Observer effects and training in underwater visual surveys of reef fishes

Abstract

Visual survey techniques are used widely to estimate abundances of target organisms in terrestrial and aquatic environments. There are a number of methodological 'errors' in almost all applications of visual surveys. Given the dependence of all visual survey data on the skill and technique of the observer, one potentially important source of imprecision andlor bias is vanation among and within observers. In studies involving large amounts of fieldwork over great geographic range and many years, it is inevitable that observers will change from place to place and through time at any single site as they are replaced or gwn experience. We present the results of 3 observer tralningkalibration exercises that lnd~cate that observational studies in which multiple observers must be employed may be subject to considerable observer-related biases and imprecision. We found that careful training and calibration of observers ameliorated such effects for most taxa, but non-trivial levels of bias for some taxa and imprecision m estimates for several taxa remained even after thorough training. It is essential that the influence of observer bias and impreclslon be well documented in multi-observer monitoring studies, so that (spurious) patterns related to differences among observers can be distinguished from real spatial or temporal patterns in the environment.

Community structure and biogeography of shore fishes in the Gulf of Aqaba, Red Sea

“…As discussed by Brock (1982), dwarf, cryptic and nocturnal species are underestimated by the visual census technique. The visual census technique is widely applied and accepted for fish ecological studies on coral reefs (English et al 1994), although differences in skill and technique of observers can be a source of imprecision and/or bias (Thompson and Mapstone 1997). Therefore the first author (M.A.…”

Comparison of an enclosure drop trap and a visual diving census technique to estimate fish populations in eelgrass habitats

“…However, other authors suggested that the variability between divers confounded the analysis of diving census data (Thompson and Mapstone 1997). In this study the variation between the 2 divers was tested and found not to be significantly different (except for C. maenas in October) and to be relatively low (Table 6) compared to the variability between sampling dates.…”

Impact of observers’ experience level on counts of fishes in underwater visual surveys

“…Both can be problematic. Training appears to be an effective way to reduce observer bias, at least for some taxa, but probably has less impact on inherent variability within observers' counts (Thompson & Mapstone 1997). It seems plausible that experience level might also be a factor affecting inter-or intra-observer variability (Kulbicki & Sarramegna 1999), but we are not aware of any previous study which has specifically examined this issue.…”