63 
Abstract— Visual estimates of reef- 
fish length are a nondestructive and 
useful way of determining the biomass, 
mean length, or length frequency of reef 
fish. Consequently, visual estimates of 
reef-fish length are often an important 
component of reef-fish monitoring pro- 
grams, many of which increasingly use 
volunteers. We compared estimates of 
the length of plastic fish silhouettes 
determined visually by experienced sci- 
entific and novice SCUBA divers. Novice 
divers showed a similar level of accu- 
racy ( mean error: 2.3 cm ) to that of expe- 
rienced scientific divers (mean error: 
2.1 cm). Significant improvements in 
accuracy and precision were provided 
by a stereo-video system (mean error: 
0.6 cm). After minimal training in the 
use of hardware and software, vol- 
unteers can obtain a high degree of 
measurement accuracy and precision 
with a stereo-video system, allowing 
them to assist with monitoring reef-fish 
lengths. 
Manuscript accepted 14 July 2000. 
Fish. Bull. 99:63-71 (2001). 
A comparison of the precision and accuracy 
of estimates of reef-fish lengths determined 
visually by divers with estimates produced 
by a stereo-video system 
Euan Harvey 
Department of Marine Science 
University of Otago 
304 Castle Street 
Dunedin, New Zealand 
Present address: Department of Botany 
University of Western Australia 
Western Australia 6907, Australia 
E-mail address: euanh@cyllene.uwa.edu.au 
David Fletcher 
Department of Mathematics and Statistics 
University of Otago 
539 Castle Street 
Dunedin, New Zealand 
Mark Shortis 
Department of Geomatics 
University of Melbourne 
Parkville, Melbourne 
Victoria 3052, Australia 
The marine environment is affected 
by many anthropogenic activities (e.g. 
the exploitation of marine resources 
for food, medicine, and curios) with 
the result that many environments are 
compromised by siltation and pollution. 
Reef fish, an important commercial, 
recreational, and cultural resource, are 
likewise being exploited at an increas- 
ing rate. Consequently, it is important 
that environmental managers and sci- 
entists have objective evidence of the 
magnitude and effect of these impacts. 
Data on the community structure of 
reef-fish assemblages, including precise 
and repeatable estimates of the length 
frequency and abundance of individual 
species, provide this information. Infor- 
mation on the length frequency of a 
fish population when linked with even 
a rudimentary knowledge of the biol- 
ogy of the species may allow estimates 
of recruitment to the adult population, 
fishing intensity, and rates of recovery 
from fishing (McCormick and Choat, 
1987) or other impacts. 
Accurate and precise length and 
abundance data are difficult to obtain 
because fish occupy different habitats 
and display varying behavior over a 
range of spatial and temporal scales. 
Environmental surveys are commonly 
used to determine the abundance and 
length frequency of reef-fish assemblag- 
es with SCUBA divers who count and 
visually estimate the length of individ- 
ual reef fish (Jones and Chase, 1975; 
Harmelin-Vivien and Bouchon-Navaro, 
1981; Russ, 1985; Bellwood, 1988; Bell- 
wood and Alcala, 1988; Kulibicki, 1989; 
Samoilys, 1989; Francour, 1991, 1994). 
Visual census techniques have many 
advantages, compared with other sam- 
pling techniques, in that they are quan- 
titative, quick, nondestructive, and re- 
peatable (English et al., 1994). The 
disadvantages of visual census tech- 
niques is that the observers undertak- 
ing the sampling need to be trained 
and must have experience to identify, 
count, and estimate the length of reef 
fish accurately (English et al., 1994). In 
addition, visual census techniques in- 
volving SCUBA divers are restricted to 
shallow depths owing to the constraints 
of decompression diving. 
