Evaluating the performance of methods for estimating the abundance of rapidly declining coastal shark populations

Abstract

Accurately surveying shark populations is critical to monitoring precipitous ongoing declines in shark abundance and interpreting the effects that these reductions are having on ecosystems. To evaluate the effectiveness of existing survey tools, we used field trials and computer simulations to critically examine the operation of four common methods for counting coastal sharks: stationary point counts, belt transects, video surveys, and mark and recapture abundance estimators. Empirical and theoretical results suggest that (1) survey method selection has a strong impact on the estimates of shark density that are produced, (2) standardizations by survey duration are needed to properly interpret and compare survey outputs, (3) increasing survey size does not necessarily increase survey precision, and (4) methods that yield the highest density estimates are not always the most accurate. These findings challenge some of the assumptions traditionally associated with surveying mobile marine animals. Of the methods we trialed, 8 x 50 m belt transects and a 20 m radius point count produced the most accurate estimates of shark density. These findings can help to improve the ways we monitor, manage, and understand the ecology of globally imperiled coastal shark populations.

Dermal denticles as a tool to reconstruct shark communities

“…On coral reefs, traditional fish surveys using diver transects or videos represent 'snapshots' of the standing population and, as such, can overlook rare, cryptic, nocturnal, or seasonally-ephemeral species (Sale & Douglas 1981, Edgar et al 2004, MacNeil et al 2008, McCauley et al 2012a; Table 6). They also lack the temporal resolution of some fishery-dependent records, and fail to capture natural fluctuations in populations over time (Connell et al 1998, MacNeil et al 2008.…”

“…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.…”

Assessing the effects of large mobile predators on ecosystem connectivity

“…Other researchers interpreting these top-heavy community patterns on Palmyra's reefs have described the trophic architecture of this system as an ''inverted trophic pyramid'' because the biomass of observed predators outnumbers the biomass of prey (Stevenson et al 2007, Sandin et al 2008. Methods used in these surveys have been shown to overestimate predator abundance (Ward-Paige et al 2010, McCauley et al 2012, helping in part to explain this unusual trophic architecture. The stable-isotope data we report herein provides another valuable means for making sense of these putative structural inversions.…”

Reliance of mobile species on sensitive habitats: a case study of manta rays (Manta alfredi) and lagoons

“…Thus, an important part of understanding how human pressure affects mobile species requires determining how mobile taxa use and rely upon the various habitats that they may traverse. This task is made difficult, however, because detailed descriptions of the habitat dependency patterns of mobile fauna are often lacking (Block et al 2001(Block et al , 2011Ferraroli et al 2004;James et al 2005), particularly in marine settings where mobile study subjects are especially difficult to observe and enumerate (McCauley et al 2012b).…”