Spatial capture-recapture (SCR) modelling
Spatial capture-recapture (SCR) models allow one to estimate the density of an animal.
SCR models rely on data in which individual animals are observed on multiple occasions, either being captured, marked and released, or individuals being identified on multiple occasions via specific diagnostic features (e.g., leopard print patterns, dolphin fin markings, etc.).
Greg Distiller provides a comprehensive introduction to SCR models and how to perform these in R.