MEPS 482:153-168 (2013)  -  doi:10.3354/meps10290

Trends in sightings and environmental influences on a coastal aggregation of manta rays and whale sharks

C. A. Rohner1,2,3,*, S. J. Pierce1,4,5, A. D. Marshall1,5, S. J. Weeks2, M. B. Bennett6, A. J. Richardson3,7,8

1Manta Ray & Whale Shark Research Centre, Marine Megafauna Foundation, Praia do Tofo, Inhambane, Mozambique
2Biophysical Oceanography Group, School of Geography Planning and Environmental Management, The University of Queensland, St Lucia, Queensland 4072, Australia
3CSIRO Marine and Atmospheric Research, EcoScience Precinct, Dutton Park, Queensland 4102, Australia
4All Out Africa Research Unit, PO Box 153, Lobamba, Swaziland
5ECOCEAN USA, Praia do Tofo, Inhambane, Mozambique
6School of Biomedical Sciences, The University of Queensland, St Lucia, Queensland 4072, Australia
7Centre for Applications in Natural Resource Mathematics (CARM), School of Mathematics and Physics, The University of Queensland, St Lucia, Queensland 4072, Australia
8The Ecology Centre, School of Biological Sciences, The University of Queensland, St Lucia, Queensland 4072, Australia

ABSTRACT: Sightings of planktivorous elasmobranchs at their coastal aggregation sites are often linked to biological, environmental and temporal variables. Many large planktivorous elasmobranchs are also globally threatened species, so it is necessary to try and separate population trends from environmentally driven, short-term fluctuations. We investigated the influence of environmental variables on sightings of 3 species of planktivorous elasmobranchs off Praia do Tofo, Mozambique: the reef manta ray Manta alfredi, giant manta ray M. birostris and whale shark Rhincodon typus. We used 8- (2003 to 2011) and 6-yr (2005 to 2011) logbook data for manta rays and whale sharks, respectively, and constructed a generalised linear model with animal sightings as the response. Predictors included temporal (year, month, time of day), biological (plankton categories), oceanographic (water temperature, time from high tide, current direction and strength and wave height) and celestial (moon illumination) indices. These predictors best fitted reef manta ray sightings, a coastal species with high residency, but less so for the wider-ranging giant manta rays and whale sharks. We found a significant decline in the standardised sightings time series for the reef manta ray (88%) and whale shark (79%), but not for the giant manta ray.


KEY WORDS: GLM · Generalised linear model · Decline · Population trend · Seasonality · Environmental variability · Manta alfredi · Manta birostris · Rhincodon typus


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Cite this article as: Rohner CA, Pierce SJ, Marshall AD, Weeks SJ, Bennett MB, Richardson AJ (2013) Trends in sightings and environmental influences on a coastal aggregation of manta rays and whale sharks. Mar Ecol Prog Ser 482:153-168

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