Monitoring elasmobranch populations using scuba divers : patterns, trends and potential biase
Ward-Paige, Christine A.
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Around the world, elasmobranch populations have undergone dramatic shifts - many large sharks have reached such low numbers that they may be at risk of extinction, while many smaller sharks and rays have increased in abundance. Over the past decades, the majority of data used to assess population trends in elasmobranchs has come from commercial fisheries and research surveys in major fishing zones, while there is a paucity of information for other areas like coral reefs. Because elasmobranchs are highly vulnerable to exploitation and many species are of increasing conservation concern, it is desirable to use non-extractive monitoring methods. The goal of this thesis is to examine the accuracy, precision and value of data collected by scientific and recreational scuba divers for analyzing trends in abundance and distribution of elasmobranch populations. Underwater visual censuses (UVC) have been deployed to monitor marine fishes for decades; however, they have only recently been used to study communities that include sharks. Using a simulation model I show that non-instantaneous UVC can produce large overestimates in the counts of mobile fish, and that density estimates need to account for animal mobility and survey methods. This has important implications for descriptions of abundance, biomass and community structure as well as conservation and management targets. Because it is not feasible for scientists to conduct UVC over broad spatial or temporal scales, recreational divers may be a valuable source of data. I used simulations and field studies to demonstrate that methods deployed by non-scientific divers detect fish at lower densities than scientific methods, and that inexperienced divers detect and count elasmobranchs as precisely as experienced divers. Finally, using a volunteer collected database in the greater-Caribbean, I demonstrate the value of nonspecialist data for revealing (i) previously undocumented spatial and temporal trends in the commonly sighted yellow stingray, and (ii) contemporary shark distribution and sighting frequency in relation to human population density and exploitation. Overall, this thesis contributes to our knowledge of temporal and spatial trends of elasmobranch populations and provides insight into the precision and limitations of UVC methods conducted by scientific and non-scientific divers for monitoring fish around the world.