174 U. S. BUREAU OF FISHERIES 



knowledge that the treatment was somewhat dark and heavy for the 

 purpose intended. By accident the proportion of cuprous oxide was 

 nearly doubled, thus rendering the nets still darker. In spite of this 

 the cuprous oxide treatment showed itself to be of value, and it can 

 be confidently expected that treatment uith a more suitable modi- 

 fication will be even more satisfactory. No great amount of deterio- 

 ration took place in the twine for the reason that the total length of 

 time the nets were in the water was only 30 days.^ 



Later tests made under laboratory conditions made it possible to 

 compare the preservative action of this treatment with others. It is 

 hoped that other treatments lighter in color than 2850 will be found 

 for gill nets, but a preservative treatment described below gives 

 results so satisfactory that it must be considered seriously before 

 successors which are untried by practical tests are adopted. A 

 treatment proposed as an alternative to preservative 2850 consists of: 



3 pounds cuprous oxide, 98 per cent passing through a 350-mesh sieve. 



4 pounds pine tar. 

 8 pounds coal tar. 



10 gallons water-gas-tar oil. 



This treatment can be safely recommended as equal in catching 

 power to untreated gill nets. It might possibly catch fewer whitefish 

 than white nets, but a difference in "fishing power" has not been 

 proved yet. 



Copper resinate is an effective treatment surpassing copper oleate 

 m preservative action. It is also cheaper and does not allow loiots to 

 slip. While no practical fishing tests have been made with copper 

 resinate, it seems that a solution of it containing 1}^ to 3 pounds per 

 gallon of kerosene would be equally good as a preservative as copper 

 oleate and have none of its disadvantages. 



SUMMARY 



In this study the effect of net fouling in Lake Erie has been studied, 

 from which it appears that "red slime" and similar material do not 

 influence the strength of the netting greatly, except in that they harbor 

 destructive bacteria which attack the cotton fibers and cause the net 

 to rot. It has been proved that bacterial attack is the prime cause 

 of the deterioration of nets. Simple methods for preventing bac- 

 terial rotting are described. 



Since drying, brining, etc., are not always practicable, other 

 preservatives have been tested in Lake Erie. Gill nets treated with 

 two different preservatives have been proved, by practical fishing 

 tests, to catch virtually as many fish as white nets, and to retain 

 their strength longer. These treatments are described on pages 172 

 and 173. 



Nearly all of the treatments used in the test shown in Table 5 are 

 suitable for pound nets. They are all excellent preservatives, though 

 2811 will decrease in strength rapidly with time, as past experience 

 has shown. Treatment 2811, the usual hot-tar treatment, is stickv; 

 2818 is not nearly as bad, yet nearly as stiff as tar alone. In physical 



•At the end of winter storage after this season, the untreated nets had a tensile strength per thread of 2.73 

 and 2.75 pounds, respectively. The modified far-kerosene treatment showed a tensile strength of 3.07 

 pounds, and treatment 2850 tested 2.95 pounds. 



