200 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



found that the value increases from 5.299 to 5.378, indicating that the point for 3-mm. 

 in figure 17 should probably be raised by an amount nearly equal to the diameter of 

 the dot representing it. Similar examination of catches of 4-mm. larvae indicates 

 that the total for cruise V might be raised from 8,236 to 9,945, a change that becomes 

 imperceptible when worked through to the values on the graph of survival. 



On cruise VIII the stations at Fenwick, Winterquarter, and Chesapeake were 

 omitted. At the very most these could have contributed nothing to any of the 

 averages involving this cruise, excepting possibly a very few individuals in the 7-, 

 8-, and 9-mm. classes. These would not cause a perceptible change in the survival 



curve. 



By the time of cruise IX, only one larva was found along the New York section, 

 and it was so probable that none at all remained south of that locality that the omisson 

 of all stations from there southward could not have had any effect on the survival 



curve. 



Hence it may be concluded that the use of cruise totals introduced no errors other 

 than a slight lowering of the 3-mm. point on the survival curve. 



Turning now to the possibility that errors were introduced by the selection of 

 certain cruises, it will be recalled that the successive points on the survival curve con- 

 sist of averages of the catches in groups of cruises, using successively later cruises for 

 the successively older larvae so as to follow the main population through the season 

 from egg stages to late post-larvae. Owing, however, to exigencies of boat operation, 

 the cruises toward the end of the season were separated by wider intervals of time, so 

 that the average numbers of older larvae were calculated from samples more widely 

 spaced in time. This would tend to include relatively more submaximal values for 

 the older larvae than were included for the eggs and younger larvae. Although the 

 effect of this cannot be directly measured, it is possible to deduce the extreme amount 

 of distortion to be expected from the inclusion of submaximal values. 



This can be done by restoring submaximal values to the computation of the 

 average number of young larvae. For instance, for 5-mm. larvae, the average of the 

 catches for cruises III to VII, which were the ones used in the mortality determina- 

 tion, was 1,760. Inclusion of cruises I, II, and VII would restore submaximal values 

 and produce an average of 1,220. Substituting the latter figure in column 3 of table 

 7 and carrying the computation over to column 5 gives a figure of 4.387 instead of 

 4.547 for the 5-mm. class. This would lower the point for 5 mm. in figure 17 by 

 about 1H times the diameter of the dot representing that point in the graph. This 

 is a very small alteration brought about by a relatively large increase in submaximal 

 values. Therefore the inclusion of what was probably a relatively small number of 

 submaximal values for the older larvae by the method used in averaging cruises to 

 obtain the mortality curve could have lowered the points representing the older 

 larvae very little indeed, and therefore have altered the curve by only the slightest 

 amount. 



Next may be examined the distortion that could be connected with the growth- 

 rate data employed in computing the mortality curve. Evidences of the reliability 

 of the growth-rate determination were given in the section on that subject, and it 

 was concluded that the general course of the growth curves must be essentially 

 correct. It remains to be considered here whether there might nonetheless actually 

 have been irregularities in growth, and because they were not reflected in the growth 

 statistics used in computing mortality rates, they could have produced the observed 

 peculiarities in the survival curve. 



