Diel vertical migration of zooplankton and meso pelagic fish in the Arabian Sea

26 Pages·2005·0.78 MB·English

Not to be cited without prior reference to the authors
International Council f o r C M 1998/S:7
the Exploration oft he Sea Visualization of Spatial Data
Diel vertical migration of zooplankton and meso pelagic fish
in the Arabian Sea
by
Jiangang Luol Peter B. Ortner, David ForcuccP, and Shailer R. Cummings2
,
1. Rosenstiel School of Marine and Atmospheric Science, Division of Marine Biology of
Fisheries, 4600 Rickenbacker Causeway, Miami, FL 33149, USA.
2. NOAAlAOML/OCD, 4301 Rickenbacker Causeway, Miami, FL 33149, USA.
ABSTRACT
Acoustic (153 kHzADCP and 12 kHz hull transducers), and MOCNESS (MOCI and
MOCI0) tow data collected in the Arabian Sea during intermonsoon (AprillMay) and monsoon
(August) periods document substantial diel migrations of fish and zooplankton despite the year
round presence of an oxygen minimum «0.2ml/l at 125-150 m). Fish and zooplankton layers
were separated by comparing the backscatter layers between 12 kHz sonar and 153 kHz ADGP,
which confirmed that most of the migrating layers are fishes. Fish vertical migration speeds
were independently estimated from both the slopes of principal volume scattering layer and the
vertical velocity component of ADCP, resulting in average speeds of 4 and 3 cm S·I and
maximum speeds of 13 and 10 cm S·I, respectively. A few migrating zooplankton layers were
identified with an average speed at about 2 cm S·I and maximum speed as high as 8 cm S·I.
MOCNESS zooplankton displacement volumes indicate that at the sites discussed zooplankton
biomass measured during in the monsoon season cruise was as much as' fivefold greater than that
in the intermonsoon season cruise, which can be generalized to the region by systematic
differences of daytime volume backscatter intensity of ADCP between cruises. Migration depth
was seasonally determined by local hydrographic conditions (DO and temperature vertical
gradients) .
INTRODUCTION
Diel vertical migrations of zooplankton and mesopelagic fish in the open ocean are well
known (Emight 1977, Buskey and Swift 1983, Williams and Conway 1984, Forward 1988,
Fragopoulu and Lykakis 1990, Uye et al 1990, Anderson and Sardou 1992, Atkinson et al
1992a, 1992b, ), but until recently most of these studies were done with integrating-sampling
methods (plankton nets and trawls), which lack sufficient of spatial and temperal resolution to
give precise description of spatial distribution or estimate of vertical migration velocity .. In the
1
past decade, the Acoustic Doppler current profiler (ADCP) has been increasingly used for
estimating zooplankton biomass and distribution pattern (Flagg and Smith 1989, Heywoodet al
1991, Zhou et aU 994, Ashjian et al 1994) as well as the vertical velocities of the dielmigrating
zooplankton (Enright 1977, Plueddemann and Pinkel 1989, Fishcher and Visbeck 1993,
Buchholz et at! 1995, Heywood 1996).
The ADCP was designed primarily to measure the horizontal components of velocity by
measuring the speed of suspended particles (plankton) in the water. This assumes that the
particles are advected passively with the water and that the swimming speed of the plankton is
negligible in comparison to the water movement. Flagg and Smith (1989) first used a bottom
mounted ADCP to estimate zooplankton abundance and vertical distribution structure over a
continental shelf by correlating relative backscattered acoustic intensity with zooplankton
abundance from net tows. However, the backscatter can result not only from zooplankton but also
from any organisms which are larger than the miIiimum detection size of the acoustic frequency
employed (Greenblatt 1981), for example fr()m squids and pelagic fishes. ADCP estimates of
zooplankton abundances and distributions can be accurate only where or whenthe larger
organisms are exceedingly scarce compared to zooplankton. Where (or when) pelagic fishes are
relatively abundant, the current method of using ADCP to estimate zooplankton abundances and
distribution can produce erroneous results. More sophisticated acoustic methods can separate
smaller scatterers from larger ones. Multifrequency acoustics systems (Holliday et al 1989,
Costello et al 1989, Pieper et a11990, Nappet al 1993, Holliday and Pieper 1995, Barans et al
1997) have been used to estimate zooplankton abundance by size. Due to the high frequencies
(100 kHz-6~OMHz) used inmuItifrequency systems, their sampling range is typically limited to a
short distance «16 m, Barrans et al 1997) from the transducers, aIld their sample volume is
typically quite small «1 m·l
).
In this paper, we discuss a simple comparatively crude acoustic alternative. We used a
low frequency (12kHz) hull-mounted acoustic system in conjunction with a high frequency (153
kHz) ADCPto separate the sound scattering layer (SSL) oflarge organisms (i.e., fish) frorn that
of srnall organism (Le., zooplankton). We then use the layer structures'to estimate vertical· ..
migration speeds of zoo' plankton and fish from the slope of the principal volume scattering layer
and compare it to the vertical velocity component of ADCP.
Methods
Data collection
The data discUssed in this paper were collected during tWo GLOBEC cruises aboard
NOAA Ship Malcolm Baldrige in the Arabian Sea in 1995 (Fig. 1). The first cruise was from
April 27 to May 24 late in the intermonsoon season, and the second cruise was from July 31 to
August 27 during the south-west monsoon.
. A calibrated 12 kHz transducer was mounted in the transducer well of the ship and
operated intermittently during bothctuises. Transmit and data logging were controlled by a
PCIDOS baSed computer running a compiled BASIC program. A trigger pulse was seht to a .
Raytheon PTR-l 05B that transmitted a 3 millisecond pulse of 12 kHz sound. The trigger pulse
was keyed at 3 second intervals. A specially designed and built log receiver used two AD" .
640BD amplifiers to obtain 80 dB gain. One channel of a National Instruments (DAS"50) data
2
acquisition board and associated tenninal panel performed !Ill AD conversion of the sonar signal
return. The digitized data was stored on hard disk and daily moved to a Fujitsu Floptical
(DynaMo 135) media for backup. The digital data files were reformatted from the original binary
file structure into an ASCII file structure, which was then analyzed in by a MatLab program into
20-ping averages (I minute) data intervals. Bin sizes were varied between 2 to 8 meters during
different segments of the cruise.
The ADCP used was a 153 kHz multi-beam Doppler sonar (RD Instruments) also hull
mounted on the ship. It was in operation continuously during both cruises, and was set to collect
data in 8-m depth bins. The starting depth was set 2 bins below the keel at 22 m depth(l6 m +7
m ship draft). Acoustic backscatter and x-y-z velocity component data from the ADCP were
collected at lOs intervals and then averaged over 5 min intervals. Raw data were processed with
the Common Oceanographic Data Access System (CODAS, version 3.1, Firing et aI1995).
10-m MOCNESS (MOC I 0) tows were taken to collect small fish in the upper 1000
meters with some tows as deep as 1500m. The MOCIO net had a 10 square meter nominal
mouth opening. Five nets, 3mm mesh, were fished. Net I acted as a drogue to stabilize the net
during decent and the remaining four riets commonly sampled intervalslOOOm to 750m, 750m to
500m, 500m to 150m and 150m to surface. Day artdnight tow sequences were done at sample
sites along the cniise track. The total volumes filtered ranged from 3000 to 45,000 cubic-meters.
These data have be enUmerated at WHO! by Dr. Larry Maden. I-m MOCNESS (MOCl) tows,
fitted with 150 micro-mesh nets, were taken to various depths. We used only data collected
shallower thart 200 m for comparison with ADCP data. Displacement volumes were measured at
AOML by the late Leonard C. Hill.
Data analysis
ADCP data processed with the CODAS software were exported into a Digital Alpha
workstation for further analysis. Current velocities measured by ADCP actually represent the
velocities of the scaiterers rather than the water itself. Upwelling or downwelling velocities are
mcp
usually small, therefore, the vertical velocity (Vz) measured by can be interpreted as the
vertical switnrningvelocity oforgarusms (Heywood 1996). The values of vertical velocity from
ADCP are still noise even after CODAS processing. To solve this problem, we filtered the ADCP
measured vertical velocity (Vz) by setting the background velocity to as close to zero as possible.
Vertical velocities were also estimated from the slopes of the principal volume scattering
layers (Vs). During the diel migrating time periods (dawn or dusk), individual sound scatter layer
(SSL) was isolated interactively ondisplay using IDL software (Interactive Data Language,
Research System, Inc). From the isolated SSL, the depth of the maximum backscatter intensity
was calculated as a function of time, D(t}. Then the vertical migration velocity Vs was estimated
by the rate of change in SSL as the derivative of D(t) with respect to time(lj: Vs=dD(t)ldt.
RESULTS
In 50 days of ADCP data, we have 38 dawn/dusk segments with indications ordiel
vertical migrations. From the 38 segments, we calculated and plotted velocities as a function of
time for 59 isolated layers (see for example, Fig. 2). For each layer,we then calculated the mean
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and the maximum velocity during the migration period from both direct measuremellt .of ADCP
(Vz) and the slope of the principal volume scattering layer (Vs) (see Table 1). The mean and
maximum of Vs .are significantly larger (paired t-test 0:=0.05) than those of Vz for both dawn and
dusk migration (Table 2). The maximum vertical migration velocities estimated during th\l two
cruises are-IO.3cm S·l from Vz and -13.0 cm s" from Vsfor downward migrations,.and 9.2 cms"
from Vz and 10.6 cm s" from Vs for upward migrations. There are no significant differences of
maximum velocities between dawn apd dusk for both Vz (p=0.0567) and Vs(p=0,4687), but there
are significant differences of mean velocities between dawn and dusk for both Vz (p=0.0090) and
Vs (P=0.0066). Unfortunately, only 9 of the 38 segments overlap times with good quality 12 kHz
sonar data due to frequent interrupti nsof 12 kHz sonar data collection by other research
activities and ship operations. Belo we will describe patterns of migration in detail for 4 .•
dawnldusksegments, which have sy optic ADCp and 12 kHz data, two from each crnise ..
May 13 (IS030'N, 61°19'E)
. At dusk, two distinct layers e seen migrating towards the surface by both the ADCP
(Fig. 2a) and the 12klIzsonar(Fig. c). The first l~yer started at about 350 m depth and the.
second layer started at about 250 m d pth. Since both layers are seen both the 12 kHz sonar .and
the AIJCP, we can conclude that the' were fishes In these layers, but we can not determine
zooplankton were also present. Ther is a third layer between 50-70 m depth during the day in the
ADCP data.(Fig. 2a}that is not seen by the 12 kHz sonar; this suggests this is a zooplankton
layer. MOC 1 biomass data indicate at zooplankton remain above 100 m (Fig. 3a), while ..
MOClO tows show the majority offi h distributed below 150 mduring the day and above 150m
during the night (Fig. 4a). These dat confirm our interpretation that primary migrating layers are
fish and the shallow resident layer is ooplankton.
The vertical velocities are sho in Figure 2b and 2C. The ADCP vertical velocity
component (Fig.2b) shows higher ve pcities generally corresponding to the migrating layers in
the backscatter plot (Fig. 2a). The m . um and mean velocities estimated directly from the ..
ADCP vertical component are 9.1 cm s" ~d 3.3 cm 5" for layer-I, and 3.7 cm s" and 1.4 cm s·,
for layer-2 (Fig. 2d). The maximum d mean velocities estimated from the slopes of the
principal volume scattering layers are 7.6 cm s·land 4.0 em .s·, foriayer-I, and 6.7. cm s·, and3.2
cm s·, forlayer-2 (Fig. 2d). In both c es, the maximum velocities occurred just after sunset, and
the upward migration was accomplis ed in about one hour. .
The hydrographic data show a relatively shallow mixed layer with a very rapid decline in
temperature, salinity and dissolved 0 gen (~O) from 120to 150 m (Fig. Sa). The DO level
decreased from 4.8 mg I" at 100 m to less than 0.1 mg 1" at 150 m. Fishes dwelt in th\l low DO
and low temperature (15 0c) wat~r below 240 m during the day and after sunset migrated to the
DP
high and high temperature (25°C) water above 120 m (Fig .2a, 2b). Zooplankton popull!-tion
gen~rally resided in the warm weIl~oxygenated surface waters and accounted for the daytime near
surface ADCP backscatters (Fig 3a). . '
May 16 (15°57'N,60007'E)
. At dawn, two days later at the same site, two dense closely spaced layers are seen : '
migrating down towards deeper water by the 12 kHz sonar (Fig. 6c) while one. thick layer, which
4
covers the spatial range of the two layers, is seen bytheADCP (Fig. 6a). This difference can be
attributed to the difference in the sampling resolution of this 12 kHZ sonar data (2m bin) and the
ADCP data (8m bin). One layer started at about 1Q O mdepth during the later night and ended
below 350 m depth at sunrise, while the second layer started at about 50 m depth during the late
night and ended at around 350 m depth at sunrise. There are some backscatter signals seen
between 50-100 mdepth during daylight on the ADCP image that are not apparent in on the 12
kHZ image. We conclude that these signals are from zooplankton, which is confIrmed by MOCI
data indicating that 95% of zooplankton biomass remains above 120 m depth (Fig 3b). MOClO
are
data indicate fish generally below 150 m depth during the day and are above ISO m depth
during the night (FIg. 4b).Agitin we can verify that the migrating layers seen on ADCP and 12
kHz images are fIshes and'the non-migrating new surface layer is zooplankton. .
The ADCP vertical velocity component (Fig. 6b) shows layers of negative velocities
(downward migration) correspondingto the migrating layers in the backscatter image (Fig. 6a).
The maximum and mean velocities estimated directly by the ADCP are -8.3 cm S·I and -4.1 cm S·i
forlayer-I, and -8.3 cm 5-1 and -3.5 cm S-I for layer-2 (Fig. 6d). The maximum and mean
velocities estimated from the slopes of the principal volume scattering layer are -lOA cm S-I and-
4.8 cm S-I for layer-I, and -10.8 cm S-I and -5_0 cm S-I for layer-2 (Fig_ 6d). The maximum
velocities occurred just before sunrise and the downward migration again took about an hour.
The hydrographic proflle on May 16 was very similar to the proflle on May 13, indicating
a rapid decline in temperature, salinity and DO from. 100 to 150 m depth (Fig. 5b). The DO level
is reduced from 5mg I-I at 100 m to less thanO.l mg I-I at 150 m. During the night, fIsh were
distributed in highDO (5 mg 1-1) and high temperature (above 25 DC) water above 120 m. After
sunrise, fIsh migrated down to the low DO and low temperature (below IS DC) water .below 300 m
(Fig .6a, 6b). Zooplankton generally remained in the warm-well oxygenated waters and !lgain
accounted for the correspondent near surface ADCP daytime backscatter(Fig. 3b).
August 9 (13°55'N, 50"40'E) .
At dawn, two distinct layers are seen migrating downwards by the ADCP (Fig. 7a) while
one layer is far more prominent in the 12 kHZ data (Fig .7c). The dominant layer in the 12 kHz
image started near surface at dark and moved to -250 m depth at daylight, which matches the
lower layer in the A[)CP image. The strength of the 12 kHZ signal suggests this is a fIsh layer.
The second layer On the ADCP image also started near surface in the dark but migrates to only
200-220 m depthat daylight. Since this layer is barely apparent in the 12 kHZ image, we suspect
this is a mixture of zooplankton and fIsh dominated by zooplankton_ This is generally consistent
with our MOClzooplankton tOw dau.which indicate a signifIcant proportion of zooplankton
biomass distributed below 100 m depth during the day (Fig- 3c).
The ADep vertical velocity component (Fig. 7b) contains patches of negative velocities
surrounding the depth of downward migrating layers around sunrise_ The maximum and mean
velocities estimated directly by the ADCP were much slower than at the other sites, -4.3 cm S-I
and -2.4 cm S-I forthe fIsh layer, and -4.0 cm S-I and -1.8 cm S-I for the zooplankton layer (Fig.
7d). The maximum and mean velocities estimated from the slopes oft he principal volume
scattering layer ard -6.7 cm S·I and -3.2 cm S-I for the fIsh layer, and -4.5 cm S·I and -2.4 cm S-I for
the zooplankton layer (Fig. 7d).· The maximum velocities for both fIsh and zooplankton occurred
5
before sunrise and the relatively slow migration took nearly an hour and half to accomplish even
though the depth attained was less. . '
. The hydrographic profile at this site is different from the other site at the same seas~n.
(compare Fig. Sc and Sd).· The geographical location (Gulf of Aden, Fig. I) is the primary cause'
has.
of this difference. The shallow thermocline is at moved up to 20 m depth, and is
accompanied by marked temperature, salinity and DO gradients at corresponding shallow depths
(Fig. Sc), The low temperature (IS 0C) water begins atonIy .160 m depth as comparing to the
other sites (about 300 m). The DO minimum is a less acute. DO decreased from 6.7 mg I". at 20m
to 2.5 mg I" at 40 m,andto 1.0 mg I" at 80 m. There is a secondary higher DO layer (1.8 IpgJ-')
associated with a.layer of lower salinity «3S.6) water between 10 0 m and 180 m depth. This'
water represents the intrusion of a very different water mass. From 180 t0300 m, DO .
i·'.
concentration stayed above O.S mg 1-' as compared to the other sites where it felIto 0.1 mg
During the night, the fish and zooplankton were distributed above. the low salinity water at <100
m depth. At day break, the fish layerhad less distance. to descent to reach low temperature and
low DO water which began resent just below the low salinity layer. At least, some zooplankton
migrated into cooler oxygen-depleted waters along .with the fish.
AugustI9 (lsoS7'N, 61°54'E) .
At dawn of August 19, .a complex network of SSL layers are seen migrating downward'
by the ADCP (Fig. 8a). In the 12 kHz sonar data (Fig. 8c), two layerspresent at 20-90 m depth.
that later jointogethe~into one layer during the twilight downward migration period. By s~se,
this coribined layer has moved to below 300 m depth. This layer closely matches the lowest layer
seen by the ADCP. In.the ADCP image, one layer started near the surface at dark and migrated to
200 m depth by daylight. That this layer is not seen by the 12 kHz sonar, suggests this is a
zooplankton layer. Anotherlayer is seen at 40-90 m depth by the ADCP during the day that is
also not seen on the 12 kHz image. MOCI data indicate that the majority of zooplankton are
distributed above SO m (Fig. 3d), but there are some zooplankton below 100 m depth, which is
again consistent with our inte~retationoftheacousticdata. The MOCIO data confirm that the
majority of fish are distributed below ISO mduring the day and apove ISO m during the night
(Fig. 4d)whi,ch is. also consistent with our acoustic cfuta.
Th~ ADCP vertical velocity data show a large patch of negative velocities just before
sunrise at the depth of the mixed (ayer(8b). The llllIfclmum and mean velocities estimated
00 •
directly by the ADCP are -10.3 cm s·, and -4,9 cm s') for whatwe conclude is a fish layer, and - •
8.1 cm s"and -4.3 cm s·, for what we conclude is a zooplankton layer (Fig. 8d) .. The maximum
andmean~elocitiesestimated from the slopes of the principal volumescattering layers are-"11.9 .
cm s·, and -4.S cm s" for the fish layer, and -7.3 cm s:' and -2.S cm s" for the zooplankton layer
(Fig. 8d). The maximum velocities for both fish and zooplankton occurred just before sunrise but .
the migration was much more rapid than at the Gulf of Aden site and was accomplished in3pc40 .
min.
.This site is located close to the May 13 and May 16 sites in the middle of Arabian Sea.'
The differences in hydrographic profiles result from a combination of monsoonal overturning of
the upper water column, the possible advection of coastal waters emanating from the Omani coast
as weI1as the proximity of an eddy f~ature. A rapid decline in temperature and dissolvedoxygen
0
6
(DO) occurs at 50 to 70 m depth (Fig. 5d) ~hich do~~ ~~t~~rrespondto a significant change in
salinity. The DO level is reduced from .6.8 mg I-I at 50 Ip. to 1.5 mg 1-1 at 70 m, then to O. I mg I-I
by 120 m. During the night, the majority of fishes are distributed above 50 m depth (Fig. 7a)in
high DO (6.8 mg 1.1 and high temperature (above 25°C) water.· At sunrise, fishes start migrating
)
to the low DO and low temperature (below 15°C) water .below the 300 m (Fig. 8a, 8b). The
migrating zooplankton lay!:r also enters at least the upper reaches of the cool oxygen-depleted
deeper waters.
Zooplankton biomass ~n May 13, May 16.and August 19 are compared to investigate
seasonal changes in zooplankton abundance since these sites arein.the same general area
representative of the central basin (Fig. I). Zooplankton biomass was integrated from the surface
to 200m depth. Integrated zooplankton biomass is 5.51,5.10, and 30.80 gm" for May 13, May
16, and August 19, respectively. There is not much difference in biomass densities between days
in May but the Augustbiomass density Wl\S 5.80 timesthat of May. Similarly, the average
daytime ADCP.volume backscatter intensities from the same days were compared. The average
intensities are 2.17e-8 (~76.6 db), 2.04e-8 (-76.9 db), and 13.78e-8 (-68.6 db) for May 13, May 16,
and August 19, respectively. The August backscatter intensity is about 6.5 times that of May,
confirming that at least the daytime APCP backscatter is a reasonable measure of zooplankton
biomass.
DISCUSSIONS
Our results indicate that it is possible to distingusish fish and zooplankton layers by
comparing backscatter signalsJrom a 1.53 kHz ADCP and a 12 kHz sonar. The proximate
minimum size that each frequency can detect can be estimated based on the work of Greenblatt
(1981). Using Greenblatt's (1981, equation 6) formuIationand substituting f=153 and 12 kHz, we
estimate minimum detection sizes of 0.2 and 2.4 mrn equivalent spherical radius, respectively.
Equivalent spherical radius (a) can be converted to organism length(L) by an approximate
relationship L=5a (Holliday arid Pieper 1980, Greenlaw and Johnson 1982). Thus, the minimum
length ofo rganisms detected by 153 kHz ADCP is 1 mm, and by the 12kHz sonar is about 12
mm, On the other hand, there are no simple upper limits for what size each frequency can detect,
although there appears to be some dispute on this point (plueddemann and Pinke11989, Heywood
1996). Our view is that backscatter signals from a 153 kHz ADCP are from organisms larger than
I mm while the signals from 12 kHz are from a subset of those organisms (i.e., those larger than 12
mm). Our dataand supporting tow samples confirm that while a layer of zooplankton is seen as a
153 kHz backscatter layer without a 12 kHz signal, and a fish layer by a 12 kHz backscatter layer
but by itself a 153 kHz backscatter layer is not an unambiguous indication of a zooplankton layer.
The average vertical migration speeds of fish estimated from the slope of the principal SSL
range from 2.1-6.6 cm S-I for downward migrations at dawn, and from 3.0-6.2 cm S-1 for upward
migrations at dusk. These speeds are comparable with downward speeds of 5 cm S-1 and upward
speeds of 2.8 cm S'I estimated by Gjflsaeter (1984) in the Arabian Sea. Also, her noted a
systematic difference in upward and downward migration speeds we too saw in our mean velocity
ata. The average vertical migrating speed of zooplankton estimated from the slope of the
7
8
." __ , h
change in zooplankton biomass is also reflected in our ADCP volume b~ckscatter intensity data
above 200 m dUring the day. That data generalizes the site specific res.uIts to virtually the entire
. cruise track sampled. The agreement between MOCNESS biomass and ADCP dUring the day is
possible because throughout the region most fishes are in deeper water (> 250 m) dUring the day;
day-time ADCPbackscatter signals are from smaller non-migrating organisms or organisms
undertllking only limited migrations. As fishes migrate to the surface water, the ADCP backscatter
signal results from a mixtui-e ofzooplanIcton and fish (the later can resonating due to their swim
bladders). Therefore the ADCP volume backscatter intensity increases dramatically dUring the
night. There is no easy acoustic method for separating fish and zooplankton in a mixed layer other
at
than useing a muHiple frequencies (Hollildayet al 1989, Costello et 1989, Pieper et a11990,
Napp et al 1993, Holliday and Pieper 1995, Barans et al 1997). Although the ADCP alone has
been frequently used to estimate zooplankton biomass, distribution pattern, and v~rtical velocities
(Flagg and Smith 1989, Plueddemannand Pinkel1989,Heywood et al 1991, Fishcher and
Visbeck 1993, Zh6u et al 1994, Ashjian et al1994, Buchholz et al 1995, Heywood 1996),. one must
be concerned about the identity of backscatter population. A simple solution, as this study has
successfully demonstrated, is the use of high frequency ADCP (153 kHz) in conjunction with a
low frequency sonar (12 kHz) to separate backscatter layers offish and zooplankton.
ACKNOWLEDGMENTS
This research was funded by the Office of Naval Research (N000149710156). We thank the crew
of NOAA Ship Malcolm Baldrige and a multitude of scientific personnel for their helps at sea;
John Prom (NOAA/AOML) for providing the 12 kHz sonar deck unit; Paul Dammann
(AOMLINOAA) for writing the original 12 kHz data acquisition program; Charles Greenlaw
(TRACOR Applied Sciences), and Charles Flagg for their help with 12 kHz and ADCP data
processing respeCtively; and Larry Madin (WHOI) for supplying unpubliShed MOC-IO data.
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