Scientific Encounters with Whale Sharks

On June 26, 2006, GCRL and NOAA National Marine Fisheries Service (NOAA Fisheries) scientists encountered an aggregation of 16 whale sharks approximately 50 miles southwest of the Mississippi River Delta. The encounter lasted more than four hours. The sharks remained within an area approximately 1.0 km2 and continuously ram-filter fed at the surface. See Feeding Ecology. They were skimming the surface of the water as they swam with their lower jaws positioned slightly under the surface, an activity that was interspersed with periodic gulping and contraction of the buccal cavity which caused lateral displacement of the gill slits. Additionally, there was an isolated incident of a whale shark exhibiting a “coughing” behavior, which may be a mechanism to clear or flush gill rakers of accumulated food particles. Frequently some of the sharks appeared to pair off and swim parallel and adjacent to each other (See photo below). The whale sharks fed continuously during the entire encounter. The estimated lengths of the whale sharks ranged from 6 to 12 meters Total Length (TL), with most being greater than 8 meters TL.

Plankton nets towed in surface waters where the sharks were feeding produced large volumes of fish eggs, which comprised two distinct types of eggs, molecularly identified by Joe Quattro at the University of South Carolina as little tunny (Euthynnus alletteratus, 98%) and crevalle jack (Caranx hippos, 2%). Fish egg densities were more than 100-fold higher at the study site compared to fish egg densities from a nearby control site. This encounter provided the first documented observations of an aggregation of whale sharks feeding on fish eggs in the Gulf of Mexico.