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Whale Distribution and
Abundance
Blue whale sightings from whale-watch trips between
1992-1996 were concentrated along the edge of the
Monterey Submarine Canyon (Fig. 2), and were
seasonally present in Monterey Bay between June and
November (Fig. 3). Relative abundance estimates
from whale-watch trips qualitatively tracked
abundance estimates from systematic surveys (Fig.
4), and thus probably accurately reflect seasonal
abundance of blue whales between 1992-1996.
Systematic surveys revealed that blue whale density
during the time period of peak abundance (August)
was 0.034 whales km2 (+0.056, -0.204 S.E.).
Whale Foraging and
Diet
Two whales foraging during daylight hours
(1100-1700) on the edge of Monterey Submarine
Canyon were tagged with TDRs in 1996 (August 19 and
22, 1996). To account for short duration shallow
dives associated with respiration, only dives that
exceeded 2 min in duration and 30 m depth were
included for analysis of foraging dives. This
convention was supported by three observations: 1)
all short duration shallow dives took place within
a series of surface respirations, 2) no echo
returns attributable to euphausiids were observed
in water less than 30 m deep, and 3) zooplankton
net hauls to less than 30 m did not contain
euphausiids. Maximum foraging dive depth ranged
from 144 to 176 m and 142 to 193 m in the two
tagged whales, respectively. Mean maximum dive
depth was 155 m (±9.8) and 172 m (±14.7),
and mean dive durations were 8.8 min (±0.8)
and 8.3 min (±1.4), respectively. Both whales
showed a stereotypical pattern, diving consistently
and directly down to the 150-200 m layer in the
water column and performing a series of 1-4 20-30m
vertical excursions on each dive (Fig. 5). Surface
tracks of both whales remained within 5 km of the
canyon edge throughout the tag deployment period,
moving parallel to the canyon edge.
In August, 1996, blue whales fed exclusively
upon euphausiids in proportions (percent by number)
significantly different from the composition of
euphausiids in net samples (C2 =455.55, df=2,
P<0.001). Fecal sample analysis (N=5) revealed
that blue whales in Monterey Bay fed primarily upon
Thysanoessa spinifera (80% ± 22.6%) and
Euphausia pacifica (13% ±26.4%); other
euphausiids accounted for 7% (±4.7%) of diet.
Euphausiid species composition within net samples
collected during the same period consisted of
30.17% (±34.95%) T. spinifera, 68.32%
(±34.75%) E. pacifica, and 1.51% (±2.56%)
other species. The mean size of T. spinifera and E.
pacifica taken by whales was 19.3 mm (±1.53,
n=162), and 16.0 mm (±2.05, n=82),
respectively. This was significantly larger than
the size of T. spinifera and E. pacifica taken in
net samples (16.3 mm ± 3.71, n=100 and 11.8 mm
± 3.32, n=100, respectively). T-test; T.
spinfera: t=9.12, d.f.=260, P<0.001; E.
pacifica: t=9.99, d.f.=180, P<0.001 (Fig.
6).
Euphausiid
Distribution, Abundance, and
Composition
The large-area survey conducted on 13-14 August
1996 revealed euphausiid schools were located close
to the offshore edge of the submarine canyon (Fig.
1). Mean backscattering strength of euphausiids in
this survey was 2,528 m2 nmi-2 (±461.7 S.E.),
which is equivalent to a mean density of 1.3 g m-3
(±6.35 S.E.), 39 individuals m-3, or 260 g m-2
Blue whales encountered during this survey were
also located over or close to edge of the submarine
canyon (Fig. 1). Small-area surveys revealed these
canyon-edge euphausiid schools were concentrated
between 80 and 180 m, averaged 15.1 m (±8.38)
in height (N=226, 0.5 nmi samples), with most
euphausiid schools located between 120 and 160 m
(Fig. 7). The mean integrated backscattering
strength of these canyon-associated schools was
20,385 m2 nmi-2 (±1873 S.E.), which is
equivalent to a mean density of 145.3 g m-3
(±11.51 S.E.), 4,403 individuals m-3, or 2201
g m-2 (integrated over the mean 15.1 m school
depth).
There was considerable variability between
spatially distinct euphausiid schools both with
respect to species composition and individual size
structure. Percent compostion and mean size for T.
spinifera and E pacifica within the seven targeted
net samples conducted between August 14-21, 1996
are summarized in Table 1. The overall mean size
for T. spinifera and E. pacifica individuals was
16.3 mm ± 3.71, n=100 and 11.8 mm ± 3.32,
n=100, respectively, however, there were
significant differences in the mean size of
individuals between spatially distinct schools
(Kruskal-Wallis T. spinifera H = 89.91 p < 0.001
df = 2, E. pacifica H = 380.23 p < 0.001 df =
5).
Net samples from May, August, and September,
1996 revealed strong seasonal recruitment and
growth for both T. spinifera and E. pacifica
populations within Monterey Bay (Fig 8). Juveniles
made up the bulk of both populations in May of
1996, while adults became numerically dominant in
August and September, though the persistent
presence of juveniles within all samples indicated
that recruitment was continuous throughout the
summer.
Seasonal Abundance of
Zooplankton
ADCP backscatter was averaged between 1992-96 to
provide a seasonal climatology of zooplankton in
Monterey Bay (Fig. 9). Early in the year (Jan-Feb)
backscatter intensity is relatively high, with
layers appearing near the surface and below 150m.
By mid-March the deeper layer has disappeared and
overall backscatter is at a minimum. Water column
backscatter abruptly increases in July, and the
deeper layer of zooplankton reappears. High levels
of backscatter persist until mid-October when
backscatter levels diminish, but the deeper layer
persists.
Seasonal Patterns in
Oceanography
Oceanographic climatology for Monterey Bay between
1992-1996 is summarized in Fig. 3.
Upwelling-favorable winds lead to a shift in the
upwelling index from negative values (downwelling)
to positive values (upwelling) in late February.
Upwelling persists until late summer. This
upwelling leads to a sharp decline in sea surface
temperature, indicating cold nutrient-rich water
has reached the surface by mid-March. This is
linked to a mid-March increase in primary
production, which is also reflected in surface
chlorophyll-a values.
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