Factors Affecting Distribution and Reproductive Success in Elephant Seals (Mirounga Angustris) at Point Reyes

Pettee, Jessica

A report on a study that was done to determine environmental and demographic factors affecting annual pup mortality and individual female reproductive success. In 1981 a breeding colony of northern elephant seals, Mirounga angustirostris, became established at Point Reyes National Seashore (PORE). This population has grown from one breeding pair to over 1,000 individuals, and is expanding rapidly onto new beaches within PORE. The study goals were to determine environmental and demographic factors affecting annual pup mortality and individual female reproductive success, update existing information regarding age structure, immigration, and internal recruitment of breeding females, and describe the pattern of elephant seal colony expansion at PORE. All PORE beaches used by elephant seals were surveyed three times per week during the 1997 and 1998 breeding seasons. Environmental data, including rainfall, swell height, and tide level, were also collected. In addition to census surveys, surveys for marked animals were conducted at least three times per week. Stepwise multiple regression was used to determine possible relationships between pup mortality and environmental variables at several breeding sites. Discriminant analysis was used to create a predictive model of individual female reproductive success based on resight data. Descriptive statistics were used to evaluate data on age structure, immigration, and internal recruitment of females breeding at PORE. Stepwise multiple regression was also used to identify which environmental variables were associated with colony formation. A significant negative relationship was found between pup mortality at PRH in 1998 and date, swell height, and high-high tide level. In both the 1997 and 1998 breeding seasons, females reliably separated out by groups (successful and unsuccessful) according to their discriminant analysis factor scores. Ninety-three percent of females were classified correctly by the 1997 model and 100% of the females were classified correctly by the 1998 model. When environmental conditions became severe, as happened during the 1998 El Nino Southern Oscillation (ENSO), those variables associated with successful females change. The majority of breeding females at PORE were younger than nine. The mean age of breeding females during the study period was 5.9+/-2.4. This is unchanged from the 1993 breeding season (5.9+/-3.0). During the 1998 ENSO driven storm season dramatic population increases occurred at the newer sub-colonies and another colony formed. Heavy swell at exposed sub-colonies was correlated with decreased numbers of females at those sites and increased numbers of females at adjacent, more sheltered locations. In Summary, sheltered sites were not as susceptible to the effects of extreme weather that often resulted in increased pup mortality at the more exposed beaches. Female reproductive success was higher in low-density harems, increased predictability though the prime-age class, up to 8 years old, and then became variable. Random environmental events, like ENSO driven storms, affect the relationship between female reproductive success and variables considered important to that success, such as swell height and tide level. The elephant seal colony at PORE is still expanding and internal recruitment is increasing, as indicated by high numbers of young breeding females present. The slow but steady growth of newer sub-colonies may result from the density dependent factor of over-crowding as a main source colony. Overcrowding coupled with density independent factors like severe weather events may affect sub-colony formation and recruitment rate, as measured by female habitat use.