Pleanary Speaker II

Grace Casselberry, UMass Amherst 

Title : Quantifying depredation rates and spatial overlap between great hammerhead sharks (Sphyrna mokarran) and Atlantic tarpon (Megalops atlanticus) in a recreational fishing hotspot 

Abstract : Studies of predator-prey interactions in marine ecosystems are difficult to conduct but essential for effective resource management. Human activities, like recreational angling, can alter the behavior of individuals within populations and facilitate predation events. The Atlantic tarpon (Megalops atlanticus) fishery in Florida is primarily catch-and-release, but fishing guides in the Florida Keys are increasingly reporting the loss of hooked tarpon to depredation, particularly by great hammerhead sharks (Sphyrna mokarran). This appears to be a serious issue, particularly in Bahia Honda, a putative tarpon prespawning aggregation and famous fishing hotspot. To better understand this issue, we used visual surveys to quantify depredation rates of tarpon by sharks in Bahia Honda, and acoustic telemetry to examine spatial overlap between predator and prey. Seventeen great hammerheads were tagged with acoustic transmitters and color-coded cattle tags to aid in visual identification of individuals, and 200 tarpon were implanted with acoustic transmitters. Results from standardized visual observations of fishing activity in spring, 2019 indicate nearly 15% of hooked tarpon suffer depredation mortality by great hammerhead sharks, and that water current direction and angling intensity contribute to depredation susceptibility. Moreover, analyses of movement patterns of both great hammerheads and tarpon indicate that hammerheads modify their space use in Bahia Honda channel with increasing tarpon presence. This, coupled with the visual survey data, will be the foundation for potential solutions to reduce depredation events and decrease the potential for angler-shark conflict.