270 PLANKTON STUDIES IN RELATION TO 



In certain cases the samples were collected by two separate nets, of 

 coarse and fine mesh respectively. 



As this paper does not deal with plankton other than in its direct 

 bearing upon mackerel food, it has been considered desirable, for pur- 

 poses of easy reference, to include the surface hauls with those taken 

 from bottom to surface (oblique) under one column for each position 

 noted on the Plankton Tables (Tables I, II). In thus combining the 

 analyses of the surface and oblique hauls, it may be mentioned that 

 the comparative signs (which are those adopted in the International 

 Plankton Investigations) placed against the Phytoplankton species are 

 those derived from the analysis of the surface haul, whilst for Zoo- 

 plankton the higher of the two values recorded for the surface and 

 oblique hauls is used. 



In the determination of the plankton samples, a general examination 

 was first made under magnification x 48 and x 160 in a shallow 

 trough. The phytoplankton was subsequently examined under cover- 

 slip with higher powers, x 520 to x 824, etc. 



In the greater number of cases six stomachs were collected from 

 every catch and preserved in 10 % formalin. The contents of each were 

 subsequently washed out into Petrie dishes and examined in a similar 

 manner to that described for plankton. The whole of the sample, 

 unless very bulky, was examined. Other modifications of detail with 

 reference to stomach samples are explained under a later heading^ 

 page 273. 



My thanks are largely due to Dr. E. J. Allen, who has materially 

 assisted me throughout the investigation and in the preparation of this 

 paper ; also to Mr. Mathias Dunn, for much valuable help in the 

 collection of observations and material. I here also desire to express 

 my thanks to Mr. Howard Dunn, Mr. D. J. Mathews, Dr. L. H. Gough, 

 and Mr. W. Bygrave, for assistance in the collection or identification 

 of material, and to Mr. A. E. Hefford for help in the preparation of 

 this paper. 



11. THE WESTEEN SPPJNG MACKEEEL FISHEEY. 



The factors, which influence the inshore migration of mackerel in 

 greater or less abundance, have long been the subject of inquiry. That 

 the purpose of spawning is the primary object of such migration has 

 been admitted, but it still remains to show reasons for the greater or 

 less extent of this migration, which from a commercial standpoint, 

 under the present system of drift fishing, is equivalent to a good, bad, 

 or indifferent season. Many previous authors have shown that 



