450 Birge — Vertical Distribution of Pelagic Crustacea. 



The numbers vary from 984 to 4212, with an average for the 

 whole month of 2229. The next level (3 — 6 m.) contained 

 on the average 29.27 per cent, of Diaptomus. The maximum 

 was 48.6 per cent. (J. 16, 12 m. ), the minimum 16.4 (J. 17,. 

 6 p. m.). The average number was 1257, the minimum 462, the 

 maximum 3244. This level showed on the whole the smallest 

 amount of variation. 



The 6 — 9 m. level varied greatly, as would be expected, since 

 it contains the lower part of the densely populated region. Its 

 average content was 14 per cent, of the catch, the maximum 

 32.5 per cent. (J. 27, 12 midnight), the minimum 3.4 (J. 17, 

 6 p. m.). It is noteworthy that in the second and third periods 

 the percentage found in this level at night was far larger than 

 in the day, the day average in the second period being 4.8 per 

 cent., while that at night was 17.6 per cent. In the third 

 period the numbers were 12.2 per cent, and 22.9 per cent, re- 

 spectively. In the fourth period the difference was very slight — 

 practically nothing, and in the first the day catch was slightly 

 larger in percentage, though almost exactly the same in numbers.. 



The center of population for Diaptomus, that is the level 

 above which 50 per cent, of the animals are found, lay on the 

 average just above the 3 m. level, if we assume a uniform dis- 

 tribution of the animals in each level. It varied from a depth 

 of about 2.4 meters in Period II, to 3.6 m. in Period III, and 

 3.3 m. in Period IV. The depth seems to depend upon the de- 

 gree of disturbance of the water. In calm water the Crustacea 

 aggregated nearer the surface, and had a wider distribution in 

 stormy weather when the water was disturbed. 



Diurnal distribution. — It will be seen from the accompanying 

 tables and plates that no noteworthy difference exists between 

 the vertical distribution of Diaptomus by day and by night. The 

 averages of the observations for each period show that there 

 was no tendency of the Diaptomi toward the surface by night, 

 or sinking by day. A closer study of the detailed results serves 

 to strengthen the conclusions drawn from the average. No 

 single observation shows any tendency toward a vertical move- 

 ment in this direction. In the middle level (3-6 m.) the day and 

 night observations are close together — practically identical — 



