NOAA Technical Memorandum NMFS 

4 

i 

# DECEMBER 1982 

A PRELIMINARY STUDY OF DOLPHIN RELEASE 

PROCEDURES USING MODEL PURSE SEINES 

David B. Holts 

James M. Coe 

NOAA-TM-NM FS-S W FC-25 

U.S. DEPARTMENT OF COMMERCE 

National Oceanic and Atmospheric Administration 

National Marine Fisheries Service 

Southwest Fisheries Center 
NOAA Technical Memorandum NMFS 

The National Oceanic and Atmospheric Administration (NOAA), organized in 1970, has evolved 

into an agency which establishes national policies an manages and conserves our oceanic, coastal, and 

atmospheric resources. An organizational element within NOAA, the Office of Fisheries is responsible for fisheries policy and the direction of the National Marine Fisheries Service (NMFS). 

In addition to its formal publications, the NMFS uses the NOAA Technical Memorandum series to 

issue informal scientific and technical publications when complete formal review and editorial processing are not appropriate or feasible. Documents within this series, however, reflect sound professional 

work and may be referenced in the formal scientific and technical literature. 
NOAA Technical Memorandum NMFS 

This TM series is used lor documentation and timely communication 01 preliminary resutts. imterim reports. or 

special purpose inlormatlon, and have not received complete formal review. editorial control. or detailed editing 

DECEMBER 1982 

A PRELIMINARY STUDY OF DOLPHIN RELEASE 

PROCEDURES USING MODEL PURSE SEINES 

David B. Holts 

Southwest Fisheries Center 

National Marine Fisheries Service, NOAA 

La Jolla, California 92038 

and 

James M. Coe 

Northwest and Alaska Fisheries Center 

National Marine Fisheries Service, NOAA 

Seattle, Washington 981 12 

NOAA-TM-NMFS-SWFC-25 

U.S. DEPARTMENT OF COMMERCE 

Malcolm Baldrige. Secretary 

National Oceanic and Atmospheric Administration 

John V. Byrne. Administrator 

National Marine Fisheries Service 

William G. Gordon, Assistant Administrator for Fisheries 
CONTENTS 

INTRODUCTION .................................................. MODEL SCALING FACTORS ......................................... 

OPERATIONAL AND EXPERIMENTAL PROCEDURES ....................... 

OBSERVATIONS OF PURSE SEINE DYNAMICS 

Sinking Speed .............................................. Pursing .................................................... Backdown Observations ...................................... Effect of the Backdown Arc at the Apex ..................... Modifications to the Backdown Channel ...................... Possible Cause of a Common Gear Malfunction ................ 

LIMITATIONS TO MODEL STUDIES .................................. 

RECOMMENDATIONS ............................................... 

ACKNOWLEDGEMENTS .............................................. 

LITERATURE CITED .............................................. 

Tab1 e 1 . Physical parameters of full -si zed purse sei nes 

and their models .................................... 

Page 

1 

2 

4 

5 

6 

7 

8 

8 

10 

10 

11 

13 

14 

15 

i 
LIST OF FIGURES 

Figure No. Page 

1 General dimensions of model purse seines used 

in study ........................................... 16 

2 Ideal i zed net configuration during the pursing 

operation .......................................... 17 

3 The degree of turning arc during backdown influences 

the location of the channel apex: A. Backdown 

channel with proper turning arc for correct apex 

position, B. A tight turning arc rotates the apex 

around to the bow side of the channel, and C. Too 

straight of an arc moves the apex to the channel s 

stern side......................................... 18 

4 Cross section of backdown channel showing 

ideal i zed devel opment of stern-sway ................ 19 

ii 
A PRELIMINARY STUDY OF DOLPHIN RELEASE 

PROCEDURES USING MODEL PURSE SEINES 

David B. Holts 

Southwest Fisheries Center 

National Marine Fisheries Service, NOAA 

La Jolla, California 92038 

and 

James M. Coe 

Northwest and A1 aska Fisheries Center 

National Marine Fisheries Service, NOAA 

Seattle, Washington 98112 

INTRODUCTION 

Model fishing nets have been used for many years as an inexpensive aid to 

improving various types of fishing gear. The most common and successful 

model s are those of trawl s that can be observed in fl ume tanks, thus a1 lowing 

changes in rigging and construction to be tested without going to sea. 

first flume tank was built at Boulogne-sur-Mer, France in 1967. 

larger flume tank was built in Lorient, France in 1978. 

Authority (Hull, England) built a large flume tank in 1976 that can 

accommodate models of large pelagic trawls at towing speeds up to 13.5 knots 

( Noel , 1980). 

Model testing in these flume tanks has greatly increased the understanding of trawl dynamics and has led to substantial refinements in modeling 

techniques. Additionally, it has shown that model studies of net dynamics can 

be done at a fraction of the cost of using full-sized commercial fishing 

gear. Not only are the costs of net material, labor and operations lower, the 

time required to modify a model is considerably less than for the full-size 

gear. When certain modeling rules are followed and the models are scaled up 

to full size, accuracy of performance prediction can be maintained (Wileman, 

1980) 

The 

A second, 

The White Fish 

* 

The first model purse-seine studies carried out by the National Marine 

Fisheries Service (then the Bureau of Commercial Fisheries) were directed 

towards improving the overall efficiency of the purse seine. 

Green (1968) built a 1:25 scale model of a 425 fathom-long, seven-strip-deep 

(a standard net strip in U. S. tuna purse seines is 100 to 120 fathoms long by 

100, 4 l/$-inch meshes deep) tuna seine using the modeling rules of Dickson 

(1959). Ideal materials for model construction were not available, and the 

weight of the model 's leadline and web was reduced at a scale less than 

1:25. The authors were able to get reliable results, however, by fully 

testing the basic design and then modifying the model and retesting. Their 

concl usions, based on performance comparisons of "before" and "after" tests, 

were: 1) a lower coefficient of hang-in (K ) increased sinking speed and 

maintained the maximum surface area for a fonger time, 2) a longer leadline 

(than corkline) increased the sinking speed, 3) tapered ends saved webbing 

without impairing fishing depth, and 4) deeper nets maintained their shape 

Ben-Yami and 
2 

longer. These model studies led to the design of the "hybrid seine'' (Green et 

al., 1971) which was easier to handle, sank faster, used less webbing, and had 

a greater surface area for a longer time than did the more conventional nets 

of that time. 

A model purse seine designed for studies relating to the tuna-dolphin 

problem was built by Gary Loverich under the direction of Richard McNeely in 

1972 at the Northwest and Alaska Fisheries Center (NWAFC). This model was 

designed from the same tuna sei ne used by Ben-Yami and Green (1968). 

was able to locate more suitable materials that allowed improved accuracy in 

scal i ng . 

Loverich 

The McNeely-Loverich model was the scal ed equi Val ent of ni ne strips deep 

and had a mid-depth purse line installed throughout its length to test the 

feasibility of double pursing to separate the tuna from the dolphins. 

with this model showed a mid-depth purse line to be impractical and saved 

considerable time and money that might have been expended testing the idea on 

a commercial sei ner. 

general purse seine dynamics; however, the model was not used to study the 

backdown maneuver as a method of dolphin release. 

Tests 

These early tests a1 so provided Val uabl e information on 

A second model was b u i l t a t the NWAFC in the spring of 1973. This was a 

1:50 scale model of a newly designed "Large Volume Net" (LVN) scheduled for 

construction by NMFS in the fall of that year. 

model were 1) increased depth (17 strips) to prevent pre-backdown net 

collapse, provide greater surface area and increase pursing speed and 2) 

tapered ends to reduce excess webbing and attendant gear malfunctions. The 

model was set three times to evaluate general sinking and pursing 

characteristics. It performed well in these tests but appeared to sink too 

slowly due to greater-than-expected webbing resistance. 

The two main features of the 

All of the studies involving model purse seines described thus far dealt 

with the dynamics of setting, sinking speeds, hang-in coefficients and 

pursing. None has examined the process of releasing dolphins by 

backing-down. The backdown procedure (Coe and Souza 1972) is the primary 

means of releasing captured dolphins in the eastern tropical Pacific tuna 

fishery. It is also this phase in the operations that most frequently results 

in dol phi n mortal i ty. 

The purpose of this study was to evaluate the usefulness of model purse 

seines in the design and testing of dolphin-rescue gear and procedures. Of 

primary interest was the behavior of the models during normal and modified 

backdown procedures. 

such as webbing becoming tangled in the purse rings, were also examined. 

The effects on performance of various gear malfunctions, 

MODEL SCALING FACTORS 

The two models used in this study were designed to be set from a two-man 

inflatable raft in a large swimming pool. They were both scaled using the 

lUnpubl ished working notes of McNeely and Loverich. 
3 

modeling rules of Dickson (1959) and Kawakami (1964). 

The first model (Figure la) is a standard rectangular net fashioned after 

an early design described by McNeely (1961). It is a 1:25 scale model of a 

425-fathom, seven-strip seine. Two additional strips (equivalent) were added 

to the model to accommodate studies using two purse lines. The model is 31.1 

meters long, sinks to about three meters and sets in a 9.8-meter circle. 

Physical parameters for both the full-sized and model versions are shown in 

Table 1. 

The second model, the Large Volume Net (LVN), was built to test the 

hydrodynamic performance of a full-sized prototype. 

long, sinks to about 3.4 meters and sets in a 7.0-meter circle. 

parameters of this LVN model are also shown in Table 1. 

The model is 22 meters 

The physical 

The modeling theory used for scaling these nets was based on maintaining 

Froude' s and Reynolds' number equivalent. Froude' s and Reynolds' numbers as 

well as general scaling theory are described by Dickson (1961), Baranov 

( 19481, Fridman ( 1964) and Kawakami ( 1959 and 1964). Keeping Froude' s number 

constant maintains a constant flotation-to-drag ratio by reducing the velocity 

(and time scale) by the square root of the 1 inear scale (Dickson, 1961). This 

a1 so insures that the forces of gravity and inertia are scaled equal ly. 

order to maintain precise dynamic and frictional forces between the model and 

full-sized gear, the Reynolds' numbers of both must also be the same. 

cannot always be achieved in practice, and some differences in water flow 

patterns and drag coefficients usually have to be accepted. 

at which the purse seines operate (as opposed to trawls) tend to minimize the 

effects of these differences. 

In 

This 

The slow speeds 

Modeling rules used by the builders of these models to ensure a constant 

flotation-to-drag ratio were: 

1. All lengths were reduced by S, 

2. Speed and times were reduced by S, 

3. Weights were reduced by S3, 

where S is the scaling factor. 

Geometric similarity between the models and the full-sized gear was 

achieved by reducing all lengths by the linear scale and maintaining all hangin rates and tapers. Reduction in mesh size was based on the amount of 

blockage (or water resistance) offered by the web to the water flow and w$S 

less than the linear scale. The mesh on the 1:25 standard model was scaled to 

only one-seventh of the original and that on the LVN model to only one-eighth 

of its original full-sized design. This technique is valid when the surface 

areas of the twines are reduced at equal ratios. 

mesh-length ratios for the standard model and its original were 0.0216 and 

0.0296, respectively, while those of the LVN model and its full-sized design 

were 0.017 and 0.011. While these ratios are almost equal, researchers at the 

White Fish Authority in England suggest using the twine-diameter ratio (TDR) 

to determine mesh size for model nets (Wileman, 1976). Here the model mesh 

size is directly proportional to the ratio of twine diameters. 

The twine-diameter-to- 
4 

- Dm 

Df 

-- twine dia. model 

twine dia. full size 

TDR = 

and in the case of the standard 1:25 model 

The mesh size is then reduced by 1/5, or to 104.82 mn x 0.21 = 22.0 mm. 

The mesh size indicated for this model was somewhat larger than the 15.2 mm 

(0.60") used. Consequently one would expect a little more blockage and 

therefore more resistance to water flow. 

The LVN model incorporated a TDR of 0.081, indicating a preferred mesh 

size of 8.7 mm (0.344 inches). The 12.7 mm (1/2 inch) mesh size used was 

therefore too large and allowed less drag than if scaled precisely. 

The flexibility, elasticity and stiffness of the selected materials used 

in constructing model s are a1 so important factors in the hydrodynamic behavior 

of model nets. Wileman (1980) doubts that elasticity and stiffness scale down 

properly (at least in trawl s) when forces are reduced by the cube of the 

scale. These difficulties and uncertainties in building and testing the model 

nets have certainly introduced some bias in performance. These biases, 

however, appeared minimal during most aspects of the study. 

OPERATIONAL AND EXPERIMENTAL PROCEDURES 

Prel iminary work i ncl uded determining optimum operating and experimental procedures for the setting, pursing and backing-down operations. The models 

were set from a two-man inflatable raft fitted with a small electric trolling 

motor, keel, ring stripper, winch, and net box (Figure 2). During the early 

trials, the techniques for setting, pursing and backing down were established, 

and, with practice, a single raft operator could perform these functions 

smoothly. The sets were initiated by holding the bow end of the model while 

the raft completed its circle. Setting speed was controlled at 0.5 meters per 

second (about one minute to complete the circle). 

The purse winch was made by connecting two level-wind-equipped fishing reels together so that both ends of the purse line could be pulled at equal 

*Standard 4 1/8 inch mesh. 
5 

speeds. 

stripper, the raft was pulled into shallow water by hand. 

purse cable were "rolled", or pulled in by hand and stacked into the net box, 

until the stern tie-down point for backdown was reached. 

was then tied down under the ring stripper, and three bow corkline bunches 

were pulled. This net-roll procedure represents a complete interruption in 

the retrieval process and was necessary because scaling of the vessel Is hull 

and skiff drag was beyond the scope of this study and because observation 

during repeated backdown trials was more easily accomplished in shallow 

water. 

pre-backdown conditions and net configurations. 

Once the net was pursed, and the purse rings placed on the ring 

Here the net and 

The net's stern side 

Multiple-backdown tests were then carried out under varying 

Applying these procedures to the two model s a1 1 owed for pre-backdown 

observations of setting characteristics, sinking rates, pursing dynamics, 

enclosed surface area and volume seined. 

such as "stern sway" (Hol ts et a1 ., 1979; Coe and Butler, 19801, surface and 

subsurface canopies, channel collapse, changes in the radius of the backdown 

arc and tie down point locations were of primary interest. 

Various aspects of backdown dynamics 

These studies were carried out in the summer of 1980 at the swimming 

pools of the University of California, San Diego (UCSD) and at the General 

Atomic Company employees' recreational pool. Both swimming pools were 

available for only one four-hour session once a week. 

allow time for setting up support and recording equipment necessary to 

standardize procedures and measure any changes in the model s' behavior between 

trials. 

This schedule did not 

Consequently our results are largely qual i tative. 

OBSERVATIONS OF PURSE SEINE DYNAMICS 

Si nki ng Speed 

The objective of the first two sessions was to establish the operational 

and experimental procedures as well as to work out any unforeseen problems. 

Sinking speeds were measured during four subsequent sessions. 

Both orientation sets were made with the standard model. It sank very 

slowly and unevenly during the first set because the webbing was dry and 

contained numerous air spaces. Wetting the model s just prior to setting 

el iminated this problem on subsequent trials. 

The purse cable was always stacked with the model nets and did not unwind 

(under tension) from the winch during setting as is the case during actual 

fishing conditions. This may have a1 lowed the model s to sink to a greater 

extent than the full-sized gear, since tension on the purse cable holds up the 

leadline and prevents full extension of the web. 

The standard model averaged 32.3 seconds to reach its maximum depth in 

four trial sets as measured at the half-net marker (15.6 m). Times were 

recorded with a stopwatch as the leadline sank past submerged markers at one, 

two and two-and-a-half meters. 
6 

trial 

1 2 3 4 X 

Depth Seconds 

lm 11 9 9 - 9.7 

2m 23 20 25 17 21.3 

2.5m 28 30 35 36 32.3 

Scaling these data back to the full-sized gear gives a sinking time of 

2.7 minutes to reach a depth of 34.2 fathoms, or 12.6 fathoms per minute 

(fm/min). This is much too fast for full-sized gear of comparable lengths and 

depths. 

our model may have sunk nearly three times too fast. Ploeger3 ( 1973) found a 

10-strip net to sink at 2.9 fm/min over the first five minutes of setting. 

Hester (1961) measured the sinking rate of a seven-strip net at about 4.5 

fm/min for the first 5 minutes. Green et a1 ., (1971) found 5 different tuna 

purse seines had an average sinking speed of 5.7 fm/min over the first 5 

minutes. The fastest, their 10-strip hybrid nets, sank at 6.6 fm/min. 

Beltestad (1980) reported a sinking speed of 12.4 fm/min (22.7 m/min) in an 

experimental purse seine made of six-sided meshes ("hex mesh"). 

Published sinking speeds for nets with seven to ten strips indicate 

We were unable to obtain sinking rates on the LVN model because its 

webbing became entangled during each of the trials and consequently did not 

sink uniformly. 

Pur si ng 

Pursing of the models began after connection of the purse cable ends to 

During this time, the standard model sank to its maximum depth 

Both ends were pulled evenly until the purse rings came up next 

The standard model was pursed in four to five minutes; this is 

the "winch." 

and would touch the bottom of the pool if the connection to the winch was not 

made rapidly. 

to the raft. 

comparable to pursing times of the older and smaller tuna seiners with slower 

deck machinery and 400 to 450-fathom nets. 

was 435 fathoms long and took an average of about 24 minutes to purse (for the 

model, 24 min t S = 4.8 minutes). 

The seine used by Hester (1961) 

Three sets were made with the 1:50 scale model of the Large-Volume Net. 

On the first of these sets, the leadline sank to the bottom of the pool and 

the rings and bridles were dragged across the bottom as it was pursed. The 

next two sets were oriented so that the model's deeper center section was in 

the deep end of the pool and the tapered ends followed up the sloping bottom 

to the shallow end. This allowed pursing without dragging on the pool 

3P10eger, J. W. 

Bathykymograph Sumnary NMFS Cruise No. 51. Southwmshwenter, La 

Jolla, California, 26 p. 

1973. Cruise report of the M/V John F. Kennedy 
7 

bottom. This model was pursed in two to three minutes, representing the time 

for a tuna purse seiner to close a 600-fathom net. As pursing proceeded with 

both models, the rings pulled in smoothly and evenly and appeared to function 

as those observed at sea.4 The bow and stern bends that normally form around 

the seiner (or raft in our study) only partially developed during pursing, 

because we made no attempt to simulate the drag of the vessel 's hull and 

skiff-pulling characteristics. 

the raft slightly into the net, but no studies were conducted concerning this 

aspect of the operation. 

The bends could be easily created by pushing 

The corkline on both models collapsed quite badly during the final stages 

This 

The LVN 

of pursing. The corkline of the standard model completely collapsed, 

el iminating a1 1 open surface area each time the rings came out of water. 

was, to a large degree, due to its high length-to-depth ratio (1l:l). 

model had a much lower length-to-depth ratio (7:l) and remained noticeably 

more open. 

The overall operation from setting to rings-up proceeded smoothly and was 

judged a good simulation of actual fishing dynamics in both models. After 

rings-up, the model (s) and raft were moved into the shallow end of the pool , where the stern-half of the net was stacked into the net box and then tied 

down at the appropriate tie-down point. The three bow corkline bunches were 

also pulled in at this time and attached to the raft's port bow. 

Backdown Observations 

Prior to each backdown sequence, the raft was positioned in the corner of 

the shallow end of the pool so that it could simulate a long, gently arcing 

backdown path to the pool Is deep end. The corkline was opened by hand and the 

webbing allowed to sink prior to initiating each backdown sequence. 

The webbing under the bow corkline bunches of the standard model hung in 

drape-like folds with elongated meshes, producing a deep bowl-like area 

directly beneath. As backdown started, the bowl folded under the rest of the 

channel, as has been observed several times by divers in the full-sized 

gear. "Stern sway,'' a subsurface, lateral folding of loose webbing along the 

stern side of the backdown channel (Figure 41, also occurred, as has been 

observed in the field (Holts et a1 ., 1979; Coe and Butler, 1980). 

The webbing under the three bow bunches of the LVN model did not hang in 

folds or create a deep bowl as observed in the standard model. This backdown 

channel formed without canopies; however, stern-sway and a "sausage-1 ike" roll 

of webbing did develop. 

The main stresses or pulling forces during the backdown procedure in both 

models occurred down the mesh row from the apex corks to the chain directly 

below. At first, this area of tight meshes formed the floor of the channel, 

4Holts, D.B., R. McLain, F.G. Alverson, and J. DeBeer. 1979. Summary of 

research results from the fifth cruise of the Dedicated Vessel. Southwest 

Fisheries Center Admin. Report No. LJ-79-20, 47 p. 
8 

but as backdown proceeded, it continued to rise and contacted the stern side 

wall of the channel. The gathering of webbing combined with the slack webbing 

from the stern wall to form a sausage-like roll of webbing. The channel floor 

was then primarily made up of the webbing on the bow side of the channel 

(Figure 4). Observations of both models were similar to the underwater 

observations made in full-size seines (Coe and Butler, 1980). 

Effect of the Backdown Arc at the Apex 

During backdown trials, the channel Is intended apex was susceptible to 

shifting towards the bow or stern side of the channel, depending on the 

turning radius of the raft as it moved backwards (Figure 3). 

model a straight backdown moved the intended apex about 20 cm (equivalent to 

3-4 fathoms on a full-sized seine) around to the stern side of the channel. 

Too tight an arc moved the intended apex a similar distance toward the bow 

side. Configuration of the LVN model Is backdown channel was highly dependent 

on the curve of the backdown arc. Minor changes in the degree of the backdown 

arc would rotate the apex corks out of their optimum position, and the channel 

would collapse,. if the backdown arc was not corrected. 

models could easily be opened or collapsed by changing the arc. 

Unfortunately, we had no method of standardizing the turning radius during 

repeated trials. The result was some inconsistency in the location of the 

apex when the channel first became fully developed. 

minimized by the conscious effort of the raft driver to execute the backdown 

trials as uniformly as possible. 

stern-sway development was obvious. 

backdown may also play an active roll in the backdown operations of commercial 

gear, and additional investigations of its effect on net configuration and 

dol phin-release efficiency would be warranted. 

In the standard 

The channels of both 

This problem was 

However, some variation in canopies and 

The factor of turning radius during 

Modifications to the Backdown Channel 

Four different modifications to the backdown procedure were made during 

this portion of the study. 

modifications were designed around the concept that a1 tering the distribution 

of forces on the excess webbing in the sides of the channel could create a 

larger, deeper, more canopy-free channel. 

a1 tering the ol d points normally at work in the backdown channel, we hoped to 

eliminate some of the slack webbing along the stern side. 

Based on past experiment and observation, these 

By creating new stress points or 

Trial 1 

In our first attempt, four purse rings (the ring directly below the apex 

cork and three adjacent sternward rings) were pulled inboard to the leadline 

to simul ate a four-fathom pull. The stern-sway and "sausage" apparently 

developed faster and involved all the webbing along the stern-going bar 

markers from the apex to the chain. 

channel, nor did it reduce the amount of the slack web responsible for 

stern- sway. 

This did not increase the depth of the 
9 

While these observations were not encouraging, it was decided to test the 

concept at sea (Butler and Foster, 1981).5 On board a chartered seiner, the 

purse rings, bridles and leadline were pulled inboard of the ring stripper 

varying distances from two to four fathoms. Underwater observations indicated 

substantial variabil i ty as to where the "sausage" actually contacted the stern 

wall of the channel; stern-sway however still existed. 

Trial 2 

The second modification to the backdown procedure was to pull in extra 

webbing at the stern tie-down point. 

corkline and leadline had fallen behind the webbing during net-roll, i.e., as 

if the webbing were rolled aboard at a greater speed, thus potentially 

reducing the amount of slack web available for stern-sway. 

model this reduced the amount of slack webbing in the stern wall of the 

backdown channel and el iminated the ''sausage" but created some minor folds 

along the apex mesh row at the center and bottom of the channel. 

pulled in 50 cm of extra webbing at the stern tie-down point of the LVN model, 

which represented 14 fathoms on a full-sized net. 

and sausage more toward the stern side and resulted in a deeper channel. 

major stress points were along the bar markers and not down the meshes as 

observed with the standard model. 

stern-going bars apparently made the channel deeper without a1 tering the basic 

configuration. This procedural modification appeared to enhance the backdown 

channel in tests of both models, indicating a need for follow-up studies to 

evaluate possible merits of reduced webbing along the stern side of the 

channel wall . 

This simulated a condition where the 

In the standard 

We also 

This moved the stern-sway 

The 

Displacing the line of stress to the 

Trial 3 

Our third modification (standard model only) was to increase the 

channel's base length by attaching the bow bunches as far forward on the port 

bow as possible, while moving the stern tie-down point as far aft as 

possible. This approximately doubled the distance between tie-down points. 

New tie-down points had to be established for this test. 

allowed development of a wider channel, which was less inclined to collapse. 

Additionally, the amount of surface area in the channel was greatly 

increased. The floor of the channel became quite shallow, so the overall 

effect on volume was minimal. Follow-up trials should also be conducted for 

thi s aspect of backdown. 

The procedure 

Trial 4 

Researchers with the Inter-American Tropical Tuna Commission ( IATTC) 

designed a single otter board to aid in preventing stern sway and canopies. 

This ''backdown board" (7.6 cm x 12.7 cm) was attached to the standard model's 

corkline on the.stern side of the channel, about two thirds the distance from 

the apex toward the tie-down point. After some bridle and location 

sButler, R. W. and T. Foster. 

charter. Southwest Fisheries Center, La Jolla, California, 25 p-- 

1981. Cruise report of the M/V Maria C. - J. 
10 

adjustments, the board prevented the backdown channel from collapsing and 

appeared to help in reducing the severity of stern sway by removing some of 

the slack webbing in that area. 

and open, thus effecting a wider channel without any apparent loss in depth. 

This backdown board concept appears to have some merit by increasing the 

surface area and by preventing net collapse. 

model. 

The board pulled the stern side corkline out 

It was not tried on the LVN 

Possible Cause of A Common Gear Mal function 

Some of the problems encountered on full-scale nets were observed in the 

model, e.g., broken chain line, pre-backdown net collapse, winch malfunction, 

net collapse during backdown and net caught in the purse rings at rings-up. 

During two sets, webbing drifted into the bight of the purse cable as the 

rings were coming up. This web became caught, or pinched, between the purse 

rings during the final stages of pursing. In one case the current from a pool 

water jet caused the webbing to drift into the cable, and in the other an 

observer's swim fin was the cause. These observations support the idea that 

ocean currents are probably one of the major factors responsible for this type 

of mal function. 

LIMITATIONS OF MODEL STUDIES 

A major problem in interpretation of all model-net studies is whether 

hydrodynamic similarity is achieved between the model and actual fishing 

nets. 

discrepancy between the model and full -si zed gear (Dickson, 1961). We woul d 

expect some degree of discrepancy with our models due to the size reduction. 

However, some workers (pers. comm., J. A. Eikelman, Jr., SRI International, 

August 1978) maintain that scaling to 1:lOO is possible if one is willing to 

consider only the forces of drag. This is reasonable, since the behavior of 

webbing is influenced more by drag than by the effects of gravity and 

turbulence, at least at the low velocities involved in the pursing and 

backdown operations of a purse seine. 

Generally, the greater the scale reduction the greater the performance 

Three basic conditions precluded precise dynamic modeling of the purse 

seine system in our studies. First, there is substantial variation in 

operating procedures and net performance among commercial purse seiners, much 

of which has not been measured in the field. Second, model nets are rarely 

perfect miniatures of the full-sized gear, because proper materials are often 

difficult or impossible to obtain. Lastly, comparative performance tests of 

scaled gear and/or procedures before and after selective modification( s) are 

applicable to full scale only when the magnitude of the modeling error is 

understood. 

Aside from the assumptions required for scaling and the difficulty of 

obtaining suitable building materials, the validity of interpreting purse 

seine model trials is limited by the inability to precisely control starting 

configurations of the net. In these trials, for example, it was not possible 

to standardize the backdown operation so that each test was initiated from the 

same pre-backdown net configuration. This would require private pool 

facilities, so that operational and support equipment could be developed to 

allow repeatability of the backdown net configuration for each trial. 
11 

One possible way to achieve this standardization is to use a towing 

basin. Here the model could be attached to a towing board and mechanically 

pulled through the water over equal turning arcs and speeds while cameras, 

attached to the towing board, recorded dynamic changes in the backdown 

channel. However, in order to simulate the normal backdown arc, a curved 

towing basin would be necessary. Unfortunately, no curved towing basins 

exist, and the straight ones would only be good for a few limited studies. 

The question is just how reliable experimental results would be when 

extrapolated into a curved backdown arc. 

changes in apron construction, apex canopi es, ti e-down di stances and hang-i n 

rates could be valuable. However, problems such as stern-sway, canopy 

formation a1 ong the channel si des, and backdown channel coll apse are greatly 

influenced by the seiner's backdown arc and therefore cannot be simulated by a 

straight tow. 

Some results such as those for major 

RECOMMENDATIONS 

There are many di ff icul ti es and uncertainties in bui 1 di ng and testing 

model nets and some bias in performance must be accepted. Modeling, however, 

is an inexpensive tool that can provide a better understanding of mechanical 

performance and hydrodynamic behavior of f i shi ng nets, even though model s 

cannot duplicate or mimic each detail of any fishing operation. 

obtained from model net dynamics testing must be correlated with observations 

and measurements of full scale gear performance. The advantage in modeling is 

that it gives greater assurance that advanced design(s) will work. 

Results 

Resul ts of tests wi th the two exi sti ng model s, however prel imi nary, were 

informative within the limits discussed. These tests identified several areas 

where equipment or procedural modifications might be utilized in the design of 

new dolphin saving techniques. These areas include 1) alteration of channel 

tie-down and stress points to further identify factors influencing stern sway 

and canopy development, 2) apron design and modification studies, 3) tests of 

optimum hang-in coefficients, 4) channel -enlargement capabilities, 5) 

a1 ternative placement of "porpoi se-safety panel SI', 6) strategic placement of 

hexagonal mesh in the channel to facilitate optimum water flow. 

A model made of hexagonal mesh would be useful in determining its 

applicability to dolphin rescue and purse seine dynamics in general. 

Hexagonal-mesh webbing is currently being produced in Norway and has performed 

favorably in Norwegian purse seines (Beltestad, 1980). Hexagonal-mesh nets 

use about 15% less material, sink faster, purse easier, and are nearly as 

strong as the rhombic mesh purse seines currently in use. Hexagonal meshes 

are less prone to collapse when being pulled through the water. 

material may be ideally suited for the western Pacific tuna fishery, where 

nets are often as much as 1000 fathoms long and 22 strips deep. 

Nets of this 

A sui table scale for hydrodynamic studies of the purse-seine operation, 

with emphasis in dolphin rescue and release, is 1:lO. At this scale, a 

850-fathom (15551111, 18-strip seine would measure 156 meters (510 ft.) long, 

fish to approximately 13.2 meters and set a 50-m diameter circle. Greater 

precision in dynamic scaling can be achieved at this scale than at greater 

reductions, and the necessary instrumentation for measuring depth, speeds, 

volumes, surface areas, etc. is currently available. A 1:lO model is 

sufficiently large to allow attachment of the necessary instrumentation yet 
12 

small enough to allow observation of the entire operation both from the 

surface and underwater. 

necessary deck gear for setting and retrieving the model seine. 

also suitable for making any number of tailoring and design changes quickly 

and inexpensively. Using a 1:4 mesh scale would allow for making fairly 

complex modifications with minimal labor, because the larger mesh size would 

allow use of standard net-making tools. 

valuable tool where the investigation of a broad range of physical and 

procedural modifications of the tuna purse seining system could lead to 

advanced designs in dolphin release techniques at a reasonable cost. 

A1 1 net material s are commercial ly avail ab1 e, as are 

This size is 

A model at this scale would be a 
13 

ACKNOWLEDGEMENTS 

We wish to express our thanks to the University of California at San 

Diego and to General Atomic Company for generously allowing us to use their 

pool facilities for this study. Rafael Guillen and Dave Bratten (IATTC) aided 

with operational procedures and designed the backdown-door. Thanks go to Gene 

Anderson, Bill Brinkerhoff, Tod Foster and Mark Lowry for their help 

throughout the project. G. Sakagawa, W. Perrin and S. Sitko reviewed the 

manuscript. Roy Allen prepared the figures. 
14 

LITERATURE CITED 

Baranov, F. I. 1976. "Selected works on fishing gear''. Israel Program - for 

Beltestad, A. K. 1980. Purse seines with hexagonal mesh. Report- Fishery 

Scientific Translations, Jerusalem, 1976, 1:565-581. 

Techno1 ogy Research Institute, 60p. 

Ben-Yami, M. and R. E. Green. 1968. Designing an improved California tuna 

Coe , 

Coe, 

purse seine, Fishery Industrial Research,-4(5):188-207. 

J. M. and R. W. Butler. 1979. Results of the chartered cruise of the 

M/V Maria C. J. September 17 to November 22, 1979. NOAA Technical 

MemoTZ?IGm%lFS-SWFC-6, 26p. 

J. M. and G. Sousa. 1972. Removing porpoise from a tuna purse seine. 

-- Mar. Fish Review, 34(11-12):15-19. 

Dickson, W. 1961. Trawl performance - A study relating models to commercial 

trawls. Marine Research No. 1:l-50. Dept. of Agriculture and Fisheries 

for ScotEGIT 

Dickson, W. 1959. The use of model nets as a method of developing trawling 

gear. Modern Fishing Gear of the World, Fishing News (Books) Ltd., 

London, pp. 166-174. 

Fridman, A. 1973. Theory and design of commercial fishing gear. Israel 

Green, R. E., J. E. Jurkovich and B. Petrich. Development and testing of the 

Fishing News 

Program for Scientific Translations, Jerusalem, pp. 122-187. 

hybrid tuna purse seine. Modern Fishing Gear of the World. 

(Books) Ltd., London, pp. 264-273. 

Hester, F. J. 1961. Tuna Seines: How deep? Pacific Fishermen 59(12):19-20. 

Kawakami, T. 1964. The theory of designing and testing fishing nets in 

model. 

London, pp. 471-481. 

Modern Fishing Gear of the World (2) Fishing News (Books) Ltd., 

Kawakami, T. 1959. Development of mechanical studies of fishing gear. Modern 

Fishing Gear of the World (1) Fishing News (Books) Ltd., London, pp. 175- 

184. 

McNeely, R. 1961. Purse seine revolution in tuna fishing. Pacific Fisherman 

59(7) :27-58. 

Noel, S. 1980. Purse seines in the test tank. Fishing News International 

19:ll. lp. 

Wileman, D. A. 1980. White Fish Authority Field Report No. 857. ICES meeting 

Reykauik, Iceland,F1980, 13p. 
15 

Table 1. Physical parameters of full-sized purse seines and their models. 

~ ~~~ 

Net Parameters 

Standard Net Large-Vol ume Net (LVN 1 

Full size Mode 1 Full size Model 

Geometric Scale 

Length (hung) 

Length (stretched) 

Depth (stretched) 

Web (nylon) 

Mesh size (L) 

Twine size 

Twine diameter (D) 

Twine diameter to mesh 

length ratio (D/L) 

Meshes long 

Meshes deep 

Ratio twine diameter d/D 

Ratio mesh length n/L 

Hang-in coefficient 

Length 

Diameter 

Cork1 i ne 

F1 oats 

Size 

Buoy a ncy 

Density 

Number 

Total flotation 

Lead line 

Length 

Size 

Weight per meter 

Weight ratio (w/W) 

Total weight 

Purse cable 

Size 

Length 

Total weight 

1 

777.8 m 

864.1 m 

73.5 m 

treated 

1048 mn 

#42 

2.26 mn 

0.55 mn 

8242 

700 

1 

1 

.90 

Nylon,twisted 

777.8 m 

- 

spongex 

15.2x9.5 cm 

0.99 kg 

118.0 kg/m3 

7000 

6970 kg 

Chain 

11.2mm( 7/16" 1 

777.8 m 

0.32 Kg/m 

1 

372.5 kg 

1:25 

31.1 m 

34.6 rn 

3.1 m 

untreated 

15.2 mm 

210/3 

0.45 mn 

0.75 mn 

2267 

205 

0.200 

0.447 

.90 

Nyl on, bra 

27.9 m 

2.72m 

- 

ded 

19.1x11.9 mm 

3.4 g 

32 kg/m3 

3.64 kg 

1077 

Chain 

28.0 m 

1.3 mm 

2.56 g/m 

0.008 

1.03 kg 

Nylon 

2.97 mn 

30.5 m 

1 

1098 m 

1378 m 

184 m 

treated 

108 mm 

#24 

1.88 mm 

0.44 mn 

12.752 

1700 

1 

1 

.79 

Dacron,twisted 

1098 m 

15.9 mm 

spongex 

16.5x21.6 cm 

118 3*86 kg/m k! 

1098 m 

1389 kg 

Chain 

1098 m 

12.7 mm 

galvanized 

2.79 0.39 kg/m 

1 

4548 kg 

Steel 9x16 

16.0 mn 

1098 rn 

1078 kg 

1 

1:50 

22.0 m 

27.5 m 

3.8 m 

untreated 

12.7 

7/2-L2 

0.0057" 0.14 mn 

0.0114 0.29 mm 

2176 

300 

0.077 

0.1176 

.79 

Nyl on, bra 

22.0 m 

12.9 mn 

ded 

Polyel thelym 

9.6x12.4 mm 

0.00194 lbs 0.88 g 

32 kg/m3 

1.021 lbs 0.46 kg 

Chain 

528 

22.0 m 

- 

0.73 g/m 

0.05 mm 

0.17 kg 

Nylon, Mono. 

0.79 mm 

22.0 m 

33 9 

Time reduction 1 S s 
16 

9 

-0 

-0 

Lo 

-0 

-0 

co 

d 

a, 

a, 

U 

v) 

Q 

cn 

b 

U 

C 

a 

cn 

C 

0 

.- 

L 

c 

r 

- 

E 

0 

c 

a 

O 

0 

<o 

a, 

C 

a, 

cn 

a, 

3 

Q 

O 

a, 

U 

4- 

Y- 

.- 

z 

Y- 

- 

E 

0 

v) .. 

T4- 

a, 

t 

E 

3 

0 

> 

a, 

a 

-I 

- 

E) 

m 

aJ 

-0 

E! 
17 

E 

0 

c, 

m 

L 

W 

9. 

0 

cn 

E: 

*Iv) 

L 

3 

P 

W 

c 

c, 

9) 

E: 

L 

3 

-0 

E 

0 

C, 

L 

3 

9) 

cc 

E: 

0 

V 

c, 

W 

c 

-0 

W 

N 

m 

aJ 

-0 

%- 

.I- 

.Im 

.#- 

.#- 

7 

Y 
\ 

\ 

\ 

Apex 

A. 

8. 

Apex 

/ 

/ 

/ 

Apex 

C. 

Figure 3. The degree of turning arc during backdown influences the location of 

the channel apex: A. 

correct apex position, B. 

around to the bow side of the channel and C. Too straight of an arc 

moves the apex to the channels stern side. 

Backdown channel with proper turning arc for 

A tight turning arc rotates the apex 
19 

cc 

0 

c, 

S 

al 

0 

al 

> 

al 

-0 

E 

- 

.#- 

e% aJ 

-0 .- 
RECENT TECHNICAL MEMORANDUMS 

Copies of this and other NOAA Technical Memorandums are available from the National Technical 

Information Service, 5285 Port Royal Road, Springfield, VA 22167. Paper copies vary in price. 

Microfiche copies cost $3.50. Recent issues of NOAA Technical Memorandums from the NMFS 

Southwest Fisheries Center are I isted below: 

NOAA TM NMFS SWFC 15 

16 

17 

18 

19 

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23 

24 

Status reports on world tuna and billfish stocks 

STAFF, SWFC 

(July, 1981 I 

An evaluation of tagging, marking, and tattooing techniques for 

M. J. WHITE, JR., J. G. JENNINGS, W F GANDY and L H 

CORNELL 

(November 1981 1 

Local stability in maximum net productivity levels for a simple model 

T POLACHECK 

(April 1981) 

Environmental data contouring program EDMAPZ 

L EBER 

(April 1982) 

The relationship between changes in gross reproductive rate and the 

current fate of increase for some simple age structured models 

T POLACHECK 

(May 1982) 

Testing methods of estimating range and beating to cetaceans 

T D SMITH 

(1982) 

"An annotated bibliography of the ecology of co-occurring tunas 

(Katsuwonus pelamis, Thunnus albaceres) and dolphins (Stenella 

attenuate, Stenella longirostris and Delphinus delphis in the 

eastern tropical Pacific" 

small delphinids. 

of porpoise population sizes 

aboard the R/V D S Jordan 

S D HAWES 

(November 1982) 

Structured flotsam as fish aggregating devices 

R S SHOMURA and W M MATSUMOTO 

(November 1982) 

Abundance estimation of dolphin stocks involved in the eastern 

tropical Pacific yellowfin tuna fishery determined from aerial and 

ship surveys to 1979. 

R. S. HOLT and J. E. POWERS 

(November 1982) 

Revised update and retrieval system for the CalCOFl oceanographic 

L. EBER and N. WILEY 

(December 1982) 

data file.