Divers monitor reef shark numbers as accurately, and cheaper, than automated tracking tools, a study suggests.
Shark populations are thought to be declining globally, but a quick search of the IUCN’s Red List shows that for 67% of shark species the population trend is unknown.
It’s difficult to monitor shark numbers due in part to many species’ relatively large home ranges. The large scale (tens to hundreds of km) and long-term (years to decades) monitoring programs are costly. We need to find simple and low-cost methods for monitoring shark populations. One option is using observations made by professional dive guides. But are these reliable and accurate?
Over a period of five years, scientists from The University of Western Australia compared counts of grey reef sharks (Carcharhinus amblyrhynchos) submitted by 62 dive guides, with data collected by acoustic tagging. The grey reef shark is one of the many species whose population trend is unknown due to lack of data. The study was conducted at dive sites in Palau.
Divers as accurate as electronic tagging
The scientists found the number of grey reef sharks observed by dive guides matched those identified by acoustic tagging. They suggest that shark behaviour was unaffected by the divers’ presence. The divers’ data also indicates that the water’s current strength and temperature influence the abundance of sharks at the monitored sites, which corroborates previous telemetry data.
Lead scientist Gabriel Vianna comments “Our study shows that with a little bit of training and a good sampling design, recreational divers collect very useful data that can be used to monitor shark populations over long periods of time and across large spatial areas. Such programs have relatively small costs when compared with other methods currently used.”
Shark fishing unsustainable
Scientists have estimated that around 7% of all sharks are killed every year. This exceeds the average rebound rate for many shark populations and explains the ongoing declines in most populations for which data does exist. Using divers could give hard evidence of the decline and help measure whether conservation efforts are succeeding.
Citizen science – cheap and popular
“Citizen science” initiatives, where volunteers collect data as part of a scientific enquiry, are growing in popularity as alternatives to conventional scientific sampling as they offer the opportunity to gather large datasets at reduced cost. The roots of citizen science, though, go back to the very beginnings of modern science.
The divers participating in the study were local residents familiar with the identification of reef sharks in Palau. Guides did their normal days’ diving but were instructed to report the total number of individual sharks of each species observed during the dive. The researchers asked them to be conservative with counts, observing features that could permit individual identification (like pigment patterns, marks and scars) to reduce the potential for repeated counts. Standard questionnaires were completed after the day trips – during which 2 or 3 dives would have been conducted. Each questionnaire contained information on the dive site visited, date, dive time, number of divers in the group, shark species and counts of individual sharks sighted by the dive guide. Dive guides estimated the depth during the sightings, current strength and visibility.
The researchers used acoustic tagging to validate the data collected by the divers. Acoustic tags can be deployed on sharks without affecting their behaviour. Whenever a shark passes within range of an array of acoustic receivers, its presence is logged. This provides an index of relative abundance that can be used to identify trends in the populations over time. The receivers were deployed at depths between 25 and 40 m and recorded the presence of tagged sharks up to 200 m away. The scientists internally tagged 39 sharks – 38 females and 1 male. Ten of these tags were also fitted with pressure sensors, which provided a record of depth of the tagged sharks. Temperature loggers near the dive sites recorded daily temperatures.
When to see sharks
Grey reef and whitetip reef sharks (Triaenodon obesus) were present at the monitored sites throughout the year. However, there was seasonal variation in the number of individuals of both species. Less sharks were seen in May, October and November. Sharks were most frequent between March and April. Current and temperature were the key environmental factors affecting shark numbers. Visibility, moon phase and number of divers in the water had little influence on the number of sharks sighted.
The results of the study are reported in the PLoS ONE journal.
Vianna GMS, Meekan MG, Bornovski TH, Meeuwig JJ (2014) Acoustic Telemetry Validates a Citizen Science Approach for Monitoring Sharks on Coral Reefs. PLoS ONE 9(4): e95565. doi:10.1371/journal.pone.0095565
Boris Worm et al, Global catches, exploitation rates, and rebuilding options for sharks, Marine Policy, Volume 40, July 2013, Pages 194-204, ISSN 0308-597X, http://dx.doi.org/10.1016/j.marpol.2012.12.034.