EASTERN TROPICAL PACIFIC

CETACEANS 
text & photos © Don Roberson
Striped Dolphin and calf, eastern tropical Pacific
In 1989, I had the unique opportunity to conduct seabird surveys aboard the NOAA research ship McArthur during a tuna-porpoise research cruise. The seabird counts were to supplement the focused cetacean surveys undertaken by two teams of 3 mammal observers, who used 25X 'big eyes' mounted gyroscopic binoculars (as shown to the left; Gary Friedrichsen in harness). These wide-ranging surveys in the eastern tropical Pacific from 1986-1990 provided evidence that porpoise numbers were declining due to tuna fishing methods. There is a close association between tuna and groups of porpoise that forage just above them (and also Sooty Terns and other tropical seabirds with the dolphin). Two NOAA ships the McArthur and the David Starr Jordan each spent four months at sea, stopping for breaks only once a month. During my 120 days at sea, standing seabird watches 2 hours on, 2 hours off, every day dawn to dusk, provided a wealth of experience with both tropical seabirds and tropical cetaceans. While my surveys were with ordinary binoculars and involved counting birds within 300 m of the ship, I did move to an extra pair of 'big eyes' whenever pods of dolphin were encountered and the ship turned off its transect to catch up with them for standardized counts. My job in those times was to identify and count the flock of birds that is always with those tuna-porpoise assemblages.

The back of our cruise T-shirt is shown above.

We ran preplanned transects thousands of miles offshore, south of Hawaii and west of the Galapagos, eventually reaching the Equator. We then turned east past the Galapagos and into Costa Rica (port call; brief stop enroute at Cocos I.) and then parallel to and below the Equator out another thousand miles and back into Ecuador (port call). When a whale or group of dolphins was spotted from the flying bridge (33 feet above sea level) by the mammal observers on the 'big eyes,' we would turn the ship to approach the cetacean for positive identification. When there were large ponds of dolphin, the mammal observers  entered their independent counts into their on-board computers. [Bird observers also immediately entered all observations into laptop computers during transects. We also looked for sea turtles.] When the ship turned to chase down a cetacean, the bird observer was considered "off effort" and could go to the bow for photos. Most of the photos on this page were in those settings.

Here, we've spotted a Great Sperm Whale Physeter macrocephalus off the starboard bow, easily identified at a great distance by its slanted blow, canting off to the left.

We encountered 21 species of whales and porpoise offshore during the 4-month cruise. Some, like the Striped Dolphin Stenella coeruleoalba (above), were widely spread across equatorial waters, mostly in small groups. Others were found in large single-species groups.
The two primary tuna-porpoise dolphins offshore are "spinners" and "spotters." A mother and calf Eastern Spinner Dolphin Stenella longirostris orientalis (left; this may be a different species than nominate Spinner Dolphins elsewhere in tropical oceans) leap clear of the sea. This animal occur in huge groups of hundreds, and often do remarkable spins during theirs leaps. 
Pantropical Spotted Dolphin Stenella attenuata (right) is quite variable. Inshore populations are heavily spotted (hence the name) but offshore stocks are not. There may be more than one species involved. They were very shy and fled the ship's approach, likely a response to previous experiences being caught in tuna purse nets.
A very widespread dolphin throughout the eastern tropical Pacific, and inshore (such as off Baja California, Mexico, where this photo, left, was taken) is the Short-beaked Common Dolphin Delphinus delphis. Although sometimes found with tuna, it was most often encountered in large single-species groups away from fish.
Rough-toothed Dolphin Steno bredanensis (left; the mammal people called them "Steenos") is another acrobatic leaper. Because it occurs well offshore, it has not been studied very much, and little is known about its life history. It does not associate with tuna but was mostly encountered is small groups of just a couple dozen. There are specimens of vagrant Rough-toothed Dolphins as far north as Monterey County.
Bottlenose Dolphin Tursiops truncatus (right) is sometimes found with tuna, but more often on its own. These offshore populations differ genetically from the inshore populations (to which the famous "Flipper" belonged), but both groups indulge in dramatic leaps and flips from time to time. Mammal people never called them "bottlenosed dolphin;" they always called them "Tursiops" (even if the same people used common names for other species).
Rarely encountered was Fraser's Dolphin Lagenodelphis hosei (left). This is a little-known dolphin found near the Equator, sometimes in groups of thousands. Years later I was to encounter it again off the island of Dominica in the Caribbean. Even though this is a poor shot, the "raccoon stripe" through the face and down the flank is apparent.
Equally uncommon were packs of Melon-headed Whale Peponocephala electra (right). These marauding packs churn up the sea as they move at great speed. Together with False Killer Whale Pseudorca crassidens and pilot whales, they are commonly called "blackfish." Blackfish are often accompanied by seabirds; it appears that Parkinson's Petrel Procellaria parkinsoni has a very strong association with these small sociable whales.
Short-finned Pilot Whale Globicephala macrorhynchus (left) occurs in small pods or family groups. It is easily recognized by both the bulbous head and the very broad base to the dorsal fin, even in poor light conditions.
Orca (Killer Whale) Orcinus orca (right) was only encountered a handful of times. It must be difficult for these large predatory whales to find enough food in the tropics, since cetaceans are scattered so far apart. It is very different in the tropics than in, say, Monterey Bay, where upwelling concentrates foods and congregates sea-going mammals. I expect that tropical orcas eat a lot of fish.
Among large baleen whales, only the Bryde's Whale Balaenoptera edeni (above) was regularly encountered. "Bryde's" is a Scandinavian name, and on the ship we pronounced it "brew-dah's" Whale. Here we see a blow (left) on a whale headed straight way, and the other shot (above) we've just missed the head and are starting to see the back of a very close whale. These whales change direction randomly underwater and are hard to follow. We always tried to get close enough to see the three rostral ridges that separates it from the similarly sized Sei Whale B. borealis.

Other cetaceans seen offshore during the four-month cruise were Pacific White-sided Dolphin Lagenorhynchus obliquidens (off Baja), Risso's Dolphin (Grampus) Grampus griseus, Humpback Whale Megaptera novaeangliae, Pygmy Killer Whale Feresa attenuata, False Killer Whale Pseudorca crassidens, Dwarf Sperm Whale Kogia simus, Pygmy Sperm Whale Kogia breviceps, Goose-beaked (Cuvier's) Whale Ziphius cavirostris, and an undescribed beaked-whale Mesoplodon sp.nov. (a species that has been photographed and described but not yet collected; full details are in Pitman et al. (1987) "Observations of an unidentified beaked whale (Mesoplodon sp.) in the eastern tropical Pacific," Marine Mammal Science 3:345-352. This is a distinctive beaked whale, with a prominent pale grayish blaze in front of the dorsal fin, is now known as "species A" (e.g., P. Folkens, 2000, Marine Mammals of the Eastern North Pacific: a waterproof guide). I have details and a sketch in my field notes.
 

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