THE WESTERN MACKEREL FISHERY. 273' 



gested, that, until individual effort on the part of the fish to secure 

 food becomes necessary, the mackerel remain in shoals. 



These results tend to support the theory that mackerel feed upon 

 plankton only when shoaling. 



Owing to a certain general similarity in the samples examined, it 

 was found possible to tabulate the results of the analyses of the 

 stomach material. These are given in the Plankton and Food Tables 

 (III, IV, and V) which are appended. From nearly every position 

 noted therein the contents of six stomachs were examined, and, 

 by a system of adopting the highest comparative symbol placed 

 against the different species throughout the series, a single sample was 

 formed. This, in certain instances, is shown for comparison with a 

 plankton sample collected on the same position as that from which the 

 fish themselves were derived. 



It has proved convenient to arrange the results in three separate 

 tables, viz. Western Area, 1906 ; Western Area, 1907 ; and Plymouth 

 to Lizard Area, 1907. Lack of observation in this latter area in the 

 earlier year has prevented the formation of a separate table for 1906. 



It will be seen that only those principal species, which occurred more 

 or less regularly in the stomach contents, have been included in the 

 tables. These comprise seven species of Copepods, viz. Acartia clansi, 

 Calanus finmarchicus, Centropages typicus, Metridia lucens, Paracalanus 

 parvus, Pseudocalanus elongatus, and Tcmora longicornis. An examina- 

 tion of the tables will show that of these species Calanus finmarchicus, 

 Pseudocalanus elongatus, and Teviora longicornis are the most important. 

 The other species, although persistently occurring in stomach contents, 

 are hardly of sufficient importance for purposes of comparison. 



In addition to the Copepods there are included in the tables, Zoeae, 

 Sagitta hijmnctata and Oikopleura dioica. These species, together with 

 the Copepods, constitute the principal forms of zooplankton observed 

 in the stomach contents. 



For the Plymouth to Lizard area it was found necessary to include 

 young fish, or Crystcdlogohius. The last item for consideration, which 

 is noted on every table as " Phytoplankton, chiefly Phaeocystis glohosa," 

 refers to the material largely found in mackerel stomachs in the early 

 part of the season. This, upon examination, in many instances proved 

 to be diatom detritus, consisting of the shells, spines, chromatophores, 

 and the jelly-like substance associated with them, forming in a majority 

 of cases a glairy mass of a dark green tint.* Associated with this 

 material in a number of instances was a certain quantity of Phaeocystis 



* For the exact diagnosis of the uature of this material the writer is indebted to 

 Mr. A. J. Mason-Jones. 



