PARENT SESSION

(06-05) Interaction of contaminants and food composition: breeding success of cormorants in the Rhine delta.

Boudewijn, Theo^*,1, Dirksen, Sjoerd¹, Van Hattum, Bert², Ohm, Marieke³, ¹ Bureau Waardenburg, Culemborg, Netherlands² Institute for Environmental Studies (IVM), Amsterdam, Netherlands³ Ministry of Transport and Publlic Works, Rotterdam, Netherlands

ABSTRACT- The Rhine delta is one of the most polluted areas in the Netherlands. Heavy metals and organic compounds have been deposited here. They accumulate in the foodchain and negative effects could be established in the Cormorant. In the period 1987-1989 combined field and laboratory research revealed that this was caused by DDE, PCBs, PCDDs and PCDFs. From 1990 however, there was a steady increase in breeding success, but only a small improvement in water and sediment quality. Food composition was determined by pellet analysis. There was a gradually change in food composition. The load of contaminants in fish is differing between species and between size classes. Using the 2,3,7,8-TCDD equivalent factor (TEF) approach, the cormorants daily intake in toxic equivalents (TEQ) was estimated. There was a significant negative relationship between the mean daily intake of TEQs during the start of the breeding season and the mean hatching and fledging success in the colonies studied. Breeding success was also negatively related to the biomass of fish available in the main feeding area. A detailed analysis showed that large preys were selected if the amount of food available was large and smaller preys if the amount of food was relatively small. The contamination of small fish is lower than the contamination of large individuals. A reduction in food supply causes a lower intake of contaminants and it results in an improved breeding success.

Key words: contaminants, cormorants, feeding ecology, breeding success