group of fish were examined in relation to fluctuations in a range of environmental factors. MATERIALS AND METHODS Experimental site and fish The experimental site consisted of an earth pond, 18 m x 14 m x 0.8 m (250 m2, 200 m3), located in the coastal wetland of L’Houmeau (46”9’N, 1”9’W), north of La Rochelle (France). This pond was connected to a water reservoir and via an outlet pipe to a channel open to the sea. A group of 60 sea bass, was introduced into the pond on 19 April 1993 for outdoor acclimatization. The initial mean weight of the fish was 178.2g (SD 86.1) and mean length was 25.8 cm (3.6). The experiment ran from 1 June until 31 October. During this period, fish were captured, weighed and measured individually four times at irregular intervals on 3 June, 1 July, 1 August, 23 September and finally the 13 December. These measurements took less than 2 h and the fish were immediately returned to the pond. Meteorological and hydrological data collection A weather station ‘Aanderaa’ was installed on the experimental site to automatically record wind speed and direction, illumination, atmospheric pressure and rainfall every 10 min. Hydrological data were also recorded by the Aanderaa data logger using three sensors (oxymeter, thermometer and conductivity meter). The hydrological sensors were cleaned twice a week. The data were transferred every month via a portable computer to magnetic disk in ASCII format. Demand feeding system Feed was dispensed when a fish bit a piezo-electric quartz crystal (Tactile Self, MICREL) located 10 cm beneath the water surface. The demand feeder was connected to a ‘Quartel data logger which recorded feeding events continuously with dates and hours in GMT. Every week, feeding data were downloaded to a portable computer equipped with Quartel software. Feed pellets were 4-5 mm ‘Complete Meal’ for sea bass and sea bream (manufactured by Le Gouessant) and the number of pellets distributed at each activation of the demand feeder was between 30 and 50 (15-25 g). The feeding system was installed on 15 May 1993,after which a training phase was initiated. Training consisted of manual distribution of pellets just below the demand feeder twice a day for 2 weeks. By the beginning of June, fish were operating the demand feeder without the aid of manual activation. Data treatment The weather and hydrological data series have an interval time of 10 min and, the number of feeding bouts were therefore summed for each 10 min period, in order that the data would correspond. Feeding bouts were analysed in relation to environmental factors by means of ANOVA and the Tukey HSD Multiple mean comparison test. The data set was simplified by calculating daily means of environmental factors, incorporating data on daily rainfall and including the number of feeding bouts per day. These shorter series, which approximate the normal law, were then examined using Pearson’s correlation coefficients and multiple linear regression. The weight and length data gave an indication of growth rates. Means and