SEA-FISHERIES LABORATORY. 109 



The mean number of Oikopleura per haul was 1,000. The mean 

 number of copepoda captured by each mackerel was 330. 

 Two-thirds of this number consisted of Temora. The Oikopleura 

 were greatly in excess of the copepoda and the mean number 

 of the whole specimens was 717. Copepoda were much more 

 abundant in the plankton catches than Oikopleura, whereas 

 exactly the opposite was found to be the case in the stomach 

 contents. This appears to be clearly not an instance of pelagic 

 fish feeding upon the abundant organisms in its vicinity. 

 During the mackerel fishery off Walney, in 1913, when cteno- 

 phora, chiefly Pleurobrachia, were extremely plentiful in the 

 plankton we found that it formed practically the sole food 

 of the fish for at least a couple of weeks.* It may be an example 

 of selection on the part of the fish, as those gelatinous organisms 

 may have a higher food value than we have been accustomed 

 to think, but one can scarcely imagine the fish picking out the 

 comparatively few Oikopleura from amongst the large numbers 

 of copepoda round about it. Temora and Pseudocalanus were 

 three and five times more numerous in the plankton than 

 Oikopleura. The former was consumed at the rate of 1/14 of 

 its numbers in the plankton, and the latter at the rate of 

 1/160. The Oikopleura was consumed at the rate of 2/3 its 

 total in the plankton. It is possible that the mackerel caught 

 on August 6th had passed through a plankton much richer in 

 Oikopleura than was the case in the vicinity of Bradda Head. 

 This is supported to some extent when the condition of the 

 stomach contents is taken into account. Freshly eaten 

 plankton organisms are quite well preserved if the fish have 

 been feeding just previous to their capture. The semi-digested 

 stomach contents of the mackerel caught on August 6th 

 indicated that the food had been taken some time before. 

 The lateness in the appearance of mackerel off the S.W. of 



* The Mackerel Fishery off Walney in 1913, Fish. Lab. Etepfc. for 1913, 

 No. XXII, p. 19. 



