NOAA Technical Memorandum NMFS 

MAY 1980 

THE MID-NET ZIPPER RIDGE A POSSIBLE 

CAUSE OF UNOBSERVED PORPOISE MORTALITY 

David B. Holts 

NOAA-TM-NMFS-SWFC-3 

U.S. DEPARTMENT OF COMMERCE 

National Oceanic and Atmospheric Administration 

National Marine Fisheries Service 

Southwest Fisheries Center 
NOAA Technical Memorandum NMFS 

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THE MID-NET ZIPPER RIDGE A POSSIBLE 

CAUSE OF UNOBSERVED PORPOISE MORTALITY 

David 6. Holts 

National 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 
The Mid-Net Zipper Ridge 

A Possible Cause of Unobserved Porpoise Mortality 

National Oceanic and Atmospheric Administration 

National Marine Fisheries Service 

Southwest Fisheries Center 

P.O. Box 271 

La Jolla, California 92038 

navid R. Holts 

Each year thousands of oceanic porpoises are killed incidently during 

purse seining operations for tuna. Nhile the U.S. fleet has greatly reduced 

its estimated mortality rates during the past several years, further 

reductions are possible. 

It has long been suspected that unobserved porpoise mortality occurs in 

deep ar as of the seine prior to backdown. 

Program , NMFS divers observed that the mid-net zipper held up a wall of 

webbing 30 feet or more above the floor of the net (Fig. 1 A&B). 

mid-net zipper ridge, formed during the pursing operation and persisted until 

it was rolled aboard. These observations were further documented by 

fathometer tracings collected from a small rubber raft crossing the net (Fig. 

During the Dedicated Vessel f 2 

The wall, or 

1,C). 

Captured porpoise usually position themselves over the deepest area of 

the net. When large numbers of porpoise and tuna are captured, or when 

porpoise must cross over the zipper ridge, the probability of entanglement is 

greatly increased. NMFS divers have observed porpoise entangled in the 

overhanqing wall of webbing and lesser folds associated with this area. 

ridge has also been observed in a number of purse seines during local water 

hauls and has been noticeably absent in other seines. 

The 

The formation of the zipper ridge most commonly results from improper 

The U.S. tuna industry and the Federal Government sponsored 

the tuna seiner M/V Queen Mary during 1978 as a research platform for 

studies related to tmn~mrpoise problem. 

The mid-net zipper is a line that extends vertically from the 

corkline to the leadline. It is occasionally pulled to divide the net 

into equal sections. If pulled when the tuna are crossing the net, the 

school will be divided into two smaller, more easily handled schools. 

This is usually only done on extremely large catches to reduce tensions 

and pressures on the fishing gear. 
-2- 

installation of the "stopper" webbinq. 

heavier, stronger twine and used to stop net tears, should they occur, and for 

attaching the zipper line guide rings. This material is usually 5-inch mesh 

selvage, 10 meshes wide and made of Mo.!l6 or No.120 nylon twine. 

installed vertically from the corkline, through the entire body of the net to 

the leadline. 

and incorrectly i ns tal 1 ed gui de rings. 

The stopper material is made of 

It is 

Less common causes for the ridge include a short zipper line 

The zipper ridge occurs when the stopper webbing is prevented from 

extending as far as the adjacent lighter web in the main body of the purse 

seine. 

advantage for those seines in which the stopper was i n s t a l l e d w i t h a greater 

amount of slack. While other factors may contribute to restrict full 

extension of the stopper, one fairly simple adjustment can eliminate them. 

This adjustment is most easily made at dockside but can be done at sea. 

A close look at zipper installation techniques has shown a definite 

The five inch mesh stopper must be re-hung to the 4 1/4-inch mesh with 

enough extra slack to permit full extension of the stopper. 

accepted methods by which full extention can be achieved. 

There are two 

The first method is the simplest and the only one that should be 

attempted at sea. Here the existing five inch mesh stopper is cut out of the 

seine and re-hung, mesh-for-mesh to the 4 1/4-inch mesh. This provides 

approximately 30% slack in the stopper webb3ng. 

meshes of stopper web should be "killed-in" to each strip of the 4 1/4-inch 

mesh. For example, two fathoms of additional slack are provided when only two 

5-inch meshes are killed-in for each strip of a 14-strip net. This proceedure 

will ensure a problem free zipper line by providing for differential shrinkage 

rates between the five inch and 4 1/4-inch mesh, should it occur. 

Additionally, there is no loss of strength in this area since tensions remain 

uniform throughout these meshes. 

An additional one to three 

The second method is the technique of stretching the five-inch stopper 

web evenly with the 4 1/4-inch mesh and tacking them together prior to lacing. 

This should only be done with pre-shrunk material and only on one side of the 

stopper at a time. Care must be taken to ensure that both sides are stretched 

equally tight prior to tacking. An additional three to five meshes of stopper 

web should be killed-in for each strip of 4 1/4-inch mesh to ensure enough 

slack in the stopper to prevent formation of the ridge. 

Another common cause of the mid-net zipper ridge is a short zipper line 

which restricts the full extension of the stopper webbing. This case is 

easily observed at rings up by noting if there is a greater tension on the 

zipper line than on the chains. Often an indentation of the corkline, at the 

zipper, protrudes into the net and towards the seiner (Fig. 1,AI. 

problem is easily corrected by splicing an appropriate length o f l i n e into the 

existing zipper line. 

This 

Kill-in is a common term used by net builders and net 

repairmen. It refers to the process of lacing two or more meshes of 

webbing to only ofTe mesh of adjacent webbing. 
-3- 

Improper installation of the zipper guide rings can effectively reduce 

Guide rings should be lashed to a single knot down the stopper’s 

the depth of the center section of a 14- strip net by as much as 3/4 of a 

strip. 

center knot row. 

meshes apart. 

These rings should be evenly spaced, approximately 18-20 

The immediate fleet-wide effect in reducing porpoise mortality cannot be 

determined at this time. Individual vessels plagued by zipper problems, 

however, do stand a good chance of achieving lower mortality rates by 

correctly modi fyi ng the mid-net zipper and/or stopper web. 

A major trend in recent years has been toward deeper and faster sinking 

Heavily weighted, 14-strip nets are now 

nets. 

than the shorter 10-12-strip nets. 

common throughout the U.S. fleet and a few vessels are equipped with nets 18- 

20 strips in depth. The overall efficiency of those purse seines with zipper 

problems may be improved by allowing the mid-section to sink faster and 

deeper, thus preventing potential loss of fish under this area. 

These have proven more efficient in capturing traveling schools of tuna 

A Mid-net zipper 

C (--92m--+ 

C 

0- 

6- 

v, 

LT mid-ne t ridge 

E 

I 

30- 

% 36’ - 

42 - 

’. 

b 

Figure 1. (A) Mid-net zipper ridge when fully pursed. 

vessel. 

trace when fully pursed. 

Note indentation of corkline opposite 

(B) Zipper ridge with one-fourth net retrieved aboard vessel. (C) Fathometer