NET DETERIORATION IN LAKE ERIE 171 



results from experiments, which have been conducted for the purpose, 

 show that an excess of rosin used in the manufacture of copper 

 resinate does not cause any deleterious effect upon the twine. When 

 an excess of oleic acid is used, however, the twine decreases rapidly 

 in strength.^ 



When one examines carefully the curves representing the changing 

 copper content of twines using cuprous oxide as the active ingredient, 

 one notices that their main characteristic is a rapid decrease from a 

 relatively high copper content to a lower one, which is followed by a 

 very gradual decrease in concentration dating from the fourth week. 

 The explanation of this fact seems to be that the first period marks 

 the time when the coating on the twine covers and projects beyond 

 the outside fibers. This outer coating is not anchored very firmly 

 and is rapidly dislodged. After the greater portion of the loosely 

 adhering toxic material is dislodged, the preservative treatment 

 inclosed within the boundaries of the twine itself has a chance to per- 

 form its function of protection. The protection is accomplished 

 partly by solution of the cuprous oxide by the water, but is also 

 accompanied by mechanical dislodgment of some of the toxic mate- 

 rial. The latter process is very slow compared to the first "shedding " 

 of active material, since the cuprous oxide particles are entangled 

 between the fibers of the cotton twine and can not fall off or be carried 

 away easily. 



The experiments, which we are considering did not extend past the 

 time when most of the toxic material would have been removed from 

 within the twine, and hence it is not possible to study the whole phase 

 of the deterioration of the test material. However, that is not 

 nearly as important as the first phase. From what has been said 

 heretofore, one can safely predict that treatments containing more 

 cuprous oxide than those that have been tried will not be very eco- 

 nomical. They, of course, will protect the twine as well as or better 

 than the present treatments, for the time of the preliminary period, 

 which has been extended by reason of the large amount of active 

 ingredients which have been added. However, when this first period 

 of "shedding" is past these newer treatments will be on an equal 

 footing with those using less toxic material. One can say, therefore, 

 that the production of cuprous oxide treatments producing a treated 

 net containing more than about 30 milligrams of copper per lineal 

 foot of No. 24 hard-laid twine can not be very economical unless the 

 shedding which has been encountered with the treatments used in 

 this series of experiments can be decreased very markedly. 



PRACTICAL FISHING TESTS 



The aid of the fishermen in Mr. Kolbe's crews, and especially the 

 assistance rendered by Capt. Charles Hoskins, made it possible to fish 

 with experimental nets — some treated, some untreated to serve as a 

 basis of comparison — under workaday conditions. This had not been 

 done before and was considered to constitute a most valuable part of 



« See Holman and Jarrell, quoted on p. 435, Document No. 998. These investigators found oleic acid 

 alone to have a marked deleterious eSect on cotton twine exposed to sunlight. Linseed oil and maiiy 

 other substances also caused the twine to weaken. Rosin was found to exert a detrimental influence, which 

 is contrary to our experience. The results of the two investigations can not be compared too closely, how- 

 ever, for in their experiments the twine was exposed to sunlight, and in ours was immersed in water where 

 only diffused light struck it. 



