194 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



instance, one day. The resulting values are given in the fourth column, and the 

 logarithms of these (column 5) of table 7 are plotted in figure 17. 



Reliability ojihe survival curve. — The determination of the survival curve was based 

 on plankton hauls generally considered to be only approximately quantitative, it 

 utilized only selected portions of the original material, and it involved extensive 

 computations. The reliabdity of the result therefore depends not only on quantitative 

 adequacy of the original material, but also on whether the subsequent procedure in- 

 troduced any biasing influences. The following discussion will draw attention to 

 the facts which appear to have an obvious bearing on reliability. Unless some 

 pertinent features have escaped notice, the conclusion is inevitable that this survival 

 curve has surprisingly high reliability for all stages up to the length of 22 mm., or, 

 for the first 60 days of life. 



Considering first the collection of material, attention may be confined to those 

 influences that might possibly cause large larvae to be caught in relatively greater or 

 lesser proportion than small larvae, for it is only by such "size selection" that the 

 slope of the survival curve, and hence the conclusions as to mortality rates, could be 

 affected. On this score there are two possibflities: the nets' catching ability might 

 differ for different sizes of larvae; or the distribution of the larvae might vary in such 

 a way as to cause a less complete sampling of one size than of another. 



In the appendix (p. 215) there is given evidence which appears to be indicative, if 

 not conclusive proof, that the nets caught practically all the larvae in the paths of 

 their travel, at least up to the 22 mm. size; hence net selection was probably not a 

 biasing influence in this size range. 



Since the nets were fished from surface to below the thermocline, and since the 

 larvae probably do not descend below that point (p. 173), and since straining was sub- 

 stantially uniform for all levels fished, there is little likelihood that differential vertical 

 distribution was a biasing factor. There remains, then, the possibility that larvae of 

 different sizes had different horizontal distributions, and that these distributions 

 differed in a manner which would have affected the relative adequacy of the sampling 

 of the various sizes. 



For small larvae up to 10 or 12 nun. in length, the drift was determined (pp. 

 183 to 191) with sufficient precision to establish the fact that the population of these 

 sizes did not drift out of the area sampled. The majority of large larvae 22 to 53 

 mm. long, however, taken off eastern Massachusetts on the final (ninth) cruise, were 

 outside the area covered on earlier cruises. Could, then, a portion of the population 

 of medium sizes (12 to 22 mm.) have left the waters south and west of Nantucket 

 Shoals, that is, the area of survey, prior to the ninth cruise, and thus have been under- 

 sampled? If so, they should have been found in the intervening area during the eighth 

 cruise, which, fortunately, included that area. This cruise took place shortly after 

 the main portion of the larval population was in the 12- to 22-mm. size range. It 

 included stations around Nantucket Shoals and on the portion of Georges Bank just 

 east of the Shoals; M hence, in the area through which larvae would have been drifting 

 or swimming if they had, by this time, begun their movement north and east past the 

 Shoals. Since no larvae of these sizes were taken there, it seems unlikely that these 

 sizes were undersampled as a consequence of emigration from the area south and west 

 of the Shoals. In other words, the intermediate, as well as small sizes of larvae, were 

 sampled in approximately then- true proportions. 



M These stations of cruise VIII have not been included in any of the tables because the hauls there lacked pertinent material. 



