|
Study
Area
Monterey Bay, located on the central coast of
California (36°45'N, 122° 00'W) (Fig. 1),
is the largest bay (approximately 1,200 km2) on the
West Coast with unrestricted access to the open
ocean. It is further distinguished by the presence
of the Monterey Submarine Canyon, a canyon of
similar dimensions as the Grand Canyon (Shepard
1973).
Whale Distribution and
Abundance--Opportunistic Surveys
General patterns in the distribution and abundance
of blue whales in Monterey Bay was assessed using
data gathered opportunistically between 1992-1996
in regular commercial whale-watching trips. Trips
departed from Monterey and were usually 4-5 hours
in duration. Typical cruises included 60-120 min in
areas where whales, if present in the Bay, were
generally found. Whale watches usually emphasized
the southern regions of Monterey Bay. Experienced
observers recorded the location, number, and
species of all cetaceans sighted. Bi-monthly
averages of the number of blue whales sighted
trip-1 were calculated for comparison with
systematic survey data. For seasonal patterns in
whale abundance, monthly averages of number of blue
whales sighted trip-1 were calculated for all trips
between 1992-1996.
Whale Distribution and
Abundance--Systematic Surveys
Between August-November 1996, a total of five
systematic whale surveys were conducted for
comparison with relative abundance estimates from
whale-watching trips. Seven random-systematic track
lines 10 to 25 km in length and separated by 5.6 km
were run at a ship speed of 18.5 km hour-1 (10
knots) (Fig. 1). Surveys were conducted using
standard line transect methods for marine mammals
developed by the U.S. National Marine Fisheries
Service (Barlow 1994). Three marine mammal
observers recorded the location and number of all
blue whales encountered from the track line out
90° abeam using 7X50 reticle binoculars from
the flying bridge (5 m above sea level). Species,
number of individuals, sighting cue, behavior,
location, time, and weather conditions were
recorded at the time of each marine mammal
sighting. In addition, ship position along the
track line was recorded every 10 minutes. Because
all surveys were conducted in similar sea states,
no adjustments were made for sea state. Whale
density estimates were calculated using standard
marine mammal line transect methods (Burnham et al.
1980, Buckland et al. 1993, Barlow 1994).
Whale Foraging
Behavior
To examine whale diving behavior in relation to
prey distribution, we attached
microprocessor-controlled time-depth recorders
(TDRs) to two blue whales foraging in the study
area (Croll et al. 1998). Dive depth was sampled
every 1 sec. Sampled depths were binned into 10 m
bins and percent time at depth was calculated for
each depth bin, excluding depths shallower than 20
m. Shallow depths were excluded as we assumed that
time spent near the surface (£ 20 m) is more
likely associated with respiration than feeding.
This is supported by the observation that no
euphausiid swarms were observed in water £ 20
m. The diving behavior of foraging whales was
correlated to the distribution and density of
euphausiid prey schools by a series of small-area
transects approximately 5.6 km long (3 nmi)
covering an area of approximately 100 km2, centered
on the tagged, foraging whales (Croll et al.
1998).
Whale Diet
The species of prey taken by whales was
determined through analysis of whale fecal samples
collected opportunistically in the study region in
August 1996. Samples were collected with a dip net
and preserved in 70% ethanol. In the laboratory, an
aliquot was taken of a well-mixed sample and all
right mandibles of euphausiids were removed and
classified to species using methods developed by
Kieckhefer (1992).
Euphausiid
Distribution, Abundance, and
Composition
The horizontal distribution of euphausiids was
measured concurrent with a systematic whale
abundance survey conducted August 13-14 1996.
Acoustic backscatter was measured using a Simrad
EY-500 echosounder operated at 200 kHz. The
echosounder system was calibrated before and after
the study using the standard sphere method
(Johannesson and Mitson 1983). Detailed description
of echosounder data analyses are presented in Croll
et al. (1998) and Hewitt and Demer (1993). For
plotting of prey distribution, backscattering area
per 3.42 km2 (1 nmi2) of sea surface integrated to
200m (sA) was calculated from SV values for every
0.9 km (0.5 nmi) of survey line.
From these large area surveys, we identified a
region of high euphausiid and whale density. Within
this region we conducted a series of small-area
surveys to measure euphausiid density and vertical
distribution between 19-22 August 1996 (Fig. 1).
Twenty-three, 3.7 km (2 nmi) lines separated by
1.85 km (1 nmi) were run at a ship speed of 18.5 km
hr-1 (10 knots). Acoustic backscatter strength was
measured as described above. The vertical
distribution (10 m depth intervals) of euphausiid
schools was measured using mean sA values averaged
over every 0.93 km (0.5 nmi) of survey trackline.
Euphausiid school density in the whale foraging
region was estimated using mean sA values averaged
over every 0.93 km of survey trackline and
euphausiid size distribution from net samples (see
below). These values were combined following Hewitt
and Demer (1993), with adjustment made for
transducer frequency (Greene et al. 1991).
Identification of euphausiid schools in
echograms was confirmed by targeted plankton tows
utilizing paired 0.7 m bongo nets fitted with 333
mm mesh. Euphuasiids were enumerated for the entire
sample or a sub-split of the sample (minimum 200
individuals), identified to species and life
history stage, and measured to the nearest mm..
Euphausiid species composition from net samples was
compared with species composition from whale fecal
samples. In addition to these targeted tows in
August 1996, 200 m oblique net samples were taken
in May, August, and September 1996 at the edge of
the Monterey Submarine Canyon to examine seasonal
changes in age composition of euphausiids.
Euphausiid size distribution from August net
samples was used for hydroacoustic biomass
estimates. Biomass estimates were calculated
following the techniques described in Hewitt and
Demer (1993), incorporating euphausiid size
distributions measured in bongo net tows.
Adjustment of biomass estimates for transducer
frequency was made following Greene et al. (1991).
Numerical densities of krill (individuals m-3) were
estimated from acoustic estimates of biomass
density using species composition and size
distribution from net tows and the allometric
conversion of standard length to euphausiid weight
derived for E. superba (Hewitt and Demer 1993).
Seasonal Abundance of
Zooplankton
The relative seasonal abundance of zooplankton in
Monterey Bay was tracked using hourly averages of
acoustic backscatter measured by a 75 kHz Acoustic
Doppler Current Profiler (ADCP) permanently mounted
on a mooring (located at 122°01' W,
36°36' N, Fig. 1), operating at 75 kHz. ADCP
data have been used to provide relative estimates
of zooplankton abundance through backscatter
strength (e.g. Buchholz et al. 1995, Griffiths and
Diaz 1996), but are not able to accurately provide
quantitative estimates of zooplankton abundance
(Brierly et al. 1998). Daily averages of 1996 ADCP
volume backscatter were binned into 1 m bins and
used to generate a seasonal comparison of
zooplankton densities.
Oceanographic
Sampling
Detailed methods for oceanographic
sampling is described in detail elsewhere
(Pennington and Chavez in press). Briefly,
shipboard time-series data were collected twice
monthly between 1992-1996 aboard the R/V Point
Lobos on single-day cruises. In this paper we
report results from two of the stations occupied in
Monterey Bay (C1, M1; Fig. 1).
Conductivity/temperature/depth (CTD) casts were
made to at least 200m with a Sea-Bird 911 or 911+
CTD mounted in a General Oceanics 12-place rosette
with 5-l Niskin bottles (silicon o-rings).
Conductivity and temperature sensors were
calibrated annually. Downcast data were binned to 1
m depth intervals, and upcast data were averaged
following each bottle trip.
Rosette Niskin bottles were filled at the
surface, 5, 15, 30, 50, and 100 m. Water from these
samples was used to calculate integrated
Chlorophyll and primary production measurements.
Chlorophyll a concentration (hereafter-termed
'chlorophyll', mg-chl m-3) was assayed with the
standard fluorometric procedure of Holm-Hansen et
al. (1965). This method was modified such that
plant pigments were filtered onto 25 mm Whatmann
G/FF filters and extracted in acetone in a freezer
for 24-48 hr (Venrick and Hayward 1984, Chavez et
al. 1991). Measurements were made on a Turner
Model-10 fluorometer calibrated with commercial
chlorophyll a (Parsons et al. 1984). Primary
production was estimated as carbon fixation
(hereafter-termed 'primary production', mg-C m-3
da-1) for 100% light penetration depth (surface)
using 14C uptake methods described in Penington and
Chavez (in press).
Upwelling indices for the study region during
the study period were obtained from the Pacific
Fisheries Environmental Laboratory/NOAA web site
for 36°N 122°W (www.pfeg.noaa.gov).
The indices are based on estimates of offshore
Ekman transport driven by geostrophic wind stress
derived from six-hourly synoptic surface
atmospheric pressure fields (Bakun and Nelson
1991). Bi-monthly means of the daily upwelling
indices were calculated.
Unless otherwise noted, means ± S.D. are
reported.
|
