Movement and Dispersion Patterns of Aggregating Leopard Sharks (Triakis semifasciata) off San Diego, California
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Description
Aggregation behavior is ubiquitous in elasmobranch fishes (sharks and rays), as hundreds to even thousands of individuals may gather at a particular location. However, the mechanisms underlying this phenomenon are poorly understood, specifically the contribution of social and non-social forces. That is, are the animals attracted to each other (social) or are they attracted to some environmental feature of the aggregation site (non-social), such as the presence of food, shelter, or favorable water temperature? In this study, the leopard shark (Triakis semifasciata) was used as a model species to begin exploring these questions by quantifying their fine-scale movement patterns within a seasonal aggregation that forms along La Jolla Shores Beach in Matlahuayl State Marine Reserve, San Diego County, California. In August 2019, we captured aerial video footage of a shark aggregation using an unmanned aerial vehicle (UAV). We then used an experimental software program to simultaneously track every shark in the video frame-by-frame. Using the sharks' trajectories, we examined individual shark positions and swimming speed relative to nearest neighbors in the aggregation. The aggregation consisted of approximately 150 sharks, exhibiting a non-uniform distribution and higher density near the center of the aggregation. Additionally, swimming speeds were slowest at the center of the aggregation. This work demonstrates the efficacy of using UAVs and novel software to non-invasively track large marine organisms in situ and lays the foundation for investigating complex social interactions within shark aggregations.
Movement and Dispersion Patterns of Aggregating Leopard Sharks (Triakis semifasciata) off San Diego, California
Aggregation behavior is ubiquitous in elasmobranch fishes (sharks and rays), as hundreds to even thousands of individuals may gather at a particular location. However, the mechanisms underlying this phenomenon are poorly understood, specifically the contribution of social and non-social forces. That is, are the animals attracted to each other (social) or are they attracted to some environmental feature of the aggregation site (non-social), such as the presence of food, shelter, or favorable water temperature? In this study, the leopard shark (Triakis semifasciata) was used as a model species to begin exploring these questions by quantifying their fine-scale movement patterns within a seasonal aggregation that forms along La Jolla Shores Beach in Matlahuayl State Marine Reserve, San Diego County, California. In August 2019, we captured aerial video footage of a shark aggregation using an unmanned aerial vehicle (UAV). We then used an experimental software program to simultaneously track every shark in the video frame-by-frame. Using the sharks' trajectories, we examined individual shark positions and swimming speed relative to nearest neighbors in the aggregation. The aggregation consisted of approximately 150 sharks, exhibiting a non-uniform distribution and higher density near the center of the aggregation. Additionally, swimming speeds were slowest at the center of the aggregation. This work demonstrates the efficacy of using UAVs and novel software to non-invasively track large marine organisms in situ and lays the foundation for investigating complex social interactions within shark aggregations.