EFFECTS OF SAWDUST ON FISH LIFE 



47 



SESSIONAL PAPER No. 22a 



entered the tank and left it continually. A few flat stones were placed here and there 

 on the bottom. 



The larger and more pugnacious ones took shelter beneath the stones, the smaller 

 and more timid ones were forced into the corners of the tank, driven away from the 

 stones by their bigger neighbours. I fed them regularly on small and well washed par- 

 ticles of meat, obtained by mincing small earthworms. These fine particles were flipped 

 into the water. As they slowly sank towards the bottom they were seized by the fry 

 and eaten with great avidity. The tank was always clean, and I had no trouble in 

 keeping the fry alive and healthy. 



In catching them for the experiments, I used a dip net. The slower ones were, 

 of course, caught first. At the end of three weeks the survivors had become so expert 

 in dodging the net that they were very difficult to capture. They had grown to about 

 inches in length and correspondingly heavy. The last few could be caught only by 

 drawing the water off from the tank. 



CONTROL EXPERIMENTS. 



The general method of conducting the experiments has been already indicated. 

 It consisted in immersing fish eggs, fry, fish food (such as aquatic larvae, worms, tad- 

 poles) and adult fish, in varying strengths of sawdust solutions and noting results. In 

 the vast majority of cases a control animal in tap water accompanied the regular ex- 

 periment, and observations were made upon both at the same time. Hundreds of small 

 minnows were used as well as the black bass fry already referred to. In some experi- 

 ments the minnows appeared to be the more robust, in other cases the fry. 



CRITICISMS. 



In some newspaper criticisms of my work at St. Andrews in 1900, objection was 

 made to the statement that sawdust poisoned the water. The writers held that there 

 was no poison in sawdust, and that it killed fish solely by taking out the oxygen dis- 

 solved in the water. They asserted that fish eggs and all forms of fish life were killed 

 by suffocation. To test this statement I took some of the yellowish brown sawdust 

 water and made a large quantity of air to bubble through it. When the air was thus 

 passing through the solution I frequently placed fish eggs, and adult fish in this aerated 

 water, but in every instance eggs and fish alike died. They died, therefore, not from 

 suffocation, but from the effects of the poison passing from the water through their 

 gill filaments, and into their blood. When not kept too long in the extract the fish 

 could generally be resuscitated by placing them in fresh water. 



DECAYING SAWDUST. 



One objection frequently urged against the practice of throwing sawdust into 

 streams and rivers is that the decaying sawdust imparts such a disagreeable odour to 

 the water that sensitive fish are driven away to other waters not so polluted. It 

 seemed to me, therefore, that some progress might be made towards a definite con- 

 clusion in this matter, if sawdust were allowed to stand for several weeks in an aquar- 

 ium and tested from time to time as to the changes going on in it, and the influence 

 of these upon fish. 



With this end in view about 1,000 grams of white pine sawdust were placed in an 

 aquarium three feet four inches long, fifteen inches wide, and filled up to sixteen and 

 a half inches deep with fresh water. This was done June 24. No water was allowed 

 to enter or leave the vessel. No direct sunlight fell upon it. 



The usual results followed, viz., a well defined layer of pale, yellow water about 

 three-quarter of an inch deep formed in a few hours and lay at the bottom. On top of 

 this was the perfectly clear layer about fifteen inches deep. 



