Estimating cover of benthic organisms from underwater video images: variability associated with multiple observers

Abstract

Using trained observers and video images of reef transects from many parts of the Great Barrier Reef, we investigated (1) accuracy of classification of benthos and (2) variability contributed by observers to the precision of estimates of benthic cover obtained from video tapes. In order to esti- mate accuracy of identification, benthic organisms were identified twice, first in the field and later from video images. These identifications were then compared. The effect of observer error on preci- sion of benthic cover estimates was examined by having 2 observers sample the same video images on 3 separate occasions. These estimates were then compared at the level of different benthic groups (hard coral, soft coral and algae) and for different hierarchical levels of classification of hard corals (life form, family, genus and species). 'Benthic groups' (mean accuracy of 90 ± 8%) and 'families of hard coral' (91 ± 7%) were identified most accurately and least variably from video images, although many genera and some distinctive species were also identified reliably. Life forms of hard corals proved to be the least accurate and most variable level of classification, with a mean accuracy rating of 74 ± 16%. There was little additional variation in estimates of cover when 2 trained observers sampled images, compared with variation in estimates of cover from repeated samples of images by a single observer. At 10% cover, variability in estimates made by a single observer resulted in mean CIs of 7.9 to 12.1%. Inclusion of variation between observers expanded CIs by only ±0.22%. Furthermore, total observer error was small relative to estimates of cover. For example, at 30% cover, the mean CI due to both between- and within-observer variability was 27.2 to 32.8%.