Estimating Population Size in Asynchronous Aggregations: A Bayesian Approach and Test With Elephant Seal Censuses

Abstract

Many organisms reproduce in temporary aggregations where estimates of colony size can be made by direct counts. When individuals are not synchronous, however, early breeders depart before the last arrive, so counts underestimate the total breeding population. We present a model describing a colony's census as a function of arrival, breeding tenure, and the correlation between them, and we use it to illustrate how variance in arrival and tenure affect the census. Counts of breeding female northern elephant seals (Mirounga angustirostris) from 1975 to 2007 were used to test the model. Four of the model's parameters•population size, mean and variance of arrival date, and the correlation between arrival date and breeding tenure•could be estimated from census data using a Bayesian approach; prior estimates of two other parameters•mean tenure and its variance•had to be used to avoid overparameterization. The model's predictions fit observed censuses well and produced reliable estimates of population size and arrival behavior, showing that the maximum census was 8%•16% below the total number of breeding females. This method could be used for estimating abundance in any asynchronous aggregation, given independent information on one of the defining distributions: arrival, tenure, or departure. 834

The Northern Elephant Seal (Mirounga angustirostris) Rookery at Año Nuevo: A Case Study in Colonization

“…Census categories included adult males (8 to 14 y of age), subadult males (4 to 7 y of age), adult females (3 y or more), suckling pups, weaned pups, dead pups, and juveniles. Additional details on census methods are provided in Condit et al (2007).…”

“…Estimating Pups Born from Parous Females Present-In larger harems, we first estimated the number of females in attendance during the breeding season using the model described by Condit et al (2007), which acknowledges that the total females that give birth on site must be estimated because all females are not present at once. On average, the number of females using the colony exceeded the peak female census in late January by 18% on the island and by 10% on the mainland.…”

“…We used the model of Condit et al (2007) to produce a mean ratio r(T) = C(T)/N, where C(T) is the count on date T and N is the total number of females using the colony-that is, r(T) is the proportion of the population present on T. We applied this ratio to estimate the number of females in 1985, 1986, 1997, and 2002 because too few counts were made in those years to use the model of Condit et al (2007). In two other years, 1983 and 2010, females moved from the island to the mainland en masse in response to bad weather.…”

“…Each female nurses her pup daily for a mean of 25 d, while fasting from food and water, and then weans the pup by returning to sea. Since early-arriving females depart the rookery, weaning their pups, before late arriving pregnant females come ashore, the peak female count is less than the total females using the colony (Condit et al, 2007). The rookery contains both suckling and weaned pups from mid-December until early March (Le Boeuf et al, 1972;Le Boeuf & Laws, 1994b).…”

Wildlife Viewing Spectacles: Best Practices from Elephant Seal (Mirounga sp.) Colonies

“…We reasoned that if seal numbers declined as tourists increased over the years, this would suggest a negative impact of viewers on pup production and restrict colony growth, given that other factors such as weather and predation could be ruled out. We obtained data on the number of pups born annually from our own research at AN and PV, from the literature (Condit et al, 2007;Ferrari et al, 2012), and from colleagues (R. Condit, P. Morris, and G. Oliver for recent counts at AN; and B. Hatfield via Friends of the Elephant Seal for PB). The number of pups born reflects the status of a colony, from which one can calculate the number of total animals associated with the colony (Condit et al, 2007).…”