BIOLOGY OF THE ATLANTIC MACKEREL 



209 



Table 13. — Size of mackerel at maturity as indicated by 1,116 individuals taken by traps in the 

 vicinity of Woods Hole, Mass., and in Massachusetts Bay during the period, June 24 to July 21, 

 1925.— Continued. 



METHODS OF COLLECTING EGGS AND LARVAE 



Mackerel eggs and larvae were collected during the spawning season in the spring 

 of each year from 1926 to 1932, inclusive. The initial work was exploratory and quali- 

 tative in nature. Tows during the period 1926 to 1929 were drawn horizontallv at 

 the surface, mid-depth, and just above bottom. In 1930 and 1931 oblique hauls were 

 employed. In 1932, oblique hauls were continued, and a device employed to measure 

 the quantity of water strained through the nets. The following description refers to 

 the collections made during 1932. 



Nets used. — The plankton net used during the first 7 cruises was 1 meter in diame- 

 ter at the mouth, and 4 meters long. The first meter of length was cylindrical and 

 composed of No. millers' gauze with 15 meshes per lineal centimeter, and for the last 

 3 meters the shape was conical and the material of No. 2 gauze, with 21 meshes per 

 lineal centimeter. At the end of the cone, attached by a coupling device, was a 

 "cod-end" 5 inches in diameter and 10 inches long, of No. 12 gauze, in which the catch 

 collected. During the ninth and tenth cruises, a stramin net was used, which was 

 2 meters in diameter (at mouth), and of the same proportions as the meter net. 



Method oj towing. — To sample uniformly throughout the range of vertical dis- 

 tribution of eggs and larvae, the method of oblique towing was used. This consisted 

 of paying out an amount of line appropriate for the maximum depth to be reached by 

 the particular tow, then hauling back a certain amount of line at fixed intervals of 

 time, usually 5 meters every 2 minutes or 2 meters every 1 minute, until completion 

 of the haul. During the period of hauling, the speed of the ship was kept as nearly 

 uniform as possible. 



During the first seven cruises, when 1 -meter nets were used, one net was towed 

 at the shoal stations where the water was nearly uniform from surface to bottom, and 

 two nets at the deeper stations where thermal stratification of water was prevalent. 

 At the stations where two nets were used they were attached to the towing cable at 

 intervals estimated to be appropriate for the upper net to sample down to the thermo- 

 cline and the lower net a nearly equal distance below the thermocline. In a typical 

 instance, with a sounding of 50 meters, the lower net would be attached at the end of 

 the line, the upper net 25 meters from the end, and another 25 meters payed out, 

 making 50 meters of line all told. Towing at the usual speed, the line would sfr&£j 1. , . / 



