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NOAA Technical Memorandum NMFS 

JULY 1980 

RESULTS OF THE CHARTERED CRUISE OF THE M/V MARIA C. J. 

SEPTEMBER 17 TO NOVEMBER 22, 1979 

JAMES M. COE 

AND 

RICHARD W. BUTLER 

CO-SPONSORED BY THE NATIONAL MARINE FISHERIES SERVICE 

AND THE PORPOISE RESCUE FOUNDATION 

NOAA-TM-NM FS-SW FC-6 

U.S. DEPARTMENT OF COMMERCE 

National Oceanic Atmospheric Administration 

National Marine Fisheries Service 
NOAA Technical Memorandum NMFS 

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NOAA Technical Memorandum NMFS 

This TM series IS used for documentation and timelv communication 01 preliminary resulls. imterim reports. 01 

special purpose inlormation and have not received complete formal review editorial Control, or detailed editing 

RESULTS OF THE CHARTERED CRUISE OF THE M/V MARIA C. J. 

SEPTEMBER 17 TO NOVEMBER 22, 1979 

JAMES M. COE 

AND 

RICHARD W. BUTLER 

NASO Contract 79-ABC-00233 

NMFS Cruise 565 

Nationd Marine Fisheries Service 

Southwest Fisheries Center 

La Jolla, California 92038 

U.S. DEPARTMENT OF COMMERCE 

Philip M Klutznick, Secretary 

National Oceanic and Atmospheric Administration 

Richard A Frank, Adminisirator 

National Marine Fisheries Service 

Terry L Leitzell, Assistant Administrator for Fisheries 
Table of Contents 

Page 

Introduction .............................. 1 

Objectives .............................. 1 

Results .............................. 2 

Net Surface Area Measurement ................... 3 

Backdown Speed and Channel Depth Measurements ........... 4 

Mid-Net Zipper Line Observation .................. 5 

Backdown Channel Widening Experiment. ............... 6 

Backdown Net Tension Measurements ................. 7 

Underwater Backdown Observations ................. 8 

Pursing Rate Measurements ..................... 9 

Discussion and Conclusions ...................... 9 

Personnel ............................... 14 

Tables ............................... 15 

Figures ............................... 19 

i 
INTRODUCTION 

The M/V --- Maria C. J. was chartered by the National Marine Fisheries Service 

to conduct fishing gear dynamics research aimed at reducing hcidental mortality 

rates of porpoise during commercial purse seining for yellowfin tuna. 

vessel is 68 m in overall length with a beam of 11.9 m and a draft of 5.5 my and 

is equipped with all standard equipment necessary to purse seine tuna in 

association with porpoise. The net is 14 standard strips of 108 mm mesh deep, 

1316 m in length and incorporates a double depth safety panel and super apron of 

31.8 mm mesh in the backdown area. For the period of this charter, Joe Brenha 

was the vessel Master and Joe Jorge was the Fishing Captain. 

The 

The charter was conducted in two segments, a local segment in the coastal 

waters off San Diego, California, and an eastern tropical Pacific Ocean (ETP) 

segment off Mexico and Central America. The local segment of this charter 

covered the period from September 17, 1979 to October 1, 1979. 

of this charter commenced on October 8, 1979 and was completed in San Diego on 

November 22, 1979. 

The ETP segment 

OBJECTIVES 

The purpose of this charter was to establish and verify measurement techniques 

leading to quantification of various aspects of purse seine behavior which may 

be relevant to incidental porpoise mortality. The specific objectives were: 

A. Repeat and verify a method presently in use to measure net surface 

area and establish the relationship between net depth in strips and 

surface area encompassed over time. 

B. Make as many simultaneous measurements of backdown speed and channel 

depth as possible to verify and/or modify a mathematical simulation of 

the backdown process. 

C. To measure the mid-net zipper line ridge and eliminate it without loss 

of the zipper function. 
-2- 

D. 

E. 

F. 

G. 

To perform repeated backdowns with the third bow bunch pulled to the 

starboard bow in order to assess the likelyhood of decreasing the 

probability of net collapse in late backdown. 

To measure strains and tensions at points of interaction between the 

vessel and its net as input to the simulation model of the purse 

seining process. 

To mark the net and dive during backdowns to observe the location on 

the net of lines of force and their movement as backdown progresses. 

During the ETP segment of the charter the technician aboard was 

instructed to measure pursing speeds for input into the simulation 

model of the purse seining process. 

RESULTS 

During the local segment of this charter cruise 13 net sets were made 

during which combinations of the above listed objectives were pursued. 

displays the chronology of events and the distribution of studies by set. 

porpoise or tuna were captured during the local segment of this charter, all 

sets were water hauls by design. 

Foundation representative participated in the cruise and enabled a number of 

experiments to be carried on simultaneously, particularly during backdown. 

first day of the charter was used to make a set to establish correct net tiedown points for backdown through alignment of the super apron. 

also used as a shakedown for all the other experimental procedures (flowmeters, 

photographic methods, dynamometer set-up, diving coordination, etc.) . second day was spent at dockside measuring and placing markers on the rings, 

across the net and along the corkline. 

to permit its lengthening and shortening to accomodate consecutive backdowns in 

single sets with the third bow bunch in different locations. 

third day (September 19, 1979) at least one set was made each day except Sundays 

and Saturday the 29th. By the afternoon of October 1, the local segment of this 

charter was compl eted. 

Table 1 

No 

Eight NMFS scientists and one Porpoise Rescue 

The 

This set was 

The 

Also, the third bow bunchline was modified 

Beginning on the 
-3- 

SCUBA diving teams entered the water either inside or outside of the net 

for backdown in sets 2-13. 

ridge, and observed the development of the backdown channel from several vantage 

points. 

in the Cruise Announcement. No accidents or mishaps of any kind occurred during 

diving activities. 

Divers placed BKG's, checked for the zipper line 

All dives were carried out in accordance with diving procedures outlined 

A. NET SURFACE AREA MEASUREMENT 

The area of the water surface encompassed by the net was estimated for 

sets 3 through 13 at the time of encirclement, at approximately one-half pursed 

and at rings up (Table 2). Seven large numbered floats were attached at regular 

intervals along the corkline so that at selected times in the set, distances and 

angles to each balloon could be estimated (Figure 1). 

using a prismatic range finder while angles were measured with a polaris relative 

to the longitudinal axis of the vessel. 

Distances were estimated 

For each set, at each time period (encircled, 1/2 pursed, and rings up) , a 

series of triangles may be constructed which approximate the surface area within 

the corkline perimeter. 

the formula A = 1/2[dl'd2'sin 03, where A = area of triangle, o = the angle 

between the adjacent balloons in degrees and dl and d2 are the distances to the 

adjacent balloons in meters. The areas of the triangles are then summed to 

approximate the net enclosed area. 

measured on day 2 to be 1316 m (720 fathoms) long and 14 standard mesh strips 

deep, the average area at encirclement was 89,497 m2, at approximately 1/2 

pursed, 51,175 m , and at rings up 17,429 m 

enci rcl ed area. 

The areas of the triangles are then calculated using 

For the net of the M/V --- Maria C. J. which was 

2 2 which was 19% of the initially 

Due to a delay in the delivery of four newly purchased bathykymographs 

(BKG) no depth/time records were made during the charter except below the apex 

of the backdown channel. The two BKG's available had a maximum depth range of 

50 m and could not be used to record depth/time traces below about the seventh 

strip of webbing. The results of the BKG traces are reported in section B 
-4- 

below. 

measurements used in previous cruises so that net volume was not approximated. 

Neither surface area nor volume parameters were measured during the ETP segment 

of the charter. 

This lack of BKG's prevented a complete duplication of the depth/area 

B. BACKDOWN SPEED AND CHANNEL DEPTH MEASUREMENTS 

Average vessel speed during backdown was measured in at least one 

backdown for each of the 13 sets made in local waters (Table 3). 

speed at the channel apex during backdown was measured in 12 of the 13 sets and 

found to be 38% (0.43 m/s; SD = 0.07) of the average vessel speed (1.13 m/s; 

SD = 0.24). When the channel apex speed is adjusted for hysteresis while the 

net is being stretched out (i.e., the vessel is moving but the channel apex is 

not) it rises to only 55% (0.62 m/s; SD = 0.07) of the measured average vessel 

speed. 

ETP and averaged 1.06 m/s (SD = 0.24). 

Average net 

Vessel speed was measured from the bow during 30 porpoise sets in the 

The method for measuring average speed during backdown in local waters 

involved the synchronized submersion of calibrated flometers. A technician in 

a raft at the backdown apex and another at the bow of the vessel recorded start 

and end counts on the flowmeters, elapsed time in seconds and lag time for the 

flowmeter to begin turning at the apex station. 

flowmeters recovered simultaneously by signaling with the ship's horn. 

method yields only the total distance traveled by the flowmeter and the elapsed 

time, which when divided yields the average speed of backdown. This method 

provides no information on the speed of the net or vessel as backdown progresses 

or as power (shaft RPM) is increased or decreased. 

during backdown was not controlled during the first 6 net sets to allow divers 

to compare net shape and dynamics under pseudo-normal conditions and to establish 

background data on net drag at various positions on the chain (Section E, below). 

The shaft RPM was controlled in three seperate backdowns executed in Set 7 for 

the specific purpose of obtaining data on the relationship of RPM, vessel speed, 

net speed and net floor rise rates, and tensions on specffic rings on the ring 

stripper. These results are shown in Table 3 and Figures 2 and 3. Shaft RPM 

was controlled and recorded in sets 3-33 during backdown but were not held 

cons ',aiii. irt order to facil itate other experiments. The speeds recorded in set 

7 are the only ones which can be related to net floor rise rates as estimated 

Backdowns were ended and 

This 

The shaft RPM of the vessel 
-5- 

from bathykymograph traces (Figure 2). 

During set 3 divers placed 0-50 m range bathykymographs at the bottoms of 

the third and sixth net strip (not counting the super apron) directly down the 

meshes from the apex of the backdown channel and placed permanent marks at each 

place to enable shipboard attachment in subsequent sets. 

plotted and the change in depth during backdown and the elapsed time were interpolated to yield the average rise rate in m/min of the corresponding area of net 

floor (Table 3). For all local segment measurements the net floor directly 

below the backdown apex 3 strips down rose from an average depth of 16.45 m to 

5.00 m in an average of 11.09 min yielding an average rise rate of 1.03 m/min. 

The same averages for 6 strips down were 20.63 m to 4.68 m in 10.54 min for an 

average rise rate of 1.51 m/min. 

The BKG traces were 

These measurements were repeated on five sets during the ETP segment. On 

the fishing grounds, the third strip under the apex rose from an average maximum 

depth of 25.20 m to an average minimum of 4.00 m at an average rate of 2.41 

m/min. 

3,70 m at a rate of 2.74 mlmin. 

134% and 81% increases, respectively, on the ETP segment. During the sets 

involved (3, 5, 6, 7, 8, 9, 10, and ll), an average of 10.6 tons of tuna were 

landed and 341 porpoise were encircled. 

ETP segment. 

The sixth strip averages show the channel floor rising from 32.70 m to 

The third and sixth strip rising rates show 

Shaft RPM was not recorded during the 

C. MID-NET ZIPPER LINE OBSERVATION 

NMFS divers have observed that the mid-net zipper line in several nets 

This ridge stands high in the net and reaches well into the zone occupied 

causes a 4 to 10 meter high ridge across the floor of the net when the rings are 

up. 

by captured porpoise. The ridge persists until it is rolled onboard and may or 

may not be the cause of some heretofore unobserved pre-backdown mortality. 

Observations by SCUBA divers during the local segment of this charter indicated 

that the purse seine of M/V --- Maria C. J. is completely free of a "zipper-line 

ridge." That, in fact, none of the components of the zipper-line system caused 

distortion in the floor or walls of the pursed seine. 
-6- 

During an examination of the zipper-line installation, on day two at 

dockside, the following points were noted: 

Strips of 108 mm (4 1/4 inch) mesh of number 42 nylon twine were 

separated at mid-net by a ten mesh wide 127 mm (5 inch) stretched mesh 

selvage, of number 120 nylon twine. 

The heavier webbing was laced mesh-for-mesh with the adjacent 108 mm 

(4 1/4 inch) strips. 

Three-inch diameter steel rings were spaced 18-20 meshes apart along 

the selvage and each lashed to a single knot. 

all, with 19 mm braided nylon rope threaded through each from corkline 

to leadline. 

There were 70 rings in 

BACKDOWN CHANNEL WIDENING EXPERIMENT 

an attempt to create a wider backdown channel with improved widthholding characteristics, backdowns were conducted with the third bunchline 

pulled around the starboard bow. 

against the bow. 

five, and six. 

throughout the backdowns for comparison with normal backdowns. In addition, 

observations were made from the deck, and during one set, divers observed the 

bow side of the channel from outside of the net. 

The third bunch itself was pulled up tight 

Backdowns with this configuration were made on sets four, 

Serial photographs were made of this modified backdown channel 

The photographs of the backdown channel during backdown were taken at 60 

second intervals from the crow's nest. 

pull the bunches and haul in the net to the normal backdown configuration, then 

backdown. 

shortened and brought around the starboard bow, the bunch was next pulled so 

that the bunched corks were held tightly against the prow at the water line. 

With these modifications complete and sufficient time elapsed for webbing to resink (as verified from BKG traces) the second backdown was conducted. 

The procedure used was to set the net, 

After this first backdown was completed the third bunchline was 

To analyze the backdown photographs, each frame was projected onto a sheet 

of paper and the outline of the corkline traced onto the paper. Buoys attached 

to the corkline at each end of the apron and each end of the apron extension 
-7- 

piece were also traced onto the paper (Figure 4). 

backdown channel was measured from the slide projections on three lines. 

Designated as "A", is the straight line distance between buoys marking the bow 

and stern ends of the super apron. "B'I is the straight line distance between 

the two points on the bow and stern of the super apron that are one-half the 

corkline distance between the apron ends and the apron extension piece ends. 

*'C" is the distance on the line between the buoys marking the bow and stern ends 

of the super apron extension piece (Figure 5). 

that pulling the third bunchline from the starboard bow does create a wider 

backdown channel throughout backdown. However, this configuration caused a 

greater area of webbing to lie perpendicular to the relative water flow allowing 

the channel to shallow up more rapidly, and create large canopies of webbing on 

the bow side of the channel. 

further. 

The distance across the 

Analysis of these data indicate 

For these reasons the concept was not pursued 

E. BACKDOWN NET TENSION MEASUREMENTS 

During backdown dynamometers were used to measure the tension on specific 

rings, on the stern corkline, and on the third bow bunchline. 

attached with skackles and slings to selected ring bridles in order to measure 

tension directly. 

down point to measure corkline tension, and a similar arrangement was employed 

to measure the strain on the third Bow bunchline during backdown. The tension 

data from sets 3-6 and sets 8-13 are averaged and presented with ranges in 

Figure 6. 

is exerted on the corkline and through bridles directly down the meshes from the 

super apron apex. 

Dynamometers were 

The stern corkline was attached to a dynamometer at the tieThese data indicate that the majority of the backdown pulling force 

Since uncontrolled backdowns were noted to require approximately 100 RPM, 

RPM's of 75, 100, and 125 were selected for strain measurements for three 

backdowns during set 7. 

(Figure 3). 

As expected, the tensions increased with increasing RPM 

On the fishing grounds, the tension on the stern corkline at the tie-down 

point was measured during backdown in six sets (19, 20, 41, 45, 53, and 56). 

these sets, an average of 600 porpoise were encircled and 9.67 tons of tuna 

landed. 

between the two sets of data. 

In 

Again, as with the backdown channel floor rising rates, there is disparity 

The average stern corkline tension during backdown 
-8- 

for the local segment was 3237 lbs, while in the ETP segment the tensions 

averaged 41 90 1 bs. , a 29% increase. 

F. UNDERWATER BACKDOWN CHANNEL OBSERVATIONS 

During the second day of the local segment, a day at the dock, the net was 

marked with colored polypropylene line. A white marker from the corkline at the 

super apron apex was laced in the webbing going straight down the meshes (knots) 

to the leadline. Two yellow lines began at the corkline nine meters to either 

side of the super apron apex and were laced into the bars running sternward to 

the leadline. A fourth line, white, began at the corkline at the stern tie-down 

point and was laced in straight down the meshes to the leadline (Figure 1). 

These markers were used by divers throughout the backdown maneuvers as reference 

points from which relative net configuration could be deduced. 

During sets 2-13, divers observed the location and orientation of these 

lines. 

the meshes from the apex were repeatedly observed to come to lie in close 

parallel proximity of each other during backdown. These three lines and the 

webbing between them would become wrapped up in a long cylinder of webbing. 

This sausage-like roll of mesh extended from the vessel to the last one-quarter 

of the backdown channel length and was 30-60 cm in diameter. Most of the 

webbing under the stern side corkline during backdown was involved in this 

I' s au sa ge . 

The two parallel lines running down the bars and the line running down 

Divers observed that the "sausage" of webbing probably plays an important 

roll in the development of the inward billowing of webbing on stern side of the 

backdown channel, commonly termed stern sway. 

backdown its contact with the stern wall of the backdown channel appears to 

limit the amount of webbing involved in stern sway. 

divers placed colored surveyor's flagging on the outermost layer of webbing on 

the stern edge of the "sausage." When the net was rolled aboard, and hanging 

from the power block, the flagging was observed to form a straight line very 

nearly parallel to the stern-going bar markers. 

As this "sausage" rises during 

During backdown in set 13, 
-9- 

Diver observations were used to construct Figure 7 which is a cross section 

of the backdown channel looking towards the apex and illustrates the development 

of stern sway as viewed from underwater about half way down the channel. Figure 

8, also from diver observation, shows the location and magnitude of webbing area 

involved in different parts of the backdown channel. 

G. PURSING RATE MEASUREMENTS 

To provide input for a simulation model of the purse seining process the 

speed at which the purse cable was retrieved was measured on an opportunistic 

basis. 

considered separately, and measured periodically during the pursing operation 

(Table 4). 

known distance from the purse block to the cable guide on the winch. The 

instantaneous speeds for all ten sets were fixed relative to the time pursed, 

i.e., time pursed equals zero, and grouped into four minute intervals. 

speeds falling within the same interval were averaged and plotted in Figure 9. 

The pursing speeds of the bow and stern portions of the purse cable were 

A stopwatch was used to time a mark on the cable traveling the 

All 

Figure 9 illustrates that as pursing nears completion, pursing speed is 

reduced. 

in pursing speed can be attributed to the conditions of the set, and the winch 

operator. 

its change during the operation. 

This is probably due to increased load on the pursing winch. Variations 

The objective here was to measure the magnitude of pursing speed and 

DISCUSSION AND CONCLUSIONS 

The ultimate goal of this charter was to discover methods of investigation 

The results presented here, when considered in that context, 

that would lead to gear and techniques to reduce incidental porpoise mortality 

in purse seining. 

have varying impacts upon future research. For example, the problem of the midnet zipper line ridge has been considered and a means to alleviate the problem 

can now be reported to vessel operators, thus potentially reducing porpoise 

mortality. In the long range, the data gathered on pursing rates, backdown 

speed and net surface area will be used to verify and set standards for both 

scale model and computer simulations of numerous net designs. 

these models as tools for gear research will greatly enhance the range of feasible 

investigations. 

Development of 

The development of adequate field methods for pursuing potentially 
- 10 - 

important lines of investigation was of primary concern during this charter. 

The surface area measurement methods were developed and used for a previously 

completed experiment. The results reported above are comparable with calculations 

made for a 1116 meter by 14 strip net using the same methods (Cabrillo 

Cruise No. 552), and verify the methods and calculations used to approximate 

surface area. 

- NMFS 

The average areas for the 14 strip nets of the M/V Maria C. J. and the M/V 

2 2 2 and 45,396 m and (3) at rings up; 17,429 m 

2 2 Cabrillo were (1) at encirclement; 89,497 m and 80,494 in , (2) at approximately 

half pursed; 51,175 m and 19,541 

m2, respectively. These measurements are consistent in that the --- Maria C. J.'s 

net had a slightly greater surface area at encirclement at half pursed because 

it was longer and a smaller surface area at rings up because bow bunches were 

pulled. 

allowing for more corkline in the net perimeter. All these averages are within 

two standard deviations of one another. This method will be used as a standard 

to assess area effects of future experimental net modifications. 

In the Cabrillo experiment no bow bunches were pulled at rings up, thus 

There is concern as to whether experimental results from net sets conducted 

locally, without fish or porpoise (i.e., local water hauls) are comparable with 

similar experimental results from sets made on the fishing grounds. 

channel floor rise during this experiment for example, shows increased rates in 

sets in the ETP when compared with sets conducted locally. 

values were recorded on the fishing grounds for the tension on the stern corkline 

during backdown. This could be due to the presence of fish and porpoise, or to 

a more deliberate execution of the backdown maneuver in the real situation. In 

either case, the differences recorded do not invalidate experiments conducted in 

the waters off San Diego since they are consistent with expected increased 

strain on the net due to concentration of the weight of tuna and porpoise against 

the backdown apex. 

Backdown 

Similarly, higher 

Some of the work presented here was a first time effort and analysis of the 

data indicates a need for more accurate and continuous measurements. 

of vessel and net speed proved feasible enough in the field, but yielded only an 

average speed for an entire backdown that includes some error due to wind causing 

the flowmeter propeller to revolve during deployment and retrieval. 

Measurements 

While the 
- 11 - 

effort yielded satisfactory data on vessel and net speed, the primary value was 

in identifying the need for refining procedures and equipment for more accurately 

repeating these measurements in the future. The method for recording the rise 

rate of the backdown channel floor was straightforward, i.e., by BKG's. However, 

the relationship between channel floor rise and backdown speed or distance 

traveled is not clear from these data. 

in local waters when shaft RPM during backdown was held constant at three 

different RPM levels, there appears to be a constant relationship between 

average vessel speed and backdown channel floor rise rates. Division of the 

rise rate in m/sec by the average vessel speed in m/sec yields the following 

dimensionless resu'l ts which indicate a possible constant relationship: 

It is interesting to note that in Set 7 

75 RPM: 0.02065 

100 RPM: 0.02076 

125 RPM: 0.02020 

If verifiable, this relationship may be a function of individual nets that can 

be used in future experiments as one measure of the effect of net modifications 

on backdown performance. The evaluation of this relationship will require more 

controlled experiments with continuous vessel speed data over a broad range of 

propel 1 or shaft speeds. 

The value of observations by divers of underwater net configuration cannot 

be overstated. Surface measurements and observations, no matter how refined, 

are limited in describing the total picture. These observations continually 

help to refine and focus our understanding of the mechanics of the purse seining 

process. The relationship between net behavior and applied forces cannot be 

fully understood without subjective underwater assessments. 

system is yet sophisticated enough to integrate the overall picture. 

No remote sensing 

Direct benefits of diving observations during this charter were to identify 

how, and to what degree various areas of webbing contribute to the make-up of 

the backdown channel during backdown. 

elucidation of the roll played by the "sausage" of webbing, described in Section 

VI, F above, in the development of stern sway. The knowledge that the "sausage" 

exists, and understanding how it interacts with the rest of the webbing during 

backdown, opens up new areas of investigation aimed at improving backdown 

The most important discovery was the 
- 12 - 

efficiency by el iminat 

The zipper-line r 

porpoise mortality and 

ng or reducing stern sway. 

dge was identified by divers as an area for potential 

it was intended that the ridge be examined during this 

charter. The purse seine of the Maria C. J. is completely free of this problem, 

however. 

other seine nets, and was of the opinion that his particular zipper installation 

was responsible for the absence of a ridge. The conclusion drawn is that the 

zipper-line ridge is due to decreasing the working depth of the webbing along 

the zipper by uneven lacing of the selvage strip to the adjacent webbing and/or 

by lashing the rings to two adjacent knots instead of one. 

selvage in mesh-to-mesh, lines of tension along the webbing bars are transmitted 

through the selvage piece without distortion and no depth is lost. By fastening 

the rings to two adjacent knots instead of only one, one mesh is eliminated from 

net depth at each ring, which in a 14 strip net with 70 rings for the zipper, 

causes 70 meshes or 0.7 strips of net depth to be lost. 

actually a shortening of the distance from the chain to the corkline and is 

translated into a ridge across the net. 

zipper-line ridge is planned in order to verify these conclusions. 

--- 

The Fishing Captain was aware of the zipper-line ridge phenomenon in 

By lacing the 

This loss of depth is 

Future research on a seine that has a 

Often ideas are brought forward that lend themselves well to relatively 

easy and straightforward testing. 

starboard bow to affect a wider backdown channel was such an idea. As mentioned 

above, this procedure was rejected because it created new areas with the potential 

to trap and drown porpoise, and also caused the backdown channel to shallow up 

more rapidly than normal. Had the procedure shown more promise, it could have 

been refined and possibly perfected in a very short time with a minimum of 

effort. 

with simultaneous underwater observations of net behavior will greatly increase 

our ability to predict the range of results expected from future proposed net 

and procedure modifications. 

Pulling the third bow bunchline from the 

The experience gained in re1 ating surface measurements and observations 

An ancillary result of this charter was a gain in acuity of understanding 

of the relationship between net depth and backdown parameters. When lacing in 

marker lines (6.4 win colored polypropylene rope), during the dockside day, an 

important observation was made. It was noted that lines marking webbing bars 

runnir,g sternward from the super apron apex, ended at the leadline in close 
- 13 - 

proximity to the marker running down the meshes under the established tie-down 

point (Figure 1). In fact, the distance between the apex bar at the leadline, 

and the marker at the leadline under the tie-down point, measured 23.8 meters. 

This may prove to be a useful observation if the relationship between net depth 

and corkline distance from the backdown apex to the stern tie-down point can be 

formulated through future measurements. 

during backdown is critical to backdown performance; too much and the apex 

collapses and the corkline will not sink, too little and a large canopy forms at 

the apex. NMFS requires vessels with new super apron installations to make a 

trial set to establish the optimum tie-down points for backdown. Trial sets are 

not practical for vessels operating from Panama however, and therefore many U.S. 

vessels have not had this benefit. 

stern tie-down point based on net depth would eliminate mortality associated 

with gross experimentation with tie-down locations during porpoise sets. 

a stern tie-down point is established, the third bow bunchline would be pulled 

until the apron is aligned. It is hypothesized that such a formula can be 

derived and verified since there appears to be a systematic relationship between 

the stern tie-down to apex corkline distance, and stretched net depth (i .e. , 

length of bars). 

The length of corkline in the water 

An empirical formula for establishing the 

After 
- 14 - 

PE RS ONNE L 

A. Local Segment 

James M. Coe, NMFS, Chief Scientist/Diver 

David B. Holts, NMFS, Dive Master 

Richard W. Butler, NMFS, Technical Coordinator/Diver 

Dale Fell baum, NMFS, Technician 

Darrell E. Lee, NMFS, Technician 

Char1 es 01 iver, NMFS, Photographer 

Thomas B. Shay, NMFS, Technician/Diver 

William H. Brinkerhoff, NMFS, Technician 

Charles Peters, NMFS, PRF 

B. ETP Segment 

Darrell E. Lee, NMFS, Technician 

Gary Worthen, Utah State University, Contractor 
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2. Table 2. Surface area encompassed (m ) by the corkline of the net of the 

M/V --- Maria C. J. (14 strips deep by 1316 rn long) 

Set No. 

03 

04 

05 

06 

07 

08 

09 

10 

11 

12 

13 

Area 1 Area 2 % of Area 1 Area 3 % of Area 1 

Enci rcl ed %1/2 pursed Rings-up 

(No. bow bunches) 

* 75,074 

* 80,642 

* 114,590 

* 88,804 

* 95,212 

* 110,033 

114,690 

81,704 

72,587 

62,341 

88,794 

60,787 

53,610 

48,839 

77 , 565 

60 , 956 

81,482 

30,550 

45 , 276 

21,832 

33 ,Ol 7 

49,011 

(81%) 

(66%) 

(43%) 

(87%) 

(64%) 

(74%) 

(27%) 

(55%) 

(30%) 

(53%) 

(55%) 

19,319 (3) 

15,113 (3) 

17,643 (3) 

15,236 (3) 

18,984 (3) 

21,426 (3) 

21,988 (3) 

18,563 (3) 

16,223 (3) 

10,853 (3) 

16,370 (3) 

N= 11 11 I1 

X= 89,497 51,175 (57%) 17,429 

S.d. = 17,579 18,627 3,163 

26% 

19% 

15% 

17% 

20% 

19% 

19% 

23% 

22% 

17% 

18% 

19% 

* These measurements are for corkline plus towline. All other measurements are for 

corkline alone after towline is in. 
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-18- 

Table 4. Pursing speeds 

Minutes to 

Set # Pursed Time 

21 

38 

42 

44 

47 

49 

27 0650 

24 0653 

14 0703 

10 0707 

1 071 6 

0 071 7 

40 0650 

11 07 07 

8 071 8 

4 0726 

0 0730 

36 0633 

19 0650 

7 0702 

1 07 08 

0 0709 

40 0603 

28 061 5 

23 0620 

19 0624 

5 0638 

1 0642 

0 0643 

42 0540 

12 061 0 

7 061 5 

0 0622 

26 071 7 

18 0725 

13 0730 

8 0735 

2 0741 

0 0743 

Activity 

Tow1 i ne 

Rings up 

Net let go 

Rings up 

Net let go 

Rings up 

Net let go 

Rings up 

Net let go 

Rings up 

Bow 

Pursing 

Speed 

(m/s 1 

.45 

.27 

* 54 

.36 

.36 

.41 

.36 

.32 

-49 

.54 

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.30 

.19 

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-36 

Towline not in 

.55 

.53 

.36 

Rings up 

Stern 

Pursing 

Speed Commen t s 

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.46 

.58 

.46 

.46 

.46 

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.58 

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.35 

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.42 

.42 

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.68 

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-18aTable 4. Pursing speeds (cont'd) 

Minutes to 

Set # Pursed Ttme 

50 

51 

52 

55 

22 1625 

20 1627 

17 1630 

12 1635 

7 1640 

0 1647 

30 0600 

25 0605 

19 061 1 

15 061 5 

8 0620 

3 0625 

0 0628 

20 1110 

18 1112 

10 1120 

5 1125 

0 1130 

33 0537 

18 0551 

14 0556 

9 0601 

3 0607 

0 061 0 

Bow Stern 

Pursing Pursing Actl'vl ty Speed Speed Comments 

cm/s 1 (m/s 1 

Towl ine in 

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.54 .38 

.41 .57 

.41 .51 

Rings up 

Towl i ne in - 36 .46 

.36 .46 

.36 .46 

.30 .38 

Towl ine in 

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Rings up 

Net let go * 54 .67 

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Figure 3. T.ension on ring bridles for three backdown speeds. 
-22- 

Figure 4. Balloons marking Super Apron. 

Figure 5. Projection of bacKdown channel for measurement. 
-23- 

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