ABUNDANCE OF SOUTHERN CALIFORNIA COASTAL BOTTLE-NOSED DOLPHINS ESTIMATED FROM TANDEM AERIAL SURVEYS

We describe a tandem aerial survey method for bottlenose dolphins (Tur siops truncatus) that uses two aircraft and independent observer teams to conduct consecutive surveys of the same coastal strip one hour apa rt. Alternatively, one aircraft with one observer team surveys the sam e coastal strip twice over several hours. Using mark-recapture analysi s, we corrected survey counts for visibility bias resulting from missi ng dolphin groups at the surface and submerged groups. Dolphin groups were considered ''recaptured'' when we determined that both observer t eams had detected the same group. This tandem method is highly useful for estimating abundance (and visibility bias) for species where popul ation closure may be assumed between flights. We assumed population cl osure between flights and matched groups using geographic location, gr oup size, and expected travel rates. We derive a new variance estimato r of population site which incorporates group-size variability commonl y encountered in cetacean surveys. From six tandem surveys conducted f rom 1991 to 1994, we estimated the abundance of southern California co astal bottlenose dolphins to be between 78 (95% CI 60-102) and 271 (24 0-306) animals, with an average of 140 (128-154). Variability in abund ance estimates is likely due to seasonal and interannual movement of a nimals along the California and Baja California coast. Abundance estim ates from tandem surveys averaged 53% higher than dolphin counts obtai ned from individual survey flights, demonstrating the importance of co rrecting for visibility bias.