58 Part III. — Twenty-seventh Annual Report 



I have attempted to determine the rate of growth from the samples of 

 herrings here investigated. There is no simple method by which this can 

 be done. If it were possible to get a complete sample of all the herrings 

 in one area at a time, it might be possible by the measurement of the 

 same to arrive at a separation of the herrings into year-groups. But that 

 could not be done by confining attention to the length of the fi.sh. Many 

 other factors must be noted. 



Are the samples herein described representative of the shoals of which 

 they formed part ? The net is selective in that it allows the fish below a 

 certain size to escape, and may fail to take those above a certain size. 

 The seine net has a larger lunge than the drift net. The sprat net of the 

 Tay takes the very small fishes which both the seine and the drift net 

 permit to escape. The sample of fishes, therefore, is sometimes i-eally a 

 description of the net, not of the shoal. This fact seems to be markedly 

 brought out in the case of the seine net in the Clyde. Where the sample 

 contains small herrings, the smallest size is very often the same in different 

 lots. The following list gives the smallest size of the herrings occurring 

 in the samples caught by seine for each month: — ^February, 14 cm.: 

 April, 14 cm. : May, 14, 15 cm. : June, 15, 16 cm. : August, 14, 17 cm. : 

 September, 14 cm. : November, 14 cm. : December, 13 cm. It is very 

 clear that 14 cm. does not mark the lower limit of the shoal, but the lower 

 limit of the catching size of the seine net. 



A second difficulty is the following : — How far is a shoal an age-group ? 

 Is it composed of herrings of one age and class ? 



There is no fixed separation of age-groups from one another, at least to 

 the exclusion of one from the area occupied by the other. The mere fact 

 that small fishes are exposed to the attack of large fishes does not mean 

 that they cannot occupy the same area. Their survival depends on other 

 factors. 



In the case of herrings which are not liable to be devoured by each 

 other, there is nothing to hinder them mixing to any degree. And the 

 mixing of diflferent classes of herrings is very apparent in the Clyde. 

 Thus summer and winter spawners are found together. Brook said there 

 were two or more classes of herrings in Loch Fyne in summer. The fishes 

 there were a mixed collection of summer and winter spawners. Ripe 

 fishes and spents may be taken in the same haul. All the evidence 

 goes to show that herrings, wherever they meet, shoal together 

 for protection, but if food becomes scarce they separate in search 

 of it, to join up again with other herrings when their hunger is 

 satisfied. The little herrings in the tank at the Laboratory showed a 

 tendency when hungry to go individually, but when fed to rush ofi" to their 

 fellows and keep close together. The feeding shoals then may be 

 continually changing in their composition, various lots of herrings giving 

 temporary adhesion to any one body of fishes. 



If, however, it should occur that a shoal is an age-group, it is very 

 unlikely that the minute sample of herrings examined will contain the 

 complete range in size, since both the very small and the very large 

 individuals will be extremely few in number. The sample will probably 

 give a piece out of the middle of the curve of sizes, and it may give 

 approximately the mode. It is, moreover, 2:)ossible that the very big 

 individuals of an immature group might leave their smaller fellows and 

 join a shoal of adult herrings. 



The age-groups are becoming rapidly reduced in numbers as they giow 

 older, and the fishes composing them grew longer. The curve representing 

 the numbers of all herrings at each length is a steadily descending one. 

 Out of this curve, then, each net may simply nip a sample suited to its own 

 character. It will not work sharply at its limits of capture, and the 

 sample may probably tail oflf gradually at each end, giving an apparent 



