CalCOFI Reports, Vol. 30, 1989 - California Cooperative Oceanic ...

SMITH ET AL.: ZOOPLANKTON PATTERN ANALYSIS
CalCOFl Rep., Vol. 30,1989
gation persisted for 10 minutes and the distance
traveled in that half hour was 3200 m, the aggregation
was said to extend 1070 m. The vertical plane
area of each aggregation was estimated as the product
of the vertical and horizontal dimensions. The
sum of the vertical plane aggregation areas was estimated
for each 4-hour period. The vertical plane
area insonified was calculated by adding the ship’s
meters of progress in 8 half-hour intervals and multiplying
by the depth of observation or 240 m.
Comparison of the two values yielded an estimate
of “coverage. ” Since there were no perfectly rectangular
aggregations, the aggregation areas represent
an overestimate varying from about 25% for inscribed
circles to a factor of two or more for diagonally
extended aggregations. Data exist for
correcting this bias and for improving precision by
more frequent evaluation of ship’s speed.
RESULTS
Environmental Description
Vertical profiles of temperature, dissolved oxygen,
and chlorophyll a can be characterized as exhibiting
similarity in shapes and values among
repetitions. Five temperature profiles taken between
April 6 and 7 showed temperatures declining
evenly, with some structure from 17°C at the surface
to 5°C at 800 m (figure 6A). Deep oxygen profiles
made near the beginning of the cruise (1433,
April 5) and the end of the cruise (0639, April 8)
showed similar gradients, with a subsurface shallow
maximum and a deep minimum at about 600
m (figure 6B). Vertical profiles of pheopigments
showed maxima of about 0.5 pg 1-’ at or near the
depth of the chlorophyll maximum layer (figure
6C). Six chlorophyll casts had distinct subsurface
maxima of 1-2 pg 1-’ between 35 and 45 m (figure
6D).
Vertical distributions of adult females of three
species of calanoid copepods illustrate different
types of behavior that influence acoustic sampling
methods (figure 7). Because all three species were
relatively abundant and are known to store lipids
(Ohman 1988b), they should have affected backscattering
at 307 kHz. Calanus pacificus californicus
Brodskii was nonmigratory, remaining in the upper
50 m within acoustic range day and night (figure
7A). If the finer-scale vertical distribution of this
species were constant over time and distance, a C.
paci_fcus calijornicus layer would influence the mean
but not the standard deviation of echo amplitude.
Eucalanus californicus Johnson was also nonmigratory,
but remained between 200 and 400 m day and
night (figure 7B). Slight shifts in vertical distribution
of E. calijornicus of a few tens of meters would
bring it into and out of acoustic range, primarily
affecting the standard deviation of echo amplitude.
Metridia pacijka Brodskii underwent a distinct vertical
migration (figure 7C). A daytime population
mode at 200-150 entered the upper 50 m at night.
Such a pronounced shift in vertical distribution
would influence both the amplitude mean and stan-
-
Calanus pacificus californicus Eucalanus californicus Metridia pacifica
0
100
5 200
r
c
300
0
100
200
300
400
400
500
500
10,000 0 10,000 12,000 0 12.000 1,000 0 1,000
15,000 5,000 5,000 15,000 18,000 6,000 6,000 18,000 1,500 500 500 1,500
NUMBER/ 1,000 m3
Figure 7. Day (open bars) and night (shaded bars) vertical distributions of adult females of the copepods Calanus pacificus californicus (A), Eucalanus
californicus (S), and Metridia pacifica (C) on April 6,1988. “ns” indicates that no sample was taken.
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