180 Part TIL — Twenty-second Annual Report 



The Table giving the particulars of the sprats obtained in the various 

 collections is given on page 178, and a comparison may now be made 

 between the average size indicated for the various groups. 



If the difference between the average size of the groups be calculated, 

 it will be found that the amount between the first and second series is as 

 follows in the various months : — 23'lmm. for all the December collec- 

 tions combined, 33*2 for April, 42*7 for October, 19*9 for November; the 

 mean for the four being 29"7mm., or about one and three sixteenths of 

 an inch. The difference in some of the cases is considerable, and this is 

 owing in large measure to the very small numbers obtained, and to the 

 fact, still more, that the great majority of the smaller specimens escaped 

 through the meshes of the net. The large difference in the average size 

 in October is due to the fact that the first series was represented by four 

 specimens got in the tow-net and measuring from 31 to 45mm., and one 

 specimen of 54mm., and, on the other hand, to the average of the few 

 specimens in the next series being too high, as already referred to. The 

 low average for November was caused by the opposite, and especially by 

 the average for the first series being exceptionally high. From this 

 circumstance, the fact that only the larger specimens of the first series 

 were taken in the small-meshed net, comparison may also be made 

 between what I have termed the mean, which is based on the intermediate 

 size between the largest and the smallest in a group. This system has 

 also its disadvantages, unless the largest and the smallest fishes present 

 fairly represent the limits of the series, but it tends to diminish the pre- 

 dominance of the larger fishes in obtaining the arithmetical average. On 

 this basis, the respective differences between the averages of the first and 

 second groups are these : — 35*0 for April, 38 for October, 27 for Novem- 

 ber, and 30 for December, the mean of the lot being 32-5mm. The 

 presence of small fishes in the tow-net, as small as 39mm. in December, 

 and 40mm. in April in the shrimp-net, shows that the true average is 

 under that arithmetically calculated. 



The differences betv-fn the averages of the second and third series are 

 as follows :— March 21*0, April 36-6, May 37-5, October 30*4, November 

 38'9, and December 38*5, the mean of the differences being 33"8. This 

 amount is rather above the natural difference owing to the fact adverted 

 to, that the larger fishes, many of which no doubt belong to a fourth 

 group, are included in the third group, and thus the average of the latter 

 is somewhat raised. The mean of the combined differences is, calculated 

 on the other basis, 35-2mm., or a little over 1| inches, and this probably 

 represents the amount of annual growth between one series and another 

 in the sprat. 



As already stated, the imperfection of the collections of the first or 

 younger group of fishes does not allow an accurate calculation of the size 

 of that group to be made, but from the sizes obtained in April, December, 

 and September it is certain that the range and the average are under 

 what is calculated from the sizes represented. In order to throw light on 

 the subject, I have made a curve (PI. X), based on the measurements of 

 the best collections, showing the gradual growth of the sprat in the different 

 generations. From this, it appears, that at one year of age, about the 

 beginning of June, the average size of the sprat is a little over 60mm., 

 and when two year's old, at a corresponding period, about 93mm. There 

 are not sufficient data to show the precise size in the next June, but, as 

 in the end of March and the beginning of April the average size is about 

 118mm.. it is probable that at the beginning of June the average size 

 would be a little over 120mm. This would indicate an approximate 

 growth of 30mm. between the first generation and the second, and 27mm. 

 between the second and the third. 



