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J Northw. Atl. Fish. Sci., Vol. 1: 91-99

The MARMAP Bongo Zooplankton Samplers
J, A. Posgay
National Marine Fisheries Service, Northeast Fisheries Center
Woods Hole, Massachusetts 02543, USA
and
R. R, MarakNational Marine Fisheries Service, Northeast Fisheries Center
Narragansett, Rhode Island 02882, USA

Abstract
Two bongo zooplankton samplers are described and the methods of construction given. Each sampler consists of a pair of nets
mounted so that the mouth openings of the rigid body sections are on either side and in front of the towing wire. The results of tests
comparing the bongo samplers with a 1-m ring net and the Gulf III encased sampler indicate that the bongo samplers give a more
representative sample of the ichthyoplankton community than either of the other two types. The results of testing the bongo samplers
for filtration efficiency and the effects of different mouth areas, towing distances and towing speeds show that none of these factors
had a significant effect upon the catch per volume of water filtered.

Introduction

Description of the Samplers

The zooplankton samplers described in this paper
were developed primarily to collect pelagic eggs andlarvae of fishes and their forage organisms inhabiting
the upper 200 m of water over the continental shelf of
the Northwest Atlantic, The fisheries of this area
exhibit great variation in the annual recruitment ofmany species, seemingly caused by variation in
mortality during the very early life stages of the fishes.
In order to study these variations in mortality rates andtheir causes, a better sampler than those customarily
used in this type of work was needed. It was obviousthat such a study would require the use of many ships,
probably from several nations, over several years.
Sampling would have to be done year-round in all
kinds of weather. A standard quantitative sampler wasneeded that was simple, sturdy, reliable, efficient, andpreferably inexpensive, and that could be deployedfrom small vessels used in coastal waters as well asfrom large offshore research vessels.

The tests and trials described below wereconvincing enough to indicate that the bongo design
was better than either the traditional ring-net or the
more modern high-speed encased net. It was also
considered that two sizes would be useful: a rather
large sampler with fairly coarse mesh to filter a large
volume of water without using excessive ship time tocollect fish larvae which may be scarce at certain times
in certain places, and a smaller sampler of fairly finemesh to collect forage organisms. The sizes adopted
for use, 20 cm and 61 cm inside diameters, gaveroughly a 9:1 ratio to test the effects of mouth area oncatches, and construction materials for these sizes
happened to be conveniently available.

The samplers described here were inspired by one
developed by McGowan and Brown (MS 1966).
Departure from their design has been considerable but
the basic principle of a pair of nets with mouth
openings on either side of and in front of the towing
wire remains the same. They called their sampler the"bongo net" and that name has been retained for thosedescribed here.
MAR MAP is an acronym for Marine Resources
Monitoring, Assessment, and Prediction program of
the USA National Marine Fisheries Service. The
MAR MAP program and its international cooperators
have adopted our versions of the bongo net (Fig. 1) as
their standard zooplankton samplers. Smith andRichardson (1977) in a recent FAO publication
recommended the bongo configuration as the best
type of gear for ichthyoplankton surveys.

The 20-cm MAR MAP bongo
The bodies of this sampler (Fig. 2) were made ofcommercially available polyvinyl chloride (PVC) pipe,
nominally 8 inches (20 cm) in diameter with wall
thickness of 5/16 inches (8 mm). The pipe was cut into
30-cm lengths which were turned in a lathe to square
the ends, round the leading and trailing edges, andmake a groove 32 mm wide and 3 mm deep near the
trailing edge. This groove is used to fasten the collar of
the net to the sampler body with a stainless steel hose
clamp. Actual measurement of 10 sampler bodies, after
construction, gave an average mouth area of 0.0314 rn-.
The towing yoke (Fig. 2), used to fasten thesampler to the towing wire and support the two bodies,is made of Type 304L stainless steel flatbar bent andwelded to shape. The cylindrical wire fastener is a
modified swivel-top messenger. The slot in thefastener is a loose fit on 1/4-inch (6mm) wire,
permitting the sampler to rotate in the horizontal plane.
The trunnion pins permit rotation in the vertical plane

J. Northw. Atl. Fish. Sci., Vol. 1, 1980

92

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VIEW OFATTACHMENT

FOR LARGE

BONGOS

Fig. 1. The MAR MAP bongo sampler array.

so that the sampler always faces straight ahead whenbeing towed. A wire-stop is used below the sampler tosupport it on the towing wire. Stops at various
positions along the towing wire allow a series ofsamplers to be used for simultaneous collections at
different depths.
The 61-em MARMAP bongo
The cylindrical bodies of this sampler (Fig. 3) wereconstructed of fiberglass reinforced polyester resin(FRP), 30 cm long with wall thickness of 12 mm. The
inner diameter is 24 inches (61 ern), giving a mouth
area of 0.2923 rn". The leading edge is rounded and thetrailing edge has a 114-inch (6 mm) raised bead, justforward of which the net is fastened by a stainless steel
hose clamp.
The yoke (Fig. 4), also made of Type 304L stainlesssteel, is an axle between two cheek plates riding in a
split sleeve with a nylon or teflon bushing, allowing
orientation in the vertical plane. The two pad-eyes
welded to the lugs of the split sleeve are shackled to thetowing wire with good quality swivels to allow
orientation in the horizontal plane. The two sampler
bodies are bolted to the cheek plates of the yolk withreinforcing plates inside the bodies to better distributethe towing load. The round bar at the rear of the yoke isintended to reduce the bending load on the axle when
the samplers are towed at high speed.

POSGAY and MARAK: MAR MAP Bongo Zooplankton Samplers

93

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Construction details of the 20-cm MARMAP bongo sampler.

Nets and Accessories
The nets used on the bongo samplers are cylindercones (Fig. 1), this construction having been shown(Smith et al., 1968) to be highly resistant to clogging.
The practice has been to make the open area in thegauze of the conical section equal to three times themouth area and to put enough gauze in the cylindricalsection so that the ratio of the total open gauze area tothe mouth area is 8-15:1 depending on the mesh size.
The finer the mesh size used, the larger the net. Meshsizes from 0.1 to 0.5 mm have been used for sampling in
neritic and littoral waters with very few cases ofclogging, most of which were caused by blooms offilamentous algae or dense concentrations of salps or
ctenophores.
The two nets on a sampler need not have exactly
the same mesh size, but the difference cannot be too
great especially on the 20-cm sampler, as the more

rapid accumulation of catch in the net with the smaller
mesh will throw the sampler out of balance to theextent that it ultimately stops sampling. The difference
in mesh size should not be much more than 0.15 mm.
Codend cups of various types (metal, plastic,
glass, canvas, gauze) have been tried at one time oranother, and all have been more or less unsatisfactory.
The weight of most types of cups causes the rear end of
the net to sink at slow speed, and they whip arounddangerously during retrieval in strong winds, Insteadof using codend cups, the best method was to securely
tie the end of the net with twine. The nets are gently
hosed down before being brought on board, washing
most of the catches into the conic part of the codends.
Further hosing on board cleans the forward part of thenets, and the rear part of the nets are carefully washedafter the codends are placed in buckets and untied.
The catch from each net is bottled and preserved after

J. Northw. Atl. Fish. Sci., Vol. 1, 198094

straining through a fine-mesh sieve.

BONGO ASSEMBLY LARGE SAMPLER (24.00 INCH)

Fig.3

Assembly details of the 51-em MARMAP bongo sampler.

Flowmeters of various types to measure thevolume of water filtered have been used both insideand outside the sampler bodies. A satisfactory type offlowmeter used consists of a 6-digit counter encased ina 3.5-cm transparent acrylic sleeve with a brass bulletshaped nose piece. The 7.6-mm diameter rotor isprecision-molded in plastic with helical vanes fastened
directly to the shaft of the counter. The design of therotor vanes is such that they offer minimum resistanceto water flow and shed seaweed strands or other debris
which tend to clog the traditional ducted fan-type
flowmeters. All moving parts are stainless steel orplastic. Although the flowmeters are calibrated by the
manufacturer, further recalibration is carried out in atank on shore and checked against the ship's electromagnetic log at sea.

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Construction details of the yoke of the 51-cm MARMAP bongo sampler.

1.-------6.00·

16375"POSGAY and MARAK: MARMAP Bongo Zooplankton Samplers

A time-depth recorder has been used on most of
the tows, giving a strip-chart recording of the amountof time sampling as a function of depth.
A depressor of some type is required to overcome
the drag of the samplers and to maintain them at thedesired depths. A simple weight of about 45 kg was
found to be sufficient at low speeds, below3 knots. A4ft (1.22-m) V-fin depressor was used at higher speeds.
With both samplers at the end of 270 m of towing wire
(6 mm), the V-fin took the gear down to 160 m at 3
knots, 115 m at5 knots, and 100 m at? knots. Tank tests
have shown the drag of the samplers at the above
speeds to be 100, 350 and 650 kg respectively.

Remarks on Construction
The MARMAP program has adopted the designsof the bongo samplers described above, but the detailsof dimensions and materials are not unique. Anyone
desiring to modify the designs should be careful of afew points. The sampler bodies must be long enoughso that the mouths will always be forward of the towing
wire. The two mouth openings must be in the sameplane and the towing point must be exactly centered or
the sampler will not face precisely in the direction that
it is being towed. The pivot point of the yoke should be
about 25 mm forward of the balance point of thebodies. The samplers will not enter the water smoothly
and tow truly unless they are tail heavy.

95

sampler, but filtration efficiency may be reduced byturbulence inside and outside the sampler. The yoke
can be made of other metals such as steel or bronze,
but care should be taken to avoid using dissimilar
metals so that welds do not become sites of electrolytic
corrosion.
Tranter and Heron (1967) have reported someinteresting tests using sampler bodies shaped liketruncated cones. Their design giving the greatest
filtration efficiency had a cylindrical forward section,
25 cm long and 12.7 cm inner diameter, to which was
attached a conical section 10 cm long which expanded
to an inner diameter of 18 cm. With the net attached to
the rear end of the cone, they reported filtration
efficiency of 1.145 for the net mouth opening. This
amounts to 2.3 times the mouth opening of thesampler.

Filtration Efficiency of the 51-cm Sampler
The filtration efficiency, that is, the volume ofwater accepted by the sampler divided by the volume
of water presented to it, of the 61-cm sampler has beenmeasured at the USA Naval Ship Research andDevelopment Center (Smith, MS 1972). The tests weremade in the high-speed towing basin with the sampler(one side only) and the net mounted on a strut attachedto the towing carriage. A pitot-static pressure probe
was mounted on another strut which passed through aslit in the net, placing the probe approximately 6 mm
ahead of the trailing edge of the sampler body.
Pressure data were taken at velocities from 0.5 to 6.5
knots (25-335 ern/sec) in 0.5 knot increments. The
pressure probe was calibrated by running it in the freestream with the sampler removed.

The dimensions given in Fig. 2, 3 and 4 were takenfrom the construction drawings used by the MARMAP
Program Office to purchase more than 100 units whichhave been distributed to many investigators in the
United States and several other countries. As a result,components are interchangeable and a damaged
sampler can be used as a source of spare parts. There
are no exact metric equivalents of the nuts, bolts, rods
and bars that were used to fabricate the samplers, anda simple mathematic conversion from inches to metricequivalents would not be useful. A completely new"metric model" of the sampler could be designed, if
there were a need, but similar metric components canbe substituted if necessary without changing thefundamental design.

The pressure probe was first placed at the centerof the mouth opening and a complete series of data on
carriage velocity and the corresponding pressure
inside the sampler obtained. The probe was thenlowered 15.2 cm below the center point of the mouth
opening and the series of speed runs repeated.
Successive runs were then made with the probe
located 22.9, 26.7, 28.6, 29.2 and 30.0 cm from thecenter.

PVC and FRP materials were used for the sampler
bodies because these products were readily available,
relatively inexpensive, and strong enough to withstandthe rough handling at sea, but other materials might beequally satisfactory. Even the shape need not be fixed,
as square-shaped bodies were tested in some earlytrials and found to work as well as cylindrical bodies.
Some investigators have constructed similar samplers
using aluminum tubing for the leading and trailingedges with fore and aft spacers and a canvas sleeve
fastened around the frame. This type of construction
may provide a lighter. more manageable, and cheaper

Table 1 gives the results of the measurements
expressed as relative velocity, that is, the observedpressure inside the sampler divided by the free streampressure clear of the sampler, for speeds of 2.5-5.0
knots. The instruments were difficult to readaccurately at speeds less than 2.5 knots, and vibrationinterfered with the readings at speeds greater than 5.0
knots. All of the values, except those taken close to thewall of the sampler, are equal to or greater than 1.0(Table 1), indicating that the sampler actually takes inand filters about 6-7% more water than would be
predicted from the product of mouth area times

J. Northw. Atl. Fish. Sci., Vol. 1, 1980

96

TABLE 1. Relative velocities of water passing through the sampler compared to the speed of the sampler through the water at sevenradial distances from the center of the mouth ooeninq, with mean and standard deviation for each location over a range of
speeds from 2.5 to 5.0 knots.
Relative velocities for 7 radial distances (em)from center of sampler mouth opening

Samplerspeed(knots)

0.00

15.2

22.9

26.7

28.6

29.2

30.0

2.5
3.0
35
4.0
45
5.0

1.083
1.056
1.125
1.063
1.045
1.080

1.083
1.111
1.083
1.063
1.000
1.100

1.000
1.056
1.083
1.094
1023
1.080

1.083
1.000
1.083
1.125
1.091
1.080

1.083
1.056
1.083
1.063
1.023
1.060

1.083
1.111
1.083
1.094
1.000
1.040

1.000
1000
0.958
1.000
0.955
1.040

Mean
S.D.

1.075
0.028

1.073
0.040

1.056
0.037

1.077
0.041

1.061
0.022

1.069
0.041

0.992
0.032

distance towed. It should also be noted that the flow ofwater through the sampler is quite uniform. The
flowmeter, therefore, can be located anywhere it canbe conveniently mounted provided that the rotor is
kept about 50 mm away from the wall of the sampler.

Results of Field Tests
The two sizes of the MAR MAP bongo samplers
have been tested to measure the differences, if any, incatches of port and starboard nets, different mouthareas, different towing speeds, different towing
distances, and any day-night effects. Towing speeds
were 3 and 6 knots (93 and 186 m/min), and distanceswere 1 and 2 nautical miles (1,852 and 3,704 m). All nets
were 0.505 mm nylon mesh cylinder-cones withfiltering ratios of about 8:1 in the 61-cm sampler and15:1 in the 20-cm sampler. An 8.5-cm ducted-Ian
flowmeter (TSK) was mounted in each of the samplers
and another on the outside of the 61-cm sampler. The
flowmeters and the ship's electromagnetic log were
TABLE 2.

calibrated on the Provincetown, MA, measured mile.
The bongo sampler array is shown in Fig. 1.
The sampling area was a 10-mile (18 km) square in
the Gulf of Maine with average water depth of about
100 m. Four sets of eight stations each and thesequence in which the four combinations of speed anddistance were made were randomly selected, two ofeach combination in each eight-station replicate. Two
replicates were done at night and two during the day.
All tows were made horizontally at about 20 rn, using
the same length of wire for each tow.
In additon to the expected plankton organisms, agoosefish (88 cm), a silver hake (32 cm) and three
dogfish (78,62 and 57 cm) were caught, all being taken
at night in the 61-cm sampler at 6 knots; two of the
dogfish were taken in the same tow, one in each net.
Plankton samples were preserved at sea andsubsequently sorted on shore. Seven species of fishlarvae from 2 to 20 mm long were identified andCatches of fish larvae (numbers per 100 m') in the tour-net MAR MAP bongo array for the different combinations of speed(S) in knots and distance (D) in nautical miles. (The symbol ... indicates that the catch was not sorted.)
Day

Night
61-cm

20-cm
S-D

Port

Stbd

6-2
3-2
3-1
6-2
6-1
3-2
3-1
6-1

45
34
53
28

38
23

3-2
6-1
3-2
6-2
3-1
6-1
6-2
3-1

25
712
142
318
99

45
10
434

192
285
101
27
25
76
48
18
52
504

Port

20-cm
S-D

Port

Stbd

33

6-2
3-2
6-2
6-1
3-1
6-1
3-2
3-1

16
76
29
110

13
71

17
80
57
16
912
212
225
76
25
19
54
10
65

61-cm

Stbd

73
59
20
1016

25
21
47
65
62
336

3-1
6-2
6-1
3-1
3-2
6-1
6-2
3-2

154
35
39
35
lost
25
58

137
66
435
95
199

50
40
30
110
55
42

Port

Stbd
9

65
43
56
435
79

32
32
43
43
32
125
38

28
97
90
lost
33
123
47
33

120
48
42

POSGAY and MARAK: MARMAP Bongo Zooplankton Samplers

97

All analyses were made using the catch per unit volume
filtered.

counted (yellowtail flounder, Atlantic cod, fourbearded rockling, silver hake, Atlantic redfish, cunnerand shanny). The catch of one net randomly selectedfrom each tow was not sorted, as indicated in Table 2.

Port and starboard nets
Application of the Wilcoxon test for differences incatches of fish larvae by the port and starboard nets ofboth samplers indicated no significant difference forboth the 20-cm sampler (P = 0.16) and the 61-cm
sampler (P = 0.54).

Catch per tow was converted to catch per 100 rn",using the sampler mouth area and the observed
revolutions of the flowmeters inside the nets as ameasure of the distance towed. Comparison of thesereadings with the flowmeter attached outside thesampler and with the ship's electromagnetic log
showed no evidence of clogging. The 20-cm sampler
filtered about 60 m3 of water through each net during
the 1-mile tows and about 120 m3 during the 2-mile
tows, whereas the 61-cm sampler filtered about 550 m 3
and 1100 m 3 respectively over the same distances. The
numbers of fish larvae caught per 100 m 3 of water
filtered by the nets are given in Table 2.

The 20-cm and 51-cm samplers
The availability of more samples for thesecomparisons enabled the testing of the day and nightsamples separately as well as combined. All tests weremade on a diagonal of the sampler array, comparing
the catch of the port net of one sampler with the
starboard net of the other sampler. There was no
significant difference between the catch per unit
volume of the larger net and that of the smaller net
either during the day (P =0.08) or at night (P =0.86) orfor day and night catches combined (P = 0.25).

Statistical analyses
The traditional parametric tests were notconsidered appropriate for data of the type presented
here, as the number of samples is small, theirdistributions are not normal, and the variances are notindependent of the means. Logarithmictransformations on other data, not reported here, did
not materially improve the situation. Consequently, thefollowing analyses are based on non-parametric(sometimes called distribution-free) statistical tests.
These tests are not particularly new (Bradley, 1968) but
they seem to have been neglected by all but a few
investigators of marine populations. Specifically, theWilcoxon matched-pairs signed-rank test was used tocompare two sets of related samples, and the KruskalWallis test was used to compare two or more sets of
independent samples (Siegel, 1956; Conover, 1971).

Day and night effects
Having confirmed that there was no significant
difference between the port and starboard nets, thecatch rates of fish larvae by the 20-cm starboard net
and the 61-cm port net was taken as being
representative of each tow. In cases where the catch of
the selected net was not sorted, the catch of the othernet of the pair was used. Using the Kruskal-Wallis test,
the four replicate blocks were tested to see if the
population in the area sampled had changed during
the 2 days spent sampling (Table 3). Catch rates
declined somewhat on the second day but theTABLE 3. Results of the Kruskal-Wallis test comparing catches of fish larvae (number per 100 rrr') undervarious factors investigated using the two sizes of bongo sampler. (N = number of tows, ~R = sumof ranks, H = test statistic, and P = probability that differences caused by chance.)
20-cm sampler
FactorN~R

Block 1
Block 2Block 3Block 4

8
8
8
8

132
157
121
118

Night
Day

16
16

253
275

3 kt/l mi
3 kt/2 mi
6 kt/l mi
6 kt/2 mi

8
8
8
8

160
131
138
99

3 knots
6 knots

16
16

1 mile2 miles

16
16

61-cm sampler

P

~R

H

P

1.34

0.72

149
158
116
105

2.77

0.42

0.17

0.69

265
263

0.002

0.96

2.71

0.45

159
127
136
106

2.05

0.57

291
237

1.04

0.31

286
242

0.69

0.42

298
230

1.64

0.20

295
233

1.37

0.25

H

98

J. Northw. Atl. Fish. Sci., Vol. 1, 1980

difference was not significant for either net. Both nets
showed no significant day-night differences.

catch rates of the 2 samplers.

The 20-cm MARMAP bongo and Gulf III
Speed and distance effects
Application of the Kruskal-Wallis test to examine
speed and distance effects showed no significant
difference between the catch rates for the four speeddistance combinations, although both nets took the
fewest larvae during the 6 knot-2 mile tows (Table 3).
Looking at speed of tow only, catch rates were higher
at 3 knots than at 6 knots, but the difference was not
significant for either sampler. The 1-mile tows took
more fish larvae per unit volume than the 2-mile tows,
but again the differences were not significant.

Comparison of Bongo Nets with Other Samplers
Gehringer's bongo and the Working Party-3 net
Jack W. Gehringer (pers. comm., National Marine
Fisheries Service, Washington, D. C.) compared theperformance of the Working Party-3 net (Fraser, 1966)
with a bongo-type sampler which he devised. His
bongo, with mouth area of 0.172 m 2 on each side, wasfitted with a 0.947-mm mesh net on one side and 0.333mm mesh on the other. The Working Party-3 net was a
1-m ring net with 0.947-mm mesh. Both samplers weretowed simultaneously from the port and starboard
sides of the vessel at 3 knots. The nets with calibrated
ducted-fan flowmeters were towed at a depth of about
5 m.
In 27 paired tows, the bongo net with 0.947-mm
mesh filtered 7,632 rn- of water and caught 4,969 fishlarvae (65 per 100 rn-), while Working Party-3 net
filtered 31,773 m 3 of water and caught 8,672 larvae (27per 100 rn-). The catch per unit volume of the bongo
was thus about 2.4 times that of the Working Party-3
net. The Wilcoxon test on the paired tows (Table 4)shows a significant difference (P = 0.005) between the
TABLE 4. Numbers of fish larvae taken per 100 m 3 of water filteredduring 27 simultaneous hauls of a bongo net and a Working
Party-3 ring net (unpublished data from J. W. Gehringer).
Set
no.

Bongo

WP-3

1
2
3
4
5
6
7
8
9
10
11
12
13
14

11.2
349.7
234.9
185.4
225.9
48.3
7.2
6.7
29.3
24.3
90.5
0.7
4.9
2.4

7.7
40.6
155.7
47.1
9.0
6.1
0.3
3.2
33.3
32.6
29.6
0.8
1.5
1.2

Set
no.

Bongo

WP-3

15
16
17
18
19
20
21
22
23
24
25
26
27

1.3
10.1
53.2
0.7
4.6
31.1
153.5
235.2
3.5
18.5
11.4
6.5
3.5

0.3
1.8
23.4
0.5
6.3
36.3
228.8
46.2
2.1
6.4
10.4
1.8
0.6

Mean

65.0

27.2

Sherman and Honey (1968) tested the 20-cm
bongo sampler against the widely used Gulf III
encased sampler (Gehringer, 1952). Ten tows weremade at 6 knots for 30 minutes with both samplers on
the same towing wire about 25 cm apart. The mouth
openings of both samplers were the same (20 cm
diameter) and the nets used (nylon on the bongo, metalon the Gulf III) were 0.366-mm mesh. Calibrated
flowmeters were used to measure the distance towed.
The catches consisted of a mixture ofzooplankters, about 80% copepods. The mean catch of
copepods in the bongo was 14,483 per 100 rn-while theGulf III took only 1,722 per 100 rn". In a later paper,
Sherman and Honey (1971) reported that all of thesignificant differences between the two samplers
occurred among the smaller organisms (median width< 0.38 mm), while there were no significant differences
between samplers in the catches of larger organisms(median width >0.40 mm). In no case was there asignificant difference between the two nets of the
bongo sampler. They concluded that many of thesmaller organisms had been extruded through the tailof the Gulf III by high filtration pressure across themeshes.

The Standard MARMAP Sampling Array
The standard MAR MAP sampling array (Fig. 1)
consists of a 20-cm bongo, a 61-cm bongo, a timedepth recorder (bathykymograph), and either a 12.2 m
V-fin depressor or a 45 kg weight. Flowmeters tomeasure the distance towed and the volume filtered are
fitted to all nets. A wire-stop holds the 20-cm sampler
on the wire 1 m above the 61-cm sampler which isshackled with a swivel to an eye at the end of the towing
wire. The depressor is fastened to a 1-m length of chain
which is shackled with a swivel to the lower pad-eye of
the 61-cm sampler. The time-depth recorder isfastened to the wire between the samplers. Nylon nets
(Nitex) with mesh sizes of 0.253 and 0.333 mm are usedon the 20-cm sampler and mesh sizes of 0.333 and0.505 mm on the 61-cm sampler. Net sizes may bechanged for special collections, and occasionally only
one size of sampler may be used.

Discussion
It seems clear that plankton samplers of the bongo
type collect more ichthyoplankton per unit volume offiltered water than either the traditional bridled ringnet or the more modern encased samplers. Theirsuperior performance is probably the result of thesmoother water flow in front of, within and behind the

POSGAY and MARAK: MAR MAP Bongo Zooplankton Samplers

nets. The tank tests show that the rate of flow throughthe bongo sampler is quite uniform. A flowmeter
located anywhere (except against the wall) would
register within 2% of the mean (Table 1).
The bongo samplers, because of theirunobstructed mouth openings, reduce the avoidance
problems caused by the bridle, pennant and towing
wire in front of the ring net. They also reduce theextrusion problem of the encased samplers because of
the lower filtration pressure across the meshes.
However, further work should be done to reduce, or atleast measure, the effects of both avoidance andextrusion, and to study by more sophisticated hydrodynamic methods the behaviour of the net as it is being
towed through the water. Only after such studies willmore meaningful conclusions be drawn from fieldcollections.
The best results with the bongo sampler willprobably be obtained with a judicious balance of
towing speed, mesh size, and mouth area to gauze area
ratio, based on the size range of the specificcomponents of the plankton community which are ofinterest. The use of conic sections in the sampler
bodies offers opportunities for increasing the volume
of water filtered per unit of ship time, but this may
cause complications by selectively sampling planktersof different densities.
Although most tows show good agreement
between the catches of the two nets on the same
sampler, occasional tows show larger differences.
These differences seem to be random and average out
when the results from several stations are pooled, butthey may be real, rather than just sampling error, andreflect the effects of sampling patchy distributions.
Station procedures have been standardized in the
MAR MAP program so that valid comparisons can bemade between collections at different times and placesby different ships. The primary objective is to sample
all levels of the water column equally to a depth of 200
m, orto within 5 m of the bottom in shallow areas, with adouble oblique tow at 1.5-2.0 knots. Because setting
the gear over the stern into the propeller wash is highly
undesirable, the ship should have a boom long enoughto place the samplers in the water well away from theside to avoid turbulence and discharges.
Sampling under the MAR MAP program involves
maintaining a constant speed of 2.0 knots, with the shipheading into the wind as the gear is being loweredsmoothly at 50 m per minute until the desired sampling
depth has been reached, and then immediately hauledback at 20 m per minute until the samplers break thesurface. There should be no hesitation at the interface,as this would lead to over-sampling the surface layer.
In some operations, like the ICNAF Larval Herring
Survey Program (ICNAF, MS 1972), tows have been99

made at 3.5 knots because some of the ships involved
could not maneuver at the slower desired speed of 2.0
knots. On some stations, after the 2.0 knot tow, asecond tow is made at 4.5 knots, usually with the 61-cm
sampler only fitted with two 0.505 mm nets, in an
attempt to sample the larger larvae which may be able
to avoid the gear when towed at 2.0 knots. Towing thegear at high speeds is not advocated for routine
operations, although the bongo sampler has been
towed at 9.0 knots without destroying the gear.

Acknowledgements
We find it impossible to acknowledge all of theadvice and help that we received from many of ourcolleagues both in the National Marine FisheriesService, formerly Bureau of Commercial Fisheries, andin other institutions. Our collections of data were madeaboard the research vessel Albatross IV, and we thankthe crew for the care with which they handled the gear.
The samples were sorted, under the supervision of R.
R. Marak, mainly by students temporarily employedduring the summer, and we appreciate theircontributions.

References
BRADLEY, J. V 1968. Distribution-free statistical tests. Prentice-Hall,
Inc. Englewood Cliffs, New Jersey, 388 p.
CONOVER, W. J. 1971. Practical non-parametric statistics. John Wiley& Sons Inc., New York, 462 p.
FRASER, J. H. 1966. Zooplankton sampling. Nature, Vol. 211, No. 5052:
915-916
GEHRINGER, J. W. 1952. An all-metal plankton sampler (Model Gulf
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ICNAF. MS 1972. Working Group on Joint Survey of Larval Herring in
the Georges Bank-Gulf of Maine areas, 9-12 May 1972, Boothbay
Harbor. Maine (G. J. Ridgway, Convener). ICNAF Res. Doc No.
123, Serial No. 2852.
McGOWAN, J. A., and D. M. BROWN. MS 1966. A new opening-closing
paired zooplankton net. Scripps Inst. Ocean., Ref. 66-23, 54 p.
(mimeographed).
SHERMAN, K., and K. A. HONEY. 1968. Observations on the catchingefficiencies of two zooplankton samplers. ICNAF Redbook 1968,
Part III: 75-80.
1971. Size selectivity of the Gulf III and bongo zooplankton
samplers. ICNAF Res. Bull., NO.8: 45-48.
SIEGEL, S 1956. Non-parametric statistics. McGraw-Hili Co., New
York, 312 p.
SMITH, P. E., R. C. COUNTS, and R. I. CLUTTER. 1968. Changes infiltering efficiency of plankton nets due to clogging under tow. J.
Cons., 32: 232-248.
SMITH, P. E. and S. L. RICHARDSON. 1977. Standard techniques forpelagic fish egg and larvae surveys. FAO Fish. Tech. Paper, No.
175, 100 p.
SMITH, R. P. MS 1972. Hydrodynamic evaluation of a plankton
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Note
Construction drawings and specifications for the
samplers, nets and accessories may be obtained fromthe authors.