Using Known Births to Account for Delayed Marking in Population Estimation of North Atlantic Right Whales

Population estimation using capture-recapture modeling typically requires that individuals are identifiable by unique marks. North Atlantic right whales (Eubalaena glacialis) can be identified by natural callosity patterns on their heads that are established nearly a year after birth, which has facilitated population monitoring using extensive aerial surveys. A well-maintained catalog of individual sightings has been used to annually estimate population size with a Jolly-Seber (J-S) model using a Bayesian state-space framework. Given that young animals cannot enter the catalog before an established callosity pattern, the terminal-year population estimate never includes new calves despite breeding area surveys that provide a nearly complete census of births. Here, I illustrate a simple modification to the J-S likelihood whereby the number of expected entrants is a function of known births and a parameter representing initial offspring mortality. A simulation study was used as a proof of concept and indicated increased accuracy and precision of population estimates. The birth-integrated J-S model had more accurate terminal-year estimates of right whale population size that remained consistent during subsequent model fitting to additional years of sightings data. While the bias corrections were fairly small (5%) given low per capita calving rates, the demonstrated improvement in accuracy will be helpful to the conservation and management processes for this endangered species. Integrated modeling approaches make better use of available data and can improve inferences on population dynamics.