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The Montebello

Credits

• Dolphins of Monterey Bay
• Gray Whale Populations

Dolphins, with 33 species worldwide, possess a complex brain, social and communication system and are highly adapted physiologically for life at sea. Dolphin schools are composed of subgroups that include closely related individuals, with the strongest bonds among related females. Although subgroups generally remain intact, the overall school size fluctuates. In contrast to baleen whales, which migrate seasonally to specific feeding and breeding areas, most small cetaceans exhibit more subtle seasonal changes in distribution, abundance and behavior. Factors such as the availability of food resources, predation pressure, physical characteristics of the environment, sex and age class segregation, and reproductive status influence the ecology of small cetaceans.

Six species occur in Monterey Bay either year-round or seasonally. With a submarine canyon and its location within a major upwelling zone, the bay is an extremely rich and productive area, which provides food for thousands of dolphins. These include the near-shore and pelagic bottlenose dolphins (Tursiops truncatus), Pacific white-sided dolphins (Lagenorhynchus obliquidens), Risso's dolphins (Grampus griseus), northern right whale dolphins (Lissodelphis borealis), long-beaked common dolphins (Delphinus capensis), and short-beaked common dolphins (Delphinus delphis).

Central California is characterized by three oceanographic seasons: the upwelling season (March to July), when winds drive cold, nutrient-rich water to the surface creating blooms of plankton; the oceanic season (August to October), when winds relax and warmer offshore water moves in, forming coastal fronts; and the Davidson Current season (November to February), when this northward-bound warmer current surfaces. The onset of each season is variable from year to year.

The frequency of occurrence (Figure 1) and the relative abundance (Figure 2, p. 17) was compiled for the pelagic dolphin species over the past eight years (1998 to 2005; 2,924 effort trips) to look at recent trends. However, data extend back to 1987, when I began my master's thesis on Pacific white-sided dolphins.

Figure 1. The frequency of occurrence by oceanographic season

Pacific white-sided dolphins (Photo by Southwest Fisheries Science Center/NOAA)

Pacific white-sided dolphins are one of the most abundant dolphins endemic to the temperate North Pacific. In Monterey Bay they are frequently sighted near the canyon edge -- although somewhat less in the past few years. In recent years (1997 to 2005), they were found on 40 percent of days, and their mean group size was 285. This is in contrast to the 1987-1991 study period, when they were sighted on 63 percent of days, with a mean group size of 203 (range 2,000-4,000), and 51 percent of the time they were found in small groups of one to 50 dolphins. Their low abundance during the spring of 2004 was likely related to the nearly daily presence of killer whales. They were also infrequently sighted and in low numbers during the 1997-1998 El Niño, a time when the warmer-water species, long- and short-beaked common dolphins, were most prevalent. By analyzing stomach contents of stranded dolphins in Monterey Bay, I found that they contained 12 species of fish and squid and that the animals commonly fed on northern anchovy, Pacific whiting and a variety of pelagic small squid.

Figure 2. The relative abundance by oceanographic season

Risso's dolphins are also frequently sighted and highly abundant in the Monterey Bay region and are also known to frequent high-relief, heterogeneous and shelf-edge habitats. Their presence and group sizes have generally increased since the 1987-1991 period (with a mean group size of 113), compared to an average of 298 in recent years. During the winter of 2004-2005, Risso's dolphins were sighted nearly every day, often in groups exceeding 500 animals -- with the largest group estimated at 6,000. This corresponded to the known presence of large numbers of Humboldt (jumbo) squid. The squid were in the 4-foot to 6-foot range and are a known prey for Risso's dolphins. It's not unusual for groups of Risso's dolphins to occur in the shallower shelf waters of the bay. Since the deep canyon is so close here, the Risso's may venture near shore on occasion to rest or feed on spawning market squid. I analyzed the stomach of a stranded Risso's in the area and found that it was full (unusual for most Risso's dolphin strandings) and contained 13 different species of squid.

Northern right whale dolphins (Southwest Fisheries Science Center/NOAA)

Northern right whale dolphins, an unusual torpedo-shaped dolphin with no dorsal fin, have a similar distribution to Pacific white-sided dolphins, and in Monterey Bay they are frequently sighted (84 percent of the time) in association with Pacific white-sided dolphins and/or Risso's dolphins. These three species often form mixed-species groups, especially when the total group size of all three exceeds several hundred individuals.

Multi-species groups of dolphins, a common phenomenon in Monterey Bay, could provide more options in feeding strategies (e.g., to herd and capture fishes) and a protective function (e.g., to defend against killer whales) similar to large, single-species herds.

Both species of common dolphins (long- and short-beaked) are associated with warmer waters. Long-beaked commons are found most often during late fall and winter in groups of 500 or more. Short-beaked commons are infrequently sighted and were most abundant during the 1997-1998 El Niño period. The long-beaked type is often sighted from shore, as they tend to travel in circuits throughout the area -- often approaching the inner bay waters in the mornings. They may stay for several days or weeks before moving on. They first appeared during the 1982-1983 El Niño, then sporadically after that until the 1990s, when they were often a seasonal visitor. Long-beaked common dolphins feed predominantly on anchovies and market squid. With a similar diet to Pacific white-sided dolphins, these two species may alternate abundance.

Bottlenose dolphins are the only species that inhabits the shallow waters of Monterey Bay, usually just outside the surf line. They were first noticed in Monterey Bay during the 1982-1983 El Niño, and some of the dolphins were known individuals that had previously lived in warmer southern California waters. They are currently year-round residents (200-300 in population, with some moving in and out of the area) that travel in small groups (fewer than 15) and are often observed from shore throughout the inner bay.

Monterey Bay is clearly an important area for dolphins. Their frequent occurrence and high abundance suggests that this rich region provides a predictable and abundant food source throughout the year.

Nancy Black
Monterey Bay Cetacean Project

Thousands of gray whales, Eschrichtium robustus, regularly traverse the waters of the Monterey Bay National Marine Sanctuary. Their twice-annual migration between feeding areas off Alaska and breeding and calving grounds in Baja California -- a round trip of 12,000 miles -- is the longest of any mammal. The regular passage of these baleen whales along the coast provides an important opportunity to conduct population assessments.

Figure 1. Abundance estimates for the eastern North Pacific population of gray whales. These data are from a working document (Rugh et al. 2004) presented to the International Whaling Commission and should not be cited without the author's permission.

In 1999, five years after the eastern North Pacific population of gray whales was removed from the federal list of endangered and threatened species (under the Endangered Species Act), a team of scientists met at the National Marine Mammal Lab (NMML) in Seattle to review this decision and to evaluate its current status. All of the available information at that time indicated that the decision had been a sound one and that the population was continuing smoothly along the road to recovery. However, during that same year (spring of 1999) stranding rates for gray whales increased ten-fold and calf production plummeted. Stranding rates were even higher in 2000, and calf counts remained at very low levels through 2001.

These observations led to speculation that the gray whale population had already exceeded the carrying capacity of its environment and that gray whales were starving. The most recent census of southbound gray whales was conducted by scientists from the NMML (2000-2001 and 2001-2002), and the count analysis revealed what some considered at the time to be an alarming trend because the abundance estimates were declining. (See Figure 1.) Now, with some time behind us and additional years of information, things don't look nearly as grim.

Stranding rates have returned to the normal levels (about 30 per year), and calf production has recovered to or exceeded levels seen before the events of 1999 and 2000. (See Figure 2.) There is a possibility that the abundance estimate in 1997-1998 (27,958 whales) may have been too high, if missing counts were overestimated when access to the research station was lost due to the washout of Highway 1 south of Carmel. That year, the census ended with some counts from Point Lobos, but there are concerns about interpreting the data because the counts were not conducted from the standard site at Granite Canyon. The low estimates of 2000-2001 and 2001-2002 (about 18,000 whales) probably reflect a drop in abundance (as evidenced by high stranding rates and low calf recruitment) but also may be biased downward if not all members of the population migrated as far south as the survey site in those years. A careful look at Figure 1 reveals that there are several instances in which increases in estimated abundance between years (1971 to 1972 and 1992 to 1993) are larger than can be explained by reproductive output of this population over a single season. It is likely that the root cause of the 1999 and 2000 events were related to climate-driven oceanographic factors, but the exact link still evades us.

However, the relationship between calf production and climate is becoming clearer. We have an excellent time series of calf estimates (1994 to 2005), and the link between the timing of sea-ice retreat in the Bering and Chukchi Seas and calf production the following winter is statistically solid. We think that late-retreating sea ice may act as a physical barrier that keeps pregnant females, the first wave of the population to return north, from reaching prime feeding sites. We feel that the nutritive condition of these females between when they arrive on the feeding grounds and early July has a direct impact on the probability that their pregnancies will be carried to term.

Figure 2. Estimates of northbound gray whale calves based on counts made from the Piedras Blancas Light Station. Data from 2001-2005 are preliminary and should not be cited without the author's permission.

The broader question is how the steady warming trend in the Arctic, and subsequent reduction in the extent of seasonal ice, will affect gray whales in the long term. Frankly, we don't know. Primary feeding grounds and primary prey have changed since the surveys that Sue Moore at NMML conducted in the 1980s.

Her most recent surveys indicate that the Chirikov Basin, between St. Lawrence Island and the Bering Strait, no longer supports dense aggregations of feeding gray whales as it did two decades ago. We don't know how the shift in diet will impact gray whales or how prey populations will respond to increased grazing. It is clear that the Arctic is experiencing a warming trend, and we don't know how this change will impact the ecosystem of which gray whales are a significant part.

-- Wayne Perryman
NOAA Fisheries -- Southwest Fisheries Science Center