30 REPORT OF THE COMMISSIONER OF FISHERIES. 
all larval forms obtainable, and to follow, as opportunity offers, the 
life history of every available fish. When once a study has been 
carried out in a sufticient way, it will become possible to recognize 
any species of fish in any form in which it is encountered, and “then 
the data gathered from systematic collecting can be intelligently col- 
lated and used as a basis for correct inferences regarding the migra- 
tions and life histories of important fishes. 
The Bureau has been attacking this general problem in a serious 
way, and will continue to do so with the assurance that after a term 
of years sufficient knowledge will have been gained to remove some 
of the most palpable obstacles to its more effective service for the 
conservation and propagation of valuable food-fishes. During the 
early part of the fiscal year one of the regular assistants and a tem- 
porary investigator were engaged in such an investigation of the 
larval development of fishes at the Beaufort laboratory. In order 
to extend the territory of observation the investigation was resumed 
at Woods Hole in June, 1915, but the opportunities for collecting 
were found to be inferior to those at Beaufort. The director of the 
Beaufort laboratory was enabled to make observations during the 
winter season which thr ow light upon the breeding habits of such 
important fishes as the “gray trout” and the mullet. Additional 
material for study is obtained in connection with the oceanographic 
observations of the schooner Grampus and the steamer Fish Hawk. 
The same problem is being attacked from a different angle through 
a study of the markings of the scales of fishes, since recent scientific 
investigations have shown the possibility of ascertaining the age and 
of making certain inferences regarding life histories from the form 
and ar rangement of the minute markings on the scales. 
Such studies have not been confined exclusively to the marine 
species. A series of studies of the fresh-water fishes of the Missis- 
sippi River was in progress in Lake Pepin, but it has suffered tem- 
porary interruption through the transfer of the scientific assistant 
in charge to become director of the biological station at Fairport, 
Towa. In connection with the same station, one of the permanent 
assistants has been detailed to Keokuk, Iowa, during a considerable 
portion of the year. The presence of the dam across the Mississippi 
at this point creates a favorable condition for certain sorts of studies 
of the movements and habits of fishes. It is the desire of the Bureau 
to continue systematic observations in this region, not only for the 
purpose of gaining additional knowledge of the habits of fishes, but 
also with the object of learning from actual experience what is the 
effect of water-power developments upon the general condition of 
fish life in the larger rivers. 
An important field of study relates to the food of fishes. It is 
evident that the abundance of fishes in any body of water must be 
limited by the amount of available food. All fishes do not take the 
same sorts of food, nor does any given species of fish subsist upon 
the same kind of food at all stages of its existence. Furthermore, 
since fish do not lay by stores of food in time of plenty, excess at a 
particular season will not tide the fish over too extended a period 
of scarcity. It may then be said that the abundance of any fish is 
limited by the minimum quantity of its food present at the time 
when it is required. The Bureau finds itself unable to undertake 
