Estimating Hawaiian monk seal range-wide abundance and associated uncertainty

Abstract

The type and quantity of survey data, and consequently, applicable methods for estimating abundance, vary throughout the range of the Endangered Hawaiian monk seal Neomonachus schauinslandi. Here we present a new approach to combine disparate data and methods to estimate the range-wide abundance of this species, along with associated uncertainty. We quantified subpopulation abundance using total enumeration, closed population capture-recapture methods, empirically derived relationships between survey effort and proportion of the population detected, minimum tallies, or standardized land surveys corrected to account for seals in the water. We used a Monte Carlo approximation to generate a distribution of range-wide abundance, by summing randomly drawn values from distributions of site-specific abundance. Data to estimate range-wide abundance were available for 2013, 2014, and 2015; our estimates were 1291, 1309, and 1324 individuals, respectively. Although the point estimates increased over 2 yr, the confidence intervals for all estimates overlapped. We recognize that these estimates are subject to some varying degree of negative bias, which precludes drawing unequivocal conclusions regarding current population trends. However, after a prolonged history of population decline in this species, the lack of evidence for further decline during 2013 to 2015 is encouraging. Additional years of consistent monitoring will enable reliable assessment of the trend in total Hawaiian monk seal abundance.