Effects of winter storms and oceanographic conditions on survival to weaning: a 37 year study of northern elephant seals on the Farallon Islands

Abstract

Northern elephant seals Mirounga angustirostris were extirpated from California in the 19th century, and only in recent decades have they recolonized. A key demographic parameter underlying population viability is the survival of pups, from birth to weaning. We evaluated local factors acting directly on pup survival prior to weaning and basin-wide factors reflecting oceanic conditions which may impact maternal condition and behavior, using a 37 yr time series from 2 adjacent islands off central California: Southeast Farallon Island (SEFI) and West End Island (WEI). Mixed-effects logistic regression indicated that annual pup survival decreased with increasing frequency of extreme waves during January and February, which may inundate haulout locations when pups are present, and increased with the North Pacific Gyre Oscillation (NPGO) index, reflecting increased primary productivity. Moreover, the impact of extreme waves was manifest only for WEI, which may reflect a greater risk of inundation compared to SEFI. Annual pup survival was most highly correlated with NPGO values during the 6 mo period preceding and including arrival and the pupping period (fall and winter). Our findings suggest that favorable foraging conditions for females during the 4 mo prior to hauling out led to better maternal condition, which improved pup survival. Impacts of storm surges on pup survival is of concern since the frequency and severity of storms is expected to increase in the future. To support population recovery, we recommend studies consider demographic parameters such as recruitment of females to the Farallon breeding population and to adjacent coastal California colonies.