482 BULLETIN OF THE BUREAU OF FISHERIES 



in proportion to the intensity of setting, was extremely small, as, for example, at 

 Station 3, where the total number collected in a period of several weeks scarcely 

 reached 100, while in the same spot many thousands of them were found later attached 

 to the shells and brush. In studying the occurrence of the larvae in relation to the 

 stage of tide when the sample was taken, a rather definite relationship was found 

 to exist; namely, that the larvae were most abundant near and at the period of low 

 water and generally absent at the time of high water. In 1926 the methods of collec- 

 tion were changed and planned so as to permit study in greater detail of the occur- 

 rence of the larvae in relation to stage of tide. For this purpose two quantitative 

 methods of plankton collection were employed, one of which was to take 3 samples of 

 50 gallons each from the top, bottom, and middle zone at each stage of the tide, and 

 the other was to pump the water continuously from a point 1 foot below low-water 

 mark (which corresponded to the level of the spawning bed) and determine the 

 number of larvae present at each hour of the day and height of the tide. In 1926 

 the larvae were found in the samples at a much later date and again occurred in two 

 rather distinct "schools" or groups following light spawning on July 22 and heavy 

 spawning on August 1. The larval period of the first group was approximately 16 

 days at a mean temperature of 21.3° C, and of the second group was 14 days at 

 23.2° C During this summer, more than 185 plankton samples were collected, the 

 majority of which were taken in MUford Harbor within a short distance of the 

 spawning bed and especially over the area of heaviest setting. ' 



DISTRIBUTION 



The abundance of the larvae from the surface of the water to the bottom was 

 found to vary according to the stage of tide, which for convenience was divided into 

 three arbitrary periods — namely, low water, intermediate, and high water — each of 

 which covers an interval of four hours. The low-water period covers the last two 

 hours of ebb tide, slack water, and the first two hours of flood tide. The intermediate 

 period included the two hours of flood tide and two hours of ebb tide when the tide 

 was approximately halfway between high and low water marks. The high-water 

 period consisted of the last two hours of flood tide, slack high water, and the first 

 two hours of ebb tide. At mean range of tide the vertical movement of the water 

 during the low-water period is approximately IJ^ feet; during the intermediate 

 period, 3J^ feet; and during the high-water period, 1}^ feet. 



In 140 samples made with reference to the tide the larvae were found to be most 

 abundant during the low-water period, virtually absent during the intermediate 

 period, and present only in small numbers during the high-water period. In the 

 following table the average number of larvae per 50 gallons that were collected during 

 three complete tidal cycles from August 11 to 13, 1926, is given as an example. In 

 this table the counts of the larvae include only those that have passed the straight- 

 hinge stage, or, in other words, "umbo" larvae that were from 3 days old to setting 

 size; and, of these, the majority in nearly every case were found to be in late stages 

 of development and within a few days of setting. The only time when umbo larvae 

 of different ages were found swimming in the water was at the time of low slack water 

 and even then the older forms predominated. 



