The effect of maternal traits on rearing success in pacific harbor seals (Phoca vitulina richardsii)

Reproductive success in species that care for their young is affected by the rearing strategy utilized. Otariids are known as income breeders, because they continue to forage during a rearing time of about a year while leaving pups on land; their rearing success is related to attendance patterns. On the other end of the continuum, large phocids are described as capital breeders, fasting on shore during a rearing time from 4 to 50 days. Their rearing success is based on maternal body mass. Harbor seals (Phoca vitulina) don't appear to follow either of these two strategies fully and which maternal traits affect their rearing success is unknown. During two breeding seasons I observed 54 harbor seal females and their pups at Gertrude Island, USA, to describe their rearing strategy and determine how maternal traits affect rearing success. Using my data and a long-term database of individual females at the haul-out site, I modelled the effect of female age, size, experience, and attendance behavior on the health of the pup. Harbor seals reared their pups for 26.4 days ±14.3 (n= 77 pups) and took swimming trips during 35.6% of my observations, taking their pups with them on 98.6% of those trips. High pup health at weaning was best explained by increased maternal rearing time, decreased distance from other seals, previous success and increased time resting. The size of the female did not affect rearing success. My results indicate that harbor seals in south Puget Sound fell somewhere between capital and income breeding strategies on the continuum and that they required different traits than those employed by income and capital breeders to successfully rear their pups.