Texas A&M oceanographers based in College Station and marine mammal biologists based in Galveston, plus a small army of graduate students, are working together to study the distribution and abundance of sperm whales (Physeter macrocephalus) and other marine mammals in the northern Gulf of Mexico.
This GulfCet II field program was funded by the U.S. Department of the Interior to census the region's 18 common species of dolphins and whales. Our goal is to measure cetacean abundance in continental margin areas of present oil and gas development and in areas further offshore to depths of two kilometers, where oil and gas exploration will likely intensify in the near future.
We are especially interested in determining how many sperm whales might live in the northern Gulf of Mexico, because sperm whales are an endangered species of marine mammal.
To characterize the oceanographic habitats in which the sperm whales are most abundant, we need to describe the region biologically, chemically and physically. From Texas A&M's oceanographic research vessel, the Gyre, we measure the water's temperature and salinity structure, the amount of oxygen and nutrients dissolved in the water, and the velocity of the currents. We also inventory the stocks of plant and animal plankton by measuring chlorophyll fluorescence and by acoustically tracking and also trawling the Deep Scattering Layers (DSLs)-masses of fish, squid, and shrimp that reflect sound and "scatter" pulses of sonar used by oceanographers to detect them.
We can sample different depth slices in these DSLs using an electronic Multiple Opening/Closing Net and Environmental Sampling System (MOCNESS) and a larger Isaacs-Kidd Midwater Trawl (IKMT). (See "Capture a slice of sea life" for more about the MOCNESS and the IKMT.)
Aside from these nets and trawls, another important tool for oceanographic habitat studies is the satellite altimeter. Satellite data for GulfCet are provided by George Born and Robert Leben, aerospace engineers at the University of Colorado at Boulder. Day or night, in cloudless or cloudy conditions, the two satellites- TOPEX/POSEIDON and ERS-2-use microwave frequencies to sense the sea-surface height, record it, and produce high resolution maps of local highs and lows in the ocean surface.
The sea-surface lows, called cyclones, (see "Spin Cycles" for explanation) are areas of upwelling in which cool, nutrient-rich water comes close to the surface. In cyclones, we find higher than average stocks of phytoplankton and the zooplankton herbivores that prey on them. Such an abundance of life has earned cyclones the nickname "ocean oases."
The opposite of cyclones are anticyclones, which are sea-surface highs and areas of downwelling. In anticyclones, nicknamed "ocean deserts," nutrients are not renewed in surface waters and stocks of plankton are low.
Anticyclone-cyclone pairs are natural laboratories for the study of high-nutrient and nutrient-depleted surface waters.
Zooplankton are abundant in the cyclone "oases," so we expected that zooplankton predators-fish and squid-would be more abundant in the cyclones and least abundant in the anticyclones. In turn, we expected to find the sperm whales, which feed on squid, to be more abundant in the cyclones and least abundant in the anticyclones.
When we went to sea in October 1996, observers sighted 41 sperm whales and acoustic surveys detected 10 whale groups. Sperm whales were uncommon in the deep-water canyon to the east of the cyclone, and even rarer in the anticyclone. Most of the whales were off the Mississippi Delta and inside or at the perimeter of the cyclone. Within this area, on October 20th, spotters saw five young calves in the presence of about 15 adult sperm whales.
We hypothesize the presence of young calves means the whales reproduce successfully in the gulf. However, we lack sufficient baseline data on calf occurrence in the gulf to judge whether a group of adults with five young animals represents an increase in regional calf production in 1996 over previous years.
As we expected, our multiple opening/closing trawls collected more zooplankton, squid, and fish in the cyclone than in its anticyclone companion, Loop Current Eddy C, located to the south. Although our nets were not large enough or fast enough to collect adult squid, the nets did capture many smaller, younger squid. We believe that the paralarval squid-especially when they are found with large amounts of zooplankton and micronekton-are indicators of the distribution and abundance of the adult squid.
We can speculate that the sperm whales gathered in the cyclone because their prey, the squid, gathered there. Likewise, the squid probably gathered in the high-nutrient waters of the cyclone to prey upon the abundant zooplankton and micronekton.
In May and June 1997, Texas A&M GulfCet observers were at sea on the fisheries research vessel Oregon II, and in August 1997, we were back at sea on the Gyre. Again, the sperm whales were most abundant in or near the cyclone, which showed up in altimetry as a broad region of locally low sea-surface height. Observers sighted 15 sperm whales in May and June and 56 in August 1997. Most were in the cyclone or in the water flowing counterclockwise around it.
Continuous altimetric monitoring of the general circulation in the Gulf of Mexico is serving two primary purposes in ongoing GulfCet studies.
First, we use historical altimeter data to map the fronts and eddies that occurred during the 10 cruises fielded in support of GulfCet I from 1992 to 1994, as well as the present cruises in support of GulfCet II. These historical data help interpret when and where cetaceans were found during the cruises.
Second, near-real-time altimeter data assist us in planning the cruise and mapping eddies, thus allowing ship resources to be directed toward biological and cetacean research instead of locating eddies and cyclones.
Collaboration among the investigators through the World Wide Web significantly streamlined the sharing, processing, and archiving of altimeter data products coincident with surveys and has produced a variety of tools for use by GulfCet investigators. To view altimetry data in near real time on the web, visit the altimetry viewer at the University of Colorado's "Gulf of Mexico Home Page."
We are learning that sperm whales are likely to be found in cyclones and other habitats where plankton are abundant.
We speculate that the Gulf of Mexico contains-and will continue to contain-these plankton habitats, thus allowing sperm whales to reproduce and exist in the gulf. We believe that the changing patterns of whale distribution and abundance in the gulf reflect their food preference and their success at reproduction.
[24K] GulfCet scientists didn't have to rely on luck to spy a sperm whale's
fluke in the Gulf of Mexico. Satellite data and knowledge about the whale
s' feeding habits led scientists to the gulf's "ocean oases,"
where whales gather to feast.
[36K] A sperm whales opens its mouth and swims, belly up, at the surface.
[33K] A sperm whale calf about five meters long was one of five calves
seen October 20, 1996, in the presence of about 15 adult whales.
Sidebar article: Spin Cycles, Winding path of warm water creates gulf's
eddies
[18K] Cyclones are the most common
place to find sperm whales, as indicated by the stars placed on this diagram
of the gulf's eddies, made from an August 21, 1997, regional altimetry map.
(Data courtesy of Robert Leben, Colorado Center for Astrodynamics Research).
[37K] A scientist scans the sea for cetaceans using powerful binoculars
nicknamed "Big Eyes." In addition to visual surveys, GulfCet scientists
conducted acoustic surveys with arrays of underwater microphones.
