42 IT. S. BUREAU OF FISHERIES 



by ti.sli slime and subsequent atmospheric exposure of the nets leads 

 to heating due to bacterial action, which is controllable, so far as is 

 known now, only by thorough washing of the nets, followed by 

 salting. 



Preservative treatments applied to gill nets which are fouled 

 during immersion should not only minimize aquatic growths but 

 nmst also leave the webbing soft and pliable and capable of flat- 

 tening when struck by fish. In contrast, purse seines, which are 

 innnersed for relatively short periods and in which fouling is caused 

 exclusively by contact with the fish, require a preservative that 

 will maintain tensile strength and resist abrasion; a round, smooth, 

 compact thread is desirable, and a high degree of flexibility appears 

 to be of minor importance. These tAvo examples will suffice to 

 point out that no one preservative formula is applicable to all 

 types and sizes of textile fishing gear. 



In the application of preservatives in which tar is a principal 

 constituent it is necessary to provide a kettle of convenient size that 

 can be heated to 250° F., a draining board, and tackle for hauling 

 out the hot net. For the lighter treatments, in w'hich solvents are 

 used, a tank that can be heated with a closed steam coil is the only 

 essential equipment, although a draining board is sometimes used. 

 The preservative treatments listed below are on the basis of 100- 

 ])ound mixes. These mixes will have to be increased or decreased 

 according to the size of the twine and the quantity of the nets to 

 be preserved. Those treatments for pound or trap nets and gill 

 nets which contain copper compounds other than bluestone were 

 developed b}^ technologists of the Bureau of Fisheries. 



TREATMENTS FOR POUND NETS 



Xo preservative developed up to the present time has demon- 

 strated its ability to prevent absolutely the growth of vegetable 

 nuiterial on trap-net webbing in all waters. When this material 

 persists in forming it may be killed by removing the nets from the 

 water and exposing them to the bright sunlight for a day. The 

 most general treatment used for preserving pound or trap nets con- 

 sists of the following ingredients: 



I'ouiids 



Coal tar 67 



Wood tar 33 



A coal tar low in residue (benzol insoluble) is recommended. 

 This mixture is heated to fuming temperature in a kettle. The 

 net is immersed in the tar for a few minutes and then drained, 

 after which it is ready for use. This treatment is fairly satisfactory 

 for nets exposed in fresh w^ater. For nets exposed in salt water 

 the treatment becomes somewhat exhausted in about one month 

 and fouling with vegetable growth becomes quite extensive. How- 

 ever, this treatment has the advantage of low initial cost and pro- 

 duces a net witli a good resistance to abrasion, the latter being 

 an important consideration with pound or trap nets. A tan-ed net 

 is relatively heavy and inflexible, and tlie treatment is attended by 

 some fire risk and considerable discomfort to the laborers applying 

 the treatment. 



