ENERGETICS OF LACTATION IN HARP SEALS (PHOCA-GROENLANDICA) FROM THE GULF OF ST-LAWRENCE, CANADA

This study reports the findings of an integrated, comprehensive analys is of lactation energetics in harp seals conducted using longitudinal measurements of mass, body composition and milk composition from mothe r-pup pairs in conjunction with water flux measurements in pups. The n ursing period of harp seals is a short, intense and relatively efficie nt period of energy transfer from mothers to pups. The average daily m ilk intake for pups was 3.65 +/- 0.24 kg which is equivalent to 79.5 M J of energy. Eighty-one per cent of the energy received in the milk wa s metabolisable and 66% of the energy was stored by the pups as body t issue. The field metabolic rate of pups was 3.9 +/- 0.4 time basal met abolic rate. The pups were growing at a rate of 2.2 kg per day during the nursing period. The distribution of this mass gain varied in terms of tissue composition, depending on the age of the pups, but over the whole nursing period approximately half of the tissue was stored as f at. Harp seal mothers lost an average of 3.1 kg per day during lactati on which was composed of 37% water, 50% fat, 11% protein and 2% ash. M others spent half of their time during the lactation period actively d iving and only one-third of their time on the surface of the ice. Milk compositional changes followed the normal phocid pattern with increas ing fat content and decreasing water content as lactation progressed. The mean mass transfer efficiency was 73%. However, this value cannot be used without qualification because female harp seals in this study fed to varying degrees, consuming an estimated 0-4.8 kg of fish per da y. Feeding does not appear to be required in order to achieve the ener gy requirements for lactation, given the energy stores possessed by fe males, and some females do fast through the entire period so feeding m ay be considered opportunistic in nature.