During the 20th century the numbers of nesting Sandwich Terns Sterna sandvicensis in The Netherlands have fluctuated heavily. After the collapse in the 19605 the number of breeding pairs increased only slowly and stabilised at a low level. Earlier studies showed that population trends may be related to changes in food availability. Since the Sandwich Tern is a highly specialised piscivorous seabird, which feeds almost exclusively on tour species of fish (herring, sprat, sandeel and greater sandeel), the Sandwich Tern is probably highly vulnerable to changes in the availability of its prey species. To determine the key factors influencing population size the Institute for Forestry and Nature research (IBN-DLO), in co-operation with the Netherlands Institute for Fisheries Research (RIVO-DLO) and the National Institute of Coastal and Marine Management (RIKZ), started a study on the population dynamics of the species in 1995. The study was conducted on Griend, the largest Sandwich Tern colony in western Europe. The target of this study was twofold, namely: (1) To assess to what extent emigration to other colonies takes place. (2) To come to a better understanding of the importance of food availability during the breeding season for chick and post-fledging survival, and ultimately for population dynamics. In 1992-1997, the provisioning of food to the chicks fluctuated stronglv from year to year. Parallel with these changes the condition of the breeding adults and parental effort changed. These parameters can be regarded as indicative for the food situation in the local foraging areas. Changes in adult condition, the amount of food supplied to the chicks (in particular herring) and parental effort coincided with a parallel change in the number of breeding pairs. In years in which these parameters indicated a scarcitv of food, the number of breeding pairs was accordingly low. In 1995, a sampling programme was started to investigate whether the assumed fluctuations in food availabilitv as measured in the colony on Griend, were reflected by changes in food abundance in the local foraging areas. Isaac's Kidd Middwater Trawl net catches indicated that food availabilitv decreased from 1995-1997. Although a parallel decrease was seen in the terns population size, the fish catches did not reflect other changes in the colony on Griend. The used sampling method was probably not suitable to detect changes in the amount of food availability for the terns. However, the fish sampling programme made clear that fish availabilitv was greatlv influenced by water clarity. In relatively clear water the fish was present at greater depths, thus beyond the reach of foraging terns. Sandeels were even IV distributed over the five sampling locations, but herring availabilitv increased towards the northern positions. At sampling points where herring availability was high, the number of foraging terns were accordinglv high. Along with the changes in the number of breeding pairs, the age composition of the population on Griend showed large fluctuations. In 1995, the proportion of older birds was relatively high, whereas in 1996 and in 1997 a large proportion of young birds nested on Griend. These changes were related to a high local survival rate of 3- and 4-year old birds in 1996 and 1997. A considerable part of both the population nesting on Griend and the Danish population consists of birds immigrated from other colonies. However, site fidelity of the terns on Griend was much higher than in Denmark. The majority of the emigrating terns settled in colonies less than 200 km away from their original colony. |
