Data concerning the numerical density, biomass and growth of Macoma balthica (L.) on the Balgzand (Western Dutch Wadden Sea) are quite well known. This study deals with another aspect: the survival of Macoma in this area. The data used, had been collected from 1969 to 1978 inclusive. In winter many of the youngest specimens of Macoma are being transported by tidal currents away from the coastal area where spatfall in early summer is heaviest.The most important question of this paper is connected with a functional explanation for this migration. Do the chances to survive increase when they have migrated and settled elsewhere on the Balgzand? Only those individuals that were carried to the edge of the Balgzand area (more than six kilometers from the place where spatfall took place) were found to survive longer than the individuals that had remained in the coastal area. Besides, an explanatian was sought for the differences in survival that were found in different places. It appeared to be correlated negatively with the growth rate af Macoma at these places. Moreover, predation and parasitism will have an influence on survival. To investigate whether a density dependent survival took place, it was necessary to know details about the dispersion pattern of M. balthica. Generally this species appeared to be dispersed randomly over substrate of the Balgzand, tending to aggregatian specially where the character af the sediment was heterogeneous. No density dependent survival was found when a comparison of different years on one place was made. From 1972 onwards, the survival during a year was found to be correlated positively with the growth in the preceding year. A significant positive correlation was found between the survival of Macoma and biomass of Cerastoderma in a certain year. This correlation was best in regard to the year classes of the cockle that are eaten by oystercatchers. Possibly oystercatchers could change over to Macoma as a prey in years when cockles were in poor supply. The survival of Macoma after the first year of life was independent of age. So in the above mentioned cases it was justified to take these animals together. |
