Quarterdeck Magazine Online -- Texas A&M Oceanography

Texas A&M University
Department of Oceanography

Summer 1999 - Vol. 7, No. 1

'Cetacean census' continues

Texas A&M researchers aboard the R/V Gyre
spot more marine mammals in the Gulf of Mexico

by Joel G. Ortega-Ortiz

Texas A&M researchers have been intensively studying cetaceans (whales and dolphins) in the Gulf of Mexico since 1992, when the first cruise for the GulfCet project went out to sea. During 15 GulfCet cruises, Texas A&M and the National Marine Fisheries Service Southeast Fisheries Science Center collaborated to create a database of cetacean sightings.
The GulfCet database helped us understand the oceanographic factors, like water temperature and nutrient levels, which influence the distribution and abundance of cetaceans in the Gulf of Mexico. We learned that the distribution of whales and dolphins in the gulf is not random; these animals are observed more frequently in "ocean oases" where food availability is higher. Some of these oases are cold core gyres or eddies (rotating bodies of water about 100-150 kilometers wide) that draw nutrient-rich bottom water to the sea surface, thereby increasing the plankton production. Fish and squid feed on the plankton and attract cetaceans to the cold core gyres (Read Quarterdeck 6.1 for information on GulfCet project and eddies in the Gulf).
Because the continental margin of the northeast Gulf of Mexico is characterized by the temporal persistence of cyclonic eddies, the region is of considerable biological interest. However, only four GulfCet cruises were conducted in the northeastern gulf. Three months after the last GulfCet survey, we welcomed the collaboration with another oceanography project to allow us to continue our research on cetaceans in the Gulf.
The first cruise of the "Northeast Gulf of Mexico (NEGOM): Chemical Oceanography and Hydrography" project went to sea in November of 1997. One of NEGOM's principal investigators, Dr. Doug Biggs, also was a principal investigator in GulfCet. Biggs, professor of oceanography, invited me to assemble a team of graduate student volunteers to "piggyback" cetacean observations on the NEGOM cruises. This was a great opportunity to continue and expand the cetacean sightings database that we had initiated in GulfCet.
Moreover, because the objective of NEGOM is to study the seasonal variations in seawater properties-such as temperature, nutrients, chlorophyll, and currents-we will be able to use the concurrent physical data to evaluate the effects of hydrography on marine mammal distribution.
Survey for cetaceans has been conducted during the first five NEGOM cruises aboard the R/V Gyre, the Department of Oceanography's research vessel. During these cruises, a team of three observers searches for whales and dolphins using long-range, or "Big-eye," binoculars positioned atop the flying bridge. The visual survey is conducted during daylight, when the ship moves between hydrographic stations. In the database, we record the geographic position of the cetacean sightings, species, and number of individuals.
While we are scanning the seas for cetaceans, NEGOM scientists stop the ship at predetermined stations in the gulf and collect water samples for later analysis of oxygen, nutrients, and salinity. Instruments measure the water's temperature, conductivity, downwelling irradiance, percent light transmission and fluorescence.
During the five NEGOM cruises, a total of 334 hours of visual survey were completed for 5,418 kilometers of transect on the ship's track across the northeastern gulf. During this effort, 289 sightings were registered and the following species were identified: Atlantic spotted dolphin, Bryde's whale, bottlenose dolphin, dwarf sperm whale, false killer whale, humpback whale, killer whale, melon headed whale, pantropical spotted dolphin, pygmy sperm whale, Risso's dolphin, sperm whale, spinner dolphin, and striped dolphin.

Editor's note: Read Quarterdeck 6.1 (Spring 1998) for information on the NEGOM and GulfCet projects.

Joel G. Ortega-Ortiz is a Ph.D. student in Texas A&M's Department of Wildlife and Fisheries Sciences. His e-mail address is joelg@tamu.edu.

View maps of the cruise track and the splaces where cetaceans were spotted.