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


Edgar, GJ and Barrett, NS and Morton, AJ, Biases associaated 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, 308, (2) pp. 269-290. ISSN 0022-0981 (2004) [Refereed Article]

DOI: doi:10.1016/j.jembe.2004.03.004


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 ≈20% and 400-mm fishes were overestimated by ≈10%. 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 ≈13% at 200 mm fish length to ≈8% 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. © 2004 Elsevier B.V. All rights reserved.

Item Details

Item Type:Refereed Article
Research Division:Biological Sciences
Research Group:Ecology
Research Field:Marine and estuarine ecology (incl. marine ichthyology)
Objective Division:Environmental Management
Objective Group:Marine systems and management
Objective Field:Rehabilitation or conservation of marine environments
UTAS Author:Edgar, GJ (Professor Graham Edgar)
UTAS Author:Barrett, NS (Associate Professor Neville Barrett)
UTAS Author:Morton, AJ (Mr Alastair Morton)
ID Code:31073
Year Published:2004
Web of Science® Times Cited:191
Deposited By:TAFI - Zoology
Deposited On:2004-08-01
Last Modified:2005-04-19

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