Temeprature is the overall factor in determining a lot of things oceanic. In warm weather, shellfish are able to digest larger organisms. In winter, the animal's ability to uptake and digest these are greatly inhibited. Beside that, there is a lot less food in the water by volume in winter anyway. It might not be so evident in the tropics, but up here where the seasons are more extreme, shellfish will uptake calcium from their own shells (and yes... pearls) when not available in food. Winter harvesting and grafting must be done with greater care as to not crack or break the shells.