Quarterdeck Volume 6, Number 1, May 1998
Cetacean census
A&M scientists use sight and sound to count the
gulf's marine mammals
Bill Evans and Randall Davis
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[21K] Two rough-toothed dolphins were among the 19, 128 marine mammals spotted in the Gulf of Mexico by Texas A&M scientists during the GulfCet I project. |
cetacean: (si-TAY-shun) a fishlike marine mammal, including the dolphin, whale, and porpoiseTexas A&M's GulfCet studies are an outgrowth of the oil industry's interests in Gulf of Mexico resources, and a shared desire by industry and government to protect the marine mammals in the gulf. In the late 1980s, it became evident that in the deeper waters of the Exclusive Economic Zone of the Gulf of Mexico lie many unexploited oil and gas resources. New seismic and drilling technologies made these resources available to the industry. The federal government, through the Minerals Management Service (MMS), was eager to lease these new deeper water sites, but the National Environmental Protection Act (NEPA) required an environmental assessment and possibly an environmental impact statement. Little or no information was available on the population size and distribution of cetaceans and sea turtles in the deeper areas of the gulf (100 meters to 2000 meters), so surveys were needed-thus the birth of the GulfCet programs.
The programs' purpose is to conduct studies for the U.S. Department of the Interior on cetaceans in the northern Gulf of Mexico to determine their seasonal and geographic distribution and to characterize their habitat in areas potentially affected by oil and gas activities. GulfCet I, a three-year program, was completed in June of 1994. GulfCet II is a three-year extension of that program. Information on the distribution, abundance, and diversity of cetaceans in the Gulf of Mexico comes from several sources. The MMS and U.S. Fish and Wildlife Service supported aerial surveys of birds, marine turtles, and cetaceans in the gulf from 1981-82 (a pilot survey was conducted in 1979). Also, whales and dolphins periodically strand themselves on the gulf's beaches and in shallow water. Information on these strandings in the Gulf of Mexico has been systematically collected since the late 1970s. A considerable amount of research has been conducted on localized populations of bottlenose dolphins; in U.S. Gulf of Mexico waters less than 200 meters deep, bottlenose dolphins and Atlantic spotted dolphins appear to be the most abundant cetacean species. Other directed studies, historic whaling records, animal strandings, and sightings by casual observers have expanded the list of cetacean species known to occur in the gulf.
The most intense field study of cetaceans in the Gulf of Mexico to date was that of the GulfCet I surveys conducted jointly by Texas A&M University and the National Marine Fisheries Service Southeast Fisheries Science Center. For three years, we gathered information over a broad area about the distribution and abundance of common and uncommon cetaceans. GulfCet I was the first extensive census of the gulf's cetacean populations using underwater listening equipment. (See "Cetacean sound off.") This "passive sonar technology" was developed by the U.S. Navy as a part of their surface antisubmarine warfare program. Most cetaceans are vocal, so we can use acoustic detecting systems to study distribution and abundance of mammals as a supplement to the traditional method of spotting them visually. An advantage of acoustic surveys is that we can conduct them 24 hours a day, in the dark, and in bad weather. During Gulf Cet I, we combined mammal sightings with acoustic surveys to provide some of the first detailed complementary information about the presence of marine mammals in the gulf. Based on visual surveys, the overall minimum number of cetaceans in the GulfCet I study area was 19,128. (See selected results.) Acoustic sampling occurred 95 percent of the available time. A total of 487 acoustic contacts were recorded, and of that number, 124 contacts were of 12 identified species. Sperm whales were the most commonly recorded species, accounting for 56 percent of identified contacts. At least 17 cetacean species were identified during eight GulfCet I aerial surveys (each of these species also was sighted during ship surveys).
The GulfCet I program provided some information on cetaceans' habitat preferences, telling us that only two species-bottlenose dolphins and Atlantic spotted dolphins-prefer depths less than 200 meters, and the rest prefer depths of 500 meters and more. However, this first phase of the study failed to tell us more about the cetaceans' favorite habitats, including information about sea-surface temperature, salinity, water column structure and distinctive features such as warm- and cold-core eddies. We considered a number of explanations for this. First, the water conditions of the Gulf of Mexico change often with the periodic intrusion of the Loop Current from the southeast and the formation of warm-core eddies that move across the northern gulf. Second, cetaceans are large, warm-blooded mammals whose wide-ranging movements are not physiologically constrained by water temperature or other hydrographic features. They perhaps move, easily and often, from one part of the gulf to another without regard to the local conditions. But the third and most likely possibility is that the distribution and movements of cetaceans depend on the availability of prey, which may secondarily be influenced by oceanographic features. Based on our experience during GulfCet I, a better understanding of cetacean habitat preference (a primary objective of the research) could only be achieved through focal studies of the physical environment and prey availability. To study cetacean habitat preference, we launched GulfCet II. (See "Whale watch.") |
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Last updated May 1, 1998
