398 G. E. BULLEN. 



measured or weighed, but certain of both species appeared to have 

 reached sexual maturity before they died. The greatest mortality 

 with both herring and mackerel takes place during the summer, and 

 this may be due to the small quantity of water in the tanks, which are 

 furnished, however, with air tubes, which continually circulate the 

 water, day and night." 



From personal observation of some mackerel which had recently 

 been installed in one of the tanks at the Brighton Aquarium in 

 October, 1910, the present writer would estimate the average length 

 to be not more than 9 inches. In this connection it may be noted that 

 Cunningham* states that the smallest ripe specimens which he had 

 examined were 11-6 in. to 12'9 in. long. 



In specimens measuring about 9 in. in length, seined at Sidmouth in 

 1911 towards the end of July, the writer observed no sexually mature 

 fish, and the same condition obtained in several shoals taken by the 

 seine at Brighton in October, 1909-10. It is fair to suppose, therefore, 

 that the mackerel forming the subject of Mr. Cowley's observations 

 were not sexually mature fish when placed in the tanks, and according 

 to Cunninghamf about a year old. 



Now, although it may be urged that observations taken upon fishes 

 acclimatized to the abnormal conditions obtaining in an aquarium are 

 open to question, certain points of interest present themselves when 

 Mr. Cowley's remarks are considered. In the first place, it will be 

 seen that mackerel and herring can exist, for a reasonable period, all 

 the year round upon food of a type similar to that which is hunted 

 by sight — we may almost say food of a more solid character than 

 plankton. Secondly, although the mackerel is known to be an active 

 swimmer and voracious feeder, its appearance as it moves leisurely 

 round the tank at a pace of not more than two miles an hour when no 

 disturbing influence is present, might suggest that it is either aware 

 of the fact that the tank water does not support sufficient plankton to 

 form a good meal, or that it is indifferent to it in view of the fact 

 that it knows that food of a more solid nature is offered to it at 

 regular intervals. 



Eeturning to the opening question raised in the first paragraph, it 

 may be suggested that here is further evidence to show that mackerel, 

 feeding by sight upon prey of a large character, are incapable of taking 

 nourishment by means of filtration. 



Mr. Dunn's observations tend to show that the larger plankton 

 organisms are hunted by the mackerel by sight, the fish thereby 

 exercising a selective capability. 



* Cunuiugham, Hark, Mar. Fishes, p. 315. t Id. 



