DISCUSSION. 



Mr. J. J. Stranahan (U. S. Fisheries Station, BuUochville, Ga.). I would like to 

 ask Professor Birge a question or two. It is not closely related to this subject, although 

 not entirely foreign. I desire to ask if you have had any experience as to the plankton 

 and the relative amount of crustaceans and other animal life that is in soft and hard 

 waters. 



Professor Birge. Yes; although the matter can not be finally settled. It is a 

 condition well known to European investigators. The hard-water lakes are much 

 richer in plankton on the average than the soft-water, but you can not make the state- 

 ment that every hard-water lake has more plankton than any soft-water. 



Mr. Stranahan. At Guilford, in England, south of London; at Castalia, in Ohio; I 

 think at Northville, in Michigan, and wherever I have known of exceedingly hard water, 

 full of lime and other salts, there has been an excess of plankton. I collected plankton at 

 Castalia to take to the World's Fair at Chicago, and with pretty carefully conducted 

 weights the amount of crustaceans exceeded the amount of mosses and aquatic plants 

 in which they were congregated when taken from the water. That water is so rich in 

 lime and magnesia that it makes stone of a shingle in a year, and that is probably the 

 greatest trout preserve in the world. At BuUochville, Ga., where I am now, our water is 

 practically aqua pura; we can use it in photographic processes; it is exceedingly soft. 

 We have put mollusks in it, but the different little periwinkles and other shell-covered 

 species die in a few months, and it is not conducive to fish culture. We took two 

 carloads of cement and buried it in our spring, and we thought we could see a marked 

 increase in the growth of some kinds of vegetation, to say nothing of small water animals 

 that grow in it. So I am a great stickler for the idea that we ought not to put fish 

 hatcheries where there is not a large amount of calcareous material in the water. 



Professor Birge. I wanted to talk about that, but with the limitations on the 

 time I could hardly get to that part of the subject. 



Doctor NoRDQVisT. Those investigations Professor Birge has made are of the 

 greatest importance in fish culture in lakes. I am of the same opinion as Professor 

 Birge, that many of the disappointments that we have in fish culture are due to lack of 

 knowledge about the amount of oxygen and the biological conditions of the lakes. 



I would only ask Professor Birge about some slight points here in his investi- 

 gations. At what time in the day was the amount of oxygen determined? 



Professor BirgE. We have made a great many attempts to discover diurnal 

 variations in oxygen, but it is a very rare thing to find any difference in the results 

 obtained from tests made in early morning, late afternoon, and late evening. We 

 have continued the tests throughout the twenty-four hours. We have found very 

 small, hardly perceptible, diurnal differences. I have received, since I came to this 

 congress, a letter from my assistant, Mr. Juday, to whom much of this work is due, 

 telling me that he had found such a difference in Lake Mendota. 



Doctor NoRDovisT. That is just what I mean with reference to the investigations; 

 it ought to show a difference, and I think that the curve that you have given in many 

 of your lakes at the depths of six, seven, to ten meters may perhaps 



Professor BirgE [interrupting]. We have spent from two to three weeks hunting 

 for diurnal changes right on that point. I put a party on those lakes — Otter Lakes — 

 and we got negative results all the time; the curve remained substantially the same. 



