Abstract
Genetic and modelling studies suggest that seasonal aggregations of whale sharks (Rhincodon typus) at coastal sites in the tropics may be linked by migration. Here, we used photo-identification (photo-ID) data collected by both citizen scientists and researchers to assess the connectedness of five whale shark aggregation sites across the entire Indian Ocean at timescales of up to a decade. We used the semi-automated program I3S (Individual Interactive Identification System) to compare photographs of the unique natural marking patterns of individual whale sharks collected from aggregations at Mozambique, the Seychelles, the Maldives, Christmas Island (Australia) and Ningaloo Reef (Australia). From a total of 6519 photos, we found no evidence of connectivity of whale shark aggregations at ocean-basin scales within the time frame of the study and evidence for only limited connectivity at regional (hundreds to thousands of kilometres) scales. A male whale shark photographed in January 2010 at Mozambique was resighted eight months later in the Seychelles and was the only one of 1724 individuals in the database to be photographed at more than one site. On average, 35% of individuals were resighted at the same site in more than one year. A Monte Carlo simulation study showed that the power of this photo-ID approach to document patterns of emigration and immigration was strongly dependent on both the number of individuals identified in aggregations and the size of resident populations.
Highlights
Knowledge of the spatial extent and connectivity of populations is an essential element of the conservation strategy for any species, as it identifies the appropriate spatial context for management actions
No evidence of cross-ocean migration by individual whale sharks was found by our comparison of photo-ID databases from sites spread across the Indian Ocean
Our study suggests that photo-ID techniques are unlikely to be of use for the understanding of movement patterns at such scales because these broad- 12 scale movements, if they occur, are likely to be relatively rare events that we may not have the power to detect
Summary
Knowledge of the spatial extent and connectivity of populations is an essential element of the conservation strategy for any species, as it identifies the appropriate spatial context for management actions Such information can be difficult and expensive to obtain, where study species are migratory or inhabit environments where they are challenging to observe and sample. Anthropogenic impacts on whale sharks such as hunting and ship-strike may endanger the future of both ecotourism and the species itself [7,8,9,10] For this reason, understanding the movements, connectivity patterns and demography of populations are critical goals of management and conservation strategies for these animals
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