206 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
I therefore conclude that shad choose (or are impelled) to lay their eggs in the 
highest daily average temperature, a conditiou which would be realized about or soon 
after sunset, when the warmer water of the flats is intermingling with the colder 
channel water and establishing a balance. This suggestion is supported by the fact 
(at least on the Albemarle Sound and Potomac River) that the greatest number of 
ripe shad are taken off the edges of the channels. 
It is but a step further to infer that fish .so sensitively organized as to recognize 
the highest average of heat on its daily recurrence would readily perceive those tem- 
pered areas below creeks, which are relatively warmer. It is not impossible that they 
would detect an increase in heat here so slight that the ordinary thermometer would 
fail to record the advance. The air temperature during the run of the shad being on 
the increase, the creeks are naturally warmer than the river channels. The degree 
of heat increases from that of frosty mornings in the springtime to the hot days of 
June, or the time when tree foliage is full grown. The creeks, being sheltered from 
winds, absorb the sun’s direct rays in a relatively greater ratio than the deeper waters 
of the river proper. The heat is imparted to the water volumes of the creeks on the 
flood tide and to their mud flats (and conserved) on the ebb tide. 
Let the cause be what it may, it can not be denied that those river areas which 
are traversed by creek currents are the fields of preeminent adaptation for the natural 
spawning of shad. 
The reason for not attempting an application of my observations to the Susque- 
hanna spawning-grounds is that I am less familiar with the localities and have not 
made full studies of the charts in relation to the recorded receipts of eggs, the details 
of which form no part of the general office files at Washington. Apparently the large 
egg-production there (equal or perhaps superior to the amount obtained on any other 
of the rivers named) is in no way dependent upon creek currents. Since, however, the 
largest and most regular production of eggs on the Potomac is derived from the gill 
nets which are operated below and in the currents of creeks, viz, those at Moxley 
Point and White House, I can but infer that similar relative conditions are in force 
on the Susquehanna to effect such a great yield of eggs from the gill boats there. That 
the colder water in the channels and the warmer water on the contiguous flats or bars 
afford there the corresponding conditions seems altogether probable. The water from 
the two areas, commingling by gravity in the early part of the night would undoubt- 
edly establish at about that time the temperature of highest daily average. The 
numerous deep channels and vast expanses of flats seem to be sufficient to account for 
the large egg-production below Havre de Grace. 
