A two-year field program, called GulfCet, was sponsored in 1996 and 1997 by the US Department of the Interior to count sperm whales, an endangered species, in the northern Gulf of Mexico. We also studied 18 other species of cetaceans - dolphins and whales - that are commonly found there. Our goal was to see how many of these marine mammals are near areas of present and future offshore oil and gas development. My part was to describe and measure the habitats where whales are most abundant. We used research vessels to measure temperature and salinity, the amount of dissolved oxygen and nutrients, the velocity of the currents, and the amount of plant and animal plankton from the sea surface to great depths.

Figure 1. Sperm whales sounding to begin a dive.

You might be surprised to learn that this work involves not only marine biologists, but also aerospace engineers such as George Born and Bob Leben (University of Colorado in Boulder). They use TOPEX/Poseidon and ERS-2 satellite altimeter data, which gives ocean height measurements, to produce "road maps" of sea surface highs and lows. We use these maps before, during, and after our cruises to help identify ocean highs and lows. In the Gulf of Mexico, some ocean highs and lows are caused by spinning ocean eddies.

Sea surface lows - called cyclones - are areas where cool, nutrient-rich water upwells and diverges. This favors the development of phytoplankton and, in turn, zooplankton herbivores, fish and squid. So, a cyclone contains a lot of organisms and can be thought of as an "ocean oasis:" a rich food source for marine mammals. On the other hand, sea surface highs - called anticyclones - are areas of downwelling and convergence. In these anti-cyclones, nutrients are not renewed in surface waters; an anticyclone is a sort of "ocean desert." Often cyclones and anti-cyclones are found together.

Animations of TOPEX/Poseidon and ERS-2 altimeter data are used to track thelocation of the Loop Current and the eddies that shed from the current. The Loopcurrent, which is a southern part of the Atlantic's Gulf Stream system, enters the Gulf of Mexico between the Yucatan Peninsula and exits through the Straits of Florida. The Loop Current shed a series of anticyclones during 1996. At the same time, a broad area of cyclonic, lower-than-average sea surface height - "an ocean oasis" - was found in the northeast corner of the Gulf. This is seen in the animation as a dark blue region that is at least half a meter lower than the Loop Current and the anticyclones it sheds (red circular areas).

Figure 2. Sea surface height anomaly map of NE Gulf for October 1996, overlaid with location of oceanographic stations made by R/V GYRE. Locations of whale encounters are shown in black overlay on station locations. 41 sperm whales (shown as dots) were sighted by visual observers and 10 whale groups were heard by acoustic survey (shown as along-track distances for each contact) in October 1996. Most were off the Mississippi delta and at the perimeter of the cyclone. One prolonged acoustic contact was made along track in LC Eddy C at night when we could not get visual confirmation.

Figure 3. Sea surface height anomaly map of the N.E. Gulf of Mexico for August 1997, overlaid with sampling site locations (white squares) and locations of whale encounters (black circles). 56 sperm whales in 22 groups were sighted by visual observers in August 1997. As in October 1996, all but one of these sightings was in the cyclone.

Figure 4. Research Vessel GYRE. This ship is 182 ft long and can accommodate 24 scientists.

We expected that squid-eating sperm whales, and other inhabitants of an "ocean oasis," would be more abundant in the cyclones. Consequently, for our research cruises in 1996 and 1997 we decided to monitor the cyclones and anti-cyclones using the ocean height "road maps" to locate the Gulf of Mexico eddies [Figs. 2 & 3]. Note a broad area of cyclonic, low sea surface in the northeast Gulf - a dark blue region that is over 20 centimeters lower than its surroundings.

Figure 5. Our colleagues John Wormuth and John Lamkin used a multiple opening-closing midwater trawl to collect zooplankton and micronekton. They made trawls at 19 locations during the October 1996 cruise and at 18 locations during the August 1997 cruise. The 8 nets that are opened-closed at different depth horizons during each of these trawls caught a variety of juvenile (paralarval) squid, as well as zooplankton and midwater fish. However, because this trawl has a mouth area of only 1 square meter and is towed at speeds of just 2-3 knots, it is not large enough or fast enough to directly catch the big squids which are the preferred prey of sperm whales.

During our October 1996 cruise aboard the research vessel GYRE [Fig. 4], our trawls [Fig. 5] collected more zooplankton in the cyclone than in the neighboring anticyclone to the south. Most of the sperm whales [Fig. 6] that we saw or heard were also in the divergence, or "oasis," part of this cyclone-anticyclone pair. Forty-one sperm whales were sighted by observers [Fig. 7] and 10 whale groups were heard by acoustic survey [Fig. 8]. Most of these were off the Mississippi delta and inside or at the edge of the cyclone. Within the sperm whale groups we found many females and young calves [Fig. 9]; this may indicate that cyclones are favorable habitats for both feeding and reproduction.

During May and June 1997, Texas GulfCet researchers were at sea on the fisheries vessel OREGONII, and in August 1997 we were onboard the GYRE once again. As was the case in 1996, sperm whales were most abundant in or near the cyclone, which again showed up in our "road maps" as a broad region of locally low sea surface height. Observers made 15 sightings of sperm whales in May - June and 56 sightings of sperm whales in August 1997. Most were within the cyclone or in the counterclockwise flowbetween it and the anticyclone to the south.

The success of these cruises in finding rich whale habitats shows the power of making at-sea observations and measurements at sites selected on the basis of satellite data. The cyclone-anticyclone pairs are natural laboratories for the study of nutrient-rich and nutrient-poor surface waters. We have learned that sperm whales are likely to be found in cyclones and other habitats with locally high plankton biomass. We believe that these "ocean oases" are sufficiently long-lived to allow a reproducing population of sperm whales to exist in the Gulf of Mexico. For more information, see http://www.tamug.tamu.edu/gulfcet/.

Acknowledgment
This research was supported by the US Geological Survey and the US Minerals Management Service under USGS contract #1445-C109-96-004. Figures 1 and 6 are courtesy of the Marine Mammal Research Program, TAMU at Galveston; Figures 2 and 3 are courtesy of the University of Colorado.

Since then, my graduate students and I have continued ship and satellite studies of sperm whale habitat. We're currently partnering with the International Association of Geophysical Contractors to study whether the acoustic emissions from geophysical seismic exploration may lead to changes in the behavior in sperm whales. In the summer of 2002, during the first field year of this Sperm Whale Seismic Studies (SWSS) fieldwork, my colleagues from Oregon State University put radio tags on 18 sperm whales and they are currently tracking the whales to find out where they go in relation to the "ocean oases" that we can monitor using space-based radar altimetry. Other colleagues from the Woods Hole Oceanographic Institution outfitted other whales with digital acoustic tags in order to record whale vocalizations and other data as the whales dived to feed. This SWSS fieldwork will continue during the summers of 2002 and 2003. The project website gives more information. This SWSS research is supported by the US Minerals Management Service and by the Office of Naval Research and the US National Science Foundation.

Figure 6. Sperm whale “spouting” on return from a dive in the Gulf of Mexico.

Figure 7. Visual observers used 25x “big-eye” telescopic binoculars to look for sperm whale “spouts” up to 3 miles away from the ship.

Figure 8. Acoustic census teams used a towed hydrophone array to listen, out to 10 miles away from the ship, for the characteristic “clicking” noise which sperm whales make as they dive to search for squid. The number of vocalizing whales can be estimated by analyzing the sound recordings.

Figure 9. This sperm whale calf, about 5 meters long, was one of 5 calves seen October 20, 1996 in the presence of about 15 adults. We hypothesize the presence of young calves means there is reproductive success in the Gulf. However, there are still not enough baseline data on calf occurrence in the Gulf to judge whether the presence of 5 young calves indicates increased regional calf production in 1996.