SEA-FISHERIES LABORATORY. 113 



fish, such as sand eels, sprats and rockling, are hunted and 

 captured. In the early part of the summer such forms as 

 copepoda, larval decapoda, and ctenophora usually constitute 

 the main food supply. Towards the end of the summer 

 one generally finds various kinds of young fish in the 

 stomachs. The purity or mixture of the stomach contents 

 gives a fairly sure indication as to whether the plankton 

 contains a particular kind of organism in abundance, or if 

 there is a variety with none more plentiful than another. 



The mackerel caught off Walney on June 30th, the first 

 favourable day for fishing after they were reported to be in 

 the area by one of the pilot boats, proved to have been feeding 

 on a very mixed diet. The food consisted of the following : — 

 Copepoda, Calanus, Temora, Centrojpages, Isias, Acartia and 

 Oithona ; Decapoda, such as megalopa of crabs, second larval 

 stage of Nephrops and larval Pagurids ; the ctenophore 

 Pleurobrachia, the arrow- worm Sagitta, and the pelagic ascidian 

 Oikopleura. One of the stomachs was crammed with a perfectly 

 pure sample of Temora. Another contained Temora in 

 abundance along with a number of Isias. All the others con- 

 tained a general mixture of the organisms. The occurrence of a 

 fish with pure stomach contents, in a sample where the majority 

 have a mixed diet, may be explained by the fish having probably 

 passed through a swarm of Temora just previous to its capture. 

 It may also have been caught some distance away from the 

 others, as a sailing boat traverses many miles in half a day's 

 fishing. When the first sample of mackerel was examined, 

 Pleurobrachia was present in small numbers in the plankton. 

 A rapid increase took place in the first week of July. During 

 the next two weeks the sea along the north-west coast of 

 Lancashire swarmed with this organism, which was quite 

 visible to the naked eye from the deck of a boat. The abundance 

 of ctenophora evidently presented the mackerel with a suitable 

 food supply, as every fish examined during the first two weeks 



