Introduction

Monterey Bay, located along the central California coast, is affected by strong seasonal upwelling, providing rich primary production. This in turn provides a reliable food source that supports four species of pinnipeds, two species of porpoise, six species of dolphins, at least four species of baleen whales, and numerous species of seabirds. Of the three ecotypes of Killer Whales known for the eastern North Pacific, transients (marine mammal predators) are most frequently sighted.

Methods

For each Killer Whale sighting, we attempted to photo-identify all whales within a group. Data collected for each group included sighting location, movements, and behavior of transient groups. If prey species were encountered, we attempted to identify them, and noted if they were harassed, attacked, or consumed. When possible, predation events were videotaped to analyze hunting strategies and roles of specific Killer Whales.

Results and Discussion

Proportion of prey items observed for 84 events documented since 1987 included California Sea Lion (35%), Gray Whale calf (30%), and 10 % or less in descending order for Dall's Porpoise, Northern Elephant Seal, seabird, Harbor Seal, Pacific White-Sided Dolphin, and Common Dolphin. Killer Whales occurred in greatest numbers either when prey items were in abundance or their young were setting out to sea for the first time. A peak in transient Killer Whale sightings occurred during late spring when Gray Whale calves and their mothers are transiting Monterey Bay on their northward migration. Northern Elephant Seals and Pacific Harbor Seals were also weaning their young at approximately the same time. There was then a decline in Killer Whale sightings until late August and sightings increased throughout the fall corresponding with an influx of California Sea Lions and Elephant Seals at sea, then declined again until early spring. On average there were about forty to fifty sightings per year. Killer Whale group sizes varied by prey type with an average group size of three for Dall's porpoise attacks to 13 for attacks on Gray Whale calves.

California Sea Lions were the most numerous prey items, and all age classes of Sea Lions were taken. Predation events on this species were very visible due to the relatively long process of incapacitating the Sea Lion by tossing, body and tail slams, and then in most cases, ending the attack by drowning the prey. In some cases, the Sea Lion was battered, drowned and killed, then towed along with the Killer Whales for several hours.

Harbor Seals have been identified as prey items from fur fragments or brief glimpses of the seal's presence before it was killed. They are possibly under-reported because of the cryptic and quick killing process used on these seals.

All age classes of Northern Elephant Seals were taken. Adult male Elephant Seals appeared to be prevented from taking deep dives by the Killer Whales and drowning was the suspected cause of death, often lasting up to an hour. Weaners were often taken quickly at the surface and a round of pummeling with flukes led to eventual drowning.

There has not been an observed attack on Northern Fur Seals, but they are known as prey for transients in this population. Their behavior of resting on the surface or in kelp paddies with little movement for long periods of time may be an advantageous strategy to avoid detection by Killer Whales.

Both Dall's and Harbor Porpoise are present in Monterey Bay, but only Dall's Porpoise have been recorded as prey items. Killer Whales travelling through the Bay were closely associated with the edge of the Monterey Submarine Canyon, an area where Dall's Porpoise occur, whereas Harbor Porpoise inhabit near-shore shallow waters where Killer Whales rarely occur. Attacks on Dall's porpoise involved a quick rush by the Killer Whales, then the porpoise was often popped up in the air by the head of the whale, or else there was a quick chase and bite which mortally wounded the porpoise. Porpoise were taken by surprise and quickly killed.

Of six dolphin species commonly seen in Monterey Bay, only Pacific White-Sided Dolphin and Long-Beaked Common Dolphin have been attacked and killed by transients. These dolphins often travel in large groups of 100-2000+, and Killer Whales often quietly trail the group for awhile before choosing one that lags behind. Both species exhibited a strong flight response whenever Killer Whales were detected, therefore proving a difficult prey item to catch.

Four species of baleen whales are commonly seen in Monterey Bay: Humpback Whales, Blue Whales, Gray Whales, and Minke Whales. Of these only Gray Whales were observed during attacks by Killer Whales. Humpback and Blue Whales both bear many indications of struggles with Killer Whales on their bodies and flukes; i.e. tooth rake marks from Killer Whales on tail flukes. One beached Minke Whale had evidence of Killer Whale predation. Due to the extreme damage on some Humpback Whale flukes, predation attempts obviously take place, but they may not occur in California waters. Several observations of Killer Whales pursuing and harassing Blue Whales exist for the study area.

Gray Whales and particularly their calves seem to provide a significant food resource on a seasonal basis. During the mother/calf phase of the northern Gray Whale migration there appears to be a location of high vulnerability associated with particular areas of Monterey Bay and its complex canyon system.

Gray Whales that migrate through Monterey Bay either pass near shore or cut straight across the canyon. Killer Whale predation events occurred most often on the north side of Bay and began over the deep canyon before the Gray Whales reached shelf waters on the north side of the Bay. The killer whales often chased the Grays farther north or east into shelf waters.

W. Perryman (NMFS) conducted surveys of Gray Whale calves from 1994 to 2002 from a shore station south of Monterey. The number of Gray Whale calves born each year varied from a peak of 501 calves counted on survey to a low of 87 calves. In years with high Gray Whale calf counts there were more attacks by Killer Whales in Monterey Bay. During 1999, the first year of low calf numbers, there were many Killer Whale sightings over the spring migration months but Killer Whales were not finding calves. Even so there were three known attacks, possibly due to the continued presence of Killer Whales in the area.

The steep bathymetry of the canyon edge seemed to provide some advantage to the Killer Whales and provoked a radical change in surfacing and travel behavior in the Gray Whales. Passive listening by Killer Whales and possible orienting vocalizations near canyon edges by the Gray Whales may serve to increase vulnerability. There is also the possibility that communication occurs between the Gray Whale mother and calf, or the calf is inappropriately vocalizing. The Killer Whales seemed to use the Bay as a prime hunting zone and may form small groups of 5-10 individuals to converge on the location of a mother/calf within minutes to hours of their detection.

Once a mother/calf Gray Whale pair was detected, the Killer Whales grouped up and pursued them until the Grays slowed down and were surrounded by the Killer Whales. As much as six hours may pass from initial attack to kill with ramming, biting, pulling on the pectoral fins, and attempts to separate the mother from the calf. During this period the mother and calf try to dash for the safety of shallow water and the mother Gray will often roll belly up and her calf will get on top of her for brief periods of safety from the intense onslaught. If the Killer Whales are successful in driving away the mother, the calf is swiftly drowned and feeding commences. The protracted struggle involves female Killer Whales, juveniles, and calves. Adult males appear to be able to kill successfully on their own but as a male team of two, they also will participate in feeding events resulting from the attack and kill of a Gray from cooperating females.

The amounts of the carcasses that the whales fed on ranged from just the tongue and blubber around the lower jaw to extensive feeding with complete stripping of the blubber. The tongue weighs as much as 400 Kg and the blubber over several thousand Kg. Sometimes the force exerted by the Killer Whales was enough to decapitate a calf.

Even though nearly the entire transient population sighted off Monterey Bay has been present during a Gray Whale attack, they were not all directly involved. In over 60% of well-documented attacks, three core groups of whales that were frequently sighted in the study area were present. Adult reproductive females within these core groups worked together and engaged in specific roles; for example, one whale acted as separator between the Gray Whale mother and calf and others assisted around the periphery of the Grays to overcome the calf.

The ecosystem found off Central California provides a diverse prey base available to Killer Whales and mastery of a variety of hunting techniques by the whales is essential. This applies to both temporal and spatial distribution of a food resource. This implies that a young Killer Whale needs an extended period of instruction, such as locating key areas at the correct time of year, determining suitable hunting habitat, overcoming prey items of vastly different sizes, working within a group cooperatively, and absorbing cultural nuance from other group members.