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Overestimating fish counts by non-instantaneous visual censuses: consequences for population and community descriptions.

Ward-Paige C, Mills Flemming J, Lotze HK - PLoS ONE (2010)

Bottom Line: Several studies have demonstrated the effectiveness of protected areas for increasing fish abundance or provided insight into the natural abundance and structure of reef fish communities in remote areas.Based on our results, we urge that published literature state explicitly whether instantaneous counts were made and that survey procedures be accounted for when non-instantaneous counts are used.Because such studies are used to make important management and conservation decisions, incorrect estimates of animal abundance and biomass have serious and significant implications.

View Article: PubMed Central - PubMed

Affiliation: Department of Biology, Dalhousie University, Halifax, Nova Scotia, Canada. globalshark@gmail.com

ABSTRACT

Background: Increasingly, underwater visual censuses (UVC) are used to assess fish populations. Several studies have demonstrated the effectiveness of protected areas for increasing fish abundance or provided insight into the natural abundance and structure of reef fish communities in remote areas. Recently, high apex predator densities (>100,000 individuals x km(-2)) and biomasses (>4 tonnes x ha(-1)) have been reported for some remote islands suggesting the occurrence of inverted trophic biomass pyramids. However, few studies have critically evaluated the methods used for sampling conspicuous and highly mobile fish such as sharks. Ideally, UVC are done instantaneously, however, researchers often count animals that enter the survey area after the survey has started, thus performing non-instantaneous UVC.

Methodology/principal findings: We developed a simulation model to evaluate counts obtained by divers deploying non-instantaneous belt-transect and stationary-point-count techniques. We assessed how fish speed and survey procedure (visibility, diver speed, survey time and dimensions) affect observed fish counts. Results indicate that the bias caused by fish speed alone is huge, while survey procedures had varying effects. Because the fastest fishes tend to be the largest, the bias would have significant implications on their biomass contribution. Therefore, caution is needed when describing abundance, biomass, and community structure based on non-instantaneous UVC, especially for highly mobile species such as sharks.

Conclusions/significance: Based on our results, we urge that published literature state explicitly whether instantaneous counts were made and that survey procedures be accounted for when non-instantaneous counts are used. Using published density and biomass values of communities that include sharks we explore the effect of this bias and suggest that further investigation may be needed to determine pristine shark abundances and the existence of inverted biomass pyramids. Because such studies are used to make important management and conservation decisions, incorrect estimates of animal abundance and biomass have serious and significant implications.

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Illustration of an instantaneous count (snapshot) of sharks in a belt-transect UVC (photos from Fig 2A and 2C in Sandin et al. 2008).To visualize the area that would be covered by a belt-transect, we have drawn in the approximate survey boundaries of a 50 m2 transect (∼4 m wide×12.5 m long which is represented by visibility distance). Sandin et al [9] covered 100 m2 (4 m wide×25 m long) per transect – approximately double the area outlined.
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pone-0011722-g005: Illustration of an instantaneous count (snapshot) of sharks in a belt-transect UVC (photos from Fig 2A and 2C in Sandin et al. 2008).To visualize the area that would be covered by a belt-transect, we have drawn in the approximate survey boundaries of a 50 m2 transect (∼4 m wide×12.5 m long which is represented by visibility distance). Sandin et al [9] covered 100 m2 (4 m wide×25 m long) per transect – approximately double the area outlined.

Mentions: Patterns in absolute density are often extrapolated from observed fish counts obtained by UVC, yet whether or not UVC were done instantaneously is rarely reported. We illustrate this point using the data and photos shown of Kingman and Palmyra atolls – the two locations where sharks dominated the top predator biomass and where the highest top predator density and biomass has been reported for reefs [9]. Photos are essentially ‘instantaneous snapshots’ of the reef and represent counts made by instantaneous UVC techniques (Fig. 5). In both photos, one top predator (i.e. shark) occurred within ∼50 m2 – a density of 0.02 individuals·m−2. However, Sandin et al. (2008) reported densities of ∼0.2 individuals·m−2 for both Kingman and Palmyra, which corresponds to 10 individuals per 50 m2 belt-transect. If we assume that the most top-predator-rich photos were used to demonstrate their abundance on reefs at Kingman and Palmyra, then the density would have been overestimated by one order of magnitude. Although a number of factors may have contributed to the discrepancy between the data and figures shown in Sandin et al. [9] (e.g. site selection, fish behaviour, schooling), our results suggest that the difference could be explained by fish speed alone if non-instantaneous surveys were used.


Overestimating fish counts by non-instantaneous visual censuses: consequences for population and community descriptions.

Ward-Paige C, Mills Flemming J, Lotze HK - PLoS ONE (2010)

Illustration of an instantaneous count (snapshot) of sharks in a belt-transect UVC (photos from Fig 2A and 2C in Sandin et al. 2008).To visualize the area that would be covered by a belt-transect, we have drawn in the approximate survey boundaries of a 50 m2 transect (∼4 m wide×12.5 m long which is represented by visibility distance). Sandin et al [9] covered 100 m2 (4 m wide×25 m long) per transect – approximately double the area outlined.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2908695&req=5

pone-0011722-g005: Illustration of an instantaneous count (snapshot) of sharks in a belt-transect UVC (photos from Fig 2A and 2C in Sandin et al. 2008).To visualize the area that would be covered by a belt-transect, we have drawn in the approximate survey boundaries of a 50 m2 transect (∼4 m wide×12.5 m long which is represented by visibility distance). Sandin et al [9] covered 100 m2 (4 m wide×25 m long) per transect – approximately double the area outlined.
Mentions: Patterns in absolute density are often extrapolated from observed fish counts obtained by UVC, yet whether or not UVC were done instantaneously is rarely reported. We illustrate this point using the data and photos shown of Kingman and Palmyra atolls – the two locations where sharks dominated the top predator biomass and where the highest top predator density and biomass has been reported for reefs [9]. Photos are essentially ‘instantaneous snapshots’ of the reef and represent counts made by instantaneous UVC techniques (Fig. 5). In both photos, one top predator (i.e. shark) occurred within ∼50 m2 – a density of 0.02 individuals·m−2. However, Sandin et al. (2008) reported densities of ∼0.2 individuals·m−2 for both Kingman and Palmyra, which corresponds to 10 individuals per 50 m2 belt-transect. If we assume that the most top-predator-rich photos were used to demonstrate their abundance on reefs at Kingman and Palmyra, then the density would have been overestimated by one order of magnitude. Although a number of factors may have contributed to the discrepancy between the data and figures shown in Sandin et al. [9] (e.g. site selection, fish behaviour, schooling), our results suggest that the difference could be explained by fish speed alone if non-instantaneous surveys were used.

Bottom Line: Several studies have demonstrated the effectiveness of protected areas for increasing fish abundance or provided insight into the natural abundance and structure of reef fish communities in remote areas.Based on our results, we urge that published literature state explicitly whether instantaneous counts were made and that survey procedures be accounted for when non-instantaneous counts are used.Because such studies are used to make important management and conservation decisions, incorrect estimates of animal abundance and biomass have serious and significant implications.

View Article: PubMed Central - PubMed

Affiliation: Department of Biology, Dalhousie University, Halifax, Nova Scotia, Canada. globalshark@gmail.com

ABSTRACT

Background: Increasingly, underwater visual censuses (UVC) are used to assess fish populations. Several studies have demonstrated the effectiveness of protected areas for increasing fish abundance or provided insight into the natural abundance and structure of reef fish communities in remote areas. Recently, high apex predator densities (>100,000 individuals x km(-2)) and biomasses (>4 tonnes x ha(-1)) have been reported for some remote islands suggesting the occurrence of inverted trophic biomass pyramids. However, few studies have critically evaluated the methods used for sampling conspicuous and highly mobile fish such as sharks. Ideally, UVC are done instantaneously, however, researchers often count animals that enter the survey area after the survey has started, thus performing non-instantaneous UVC.

Methodology/principal findings: We developed a simulation model to evaluate counts obtained by divers deploying non-instantaneous belt-transect and stationary-point-count techniques. We assessed how fish speed and survey procedure (visibility, diver speed, survey time and dimensions) affect observed fish counts. Results indicate that the bias caused by fish speed alone is huge, while survey procedures had varying effects. Because the fastest fishes tend to be the largest, the bias would have significant implications on their biomass contribution. Therefore, caution is needed when describing abundance, biomass, and community structure based on non-instantaneous UVC, especially for highly mobile species such as sharks.

Conclusions/significance: Based on our results, we urge that published literature state explicitly whether instantaneous counts were made and that survey procedures be accounted for when non-instantaneous counts are used. Using published density and biomass values of communities that include sharks we explore the effect of this bias and suggest that further investigation may be needed to determine pristine shark abundances and the existence of inverted biomass pyramids. Because such studies are used to make important management and conservation decisions, incorrect estimates of animal abundance and biomass have serious and significant implications.

Show MeSH