10

period were coupled on a basis of growth rates reported in the
literature. In this case (31 days between the midpoints of the
cruises), we linked successive 5-mm length classes.

Atlantic cod larvae (6-10 mm) were less abundant, but more widely
dispersed during Delaware II Cruise 79-05 than 1-5 mm cod larvae during
the preceding sampling period. There was no marked transport of larvae
from Georges Bank into the Gulf of Maine as indicated by the buoy
trajectories guring Experiment II. The decrease in average abugdance
from: 8.0/10 m° for 1-5 mm larvae during March-April to 3.0/10 m* for 6-
10 mm larvae during May equates to a mortality rate of 2.0%/day.

Ammodytes sp. were the most abundant larval fishes collected during
the initial sampling period, but were found for the most part in areas
to the west of buoy deployment during Experiment II. No readily
observable pattern of drift is evident from the two sets of distribution
data, although one might theorize that the main population of larvae off
Southern New England was transported in a southwesterly direction and
out of the area of coverage. However, reported average surface current
velocities of approximately 4.5 km/day (Bumpus 1973; Hansen 1977; Han
and Niedrauer 1981) would not give rise to such an extensive
transport. Estimated mortality rates based on the relative abundance of
Ammodytes sp. cohorts during the two time periods were:

6-10 mm to 11-15 mn, 3.0%/day;
11-15 mm to 16-20 mm, 3.1%/day; and
16-20 mm to 21-25 mm, 3.1%/day.

Hake (Urophycis sp.), silver hake (Merluccius bilinearis), and
redfish (Sebastes sp.) were the dominant and most widely distributed
larval fishes collected on Belogorsk Cruise 79-01 and were chosen as
candidate species to follow dispersal and estimate mortality rates
between Belogorsk Cruise 79-01 (100 stations, mid-date: Day. 234) and
Albatross IV Cruise 79-11 (106 stations, mid-date: Day 288). On the

_Dasis of the 54-day interval between the midpoints of these cruises, we
have coupled alternate 5 mm-length classes (Figures 16 and 17).

On Belogorsk Cruise 79-01, Urophycis sp. were found in southwestern
Gulf of Maine, along the southern part of Georges Bank, and throughout
most of the area sampled in the Middle Atlantic Bight (Figure 16). The
bulk of the population was either north or west of the area of buoy
deployment in Experiment IV. On Albatross IV Cruise 79-11, no hake
larvae were found in the Gulf of Maine and only scattered patches of
larvae occurred on southern Georges and in the Middle Atlantic Bight.
Buoy trajectories during Experiment IV suggest that larvae found in the
southwestern part of the Gulf of Maine and in the Great South Channel
during Belogorsk Cruise 79-01 could have been transported in an easterly
direction along the northern edge of Georges Bank and out of the area of
coverage during Albatross IV Cruise 79-11. Buoy trajectories also
suggest that the Toss of Tarvae from southern Georges could be
associated with offshore movement (shelf water entrainment). The
situation may also account for the loss of larvae in the Middle Atlantic


31

95,

“= WIND DRIVEN CURRENT

137

Figure 11.

Buoy 405 trajectory and wind-driven-current progressive
vector plot, 31 March (Day 88)-1 June (Day 152) 1979.


27

40°...

703 Oy245

250

42°

L 40°

42°

40°

L 42°

F 40°

Figure 7.

Tracks of buoys deployed during 16-19
(Experiment IV).

August 1979


36

ove 099 289 004

99 089 00L tL
L L i 1 i 1 1 fl Ll l 1 1

1 s.
wuoz-9L dS SIDAHdON Ses:

a

wwol-9 dS SIDAHdO’N

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OO€-00L
OOl-0€
O€- Ol
ol-€
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l0-62 xS¥OOO14a

ov

T T T T T T
74 099 ool

T
ol

tl

Urophycis
y en and

ruise 79-11 (mid-date, Day 288).

sp.) during Belogorsk Cruise 79-01 (mid-date, Dai
I

Comparison of the distributions of larval hake (

Albatross

Figure 16.


The deficiencies and technical problems associated wih the use of
drogued surface buoys have been discussed both in terms of undetected
drogue loss during long-term experiments (Richardson, Cheney, and
Mantini 1977; Richardson, Wheat, and Bennett 1979) and of the utility of
window-shade or parachute drogues in reducing velocity and direction
errors due to windage and other slippage effects (Kirwan et al. 1979).
Although the small-scale features of circulation are not fully resolved
by Lagrangian tracking, it has proved to be an economical and effective
method and has found wide acceptance in oceanographic research to
determine the broadscale features of ocean circulation.

Shelf/Slope Water Front, Gulf Stream North Wall,
and Gulf Stream Ring Positions

Tne NOAA National Earth Satellite Service (NESS) produces a weekly
experimental “Gulf Stream Analysis Chart" series based on thermal IR
imagery and showing the position of thermal fronts, shelf, slope, and
Gulf Stream water, and warm and cold-core Gulf Stream rings. The NMFS
Atlantic Environmental Group (AEG) uses these charts to monitor the
position of warm-core rings and the location of the shelf/slope water
front and the Gulf Stream north wall along 12 standard bearing lines
from selected points from Cape Romain, S. C., to the Gulf of Maine. We
have used published data (Hilland 1981; Fitzgerald and Chamberlin 1981)
as well as unpublished summaries provided by AEG in an effort to
elucidate the relationship of these oceanographic variables to the
trajectories of satellite-tracked buoys.

Wind and Wind-Induced Current Regimes

Meteorological data used in this study were obtained from the
National Climatic Center in Asheville, N. C., and included daily
resultant wind speeds and directions based on three hourly observations
(six per day) taken at Nantucket Island during the period of the
satellite-tracked buoy experiments (December 1978-December 1979). Wind
speed adjustment factors were used to extrapolate Nantucket Island wind
values to Georges Bank. The techniques used in determining wind
adjustment factors, examples of their application, and the utility of
their use are presented and discussed in the Appendix. The adjusted
wind data were then converted to wind-driven current values which were
estimated to be 1.5 or 3.0% of the wind speed along a wind direction of
+15° (Bigelow 1927; Haight 1942; Godshall et al. 1980).

Ichthyoplankton Distribution

The distribution of larval fishes at the time of and subsequent to
the satellite-tracked buoy experiments was obtained on plankton surveys
conducted in continental shelf waters between Cape Hatteras and Nova
Scotia as part of the MARMAP (Marine Resources Monitoring, Assessment,
and Prediction) Program of the National Marine Fisheries Service
(Sherman 1980). The sampling gear (61-cm bongo nets) and method of tow
(double oblique, 1.5-2.0 knot tows, to a maximum depth of 200 m) have been


44

be
Table 2. Monthly and annual wind statistics for Nantucket Island and Georges Shoals, 1958-61.

Nantucket Island _ Georges Shoals Georges- Nantucket_ys. Georges Shoals
Nantucket
Direction Speed Direction Speed wind speed Vartance ratio® Correlation coefficient®

Year Month (from) (knots) (from) (knots) ratio rection Spee Tection Specs
1958 April 066° 0.6 29° 7
May 196° 2.2 181° 3.0
June 225° 4.0 209° 5.6
duly 222° 5.6 212° WT
August 221° 43 208° 7.2
September 254° 2.9 263° 5.5
October 029° 2.2 360° 2.5
November 282° 4.9 272° 8.1
December 324° 6.9 296° 9.5

Total 258° 2.2 250° 3.8 0.28 2.91 0.98 0.96
1959 January 282° 8.2 276° 13.4
February 290° 6.7 279° 9.7
March 284° 42 287° 8.2
April age 0.6 094° 0.6
May 251° 5.5 262° 7.3
dune 233° 2.4 180° 3.6
duly 225° 5.4 2iae 9.5
August 232° 2.0 254° 49
September 231° 2.6 237° 4
October 253° rest 295° 13
Novenber 296° 1.9 243° 1.6
Decenber 332° 41 318° 6.8

Total 264° 3.2 263° 49 0.16 2.75 0.69 0.96
1960 January 314° 6.8 305° 12.7
February 276° 6.2 284° wT
March 314° 5.9 3u7° 8.4
April 2iae 2.9 2i7e 3.1
May 1728 3.0 138° 3.9
dune 210° 4.0 195° 6.5
duly 220° 5.2 212° 13
August 205° 1.6 198° 2.0
September 047° 1.0 061° 25
October 287° 1.8 321? 2.0
November 263° 5a 260° 49
December 290° 5.9 283° 9.3

Total 266° 2.8 262° 35 0.00 2.37 0.97 0.91
1961 January 302° 6.7 290° 15
February 289° 2.8 281° 5.9
March 320° 3.9 308° 7.6
April 281° 3 294° a7
May 2u7e 4.2 210° 49
June 215° 15 213° 10.9
duly 193° 3.4 193° 4.9
August 220° 3.0 231° 3.4
October 314° 3.0 339° 5.5

Total 261° 2.4 265° 4.6 0.00 4.72 0.96 0.90

Total 265° 27 262° 4.20 16 0.04 12,38 0,96 0.93

@ Values at 0.05 level of significance.

> significant.


42

rc— —_— ————
WIND SPEED (KNOTS) ?
ee ee ee ee
0 10 20 30 40 H w. é
30 Ss
MONTHLY | RESULTANT
AUG 1959|" or. SPEED
ee NANTUCKET | 235° 2.0
+ GEORGES 254° $1
GEORGES .
°°? PREDICTED 234! 33

e
. MONTHLY | RESULTANT
_ ' JAN 1959 |" ir. SPEED
WIND SPEED (KNOTS) a i 2 NANTUCKET | 283° 8.
aval 's Jo ceorces 272" BS
GEORGES .
— ——

Figure 22.

Comparison of progressive vector plots of daily Nantucket
Island, Georges Shoals, and predicted Georges Bank winds,
danaury 1959 and August 1959.


14

ACKNOWLEDGMENTS

We extend special thanks to David Cook, Raytheon Ocean Systems
Company, and Daniel Frye, EG&G Environmental Consultants, for generously
allowing us to use the edited buoy position and temperature data which
are the foundation of this paper. Merton Ingham and other members of
the Atlantic Environmental Group facilitated the extraction of
meteorological and satellite imagery data. Thomas Plichta compiled the
computer summaries of ichthyoplankton abundances and Lianne Armstrong
prepared the final figures. Geoffrey Laurence and Wallace Smith
critically reviewed earlier drafts of this paper.


KILOMETERS

EXP. | _ EXP 2 ; EXP. 3 | EXP. 4
ONSHORE -100 5 ot
OFFSHORE NA
200-44 Une
400 ° *
S007 . CASCO BAY 120° \
\g
ONSHORE 100
0
OFFSHORE | ve WA
y Ws, B
400-4
7 CASCO BAY 140°
ONSHORE -100 NR
° -
OFFSHORE Pe ON A
2004 o~\, - 8
4005 " CASCO BAY I60°
OMSHORE-100 5 _ ae n . 4
OFFSHORE == \ ,-2----- - “ / -
400 NANTUCKET !80°
if T T T T T T T T T T T T T T ! T T TT
365 20 100 200 300
1978 1979

A SHELF WATER FRONT
8B GULF STREAM NORTH WALL

QoNR

f-°O8! LNYN@
0
A

Figure 12. Weekly positions of the shelf water front and the Gulf
Stream north wall relative to the 200-m isobath during the
period of the four buoy experiments.


TABLE OF CONTENTS

LIST OF FIGURES. .ecceseccccecccccccccrcescetecseseenssessseeesees
LIST OF TABLES. .ccececcccecececcecccevceseetecsecesereetecseeroes
INTRODUCTION. .cccceccccccccccecccscccsevcecssevscsesensrscsescees
DATA SOURCES ..ccccccccccccvcccccsccccscvececeesseeeneeeeseseeseee

Satellite-Tracked Buoy ExperimentS..cccscccccccescccccrccecs

Shelf/Slope Water Front, Gulf Stream North Wall,
and Gulf Stream Ring PositionS..ccccccscccceeescvcsseseses

Wind and Wind-Induced Current RegimeS...cccesscscecsccsevecs
Ichthyoplankton Distribution. ccccccccceccscvecvcccecccseece
Buoy Trajectories and Residence TimeS.cesccccecrececereccece
Experiment Levcccccecccccvccccccccsceecevcssesscceecens
Experiment Ilsesccccccscvccsccnvecccccceresevsseseseses
Experiment ILl]...ccccceccccvcvcccvcceceterccesceceseeee
Experiment [Vicccccecccccccveccccsccccceesceseerecesees
Effect of Wind on Buoy Trajectories..cccccccccecececees

Buoy Movement in Relation to Thermal Fronts
and Warm-Core Ring PoSitionS.ceccccccccscescveccereaceeves

Ichthyoplankton Distribution. ..ccecccccccnccecccccsencessees
SUMMARY AND CONCLUSIONS... ..cccecvcccccccccsccccccccensevereceeees
SUMMALY ccccccccecccvcccevccccecvecceccescesssrseceseseserees

CONCTUSTONS eecvecccrcccccnvecccncnerssenssesessresseeseesene

ACKNOWLEDGMENTS oc eccevcccccccccccsccnsessrenceeersssseessrssesees
LITERATURE CITED. ce ccccccccceccscccccscecsccacececscusserecssunes

APPENDIX: COMPARISON OF WIND REGIMES AT NANTUCKET ISLAND AND
GEORGES BANK. .ccccesceccccccccccccscvccscesssssscreseceseeee

INtrOductiOnesseccccccccccccccescnccescenseveenesseerssceeees

‘Preceding page blank

vii

ix

12
12
13
14
15

19
19


25

° 70° 68° 66°

ar 42°
BUOY 234
40-4 140°
T T T T T T T
66°
66°
iL
42°4 ma05 42°
40°4 40°

“Tr
72° 70° 68° 66°

Figure 5. Tracks of buoys deployed during 30 May-2 June 1979
(Experiment III).


11

Bight, but we have no data to support this contention, The decrease in
the average abundance of,1-5 mm larvae from 67.1/10 m* during Belogorsk
Cruise 79-01 to 1.6/10 me for 10-15 mm larvae during Albatross IV Cruise
79-11 points to a mortality rate of 1.8%/day. On Belogorsk Cruise 79-
01, the average abundance of 5-10 mm larvae was 7.4/10 ne and on
Albatross IV Cruise 79-11 the average abundance of 15-20 mm larvae was
1.7710 Te which equates to a mortality rate of 1.4%/day.

On Belogorsk Cruise 79-01, silver hake larvae were found in the
western Gulf of Maine, over Nantucket Shoals, and on southern and
western Georges Bank. On Albatross IV Cruise 79-11, only a few
scattered patches of larvae were found in the Gulf of Maine, on Georges
Bank, and off Southern New England. Mortabity rates based on the
ayerage number of 1-5 mm larvae (18.0/10 m“) and 6-10 mm larvae (5.3/10
m°) caught during Belogorsk Cruise 79-01, and the average number of 1l-
15 mm larvae (0.3/10 ny and of 16-20 mm larvae (0.3/10 m2) caught on
Albatross IV Cruise 79-11, were computed to be 1.8%/day and 1.7%/day,
respectively. The shifts in distributions and decrease in abundances
between the two time periods were not associated with any obvious
residual drift patterns observed during Experiment IV or in previous
studies of circulation.

Redfish larvae (1-5 mm and 6-10 mm) were found in appreciable
numbers of central and western Gulf of Maine and in the Great South
Channel during Belogorsk Cruise 79-01, but no larvae (11-15 mm and 16-20
mn) which could be considered the progeny of the same spawnings were
found on Albatross IV Cruise 79-11. Redfish larvae (5-10 mm) were
collected at only two stations in north central Gulf of Maine during
this latter cruise. The absence of redfish larvae during Albatross IV
Cruise 79-11 does not appear to be associated with some form of sampling
bias for we have frequently collected redfish larvae of the appropriate
size range on ichthyoplankton cruises at this time of the year. In
addition, Kelley and Barker (1961), on the basis of mid-water trawl]
samples, have shown that redfish do not move to the bottom until they
are 4.5 months old (approximately 50 mm in length) and that larvae
smaller than this occupy the water column above 100 m in depth. Larvae
25 mm and less were found in the upper 20 m of the water column.

It would appear then that the absence of 11-15 mm and 16-20 mm
redfish larvae during October 1979 was authentic and not the result of
any sampling bias. A mortality rate within the range of 2.0%/day, which
is of the same order of magnitude as that estimated for the other
species, could account for the complete loss of redfish larvae during
this 54-day time period.

In no case have we been able to establish a well-defined link
between residual drift and the dispersal of larvae. Estimated mortality
rates were similar during periods of low and high residence times of
Georges Bank surface waters and in the Gulf of Maine, Georges Bank, and
Middle Atlantic Bight. This may be due in part to the fact that
sampling during the ichthyoplankton surveys was confined for the most
part to continental shelf waters (<200 m) with few samples being
obtained in the vicinity of the shelf/slope front where entrainment of
shelf water is prevalent. In any event, the loss of larval cohorts


7

HANSEN, D. V.
1977. Circulation. MESA New York Bight Atlas Monograph 3. New
York Sea Grant Institute, Albany, New York, October 1977, 23 p.

HILLAND, J. E.
1981. Variation in the shelf water front position from Georges Bank
to Cape Romain in 1979. Ann. Biol. Cons. int. Explor. Mer 36:34-
36.

KELLY, G. F. and A. M. BARKER.
1961. Vertical distribution of young redfish in the Gulf of
Maine. Rapp. P.-v. Reun. Cons. int. Explor. Mer 150:220-233.

KIRWAN, A. D., JR., G. McNALLY, S. PAZAN, and R. WERT.
1979. Analysis of surface current response to wind. J. Phys.
Oceanogr. 9:401-412.

LAURENCE, G. C.
1974. Growth and survival of haddock (Melanogrammus aeglefinus)
larvae in relation to plankton prey concentration. J. Fish. Res.
Board Can. 31:1415-1419.

LAURENCE, G. C., A. S. SMIGIELSKI, T. A. HALAVIK, and B. R. BURNS.
1981. Implications of direct competition between larval cod
(Gadus morhua) and haddock (Melanogrammus aeglefinus) in
Yaboratory growth and survival studies at different food
densities. Rapp. P.-v. Reun. Cons. int. Explor. Mer 178:304-311.

LOUGH, R. G., M. R. PENNINGTON, G. R. BOLZ, and A. S. ROSENBERG.
1980. A growth model for larval herring (Clupea harengus L.) in the
Georges Bank-Gulf of Maine area based on otolith growth
increments. NAFO SCR Doc. 80/1X/131, 16 p.

POSGAY, J. A. and R. R. MARAK.
1980. The MARMAP bongo zooplankton samplers. J. Northw. Atl. Fish.
Sci. 1:91-99.

REDFIELD, A. C.
1939. The history of a population of Limacina retroversa during its
drift across the Gulf of Maine. Biol. Bull. 76(1):26-47.

1941, The effect of the circulation of water on the distribution of
the calanoid community in the Gulf of Maine. Biol. Bull.
89(1) :86-110.

REDFIELD, A. C. and A. BEALE.
1940. Factors determining the populations of chaetognaths in the
Gulf of Maine. Biol. Bull. 79(3):459-487.

RICHARDSON, P. L., R. E. CHENEY and L. A. MANTINI.
1977. Tracking a Gulf Stream ring with a free drifting surface
buoy. J. Phys. Oceanogr. 7:580-590.


13

Conclusions

The observed buoy trajectories and estimated surface water
residence times presented in this study and the characteristic shelf
water entrainment features observed in satellite imagery tend to support
the hypothesis that the advection of fish eggs and larvae from the
spawning areas on Georges Bank could at times have a significant effect
on the dispersal and thus survival of early life history stages and the
subsequent abundance of fishes available for commercial and recreational
exploitation. The biological data presented, however, do not support
this contention in that in no case did advective processes appear to be
clearly associated with larval fish mortality estimates. This disparity
appears to be due in part to the facts that the two data sets were
obtained in vacuo and thus caused difficulties in integrating the
physical and biological information, that the concentrations of larval
fishes occurred for the most part in areas outside the areas of buoy
deployment, and that the time intervals between ichthyoplankton surveys
were too long to firmly establish cohort linkages.

In the absence of evidence to the contrary, it does not seem
unreasonable to assume as the simplest hypothesis, that both early and
late pelagic stages of marine fishes drift passively with the current
and as a consequence are at times subject to advection from the spawning
grounds. Thus it would appear expedient to reexamine this premise in a
more detailed and concurrent manner. This approach would involve buoy
deployment at the time of ichthyoplankton surveys and in areas of known
ichthyoplankton concentrations, restricted coverage to Georges Bank
proper and slope water areas to the south during periods of maximum egg
Production of commercially/recreationally important species such as
haddock and cod and minimum residence times of Georges Bank surface
waters (February-April), the collection of ancillary hydrographic data
to accurately determine water mass distribution and density structure,
and intervals between surveys of not more than 2 weeks. In addition to
the above, it is requisite that we quantify shelf water entrainment both
in terms of volume of water and numbers of organisms (phytoplankton,
zooplankton, and ichthyoplankton). This latter project of study is now
underway in connection with the National Science Foundation-sponsored
interdisciplinary study of warm-core rings in which NMFS is obtaining
quantitative information on the vertical structure and the magnitude of
the flux of shelf water and contained organisms in the entrainment
feature and relating these measurements to surface manifestations as
indicated by satellite imagery.

If, after a comprehensive sampling program, we can unequivocally
conclude that boreal larval fish species of Georges Bank origin do not
occur in entrained shelf water, we will be faced with the more
formidable task of explaining why this is so.


26

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YEAR DAY

Figure 6.  Stackplot of daily Nantucket Island wind vectors and buoy
velocity and temperature (Experiment III).


compare in Figures 10 and 11 progressive vector plots of wind-induced
surface currents and buoy trajectories during periods when Buoy 34 was
moving in a southerly direction across Georges Bank (Exp. I, 13 December
1978-30 January 1979), and Buoy 405 was moving in a northerly direction
from the Great South Channel area into the Gulf of Maine (Exp. II, 31
March-1 June 1979). Wind driven current values were based on predicted
Georges Bank winds determined from Nantucket Island wind speed
adjustment factors (described in the Appendix) and were estimated to be
1.5% (Exp. I) or 3.0% (Exp. II) of the adjusted wind speed along a wind
direction of +15°.

During the first week after release (Exp. 1, Day 348-355), Buoy 34
and wind-driven current velocities were well correlated (vector mean
buoy velocity - 17.3 cm/sec at 119°, vector mean wind-driven current
velocity - 13.3 cm/sec at 120°. After this initial period, there was a
marked divergence in the estimated wind-driven current and observed buoy
trajectories, with the buoy moving south and southwest and the wind-
driven current being directed to the southeast. For the total time
period (Day 348, 1978-Day 30, 1979) the vector mean buoy direction and
speed were 170° and 5.5 cm/sec while the vector mean wind driven current
direction and speed were 125° and 9.5 cm/sec.

The trajectories of Buoy 405 and of the wind-driven current
diverged from the time of launching (Exp. II, Day 88) with the buoy
initially moving south while the estimated wind-driven current was to
the north. Another conspicuous deviation in buoy and wind-driven
current movement during this experiment occurred between Day 120 and
131, a period in which the wind-driven current was to the east at an
average speed of 20 cm/sec, while the buoy moved to the north at an
average speed of 3 cm/sec. For the total time period (Day 88-152), the
vector mean buoy direction and speed were 349° and 4.0 cm/sec, while the
vector mean wind-driven current direction and speed were 052° and 5.4
cm/sec.

Day (1958) observed on a basis of drift bottle returns and-
estimated weather map wind velocities that the prevailing circulation in
the Gulf of Maine-Georges Bank area may be strongly influenced by winds
at all seasons of the year. We conclude, however, that there was no
readily discernible or coherent wind effect on buoy movement during the
course of the subject experiments.

Buoy Movement in Relation to Thermal Fronts
and Warm-Core Ring Positions

Using the data of Hilland (1981) and Fitzgerald and Chamberlin
(1981), we plotted the weekly positions of the shelf water front and the
Gulf Stream north wall relative to the 200-m isobath for the four most
easterly bearing lines during the periods of the four satellite-tracked
buoy experiments (Figure 12). The only readily perceivable correlation
between thermal fronts and buoy trajectories is that the shelf water
front and the Gulf Steam north wall were furthest offshore (especially
along the Casco Bay 120° bearing line), during Experiment I at which
time all buoys moved rapidly south and off Georges Bank, and were




23

AO? foe

Ay
BUOY 405

90

200M

135

44°

t 42°

+ 40°

|
70°

!
68°

66°

Figure 3. Tracks of buoys deployed during 25-27 March 1979 (Experiment

Ill).


20

direction (Figure 20) did not differ significantly from unity, while the
slope (0.618) of the regression line for wind speed (Figure 21) was
appreciably less than unity, further demonstrating the tendency of wind
-Speeds to be greater offshore and for the magnitude of these differences
to increase with increasing wind speed.

Discussion

The wind statistics indicate that wind speed, but not wind
direction, adjustment factors are required to extrapolate Nantucket
Island wind observations to Georges Bank. One of the problems we
immediately encounter in attempting to apply such adjustment factors is
that our regression equation is based on monthly resultant wind speeds
which have a maximum value of 13.4 knots (Georges Shoals, January
1959). Daily resultant wind speeds at both Nantucket Island and Georges
Shoals often exceed 15 knots, so that it is necessary to convert daily
values to means for protracted time periods (5 days or more) or to
assume that the calculated linear relation prevails at higher wind
speeds.

To test the applicability of wind speed correction factors, we
compared progressive vector plots of daily Nantucket Island, Georges
Shoals, and predicted Georges Bank winds for January 1959 and August
1959. (Figure 22). These time periods were selected because Nantucket
Island daily wind speeds during these months were for the most part less
than 15 knots and in one month (January 1959) the observation point
coincided with the regression line while the August 1959 value showed a
substantial negative deviation (Georges Bank/Nantucket Island wind speed
ratio >2/1).

For January 1959, the predicted Georges Bank wind values closely
correspond to those observed at Georges Shoals with a notable
improvement in the conformity of wind speeds. For August 1959, however,
the adjusted Nantucket Island wind values are more similar to the
observed Nantucket Island values than to those observed at Georges
Shoals. On the basis of this limited analysis, we feel that for longer
time intervals (weekly or monthly resultant winds) that adjustment
factors would, in general, improve the quality of wind velocity
estimates, but in most cases the variability in daily wind directions
and speeds between the two locations preclude the use of adjustment
factors for shorter time intervals.


Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

9.

10.

li.

12.

13.

vii

LIST OF FIGURES

Tracks of buoys deployed during 13-29 December
1978 (Experiment I). Dashed lines indicate
MiSSINg daily POSITIONS ..ccveccececvccccevecsevressseel

Stackplot of daily Nantucket Island wind vectors
and buoy velocity and temperature (Experiment I)....22

Tracks of buoys deployed during 25-27 March 1979
(Experiment 11) ..ccscccccccccccccsccsscccccsecssssee23d

Stackplot of daily Nantucket Island wind vectors
and buoy velocity and temperature (Experiment II)...24

Tracks of buoys deployed during 30 May-2 June 1979
(Experiment II]) ..ccccccccccccccccnccscsccscasseesesld

Stackplot of daily Nantucket Island wind vectors
and buoy velocity and temperature (Experiment III)..26

Tracks of buoys deployed during 16-19 August 1979
(Experiment IV) .cccccccccccccvcccccsccnccrccccesesseal

Stackplot of daily Nantucket Island wind vectors
and buoy velocity and temperature (Experiment IV)...28

Estimated residence times in days of Georges Bank
SUPFACE WALEPSceccccccccccccraccccvcccneresceesens es ld

Buoy 34 trajectory and wind-driven-current
progressive vector plot, 14 December 1978
(Day 348)-30 January 1979 (Day 30) ..ccccecsceceeeeee30

Buoy 405 trajectory and wind-driven-current
progressive vector plot, 31 March (Day 88)-1 June
(Day 152) 1979. ..cccccceccsccccevcecsctcceceseeseceedh

Weekly positions of the shelf water front and

the Gulf Stream north wall relative to the 200-m
isobath during the period of the four buoy
EXPEPIMENtS .cceececcccccccvececccceseescevesesssese sda

Trajectory of buoy 620 during the period

20 October-6 December 1979 in relation to the
location of warm-core rings 79G and 79H and the mean
position of thermal fronts as indicated by satellite
imagery for the period 1-7 November 1979........0...33


vi

TABLE OF CONTENTS (Cont.)

Wind Regime CompariSOnS..ssccccccsccccscccccsscccescsceesees 19

DISCUSSION cevececccscccesccccccccccssccsscsccssescessceece 20


closest to the 200-m isobath during Experiments III and IV when the
buoys remained over the continental shelf for the longest time periods.

Comparisons of buoy movement with surface temperature features
depicted from satellite IR imagery show that the movement of buoys off
the continental shelf was often associated with warm-core ring or Gulf
Stream meander activity (e.g., Exp. II, Buoy 510; Exp. IV, Buoys 620,
727, and 703). At other times, however, rapid and direct buoy movement
off the shelf was not associated with any apparent warm-core ring
activity or fluctuations in the locus of the shelf/slope or Gulf Stream
surface temperature fronts (Exp. III, Buoy 433). The fact that surface
temperatures measured by the buoys and buoy velocities often changed
abruptly indicates that at times the buoys were moving from one water
mass to another (across thermal fronts) rather than being entrained
within a specific parcel of water.

In Figures 13 and 14 we have plotted the trajectories of Buoys 620
and 703 (Exp. IV) during periods when satellite imagery evidenced marked
shelf water entrainment under the influence of warm-core rings 79-G, 79-
H, and 79-I. In Figure 13, water mass distributions are based on the
“Gulf Stream Analysis Chart" for the period 1-7 November (Day 305-311)
and warm-core eddy positions are from Fitzgerald and Chamberlin
(1981). Water mass distributions in Figure 14 are from U. S. Naval
Oceanographic Experimental “Ocean Frontal Analysis Chart" for the period
21-27 October (Day 294-300). Considering the possible errors involved
in buoy positioning and the difficulties encountered in the
intrpretation of satellite imagery, there is fairly good correspondence
between entrainment features and buoy movement.

Ichthyoplankton Distribution

Data from ichthyoplankton surveys made prior, coincident, and
subsequent to the satellite-tracked buoy experiments were used in whole
or in part to delineate the distributions of the dominant larval fish
species at the time and in the area of buoy deployment and to estimate
larval fish dispersion and mortality rates. The cruise numbers and
dates were:

Albatross IV 79-U3, 28 March-9 April 1979 (Exp. II);

Delaware II 79-04, 11-30 April 1979 (Exp. II);

Delaware II 79-05, 4-29 May 1979 (Exp. II and III);

Belogorsk 79-01, 11 August-2 September 1979 (Exp. IV); and

Albatross IV 79-11, 1-29 October 1979 (Exp. IV).

In Figure 15 we compare the distributions of larval Atlantic cod

(Gadus morhua) and sand lance (Ammodytes sp.) during combined Albatross
IV Cruise 79-03 and Delaware II Cruise 79-04 (109 stations, mid-date:

Day 109), and Delaware II Cruise 79-05 (130 stations, mid-date: Day
140). As described previously, the 5-mm length classes for each time


Table 1.
Table 2.

ix

LIST OF TABLES

The consecutive number of each day of the year......43

Monthly and annual wind statistics for Nantucket
Island and Georges Shoals, 1958-61......ecscesvecoes 44


15
LITERATURE CITED

ANDERSON, V. T. and S. A. AKENHEAD.
1981, Distribution and abundance of redfish and cod larvae on
Flemish Cap in 1978 and 21979. NAFO Sci. Coun. Studies, No. 1:57-
63.

BIGELOW, H. B.
1927. Physical oceanography of the Gulf of Maine. Bull. U. S. Bur.
Fish. 40(2):511-1027.

BOYAR, H. C., R. R. MARAK, F. E. PERKINS, and R. A. CLIFFORD.
1973. Seasonal distribution and growth of larval herring
(Clupea harengus L.) in the Georges Bank-Gulf of Maine area from
1 to 1970. J. Cons. int. Explor. Mer 35:36-51.

BUMPUS, D. F.
1973. A description of the circulation of the continental shelf of
the east coast of the United States. Progress in Oceanogr. 6:111-
157.

1976. A review of the physical oceanography of Georges Bank. Int.
Com. Northw. Atl. Fish. Bull. 12:119-134,

BUMPUS, D. F. and L. M. LAUZIER.
1965. Surface circulation on the continental shelf off eastern
North America between Newfoundland and Florida. Am. Geograph.
. Soc., Serial Atlas of the Marine Environment, folio 7.

BUTMAN, B., R. C. BEARDSLEY, B. MAGNELL, J. A. VERMERSCH, R. J. SCHLITZ,
R. LIMEBURNER, and M. A. NOBLE.
1982. The mean circulation on Georges Bank. J. Phys. Oceanogr.
12:569-591.

CLARKE, G. L., E. L. PIERCE, and D. F. BUMPUS.
1943, The distribution and reproduction of Sagitta elegans on
Georges Bank in relation to hydrographic conditions. Biol. Bull.
85:201-226.

COLTON, J. B., UR.
1961. The distribution of eyed-flounder and lanternfish larvae in
the Georges Bank area. Copeia 1961(3):274-279.

COLTON, J. B., JR. and R. R. BYRON. .
1977. Gulf of Maine-Georges Bank ichthyoplankton collected on ICNAF
larval herring surveys September 1971-February 1974. NOAA Tech.
Rep. NMFS SSRF 717:35 p.

COLTON, J. B., JR. and J. M. ST. ONGE.
1974. Distribution of fish eggs and larvae in continental shelf
waters, Nova Scotia to Long Island. Am. Geog. Soc. Serial Atlas
of the Marine Environment. Folio 23.


“PBB4107899 0
ae

NOAA Technical Memorandum NMFS-F/NEC- 24

This TM series is used for documentation and timely communication of
preliminary results, interim reports, or special purpose information; and
has not received complete formal review, editorial control, or detailed
‘editing.

Residual Drift and Residence Time
of Georges Bank Surface Waters
with Reference to the Distribution, Transport,
and Survival of Larval Fishes

John B. Colton, Jr., and Jacquelyn L. Anderson

Narragansett Lab., National Marine Fisheries Serv., Narragansett, Ri 02882

i Rerrovuceoey: = NTS. |

| us. gepenrenarconmerce AE
ational Teehatea Information Service

| ‘Sprin nia. 22161

U.S. DEPARTMENT OF COMMERCE

Malcolm Baldridge, Secretary

National Oceanic and Atmospheric Administration
John V. Byrne, Administrator

National Marine Fisheries Service
William G. Gordon, Assistant Administrator for Fisheries

Northeast Fisheries Center
Woods Hole, Massachusetts

June 1983


38

1961

1

js as ee ee

it

i oe ee

14

E-W VELOCITY

4
.
I
1
Uy

1960

N~-S VELOCITY

1959

il
2

Figure 18.

---GEORGES SHOALS

—— NANTUCKET 1S.

Ty
©
‘

Components of monthly resultant wind velocity at Nantucket

Island and Georges Shoals.
east and north.

Positive components are to the

1958


and 8 of Bumpus and Lauzier (1965)], although in this connection Bumpus
(1976) cautioned,

“Because there are so few returns of drift bottles and sea-bed
drifters from Georges Bank, one cannot help but suspect that a
great number are carried offshore rather than drifting to the west
to strand. Certainly when one views the results of some drogued
drifting buoy experiments . .. , one obtains the feeling that
there are ample opportunities for exchanges between Georges Bank
water and slope water."

The likelihood of appreciable offshore movement of Georges Bank
surface waters has stimulated our interest in the question of how
populations of fishes maintain themselves under conditions conducive to
the loss of spawning products (eggs and larvae). Several attempts have
been made to elucidate the role of circulation on the drift and survival
of ichthyoplankton in the Georges Bank area. Walford (1938), on a basis
of drift bottle recoveries and the distibution of haddock
(Melanogrammus aeglefinus) eggs and larvae, concluded that in 1931 eggs
and larvae were retained in a clockwise gyre and carried northward to
Georges Shoals with no significant loss to other regions, while in 1932,
the circulation was such that large numbers of larvae were carried into
deep water to the north and south of the bank where conditions were
unfavorable for survival. Colton and Temple (1961) inferred from
observations on the time and location of spawning of haddock and
Atlantic herring (Clupea harengus harengus) and the drift of bottles and
transponding buoys that with the exception of midsummer when the Georges
eddy is most pronounced, surface drift is offshore in direction of slope
water and that under average conditions most fish larvae were carried
away from Georges Bank.

’

We emphasize that these inferences regarding the transport of fish
eggs and larvae are based in large measure on drift bottle returns and
movements of transponding buoys rather than on the drift of
ichthyoplankton cohorts. A conspicuous and significant aspect of the
larval fish distribution in the Georges Bank area is the almost total
segregation of coastal (boreal) and oceanic (tropical and subtropical)
species north and south of the coastal/slope water front which coincides
approximately with the 100-200 m isobaths (Colton and Byron 1977).
Oceanic species of larval and juvenile fishes are frequently collected
over Georges Bank and in the Gulf of Maine following intrusions of slope
water (Colton 1961; Colton and St. Onge 1974), but young stages of
boreal coastal species have not been observed in the slope water area or
in entrained shelf water, albeit sampling in areas south of the 200-m
isobath (especially in entrained shelf waters) has been infrequent.

Since about the middle 1970's, there has been a resurgence of
interest in continental shelf oceanography (especially circulation),
partly as a result of public concern over possible effects of impending
offshore gas and oi] exploration and production, and also due to
improved monitoring capabilities through technological advances in data
sensing, logging, and telemetry. The remote sensing capabilities of
satellites provide more frequent and synoptic coverage of sea surface
conditions by means of infrared (IR) and visible imagery, radar, and


INTRODUCTION

Our understanding of surface circulation in the Gulf of Maine and
over Georges Bank is based in large part on the conclusions of Bigelow
(1927) following a detailed study and thoughtful interpretation of drift
bottle recoveries, tidal current measurements, dynamic computations,
wind-driven currents, and the distribution of temperature, salinity, and
plankton. Bigelow's portrayal of the development of a two-gyre system
(the Gulf -of Maine cyclonic eddy and the Georges Bank anticyclonic eddy)
conformed well with subsequent interpretations of circulation based on
observations of the distribution and drift of zooplankton populations by
Redfield (1939, 1941), Redfield and Beale (1940), and Clarke, Pierce,
and Bumpus (1943), and of fish eggs and larvae by Walford (1938).

Later interpretations and summaries of drift bottle data by Day
(1958), Bumpus and Lauzier (1965), and Bumpus (1973, 1976) added some
detail to earlier descriptions of circulation and demonstrated marked
seasonal and annual differences in circulation patterns. These studies
showed that Bigelow's (1927) July-August pattern of a relatively closed
circulation appears to evolve with the seasons as a result of vernal
warming and the consequent formation of density structure. The residual
circulation on Georges Bank, as measured using long-term moored current
meter, aircraft-tracked surface drifter, and satellite-tracked drogue
observations made between 1975 and 1979, has been summarized by Butman
et al. (1982). These recent direct observations evidence a
quasipermanent clockwise circulation around Georges Bank, but indicate
that the circulation is not completely closed and that considerable
variability occurs in the trajectory of an individual water particle.

Several points should be kept in mind regarding inferences
concerning residual drift based on drift bottle studies which in large
part form the basis of our interpretation of circulation in the Gulf of
Maine-Georges Bank area: (1) Because pronounced seasonal and annual
variations are observed in oceanographic and meteorological variables
associated with circulation, it is reasonable to expect pronounced
seasonal and annual cycles in the velocity and pattern of circulation.
(2) Intermittent flow, which can exceed average flow by a factor of 10
or more, is a fundamental characteristic of continental shelf
circulation, so that the "average" can be expected to occur
infrequently, if at all (Hansen 1977). (3) The time between release
and recovery is the maximum that any bottle would have drifted at sea.
The speed of drift based on this period is a minimum value which may be
much smaller than the actual speed of drift (Waldichuck 1963).

(4) Interpretation of drift bottle data is understandably biased very
strongly by those bottles that happen to move shoreward and thereby are
recovered. For example, of 14,444 bottles released between 1948 and
1962, inclusive, in the Georges Bank area (40°N-41°N, 66°-68°W), only
423 (2.9%) were recovered and 17% of these were recovered from overseas
(Bumpus and Lauzier 1965). Peak recovery was in the spring (March-May),
but this amounted to only 5.5% of those released. At other seasons, the
recovery rate was 1% or less with the greater proportion of bottles
being recovered from overseas. This offshore flow from Georges Bank is
not readily discernible in the quarterly charts of circulation [Plates 7

Preceding page blank |



34

— SHELF/ SLOPE FRONT
——GULF STREAM NO. WALL
@—e BUOY TRACK

38°

300

Figure 14.

70°

T !
68° 66°

Trajectory of buoy 703 during the period 17-28 October 1979
and the mean position of warm-core ring 79I and the thermal
fronts as indicated by satellite imagery for the period 21-
27 October 1979.


DROGUE 304 DROGUE 234 DROGUE 227 NANTUCKET
rene vevocity Tene vetociry ewe vetociry wind vELoeiry
Te Temsaned ne (en/aee) er (ensane) Cenors)
a8 88808385 38838 0838 38 Bo 8

W
coat

uvaa

Ava

3 Ean 3
oroue 202 pRosue 132 DROGUE 34 NANTUCKET
sessssousss 538358 088 Sos
be & at
& iB 4

|

Ava uVvaA

Mid uk.

L-.

Figure 2.  Stackplot of daily Nantucket Island wind vectors and buoy
velocity and temperature (Experiment I).


39

“ ges
2 g
2 % 3 &
iw
eK are
Ww
BS x 20S
530 2
EZ
Z<
{iu
z2
2
ive)
—_
<x
{e)
x
Yn
Yn
(ee)
0)
[a4
e)
w 8
— a — a

Figure 19. Progressive vector plots of Nantucket Island and Georges
Shoals daily resultant winds, April 1951.


L soe

| age

L 36°

Laz

L aoe

| age

Figure 1. Tracks of buoys deployed during 13-29 December 1978
(Experiment 1). Dashed lines indicate missing daily
positions.


Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

14,

17.

18.

19.

20.

21.

22.

viii

LIST OF FIGURES (Cont.)

Trajectory of buoy 703 during the period

17-28 October 1979 and the mean position of warm-
core ring 791 and the thermal fronts as

indicated by satellite imagery for the period

21-27 October 1979. .csesececcccceccerecccesscsssesendd

Comparison of the distibutions of larval Atlantic

cod (Gadus morhua) and sand lance (Anmodytes sp.)

during combined Albatross IV Cruise 79-03 and

Delaware II Cruise 79-04 (mid-date, Day
109), and Delaware Delawarell Cruise 79-05 (mid-,
date, Day 140). ccccccccccvercccccncccnsccccecseses ee dd

Comparison of the distributions of larval hake
(Urophycis sp.) during Belogorsk Cruise 79-01
(mid-date, Day 234) and Albatross IV Cruise 79-11
(mid-date, Day 288). .cccccccccvcceccessseescsseeseeed6
Comparison of the distributions of larval silver

hake (Merluccius bilinearis) and redfish (Sebastes
sp.) during Belogorsk Cruise 79-01

(mid-date, Day 234) and Albatross IV Cruise 79-11
{mid-date, Day 288)......

Pree rere reer esi

Components of monthly resultant wind velocity at
Nantucket Island and Georges Shoals. Positive
components are to the east and north...cccceeeereeeed8

Progressive vector plots of Nantucket Island
and Georges Shoals daily resultant winds,
APril 195 Liveccccccvcccccvcccvevceevecsscsscsseese se d9

Comparison of Nantucket Island and Georges Shoals
wind directions. Dashed line has a slope of

Comparison of Nantucket Island and Georges Shoals
wind speeds. Dashed line has a slope of unity......41

Comparison of progressive vector plots of daily
Nantucket Island, Georges Shoals, and predicted
Georges Bank winds, January 1959 and August 1959....42


12

observed during these surveys appears to be due to other causes
(predation, starvation?) rather than to the transport of larvae to
extrinsic environs.

SUMMARY AND CONCLUSIONS
Summary

1. Buoy trajectories substantiate previous evidence of a
seasonal development of the Georges Bank gyre and accordingly a gradual
increase in surface water residence times from winter to late summer.

2. Once the buoys exited the continental shelf (>200 m) to the
south, they remained in slope water or were entrained in the Gulf
Stream, while occasionally buoys made brief excursions into the Gulf of
Maine before reentering the northern edge of Georges Bank.

3. The shelf/slope front and the Gulf Stream north wall were
furthest offshore during Experiment I at which time all buoys moved
rapidly south and off Georges Bank, and were closest to the 200-m
isobath during Experiments III and IV when the buoys remained over the
continental shelf for the longest time periods.

4. Comparisons of buoy, wind, and wind-driven current
velocities revealed little correlation between buoy movement and wind.
This dissimilarity was most apparent during periods when the buoys were
in the proximity of the shelf/slope front or entrained in slope or Gulf
Stream waters. The contribution of wind to seasonal circulation
appeared to be negligible compared to that of tide, density field, and
offshore forcing.

5. The movement of buoys off the continental shelf was often
associated with the entrainment of shelf water generated by warm-core
rings or Gulf Stream meanders, while at other times rapid and direct
buoy movement off the shelf was not associated with any apparent warm-
core ring activity or fluctuations in the locus of shelf/slope or Gulf
Stream surface temperature fronts. Surface temperatures measured by the
buoys and buoy velocities often changed abruptly indicating that at
times buoys were moving across thermal fronts rather than being
entrained in a specific parcel of water.

6. Shifts in distributions and decreases in numbers of larval
fish cohorts were not obviously associated with any pronounced residual
drift patterns evidenced by buoy trajectories or in previous studies of
circulation. In no case was there sound evidence of a link between
residual drift and the transport of larvae. Estimated larval mortality
rates were of the same order of magnitude during periods of low and high
residence times of Georges Bank surface waters and over Georges Bank as
well as in areas to the north and west.


40°.

2 2 2
& 3S
3 = 3
T T T 7
‘ = Fs
Pag
Zz Zz |°
° w w
= =
4 a
o & 4
a @
x 4
wo o
&
s ak
b 2
R qr
4
fs
H gH
e 85 °
eh do
8 & 8G 18
2 Ne
&
in 8c
a Sa 4
x x x
if Sw
z
° . >
% 4
2 ee)
t 2
RE 4°
. on ‘
L L Ll s 4
2 2 2
iJ oO nN
~ < ~

Figure 9. Estimated residence times in days of Georges Bank surface
waters.


30

w }
g

BUOY TRAJECTORY

Figure 10. Buoy 34 trajectory and wind-driven-current progressive
vector plot, 14 December 1978 (Day 348)-30 January 1979 (Day
30).


18

RICHARDSON, P. L., J. J. WHEAT and D. BENNETT.
1979. Free-drifting buoy trajectories in the Gulf Stream system.
Woods Hole Oceanographic Institution Technical Report WHOI-79-4,
159 p.

SHERMAN, K.

1980. MARMAP, a fisheries ecosystem study in the NW Atlantic:
Fluctuations in ichthyoplankton-zooplankton components and their
potential impact on the system. pp. 9-37. In F. P. Diemer, F. J.
Vernberg, and D. Z. Mirkes (eds.) Advanced concepts in ocean
measurements for marine biology. Belle W. Baruch Institute for
Marine Biology and Coastal Research. University of South Carolina
Press.

SMITH, P. E. and S. L. RICHARDSON.
1977. Standard techniques for pelagic fish egg and larvae
surveys. FAO Fish. Tech. Paper, No. 175, 100 p.

WALDICHUCK, M.
1963. Drift bottle observation in the Strait of Georgia and its
contiguous waters, and off the west coast of Vancouver Island.
Fish, Res. Board. Can. MS Rep. Series (Oceanogr. and Limnol.) No.
147, 104 p.

WALFORD, L. A.
1938. Effect on currents on the distribution of the eggs and larvae

of the haddock (Melanogrammus aeglefinus) on Georges Bank. Bull.
49 23} 73.

U. S. Bur. Fish.




37

72°

74°
po 4

72°
h

44°

BELOGORSK 79-01

ard

40°

ALBATROSS IY 79-11 ¢

M, BILINEARIS — 11-15mm

4a

M. BILINEARIS = 6-10 mm

16-20mm

SEBASTES SP. 1-5 mm

SEBASTES SP. N-15 mm |

4at7
NO./10m? Yr
| 1-3 |

3-10
10-30

42-4 30-100 L aoe
100-300
300-1,000

. SEBASTES SP.

6-10 mm} 7

~ °F 200m”
SEBASTES SP. 16-20mm|_

70°

- T ——T—T
7 72 68° 66°

ee ne

TTT T
7a 72° 70° 68° oor

Figure 17. Comparison of the distributions of larval silver hake

(Merluccius bilinearis) and redfish (Sebastes sp.) during
Belogorsk Cruise 79-01 (mid-date, Day 234) and Albatross IV
Cruise 79-11 (mid-date, Day 288).


35

74" 72 70° 68° oor 70° 72
L L L

70°
po yt L

44°

42-4

40-4

| DELAWARE IL 79-04 /

ALBATROSS IZ 79-03 DELAWARE IL 79-05

GADUS MORHUA 6-10 mm {|

4a

GADUS MORHUA 1-5 mm

4a

AMMODYTES SP 6-10mm

ea

AMMODYTES SP 11-15mm AMMODYTES SP 16-20mm|_

ee 200M pao"
2 “ AMMODYTES SP 20-25mm |
a a a a ae i, er

Figure 15. Comparison of the distributions of larval Atlantic cod

(Gadus morhua) and sand lance (Ammodytes sp.) during
combined Albatross IV Cruise 79-03 and Delaware II Cruise
79-04 (mid-date, Day 109), and Delaware II Cruise 79-05
(mid-date, Day 140).


described by Smith and Richardson (1977) and Posgay and Marak (1980).
For this study, we have used collections obtained from 0.505-mm mesh
nets.

The larval fishes were grouped by 5-mm length classes, and cohorts
from contiguous cruises were coupled on a basis of reported growth rates
of ca. 0.14-0.25 mm/day for haddock (Walford 1938; Laurence 1974),
Atlantic cod (Laurence et al. 1981), Atlantic herring (Boyar et al.
1973; Lough et al. 1980), and redfish (Anderson and Akenhead 1981). We
used an all-inclusive growth rate of 5.0 mm/month for all species and
developmental stages.

RESULTS

Buoy Trajectories and Residence Times

Experiment 1

Six buoys were deployed on Georges Bank during 13-29 December
1978. The tracks of these buoys (Figure 1) have been described in
detail by EG&G (1979a). Briefly, al] buoys initially followed clockwise
trajectories. Three of the buoys (34, 202, and 304) then moved
directly off the Bank, across the slope water zone, and were entrained
in easterly moving Gulf Stream water. After crossing the shelf break
(200-m contour), Buoys 132, 227, and 234 were carried southwesterly in
slope water before being retrieved (Buoy 234) or entrained by the Gulf
Stream in the vicinity of Cape Hatteras. Daily buoy velocities and
temperatures and Nantucket Island wind vectors during the course of this
experiment are shown in Figure 2. Marked variations in buoy velocities
and abrupt changes in surface temperatures coincided with times when the
buoys were moving from one water mass to another.

Estimated residence times for Georges Bank surface waters based on
observed buoy trajectories are summarized in Figure 9. Georges Bank was
delineated by the 200-m contour to the north, east, and south, and by
meridian 71°W. Residence times were based on the time in days for buoys
to reach these arbitrary boundaries from various locations along their
trajectories. Residence times were least (<5 days) along the eastern
and southern edges of Georges Bank and greatest (>30 days) in the Great
South Channel area.

Experiment _II

Five buoys were deployed on Georges Bank during 23-28 March 1979.
Buoy 416, the most southern release (40°56'N, 68°09'W), stopped
transmitting 1 day after deployment. A second buoy (332) was retrieved
by a fisherman on 21 May 1979. The tracks of the operational buoys
(Figure 3) have been described in detail by EG&G (1979b). Buoy 421,
deployed off the Northeast Peak, moved rapidly north into the Gulf of
Maine. Buoy 405, launched in the Great South Channel, moved northeast
and isobath parallel for about 35 days before moving north into the Gulf
of Maine. Buoy 510, deployed at the 200-m contour north of Georges


Daily buoy velocities, water temperatures, and Nantucket Island
wind vectors during the course of this experiment are shown in Figure
6. Estimated residence times of Georges Bank surface waters based on
observed buoy trajectories are summarized in Figure 9. In contrast to
Experiment I, residence times were least (<10 days) to the west and
northwest of Georges Bank and greatest (approximately 70 days) along the
eastern edge of the Bank.

Experiment _IV

Four buoys were deployed on Georges Bank during 15-19 August
1979, The tracks of these buoys (Figure 7) have been described in
detail by EG&G (1980). Buoy 735, which was deployed at the 100-m
contour on the north flank of central Georges, was retrieved on the
Northeast Peak by a fisherman on 26 August after having moved east and
southeast. Buoy 620, which was deployed on the northwest corner of the
Bank, moved similarly to Buoy 735 and then completed a clockwise loop
around the Bank, including a counterclockwise excursion into the Gulf of
Maine north of the Great South Channel before reentering the Bank and
following its original easterly path along the northern edge and exiting
the Bank via the Northeast Channel.

Buoy 727, which was deployed on the northeast corner of Georges
Shoals (41°47'N, 67°00'W), drifted slowly to the southwest corner of
Georges Shoals before moving to the northeast edge of Georges Bank
(41°30'N, 68°30'W). Satellite positions were not obtained from Day 278
to Day 291, but apparently Buoy 727 then made a clockwise circuit around
Georges Shoals before exiting the continental shelf south of Marthas
Vineyard (70°30'W). Buoy 703, which was deployed just south of Georges
Shoals (41°00'N, 67°40'W) apparently made a clockwise circuit of the
Bank, although estimated buoy positions between Day 250 and Day 260
indicate a direct northeast track across Georges Shoals to the Northeast
Peak. From this position, Buoy 703 moved in southwesterly direction and
exited the shelf at 67°25'W. Consequent movement was across the slope-
water zone and entrainment in the easterly flowing Gulf Stream.

Daily buoy velocities and temperatures and Nantucket Island wind
vectors during the course of this experiment are shown in Figure 8.
Estimated residence times of Georges Bank surface waters based on
observed buoy trajectories are summarized in Figure 9. Residence times
were greatest (>100 days) over east central Georges Bank (reflecting the
tack of Buoy 727) and least (<5 days) around the periphery of the Bank
and along the shelf edge south of Nantucket Shoals.

Effect of Wind on Buoy Trajectories

Visual inspection of buoy and wind velocity stack plots reveals
little correlation between buoy movement and Nantucket Island winds.
This dissimilarity in wind and current velocity patterns was most
apparent during periods when the buoys were in the proximity of the
shelf/slope front or entrained in slope or Gulf Stream waters. To
further elucidate the relation of wind-driven current to buoy drift, we


ABSTRACT

Our conception of surface circulation in the Georges Bank area and
its effect on the transport and survival of resident ichthyoplankton
populations is reexamined, using trajectories of satellite-tracked buoys
and water mass distributions inferred from satellite infrared imagery.
Buoy trajectories evidenced a seasonal development of the Georges Bank
gyre and a gradual increase in surface water residence times from winter
(maximum 35 days) to late summery (>100 days)..7Comparisons of buoy,
wind, and wind-driven current veld¢ities revealed little correlation
between buoy movement and wind. The movement of buoys off the shelf was
often associated with warm-core ring or Guif Stream meander-induced
shelf water entrainment; while at other times rapid and direct cross-
shelf movement was not associated with these features.

“shifts in distributions and decreases in numbers of larval fish
cohorts were not associated with any pronounced residual drift patterns
indicated by buoy trajectories, and in no case did advective processes
appear to be linked to larval fish mortality estimates... |

a

In the Appendix, a comparison of Nantucket Island and Georges
Shoals wind velocity data is made to determine the feasibility of using
wind adjustment factors to extrapolate Nantucket Island wind
observations to Georges Bank. The wind statistics indicate that speed,
but not direction adjustment factors, are required for these
extrapolations. Examples of applying wind adjustment factors are given
and their utility is discussed.

|Preceding page blank |


28

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VELOCITY
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225 250 275 300 325 350

YEAR DAY

Figure 8. Stackplot of daily Nantucket Island wind vectors and buoy
velocity and temperature (Experiment IV).


16

COLTON, J. B., JR. and R. F. TEMPLE.
1961. The enigma of Georges Bank spawning. Limnol. Oceanogr.
6:280-291.

DAY, G. G.
"1958, Surface circulation in the Gulf of Maine as deduced from
drift bottles. U. S. Fish. Wildl. Serv. Fish. Bull. 58:443-472,

DIXON, W. J.
1950. Analysis of extreme values. Ann. Math. Statist. 21:488-506.

DUNCAN, D. B.
1975, "t" tests and intervals for comparisons suggested by the
data. Biometrics 31:339-359.

EG&G.
1979a. Appendix A of the Tenth Quarterly Progress Report,
NEOCSPOP. EG&G Environmental Consultants, Waltham, MA, May 1979,
32 p.

1979b. Appendix A of the Eleventh Quarterly Progress Report,
NEOCSPOP. EG&G Environmental Consultants, Waltham, MA, August
1979, 27 p.

1979c. Appendix A of the Twelfth Quarterly Progress Report,
NEOCSPOP. EG&G Environmental Consultants, Waltham, MA, December

1979, 31 p.

1980. Appendix A of the Thirteenth Quarterly Progress Report,
NEOCSPOP. EG&G Environmental Consultants, Waltham, MA, July 1980,
30 p.

FITZGERALD, J. L. and J. L. CHAMBERLIN,
1981. Anticyclonic warm core Gulf Stream eddies off the
northeastern United States in 1979. Ann. Biol. Cons. int. Explor.
Mer 36 36:44-51.

GODSHALL, F. A., R. G. WILLIAMS, J. M. BISHOP, F. EVERDALE and S. W.
FEHLER.

1980. A climatological and oceanographic analysis of the Georges
Bank region of the outer continental shelf. Final Rep. to the
Bureau of Land Management, U. S. Dept. of Comm., NOAA, Washington,
D. C., 290 p.

HAIGHT, F. J.
1942. Coastal currents along the Atlantic coast of the United
States. U. S. Dept. of Comm., Coast and Geodetic Survey, Spec.
Publ. No. 230, 73 p.

HAN, G. and T. NIEDRAUER.
1981. Hydrographic observations and mixing processes in the New
York Bight, 1975-1977. Limnol. Oceanogr. 26:1126-1141.




IS. WIND DIRECTION

NANTUCKET

40

0+4——_—_—

TT 1 TT en ee ee
0 40 80 120 160 200 240 280 320 36C

GEORGES SHOALS WIND DIRECTION

Figure 20. Comparison of Nantucket Island and Georges Shoals wind
directions. Dashed line has a slope of unity.


24

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é q
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Figure 4, Stackplot of daily Nantucket Island wind vectors and buoy
velocity and temperature (Experiment III).


33

66° 64° 62°
| 1 { \ L |
ae - _- 200M ~---~
oe st > a
NO. EAST + -
_ .7 00M
CHANNEL gos 2
42° 42°
GEORGES 7306 318 295
BAN ke aoa e324
40°44 40°
— SHELF/SLOPE FRONT
m= GULF STREAM NO. WALL
-——~ BUOY TRACK
3°. &—a# WARM CORE RING TRACK 38°
I T T T T
66° 64° 62°

Figure 13. Trajectory of buoy 620 during the period 20 October-6
December 1979 in relation to the location of warm-core rings
79G and 79H and the mean position of thermal fronts as
indicated by satellite imagery for the period 1-7 November
1979,


19

APPENDIX:
COMPARISON OF WIND REGIMES AT NANTUCKET ISLAND AND GEORGES BANK

Introduction

To determine the feasibility of using wind direction and speed
adjustment factors to extrapolate the long time series of Nantucket
Island wind observations to Georges Bank, we compare here Nantucket
Island (41°15'N, 70°04'W) and Georges Shoal Texas Tower (41°41'N,
67°47'W) wind data during the period April 1958-October 1961, an
interval in which hourly wind observations (24/day) were taken at both
locations. These data, reduced to daily and monthly resultant (vector
mean) wind speeds and directions were obtained from the National
Climatic Center in Asheville, N. C.

Wind Regime Comparisons

Nantucket Island and Georges Shoals monthly and annual wind
directions and speeds, Georges Shoals/Nantucket Island wind speed
ratios, and annual wind speed and direction correlation coefficients and
variance ratios are given in Table 2. Zonal and meridional components
of monthly resultant wind velocities at the two locations are plotted in
Figure 18. Although wind speeds were greater at Georges Shoals, both
the high correlation coefficients and the similarity of the wind
component cycles substantiate the similarity between wind conditions at
the two locations.

Duncan's Multiple Range Test (Duncan 1975) at a level of
significance of 0.05 was used to test the significance of the
differences in wind speed and direction at the two locations. With one
notable exception (April 1959), wind direction at the two locations was
from the same. sectors during all months and years. In April 1959, the
monthly resultant wind at Nantucket Island was from the west (291°)
while on Georges Shoals it was from the east (094°). Progressive vector
plots of April 1959 daily resultant winds (Figure 19) show that this
aberrence was due for the most part to appreciably higher Georges Shoals
wind speeds (notably, Days 5-8, 23, 26, 28, and 29) rather than to
marked differences in wind direction. Although the April 1959 deviation
in wind direction at the two locations appears to be real, a test for
outliers (Dixon 1950) indicates that we can omit these values from our
computation.

With few exceptions (April and November 1959, and November 1960),
monthly wind speeds were greater at Georges Shoals. These wind speed
differences were statistically significant in 1961 and for total
years. Although there is little reason for considering a wind data
variate at one location as dependent on that at another location, we
have used linear regression rather than correlation analysis for
determining the relation of wind speed and direction values at the two
locations since the regression equation allows for simple calculation of
adjustment factors. The regression coefficient (1.003) for wind


altimetry. Satellite coverage has proved to be particularly useful for
monitoring surface temperatures, position of ocean fronts and eddies,
and large-scale water mass movements. Gulf Stream rings and meanders
are ubiquitous features of these data, and as a result, we now have a
much better understanding and appreciation of the magnitude, complexity,
frequency, and causes of coastal and oceanic water mass exchange. This
imagery has been especially informative regarding the frequency and
extent of shelf water entrainment from the western Scotian Shelf and
along the southern edge of Georges Bank generated by the presence of
warm-core rings and Gulf Stream meanders.

In this paper, we attempt to update and refine our conception of
surface circulation in the Georges Bank area and its probable effect on
the drift and survival of resident ichthyoplankton populations on a
basis of these recent and innovative monitoring efforts. Our main
sources of information, in addition to satellite imagery, were seasonal
observations of near-surface circulation, based on the trajectories of
satellite-tracked buoys and the distribution of larval fishes during
roughly similar time periods. We emphasize here that the two sets of
data were not obtained concurrently or with the objectives of this study
jin mind, but our interest in the problem prompted us to examine this
additional information in hopes of elucidating the role of residual
drift on larval transport and survival.

DATA SOURCES
Satellite-Tracked Buoy Experiments

Drogued buoys were deployed on Georges Bank during four discrete
Lagrangian experiments commencing in December 1978 and March, May, and
August 1979 as part of the New England Outer Continental Shelf Physical
Oceanography Program sponsored by the U.S. Bureau of Land Management.
Each experiment incorporated 4-6 buoys and lasted 8-10 weeks. The field
work and instrumentation were provided by Raytheon Ocean Systems Company
and data processing and analysis by EG&G Environmental Consultants. The
COSRAMS buoys used in these studies were manufactured by Polar Research
Corp. and were fitted with 2 x 1l-m window shade drogues centered at a
depth of 11 m, and with drogue tension and temperature sensors.
Positions accurate to a few kilometers (Richardson, Wheat, and Bennett
1979) were determined from the Doppler shift of the signals received by
Nimbus 6 satellite. Position errors were further reduced by eliminating
suspect fixes and through the use of a cubic spline function to compute
buoy positions and velocities evenly spaced in time. Buoy
specifications and data editing and processing are given by EG&G
(1979a,b,c; 1980) and Richardson, Wheat, and Bennett (1979). Computer
outputs of edited data (four evenly spaced daily positions and
temperatures provided by EG&G Environmental Consultants) were used to
compute the daily mean buoy positions and water temperatures utilized in
this study. Dates of buoy positions are given in terms of the
consecutive day of the year (Julian Day). A table is provided (Table 1)
to convert these numbers to day of the month.


NANTUCKET IS. WIND SPEED (kts)

41

a
{

z

10>

@
!

Oe a I SO Sn SS

T T T +
0 2° 4 6 8 10 12 14
GEORGES. SHOALS WIND SPEED (kts)

—

Figure 21. Comparison of Nantucket Island and Georges Shoals wind
speeds. Dashed line has a slope. of unity.


Table 1.. The consecutive number. of each day of the year.

Day Day
of of
month Jan. Feb.? Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec. Month
1 1 32 60 91 121 152 182 213 244 274 305 335 1
2 2 33 61 92 122 153 183 214 245 275 306 336 2
3 3 34 62 93 123 154 184 215 246 276 307 337 3
4 4 35 63 94 124 155 185 216 247 277 308 338 4
5 5 36 64 95 125 156 186 217 248 278 309 339 5
6 6 37 65 96 126 157 187 218 249 279 310 340 6
7 7 38 66 97 127 158 188 219 250 280 311 341 7
8 8 39 67 98 128 159 189 220 251 281 312 342 8
9 9 40 68 99 129 160 «6190 221 252 282 313 343 9
10 10 41 69 100 «130 161 191 222 253 283 314 344 © «10
ll ll 42 70 101 131 162 192 223 254 284 315 345 11
12 12 43 71 102-132 «163 «193 224 255) 285) 316 346 «12
13 13 44 72 103 133 164 «6194 «6225 «256 «= 286 317 347s 13
14 4 45 73 104 «134 165 «6195 226 «6257: 287) 318) 3484
15 15 46 74 105 «135 «166 «©6196 §6227) 258) 288) 319) 349) 15
16 16 47 75 106 «136 167) «197 «228 «= 259° 289 320 350s 16
7 17 48 76 107, 137: 168) =-198 Ss 229° 260) 290) 321 35117
18 18 49 77 108 $138 169 199 230 261 291 322 352 18
19 19 50 78 109 139: 170 «200 231) 262) 292) 323. 35319
20 20 $1 79 110. «140 «171 «©6201 =. 232, «263. 293 324) 354 20
ra 21 52 80 111-141: «1720 202) 233, 264 294) 325 355 21
22 22 53 81 112-142 «173° 203) 234, 265) 295. 326 356 22
23 23 54 82 113° 143: «174 204 235. 266) 296 = 327, 357) 28
24 24 55 83 114 «144 175) 205) 2236S 267) = 297) 328 358d
25 25 56 84 115° 145 «176 «©6206 )=— 237, 268 Ss 298) 3329) 359 25
26 26 57 85 116 «146 177° 207) 238 «6269 «299 3330 360s 26
27 27 58 86 117.147: «178 208) 239. 270s 3300s 3331S 361 27
28 28 59 87 118 148 «179 «209 240) 271) 301 332) 362) 28
29 29 * 88 119-149 180) «210 241.272) 302, 3333) 363) 29
30 30 89 120 150 181 211 242 273 303 334 364 © 30
31 31 90 151 212 = 243 304 365 31

4 In leap years, after 28 February, add 1 to the tabulated number.


Shoals, completed a counterclockwise loop into the Gulf of Maine before
reentering the northern edge of Georges Bank and moving isobath parallel
to the east and then south crossing the 200-m contour at approximately
66°W. This buoy then completed two counterclockwise loops before moving
east and then north into the Northeast Channel. Buoy 332 moved in a
clockwise direction, but remained in the Georges Shoals area and within
60 km of its deployment position before being retrieved 54 days later.
As in Experiment I, marked variations in buoy velocity and water
temperatures were associated with different water mass regimes.

Daily buoy velocities, temperatures, and Nantucket Island wind
vectors during the course of this experiment are shown in Figure 4. The
estimate of residence times of Georges Bank surface waters was not
particularly instructive as the March deployment was biased to the north
and not amenable to such an interpretation.

Experiment III

Five buoys were deployed in the Georges Bank-Great South Channel
area during 30 May-2 June 1979. A detailed summary of the tracks
followed by these buoys is given by EG&G (1979c). Buoy 556 stopped
transmitting on 26 July, Buoy 234 was picked up by a fishing boat on 6
August, and Buoy 620 was retrieved by EG&G on 14 July. The buoy
trajectories in this experiment (Figure 5) agree with the generally
accepted circulation pattern in that they evidence a clockwise gyre over
Georges Bank, a return (northerly) flow along the eastern side, and an
outflow (southerly) along the western side of the Great South Channel
before turning west around Nantucket Shoals. In addition, the
trajectories of Buoy 556 and 433 indicate a westerly drift along the
southern side of Georges Bank and across the Great South Channel before
turning northwesterly towards Nantucket Shoals and Marthas Vineyard,
then westerly along the continental shelf off Southern New England.

Buoy 620, which was deployed on the western side of the Great South
Channel, moved south and then west and northwest around Nantucket Shoals
before turning southwest to the point of recovery south of Long Island
(40°24'N, 72°40'W). Buoy 616, which was released on the east side of
the Great South Channel, moved north to approximately 42°N in 30 days
and then into the Gulf of Maine where it followed a complex gyral drift
pattern with a net northward and then southward flow before turning east
and reentering the northern edge of Georges Bank at approximately
66°30'W. Buoy 556, which was launched over north-central Georges Bank,
moved southwesterly across Georges Shoals and then westerly parallel to
the bathymetry crossing the base of the Great South Channel and then
northwesterly onto Nantucket Shoals. Buoy 443, deployed on eastern
Georges, moved southwesterly and then westerly to 72°W before turning
south and crossing the 200-m contour at which time it moved to the east
for approximately 20 days before moving south into slope water. Buoy
234, which was released on Georges Shoals, followed a clockwise path
along the 50-m isobath before being retrieved by a fisherman
approximately 85 km northeast of its deployment position.