BIOLOGY OF THE ATLANTIC MACKEREL 213 



In this particular sample the special treatment concerned the larvae of 7-mm. 

 and upward. This was not uniformly true. The completeness of removal of large 

 larvae from the remainder varied with the character of the plankton with which they 

 were mixed and also, no doubt, with the fatigue of the person sorting the material. 

 Due to this -variation each haul was treated according to the internal evidence pro- 

 vided hy the counts therefrom. More often than not the relative counts of the 

 small sample and of the remainder indicated completeness of removal of smaller 

 than 7-mm. sizes from the latter so that the length of larvae concerned in the special 

 treatment was usually 5 or 6 mm. rather than 7 mm. as in the sample given. 



COMPUTATIONS OF CATCH PER STATION 



Standard haul. — Since it was desired to have a number representing the total 

 population of eggs and larvae at each station, regardless of depth, the catches were 

 converted to the basis of a standard amount of straining per meter of depth fished. 

 The standard amount selected was the average of actual performance, as measured 

 by the current meter during the first seven cruises of 1932, which was 17.07 cubic 

 meters of water strained per meter of depth fished. The average performance was 

 taken rather than any arbitraiy amount because it involved a minimum alteration 

 of original data, and the resulting figures represent nearly the actual numbers caugbt, 

 except for the last two cruises, when the adjusted two-meter net catches represent 

 approximately one-sixth of the actual numbers taken. Where an upper and a lower 

 net were employed, the standardized catch of the lower net was added to the stand- 

 ardized catch of the upper net after a correction for contamination was applied to 

 the numbers found in the catch of the lower net. The computations are illustrated 

 in table 15. 



The procedure for 2-meter-net hauls was exactly the same as for 1-net hauls 

 by 1-meter nets except that an additional factor of one-fourth was applied to 

 offset the quadrupled cross-sectional area of the net's mouth. Other things being 

 equal, this would have resulted in standardization factors about one-fourth as large 

 as those for the 1-meter nets, but actually the 2-meter net was towed somewhat 

 faster and its oblique path was somewhat more gradual due to a higher towing angle 

 in relation to the amount of line hauled in at each time interval. Hence the average 

 amount of water strained per meter of depth fished was about 6 times, instead of 4 

 times, as great as in the 1-meter nets, and the factors for standardizing accordingly 

 averaged about one-sixth. 



For both sizes of net, therefore, the resulting factors for standardizing given in the 

 columns headed "S factor" in tables 17 and 18 are such as to convert the catches at 

 each station to the equivalent of the numbers that would be found in a column of 

 water with a cross-sectional area 17.07 square meters, and extending from the surface 

 to the deepest level reached by the nets at each station. This may also be stated 

 as being equivalent to 21.7 times a vertical haul of a 1-meter net of perfect straining 

 capacity. 



