To understand seasonal declines of investment in egg production, we studied the relationships among laying date, clutch, and egg sizes in a Great Cormorant Phalacrocorax carbo population. Seasonal changes in the amount and number of fish caught per casting net were used as an index of food abundance for Great Cormorants. Clutch size and egg volume were positively correlated and decreased as the breeding season progressed. Clutch volume (i.e. total egg volume within clutches) was explained by clutch size and laying date. However, the amount and number of fish caught per casting net increased through the season. We conclude that the observed seasonal decline in egg production investment was not a result of declining food abundance during the breeding season in Great Cormorant. The amount of egg investment appeared to depend on the fishing ability of the individual rather than fish availability during the breeding season. Early in the breeding season, only those individuals with strong fishing abilities would be able to acquire sufficient nutrition to lay eggs. Thus, egg investment and food abundance are expected to show opposite seasonal trends.