Status of the Introduced Mule Deer Population on Catalina Island, California, Based on Annual Spotlight Counts

Mule deer were first introduced to Santa Catalina Island, California, in the early 1930s and persist today. Other feral, non-native ungulates have been eradicated (goats, pigs) or significantly reduced in numbers (bison) over the past two decades. Effective management of the deer population is necessary to protect the island’s biodiversity but is dependent upon reliable estimates of population density and demography. We used annual summer spotlight counts, conducted in eight of the past ten years, to estimate deer densities in the island interior. In 2021, we also surveyed transects in the area around Avalon, the largest town on the island. Distance sampling (Program DISTANCE) was used to model density based on line transect data. Island-wide densities varied from 6.3 to 16.9 deer per km2, with an average of 10.2 per km2, and were positively correlated with July-June rainfall during the preceding year. Most (77-96%) of the identifiable deer were adults and most adults were does (58-75%). Deer were spotted most frequently in island chaparral and coastal sage scrub vegetation, the most common vegetation cover types along transects, whereas the use of grasslands and man-made and non-native habitats varied among years. The estimated deer density in Avalon (65.7 per km2) was six to nine times higher than estimates for the interior transects in 2021. The high density of deer near Avalon suggests that gardens, landscaping, golf courses, and intentional feeding subsidize deer numbers in town that have the potential to spill over into the more natural areas of the island