Environmental influences on Great White Sharks
August 16, 2013 by dyertrust
ENVIRONMENTAL INFLUENCES ON THE ABUNDANCE AND SEXUAL COMPOSITION OF WHITE SHARKS IN GANSBAAI
The Dyer Island Conservation Trust has published a new article focusing on the driving factors and cues behind the presence of Great White sharks in the Gansbaai area. This work was made possible through a 5 year long study by the DICT, and Animal Demography Unit, Cape Town University. The data was collected from the Marine Dynamics commercial shark tours vessel, Slashfin which samples water temperatures with every trip.
Factors influencing the presence of this species include seasonal prey availability and environmental conditions. The latter varies dramatically from season to season in this key aggregation site. Furthermore this study has found a link to climatic phase on the sexual composition of white sharks utilizing this bay.
There seems to be a climatic fluctuation occurring every few years, marked as a La Nina or El Nino event, which has been linked to rainfall and sea surface temperature variations in the Western Cape, this study found a correlation to sexual segregation in white sharks during these different phases.
Male white sharks seem to prefer cooler water temperatures during a La Nina event, utilizing both the Dyer and inshore areas during this period. This could be due to less larger females visiting the bay, thus less competition on a foraging basis between the sexes. Larger females may prefer warmer water temperatures to facilitate their higher growth rate demands, a behaviour known as the thermal niche hypothesis that has been described in the species in Southern Australia, and the Pacific.
Wilfred Chivell, owner of Marine Dynamics and founder of the DICT, The studies published through this research conducted in the Gansbaai area will help us understand and manage, as well as protect, Great White sharks more effectively.
The outcome of this study has shown that there is possibility of using this data to predicatively model high risk times and areas for bather safety.
This is the second scientific publication from the Dyer Island Conservation Trust (DICT) in PLOS one, a popular peer reviewed on-line journal, in the last month. The teams previous study came out in July 2013, and was the first open population estimate of Great White sharks in the region, concluding that numbers could be 50% lower than expected.
Thank you to the Marine Dynamics, Volkswagen South Africa, the University of Cape Town - ADU Department, our other corporate sponsors, and all of our of donors all across the world - YOU made this possible.