Lactation performance data for grey seals (Halichoerus grypus) from 1999-2004.

Individually marked, free-ranging grey seal (Halichoerus grypus) females were sampled longitudinally over 6 breeding seasons (1999-2004) on Sable Island, Nova Scotia, Canada. Data were collected on the proximate composition of the milk (early and peak lactation), daily milk output, lactation length, female and pup masses (early and peak lactation) and pup mass at weaning. The purpose of the data was to examine to what extent variation among individuals in components of lactation performance could be attributed to inherent differences among individuals.