In 1984 and 1985, during surveys designed primarily to census dugongs, six species of sea turtles were counted
from the air at an overall sampling intensity of 9% over a total area of 31 288 km2 within the northern sections
of the Great Barrier Reef Marine Park. The sea turtles were not identified to species. We attempted to correct
sightings for perception bias (the proportion of animals visible in the transect which are missed by observers),
and to standardise for availability bias (the.proportion of animals that are invisible due to water turbidity)
with survey-specific correction factors. The resultant minimum population estimate in November 1985 was
(mean � s.e.) 32 187 � 2532 sea turtles at an overall density of 1.03 � 0.08 km-2, a precision of 8%. We
consider this to be a gross underestimate of numbers present. Significant differences between population and
density estimates obtained from repeat surveys of the same areas were accounted for by differences in
Beaufort sea state and cloud cover. The analysis of covariance data suggested that we had not been successful
in standardising all biases. Turtles were widely distributed throughout the Great Barrier Reef lagoon from
inshore seagrass beds to mid- and outer-shelf reefs. Highest densities were observed on inshore seagrass beds
and on mid-shelf reefs, particularly between Murdoch Island and Cape Melville, and in Princess Charlotte
Bay. Maps of density and distribution are given. We discuss the value and limitations of this survey regime for
censusing sea turtles.
