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Biases associated with the use of underwater visual census techniques to quantify the density and size-structure of fish populations

Edgar, GJ, Barrett, NS and Morton, AJ 2004 , 'Biases associated with the use of underwater visual census techniques to quantify the density and size-structure of fish populations' , Journal of Experimental Marine Biology and Ecology, vol. 308, no. 2 , pp. 269-290 , doi:

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Estimates of reef fish densities made by divers visually censusing 5 m wide strip transects were
compared with capture–resight estimates calculated independently using data on the resighting
frequencies of fish marked with colour-coded tags. The difference in density estimates between
methods varied between species but with patterns consistent at the three eastern Tasmanian sites
studied. Densities of the two most abundant species, the wrasses Notolabrus tetricus and Notolabrus
fucicola, showed good agreement between methods. By contrast, populations of the two
monacanthids Meuschenia australis and Meuschenia freycineti were underestimated by an order
of magnitude in strip transects relative to capture–resight, while populations of the open-water latrid
Latridopsis forsteri were overestimated.
For all common fish species observed in strip transects, variation in density estimates between
divers was extremely low compared to variation between sites and between months. Variation in
density estimates between different days within a week and between 50 m blocks located 50 m apart
was also negligible; however, residual error associated with variation by a single diver within a block
and day was high. Biases associated with changing detectability of fishes in open versus densely
vegetated habitats were assessed by algal removal experiments, and were found to be negligible or
non-significantly low for five of the six species examined. The exceptional species—N. tetricus—
exhibited a 70% rise in fish sighted by diver following algal clearance despite trapping data
indicating no increase in fish numbers.
Diver estimates of fish length were on average 7% greater than measured lengths. Divers
possessed a clear tendency to make increasingly inaccurate size estimates as fish length deviated in

either direction from 300 mm. Lengths of 175-mm animals were underestimated by c20% and
400-mm fishes were overestimated by c10%. These changes in diver bias with fish size were
largely independent of fish species, site and diver. The precision of size estimates, as indicated by the
standard deviation of bias, also varied with fish size, with values varying from c13% at 200 mm
fish length to c8% at 400 mm length. The decline in precision at small body size largely reflected
size intervals used by divers to bin data. Divers appear capable of making more precise size estimates
than the 25-mm interval used at small fish sizes.
D 2004 Elsevier B.V. All rights reserved.

Item Type: Article
Authors/Creators:Edgar, GJ and Barrett, NS and Morton, AJ
Journal or Publication Title: Journal of Experimental Marine Biology and Ecology
ISSN: 0022-0981
DOI / ID Number:
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