72 
Abstract— We calculated the power of 
visual length estimates by novice and 
experienced scientific SCUBA divers 
and estimates generated by a stereo- 
video system to detect changes in the 
mean length of three common species 
of reef fish from New Zealand. Length 
estimates from a stereo-video system 
had much greater power for blue cod 
(mean length=33.1 cm., range 19.5-50.1 
cm.) and snapper (mean length=31.7 
cm., range 23-71 cm.). For a third spe- 
cies, red cod (mean length=42.5 cm., 
range 13-74 cm.), the statistical power 
of diver and stereo-video estimates was 
much less for an equivalent number 
of samples owing to the greater vari- 
ation in the true mean length of red 
cod recorded at different sites. At 90% 
power, a stereo-video system detected a 
15% (~5-cm) change in the mean length 
of blue cod with 63% less samples (10) 
than those required by the experienced 
scientific divers (27). Novice scientific 
divers required 28 samples. 
Manuscript accepted 14 July 2000. 
Fish. Bull. 99:72-80 (2001). 
Improving the statistical power of length estimates 
of reef fish: a comparison of estimates 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 
Visual censuses of reef fish have been 
used to monitor fish communities as 
indicators of environmental degrada- 
tion (Hourigan et al., 1988; Fausch et 
ah, 1990) and as a fisheries manage- 
ment tool for assessing the condition 
of reef fish stocks (Ault et al., 1998). 
Ault et al. ( 1998) used data on the aver- 
age length of a fish stock as an index 
of fishing effects. Information on the 
length frequency or mean length 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). 
Environmental surveys commonly 
use SCUBA divers to count and visual- 
ly estimate the length of individual reef 
fish (Jones and Chase, 1975; Harmel- 
in-Vivien and Bouchon-Navaro, 1981; 
Bellwood and Alcala, 1988; Samoilys, 
1989; English et al., 1994). These visu- 
al censuses have many advantages in 
comparison with other sampling tech- 
niques: they are quantitative, quick, 
nondestructive and repeatable (Eng- 
lish et al., 1994). Visual census tech- 
niques have been widely adopted and 
are used to monitor changes in the 
relative abundance or mean length of 
reef fish within marine protected ar- 
eas (Bell, 1983; McCormick and Choat, 
1987; Alcala, 1988; Cole et al., 1990; 
Francour, 1991, 1994; Russ and Acala, 
1996) and as a tool for assessing the 
standing stock or biomass of individual 
species of reef fish (Craik, 1981; Russ, 
1985; Medley et al., 1993; Polunin and 
Roberts, 1993; Hart et al., 1996). Bio- 
mass is estimated from the relationship 
between length and the weight of an in- 
dividual fish of a certain species (Kul- 
bicki, 1989; Kulbicki et al.,1993). How- 
ever, the question not yet addressed is 
how useful are data from visual length 
estimates for detecting changes in the 
mean length or length frequency of a 
population of reef fish? 
The advantages of assessing the sta- 
tistical power of environmental moni- 
toring programs has been discussed by 
