TIME AND ENERGY CONSTRAINTS OF FISHING BEHAVIOR IN BREEDING CORMORANTS PHALACROCORAX-CARBO-SINENSIS AT LAKE IJSSELMEER, THE NETHERLANDS

TWO Cormorant colonies in The Netherlands (Naardermeer and Oostvaarder splassen), exploiting the same water bodies but situated at different distances from them, were compared with respect to daily variations in exact fishing sites and corresponding variations in time budget and f ish consumption. Mean flying distances between colony and fishing site were estimated at 22 and 15 km respectively. Birds from the Naarderme er colony carried out less trips but of a longer duration than birds f rom Oostvaardersplassen, most markedly so in the chick rearing period (2 trips per day of 185 min vs. 3 trips of 165 min). Daily fluctuation s in time spent away from the colony were clearly dependent on daily s hifts in main fishing sites. On days when fishing was concentrated at larger distances, individual foraging trips lasted longer, due to the increase in flying time. Neither net fishing time nor daily fish consu mption, as estimated by pellet analyses, compensated for the increment in time and energy expenditure on those days. It was estimated that t he average daily energy expenditure would amount to about 2.8 . BMR (b asal metabolic rate) in birds from Naardermeer and to about 2.7 . BMR in birds from Oostvaardersplassen. Fish consumption estimates based on pellet analyses led to an estimated DME (daily metabolisable energy) of 2 . BMR for both colonies. Thus, an overall negative energy balance became apparent, resulting in estimated mass losses throughout the br eeding season of on average 980 and 860 g for Naardermeer and Oostvaar dersplassen birds, respectively. Mass losses are likely to be higher w ith increasing travelling distances, indicating that travelling distan ce may influence reproductive output. This could be one of the factors causing consistently lower reproductive outputs at the Naardermeer th roughout the years.