Variation in life history traits is directly linked to individual fitness. This interplay is complicated by environmental perturbations in an unpredictable habitat. To maximise fitness, individuals react to environmental changes by reallocating resources between maintenance, growth and reproduction. Disentangling these factors is complicated as traits are interlinked by trade-offs between current reproduction and future survival and reproduction. This study provides first estimates of life history traits and trade-offs of a tropical apex predator, the Galapagos sea lion (Zalophus wollebaeki), in an unpredictable habitat, the Galapagos archipelago. Thirteen years of individual data on birth mass, early growth and offspring, and environmental data allowed the examination of factors influencing reproductive performance of adult females and calculation of pupping rates. Females became primiparous between ages 4 and 9. Neither oceanographic nor body condition in the females’ first year of life influenced age at primiparity. Age at primiparity had no effect on a female’s birth rate, on average one pup every 2 years. Sex of a pup did not influence the subsequent inter-birth interval, but first-year pup survival lengthened it. Until age 6, females showed lower birth rate (< 0.40). Fecundity was higher between age 6 and 14 (birth rate 0.40–0.48). We could not detect an influence of inter-annually differing oceanographic conditions on pupping rates. Female Galapagos sea lions appear to deal with variation in early-life history traits and environmental unpredictability by a low but stable reproductive output modified only by the trade-off between current and future reproduction.