A Longitudinal Study of Steller Sea Lion Natality Rates in the Gulf of Alaska with Comparisons to Census Data

Abstract

Steller sea lion (Eumetopias jubatus) numbers in the Western Distinct Population Segment are beginning to recover following the dramatic decline that began in the 1970s and ended around the turn of the century. Low female reproductive rates (natality) may have contributed to the decline and remain an issue of concern for this population. During the 2000s we found high natality among Steller sea lions in the Gulf of Alaska indicating a healthy population. This study extends these previous estimates over an additional three years and tests for interannual variations and long-term trends. We further examine the proportions of pups to adult females observed on the rookery and nearby haulouts during the birthing season to assess whether census data can be used to estimate natality. Open robust design multistate models were built and tested using Program MARK to estimate survival, resighting, and state transition probabilities in addition to other parameters dependent on whether or not a female gave birth in the previous year. Natality was estimated at 70% with some evidence of interannual variation but a long-term increasing or decreasing trend was not supported by the data. Bootstrap and regression comparisons of census data with natality estimates revealed no correlation between the two methods suggesting that census data are not an appropriate proxy for natality in this species. Longitudinal studies of individual animals are an appropriate method for estimating vital rates in species with variable detection over time such as the Steller sea lion. This work indicates that natality remains high in this region and is consistent with a population in recovery.

Population Trend and Elasticities of Vital Rates for Steller Sea Lions (Eumetopias jubatus) in the Eastern Gulf of Alaska: A New Life-History Table Analysis

“…Fecundity (also referred to as natality) and survival rates, previously estimated for this rookery [ 16 , 17 ], were used as input data to a life table matrix generally following recommendations by Caswell [ 5 ] ( S1 Table ). The year 2002 starting population was based on pup and non-pup population counts at rookeries and haulouts in the EGOA between 2000 and 2002 by the National Marine Fisheries Service [ 38 , 39 ], with non-pup numbers following sex-specific age distributions based on survival probabilities outlined below.…”

“…Juvenile survival from ages 1 to 4 was specific to age and sex ( p x j and q x j for females and males respectively), but did not vary across years [ 16 ]. Adult female survival ( p x a , ages 5+) also did not vary across years and was modeled using a beta distribution with a peak at 8 yrs of age and an average of 88.1% between 5 and 20 years of age [ 17 ], after which survival dropped off sharply. Subadult and adult male survival estimates are currently unavailable for the EGOA.…”

“…However, such estimates can be highly imprecise because they do not account for, or poorly account for, detection probabilities, sampling variation, and biological process variation [ 11 , 12 ]. Estimates of Steller sea lion vital rates based on longitudinal studies, which do account for detection probabilities and some sources of sampling and process variation, are now widely available [ 13 – 17 ]. These data can be modeled in life history matrices to understand population trends and potential threats to population recovery that are, in some ways, more precise than using snapshot counts of population age structure [ 1 , 12 ].…”

Maternal and offspring effects on the timing of parturition in western Steller sea lions (Eumetopias jubatus)

“…Regarding birth in a previous year which was scored as 0 or 1, we had a very high sighting probability (99.9%) of mature females that give birth in our study sample each year due to excellent spatial and temporal coverage of the rookery throughout the pupping season (Maniscalco et al 2014). However, some females lose their pups at a very early age due to a variety of reasons (Maniscalco et al 2008), and if a female lost her pup within one month of its birth, she received a score of 0 for a previous year's birth because she would not have endured the year-long burden of lactation which is energetically the most costly aspect of reproduction among pinnipeds (Oftedal et al 1987).…”

“…However, reproductive rates of adult females have been very good (ca. 70%) in recent years (Maniscalco et al 2014). Beginning in 1999, up to six remotely operated video cameras were used to conduct observations at Chiswell Island.…”