Observer effects in shipboard sighting surveys of dolphin abundance

We Investigated observer searching patterns and the effects of observer experience and environmental factors upon observer’s abilities to detect dolphin schools and estimate dolphin school size and composition during two research vessel sighting surveys conducted in the eastern tropical Pacific in 1982 and 1983. During the 1983 cruise, observers with exclusively research vessel experience displayed a higher mean dolphin detection rate than observers with exclusively tuna vessel experience. During the 1982 cruise, there was no difference between mean detection rates of the two types of observer. The two observer types did not differ in their estimates of average school size or species proportions. Watch lengths from 1 to 3 hours did not significantly affect detection rates. Observers positioned on both sides of the ship searched through mounted binoculars from abeam to across the bow; the observers concentrated their effort around the trackline. Sun glare off the bow resulted In a decrease, though not complete curtailment, of searching on the trackline. 1984 NMFS (National Marine Fisheries Service) SWFSC (Southwest Fisheries Science Center) Library Public Domain 1930