Abstract
Diving with sharks, often in combination with food baiting/provisioning, has become an important product of today’s recreational dive industry. Whereas the effects baiting/provisioning has on the behaviour and abundance of individual shark species are starting to become known, there is an almost complete lack of equivalent data from multi-species shark diving sites. In this study, changes in species composition and relative abundances were determined at the Shark Reef Marine Reserve, a multi-species shark feeding site in Fiji. Using direct observation sampling methods, eight species of sharks (bull shark Carcharhinus leucas, grey reef shark Carcharhinus amblyrhynchos, whitetip reef shark Triaenodon obesus, blacktip reef shark Carcharhinus melanopterus, tawny nurse shark Nebrius ferrugineus, silvertip shark Carcharhinus albimarginatus, sicklefin lemon shark Negaprion acutidens, and tiger shark Galeocerdo cuvier) displayed inter-annual site fidelity between 2003 and 2012. Encounter rates and/or relative abundances of some species changed over time, overall resulting in more individuals (mostly C. leucas) of fewer species being encountered on average on shark feeding dives at the end of the study period. Differences in shark community composition between the years 2004–2006 and 2007–2012 were evident, mostly because N. ferrugineus, C. albimarginatus and N. acutidens were much more abundant in 2004–2006 and very rare in the period of 2007–2012. Two explanations are offered for the observed changes in relative abundances over time, namely inter-specific interactions and operator-specific feeding protocols. Both, possibly in combination, are suggested to be important determinants of species composition and encounter rates, and relative abundances at this shark provisioning site in Fiji. This study, which includes the most species from a spatially confined shark provisioning site to date, suggests that long-term provisioning may result in competitive exclusion among shark species.
Highlights
Sharks are an important component of coral reefs, both in terms of the ecological role they play in reef ecosystems [1,2,3], and increasingly because of their value for shark-diving tourism [4,5]
In order to determine changes in species composition and relative abundances at the Shark Reef Marine Reserve, we asked the following questions: 1) Did species composition and/or encounter rates change over time at the Shark Reef Marine Reserve? 2) Are there seasonal and/or long-term changes in relative abundance of the different shark species at the feeding site? 3) Does the presence of a competitor operator conducting shark feeds at a nearby reef have an effect on shark abundance at the Shark Reef feeding site? In answering these questions, our results provide baseline data on the long-term trends in relative abundance and seasonal cycles, and help elucidate whether the numbers of the eight species of sharks visiting the Shark Reef Marine Reserve changed over the years
Our findings show a dynamic and variable picture clearly indicating that not all shark species that visit Shark Reef Marine Reserve show similar responses to provisioning, and that the responses can change over time
Summary
Sharks are an important component of coral reefs, both in terms of the ecological role they play in reef ecosystems [1,2,3], and increasingly because of their value for shark-diving tourism [4,5]. Watching sharks in their natural habitats, and diving with sharks, has become an important product of today’s recreational dive industry [6]. The number of C. falciformis declined considerably, leading the authors to suggest that declines were related to local fishing pressure rather than competitive exclusion [15]
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