Overestimating Fish Counts by Non-Instantaneous Visual Censuses: Consequences for Population and Community Descriptions

Abstract

BackgroundIncreasingly, 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·km−2) and biomasses (>4 tonnes·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 FindingsWe 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/SignificanceBased 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.

Dermal denticles as a tool to reconstruct shark communities

“…Cryptic behavior, rarity, and diurnal and seasonal movement patterns prevent sharks from being meaningfully censused in many regions (Sale & Douglas 1981, MacNeil et al 2008, Ward-Paige et al 2010a, McCauley, et al 2012a. Time series or replicated surveys have also shown conflicting trends for the same area depending on the survey method used and its associated biases (Burgess et al 2005, Ward-Paige, et al 2010a, Nadon et al 2012, leading to misrepresentations of the status of shark populations and their unfished baseline conditions (Heupel et al 2009, Rizzari et al 2014.…”

“…We observed a high degree of correspondence between the denticles found in the sediments and the sharks documented in the region. We therefore propose that (1) denticle assemblages in the recent fossil record can help establish quantitative pre-human shark baselines and (2) time-averaged denticle assemblages on modern reefs can supplement traditional surveys, which may prove especially valuable in areas where rigorous surveys of sharks are difficult to perform.KEY WORDS: Dermal denticle · Functional morphology · Shark · Paleoecology · Baseline 566: 117-134, 2017 diurnal and seasonal movement patterns prevent sharks from being meaningfully censused in many regions (Sale & Douglas 1981, MacNeil et al 2008, Ward-Paige et al 2010a, McCauley, et al 2012a. Time series or replicated surveys have also shown conflicting trends for the same area depending on the survey method used and its associated biases (Burgess et al 2005, Ward-Paige, et al 2010a, Nadon et al 2012, leading to misrepresentations of the status of shark populations and their unfished baseline conditions (Heupel et al 2009, Rizzari et al 2014.…”

“…Time series or replicated surveys have also shown conflicting trends for the same area depending on the survey method used and its associated biases (Burgess et al 2005, Ward-Paige, et al 2010a, Nadon et al 2012, leading to misrepresentations of the status of shark populations and their unfished baseline conditions (Heupel et al 2009, Rizzari et al 2014.…”

The paradox of inverted biomass pyramids in kelp forest fish communities

“…Half a century of temperate and tropical reef science has yielded a wealth of knowledge regarding how species interactions shape community ecology, yet our ability to predict community size-structure remains constrained by a lack of empirical data and adequate theoretical treatment [1][2][3]. Inverted biomass pyramids (IBPs), where the biomass of large predatory fishes far outweighs biomass at smaller body sizes and lower trophic-levels, have been reported on relatively pristine reefs in the remote tropical Pacific [1,4].…”

Non-Consumptive Effects of Predators in Coral Reef Communities and the Indirect Consequences of Marine Protected Areas

“…Unlike extensive surveys within the FKNMS ), we did not find significantly greater predator biomass at protected sites compared to unprotected sites. The lack of significance, however, may be due to substantial variation in predator abundances across protected sites, relatively low sample size, or imperfect predator detection (Ward-Paige et al 2010). Territories in protected areas were smaller, consistent with the hypothesis that increased predation risk results in an increased cost of holding larger territories.…”