Cover • Figures & Tables • Introduction • Materials & Methods Discussion • Results • Acknowledgements • Literature Cited

Discussion

Whale Diet and Euphausiid Composition Blue whales foraging in Monterey Bay fed exclusively upon epipelagic euphausiids (T. spinifera and E. pacifica), in proportions that were different from the overall composition in net samples. Schoenherr (1991) found that blue whales in Monterey Bay fed primarily upon T. spinfera, and while she did not report E. pacifica in the diet, it is likely that much of the unidentifiable material in her samples consisted of E. pacifica. This is supported by the observation that E. pacifica was the second most common euphausiid species she found in deep net tows. Because her study focussed upon surface feeding whales, and T. spinfera is found in the upper portion of the water column where it often forms surface swarms (Youngbluth 1976, Smith & Adams 1988), it is possible that the importance of this species was overemphasized in her study. Similar to our study, Fiedler et al. (1998) found that the dominant species in net tows in the vicinity of foraging blue whales and blue whale fecal samples near the Channel Islands, California were T. spinifera and E. pacifica. Fiedler et al. (1998) speculated that foraging blue whales preferentially feed upon adult euphausiids. Comparison of our blue whale diet and net sample data in Monterey Bay support this hypothesis: euphausiids found in whale fecal samples were significantly larger than those taken in net samples. This observation may be due to several possiblilities including bias towards less mobile euphausiids in net samples, escape of smaller euphausiids through the baleen of foraging whales, and/or preferential targeting of adult euphausiid schools by foraging whales. A number of observations support the hypothesis of preferential targeting of adult euphausiid schools: Individual size distributions for both species of euphausiids within whale fecal samples were similar to size distributions for adults collected within net tows, though in the case of T. spinifera, there is some indication that whales were foraging on larger individuals (> 25 mm) that may have avoided capture in net tows (Fig 6). Furthermore, data on the species and size composition of euphausiid schools (Table 1) demonstrate that euphausiid schools are highly variable in species and size structure of fine spatial scales. Our direct measurement of the diving behavior of whales in Monterey Bay (Fig. 5) indicates that whales concentrate their foraging activity on deeper layers of euphausiids located between 150-200m. Several investigators (e.g. Lavaniegos 1996, Bollens et al. 1992) have found that later life history stages of euphausiids are found deeper in the water column. Wishner et al. (1995) also suggested that right whales select copepod aggregations with older lifestages. Blue Whale Foraging The distribution of blue whale sightings and euphausiid densities (Figs. 1 and 2), and the daytime vertical distribution of whale dives and euphausiids (Fig. 6) indicate that whale foraging effort is concentrated on dense euphausiid schools associated with the Monterey Submarine Canyon. Whales dove directly down to the densest aggregations of euphausiids between 150 and 200m on the canyon edge. Studies of blue whales foraging off the Channel Islands, California have reported similar behavior (Fiedler et al. 1998, Croll et al. 1998). Whales sought patches that were approximately two orders of magnitude greater than the densities generally available in the Bay. Because our measurements were directed at patches where whales were observed foraging, we feel that this provides, for the first time, an estimate of the magnitude of prey densities for large rorquals. These densities are higher than both the mean (6-73 kg m-2) and maximum (154 kg m-2) densities estimated for Thysanoessa raschi and Meganyctiphanes norvegica by Simard and Lavoie (1999) using acoustics in the Gulf of St. Lawrence, an important blue whale foraging area. These values are also higher than mean values in other regions of high euphausiid density: 2-102 g m-2 for the Scotian Shelf, North Atlantic (T. raschi, M. norvegica and T. inermis) (Sameoto 1980, 1983, Cochrane and Sameoto 1991) and 30-61 kg m-2 for Elephant Island, Antarctica (Hewitt and Demer 1993). Acoustic measurements from these other studies, however, were not directed at specific locations where large whales were concurrently foraging. How do these densities compare with prey densities observed for other zooplanktivores? Brodie et al. (1978) estimated that fin whales (Balaenoptera edeni) required prey concentrations of at least 17.5 g m-3 to meet its daily energy requirements. Although mean euphausiid densities in Monterey Bay were much lower than this, such densities were readily available at the canyon edge. Dolphin (1987) estimated euphausiid densities where humpback whales were foraging at 910 individuals m-3 (compared to our finding of 4,403 individuals m-3), and that minimum required densities were about 50 individuals m-3. Wishner et al. (1995) found that zooplankton densities in regions where right whales foraged in the southwestern Gulf of Maine were approximately 3 times the mean densities in the region (whale feeding densities averaged 3.1-5.9 g m-3, compared to 1.1-3.6 g m-3 where whales were not foraging). In a related study, Macaulay et al. (1995), using hydroacoustic surveys, estimated zooplankton density where right whales were foraging at 18-25 g m-3 (compared to 1-5 g m-3 where whales were not foraging). Sims and Quayle (1998) found that basking sharks (another large filter-feeder) preferentially feed on the richest, most profitable zooplankton patches associated with fronts. In a related study, Sims (1999) found zooplankton densities in regions where sharks foraged where 3.2 times that of median zooplankton densities. They estimated the foraging threshold for these filter-feeding zooplanktivores at approximately 0.62 g m-3. Compared to these large zooplanktivores, blue whales are seeking extremely dense aggregations of zooplankton to meet their metabolic needs. Unlike blue whales, which lunge at discrete, dense concentrations of prey, right whales and basking sharks are skim feeders, filter feeding much larger volumes of water for prey in a less selective manner. Thus, they should find patches of much lower prey density than those sought by blue whales profitable. Distribution of Whales and Euphausiids Schoenherr (1991) first reported the association of blue whales with the steep topography of the Monterey Submarine Canyon, and our study confirms this observation. This association could result from whales directly responding to physical patterns in water temperature or currents in this region, or they may be associating indirectly via patterns in the distribution of biological resources that are directly responding to the canyon edge habitat. Water temperature and current patterns in the upper portion of the water column in Monterey Bay are most strongly influenced by upwelling north of the Bay than the canyon feature itself (Paduan and Rosenfeld 1996, Rosenfeld et al. 1994), so it is unlikely whales are directly associating with the canyon. Instead, both our study and Schoenherr's demonstrate that whales are aggregating on the canyon edge as they exploit dense schools of euphausiids associated with the canyon. There are several factors that may lead to the association of euphausiids with the canyon edge. Euphausiids are generally found in regions of high primary productivity (Brinton 1962a, 1962b, Mauchline 1980). In most areas they have been studied, adult epipelagic euphausiids such as E. pacifica and T. spinifera undergo diel migrations to depths in excess of 100 m (e.g. Hovekamp 1989, Greenlaw 1979, Bollens et al. 1992). Along the central California coast, the continental shelf break occurs at a depth of around 100-150 m. Some of the most productive coastal waters along the California coast are found over inshore of the shelf break, downstream from upwelling centers (Reid et al. 1958, Wooster and Reid 1963, Rosenfeld et al. 1994, Pennington and Chavez in press). Topographic breaks in the shelf such as the Monterey Submarine Canyon bring water depths in excess of 1,000 m within 10 miles of shore, downstream from upwelling centers such as Pt. Ano Nuevo. These breaks provide euphausiids that aggregate in the canyon the ability to undergo diel migrations in excess of 100 m (presumably to minimize predation in daylight hours) while remaining in the highly productive recently upwelled nearshore waters (Rosenfeld et al. 1994). The current dynamics of the canyon may also help reduce energetic costs for swimming in euphausiids during the day. Below 100 m over the continental slope off central California, the dominant current is the northward-flowing California Undercurrent (Chelton 1984, Wickham et al. 1987, Chelton et al. 1988, Tisch et al. 1992). Ramp et al. (1997) found that northward currents at 100 m depth off Pt. Sur, California (approximately 60 km south, outside of Monterey Bay) averaged 9.5 cm s&endash;1. In contrast, currents at 100 m in the Monterey Submarine Canyon where euphausiids were aggregated average <2 cm s&endash;1 (Chavez, upub. Obs.). Thus, the Monterey Submarine Canyon habitat would provide: 1) the opportunity for high energy gain during nighttime surface feeding due to its location downstream from an upwelling center; 2) a refuge from daytime predation as euphausiids can migrate to depths in excess of 100 m in the canyon, and 3) reduced swimming energy output during daytime schooling at depth due to reduced canyon slope currents. Similar factors may be important in other blue whale foraging areas off the California and Baja California coasts where bottom topography provides deep water access downstream from coastal upwelling centers (e.g. Santa Barbara Channel, Cordell Bank, Gulf of the Farallons, Punta Eugenia, Bahia Loreto). Seasonal Patterns in Oceanographic Processes and Whale Abundance Coastal upwelling occurs seasonally along the coast of California (Reid et al. 1958, Wooster and Reid 1963, Bakun et al. 1974, Bakun 1990). Equatorward winds develop in the spring due to movements of the Aleutian low-pressure system and North Pacific high. These winds act in combination with the Coriolis force, leading to a positive curl in the wind stress that moves an Ekman layer of surface waters offshore. This draws cold, nutrient-rich deeper water to the surface that extends as a broad band of cool water 10's of kilometers along the California coast (reviewed by Hickey 1979). In some regions fronts, plumes, and eddies can develop, extending >100 km offshore (Abbot and Zion 1985, Kelley 1985, Strub et al. 1991). This seasonal upwelling of nutrient-rich water supports high primary production and, in turn, higher trophic levels (Reid et al. 1958, Barber and Smith 1981, Hutchings et al. 1995). In Monterey Bay, California, high levels of primary and zooplankton production are supported by springtime upwelling to the north of the Bay between Pt. Año Nuevo and Davenport (Rosenfeld et al. 1994, Service et al. 1998, Pennington and Chavez in press). Skogsberg (1938) defined three oceanographic periods in the Bay: 1) a spring/summer 'upwelling season'; 2) a summer/fall 'oceanic season', and 3) a winter 'Davidson Current season'. These periods have generally been accepted by subsequent studies (Barham 1957, Bolin and Abbot 1963, Pennington and Chavez in press). During the upwelling season, pulses of northwest wind lasting a few days generally develop around February, supporting pulses of high primary production which lag the initiation of upwelling by 6-10 days (Dugdale and Wilkerson 1989, Service et al. 1998). Depending on conditions, these pulses can sporadically occur into the oceanic season (Penington and Chavez in press). Fewer studies have examined seasonal changes in secondary productivity, but Barham (1957) found that zooplankton abundance was highest in the late upwelling and early oceanic seasons. Physical and biological oceanographic climatologies for Monterey Bay between 1992-1996 confirm these seasonal patterns and demonstrate linkages between physical forcing, sea surface temperature, and productivity in the Bay (Fig. ***). Increased wind forcing in February leads to decreases in sea surface temperature (upwelling) and increases in primary production and surface chlorophyll. However, it is less clear how these events are linked to seasonal patterns in zooplankton and blue whale abundance. Moderate zooplankton abundance appear to persist through February, with distinct scattering layers near the surface and below 150 m (Fig. 9). However, by March, zooplankton backscatter is considerably reduced, and the deeper backscatter layer is no longer present. It is not until July, several months after the seasonal increase in primary production (Fig. 3) and the initiation of the oceanic period, that zooplankton backscatter dramatically increases. At this time both the shallow and deeper backscatter layers reappear, persist through September and begin to taper off in October. The seasonal arrival of blue whales in Monterey Bay appears to be linked to this dramatic increase in zooplankton in July. In a long-term study of productivity off Pt. Conception, California, Hayward and Venrick (1998) similarly found close linkages between physical forcing and integrated chlorophyll values. They also found that while integrated chlorophyll peaked in spring, zooplankton biomass did not peak until one to four months later. Two non-exclusive hypotheses may explain the July increase in zooplankton density and arrival of blue whales in Monterey Bay. First, ontogenetic development of euphausiid larvae, spawned from over-wintering adults between January-February, may lead to a July recruitment of adult euphausiids. This hypothesis is supported by the changes in euphausiid development from net samples in May (mostly larval euphausiids) into August and September (predominantly adult euphausiids) (Fig. 8). Second, a seasonal decrease in the intensity and frequency of upwelling-favorable winds leading to a shoreward collapse of offshore productivity towards the coast (Abbot and Barksdale 1991). This would lead to a seasonal peak in euphausiid density as euphausiids tracking this shoreward collapse arrive in Monterey Bay. Both of these factors would explain seasonally high densities of adult euphausiids in Monterey Bay, leading to the appearance of blue whales in the Bay in July. An interesting parallel to our study is that of Robison et al. (1998). They examined the 1993-1995 seasonal abundance in Monterey Bay of Nanomia bijuga, a predatory siphonophore that feeds primarily upon the same euphausiid species as blue whales (Alvarino 1971, Mackie 1985). They found that daytime population densities were highest between 200 and 400 m, deeper than our ability to measure zooplankton abundance with acoustics. Similar to our observation for blue whales, they found that siphonophore populations peaked in July, about 3-4 months after the seasonal peak in primary production. Unlike blue whales, which seasonally migrate to Monterey Bay, Robison et al. hypothesized the seasonal increase in Nanomia resulted either from onshore advection through the intrusion of offshore oceanic water due to decreased upwelling (essentially tracking the shoreward collapse of Euphausiids), or in situ population growth of siphonophores. Prey Resources and Whale Migration Patterns The annual migratory movements of the California blue whale likely reflect seasonal patterns in productivity in other foraging areas similar to those we have described for Monterey Bay. Regions with different seasonal upwelling patterns and euphausiid species with different life history traits will show temporal differences in peak euphausiid abundance. Due to their high total prey requirements, California blue whales likely migrate seasonally between dense, ephemeral euphausiids patches that appear in southern/central California in the summer and fall, the Gulf of California in the winter, and the central Baja California Pacific coast in the spring. Tershy (1992) hypothesized that such seasonal movement patterns by rorqual whales in response to ephemeral and patchy prey resources are similar to patterns described for terrestrial ungulates. Jarman (1974) related the distribution of antelope species to gradients in food quality, patch size, and spatial/temporal distribution. He proposed that high quality food that is found in small and predictable patches is exploited by small, resident, territorial species with low total caloric requirements and high mass-specific metabolic rates. At the other extreme, low quality food that is found in large and unpredictable patches is exploited by large, migratory, non-territorial species with high total caloric requirements and low mass-specific metabolic rates. Similar patterns have been found in carnivorous, omnivorous, and frugivorous species (Bekoff et al. 1981, Clutton-Brock 1974, Fleming 1991, Jarman and Southwell 1986, Lovegrove and Wissel 1988). Our study of the relationship between the world's largest predator and the temporal and spatial distribution of its prey support Tershy's (1992) hypothesis. We found that California blue whales foraging in the coastal upwelling zone sought extremely dense patches of euphausiids aggregated on the edge of the Monterey Bay Submarine Canyon. High euphausiid densities appear to result from the habitat provided by the proximity of the deep canyon to an upstream coastal upwelling center. Dense patches seasonally develop, lagging the seasonal increase in productivity by 3-4 months. This lag may result from the temporal development of euphausiids spawned around the seasonal peak in primary production and the tracking of the shoreward collapse of productivity with decreased coastal upwelling.

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