To go or not to go with the flow: Environmental influences on whale shark movement patterns |
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Authors: | Jai C Sleeman Mark G Meekan Jeffrey J Polovina Guy S Boggs |
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Institution: | a GIS and Remote Sensing Group, Charles Darwin University, Darwin, Northern Territory 0909, Australia b Australian Institute of Marine Science, P.O. Box 40197, Casuarina MC, Northern Territory 0811, Australia c Hopkins Marine Laboratory, Stanford University, 120 Oceanview Blvd., Pacific Grove, CA 93950, USA d National Marine Fisheries Service, Pacific Islands Fisheries Science Center, Honolulu, HI, USA e CSIRO Marine and Atmospheric Research, G.P.O. Box 1538, Hobart, Tasmania 7001, Australia f The Environment Institute and School of Earth and Environmental Sciences, University of Adelaide, Adelaide, South Australia 5005, Australia g South Australian Research and Development Institute, P.O. Box 120, Henley Beach, South Australia 5022, Australia |
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Abstract: | Seven whale sharks were tracked using satellite-linked tags from Ningaloo Reef, off northern Western Australia, following tagging in April and June 2002 and April-May 2005. We investigated how the movements of those whale shark tracks were influenced by geostrophic surface currents during sequential one-week periods by using a passive diffusion model parameterised with observed starting locations of the sharks and weekly maps of surface current velocity and direction (derived from altimetry). We compared the outputs from the passive diffusion model and maps of chlorophyll-a concentration (SeaWiFs/MODIS) and with the actual tracks of the sharks using GIS and generalized linear mixed-effects models (GLMM). The GLMM indicated very little support for passive diffusion with sea-surface ocean currents influencing whale shark distributions in the north eastern Indian Ocean. Moreover, the sharks' movements correlated only weakly with the spatial distribution of sea-surface chlorophyll-a concentrations. The seven whale sharks had average swimming speeds comparable with those recorded in other satellite tracking studies of this species. Swimming speeds of the seven sharks were similar to those reported in previous studies and up to three times greater than the maximum sea-surface current velocities that the sharks encountered while traversing into lower southerly latitudes (moving northward towards the equator). Our results indicate that whale sharks departing from Ningaloo travel actively and independently of near-surface currents where they spend most of their time despite additional metabolic costs of this behaviour. |
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Keywords: | Geostrophic currents Migration Oceanography Passive diffusion model Rhincodon typus Satellite tracking |
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