REPORT OF THE COMMISSIONER OF FISHERIES. 21 



controlled results will accrue from this method than are now at- 

 tained by the infection of fish liberated in the open waters with all 

 the attendant uncertainties as to the fate of the little mussels dropped 

 fortuitously. 



Statistical and biological surveys have been conducted during the 

 year in Lake Pepin and Lake Pokegama, in both of which mussel 

 culture has been carried on for several years. In Lake Pepin there 

 has been also an alternate sectional closure to commercial mussel fish- 

 ing. The survey of Lake Pepin indicates that there has been an 

 increase in the commercially important mucket {Lampsilus lideola), 

 propagated by tlie bureau, while the other species not propagated 

 have decreased. The facts developed in this survey are the most 

 conclusive evidence of the value of mussel culture so far obtained. 



The personnel of the Fairport laboratory has been active in assist- 

 ing the State authorities in developing rational legal measures for 

 the conservation of the mussel beds, as a consequence of which the 

 States of Minnesota, Iowa, Wisconsin, and Illinois have entered into 

 cooperative closure of alternate sections of the Mississippi River 

 between Brownsville, Minn., and Keokuk, Iowa, and Arkansas has 

 announced the intention to pursue the same system of regulation oil 

 the Black and White Rivers. 



MOSQUITO CONTROL. 



Investigations relative to mosquito control by means of fish, par- 

 ticularly the top minnow Gambusia, were continued at Augusta, Ga., 

 from July 1 to November 10, in cooperation with the U. S. Public 

 Health Service. 



The problems under immediate consideration were the effects of 

 various types of vegetation on mosquito production; the effects of 

 increased competition for food among Gambusia upon the rate of 

 mosquito production; sex frequency and the relative vitality of the 

 sexes of Gambusia ; and the proper treatment of the fish in captivity 

 and under shipment. 



Tests made with various types of vegetation, including some that 

 have been considered inimical to mosquito production, show that few, 

 if any, of them prevent mosquitoes breeding, but that many species 

 of plants do not form barriers between top minnows and mosquito 

 larva and are therefore neutral in their effects on mosquito produc- 

 tion in waters containing the minnows. It was shown also that it is 

 not always the dense vegetation, as is commonly believed, that 

 forms the" best protection for mosquito larvae against fish. 



Competition for food among Gambusia is advantageous, and even 

 in the presence of a serious barrier complete mosquito control may be 

 approached if sufficient minnows are present. The observations on 

 the vitality of the sexes indicated that the males succumb more easily 

 than the females. 



Under shipping conditions Gambusia lived equally well in vessels 

 of the same diameter wlien only one-third full of water as they did 

 when the vessels were filled two-thirds full, and tests indicate that 

 shipments can be made most successfully when the temperature of the 

 water ranges from 83° F. downward. During December, 1922, 200 

 Gambusias were shipped to Palestine with a loss of but 30 en route. 



