THE WESTERN MACKEREL FISHERY. 275 



observed in the stomach material of fish from the same position, an 

 exhaustive examination of a fair number of stomach samples has 

 failed to show the presence of the same variety of organisms in the 

 stomach, as occur in the tow-nettings taken on the same positions as 

 the mackerel. Possibly this fact may be offered as additional evidence 

 in support of the food-layer theory already mentioned. 



In the early part of April, 1906, as in the two preceding months, 

 an unprofitable fishery had been carried on in the Bristol Channel by 

 the greater number of the steam drifters. From fishermen's reports this 

 condition had, it appeared, continued since the commencement of the 

 season without a single period of improved fishing. On the 20th of 

 April, however, some good shoals were struck to the south-west of the 

 Wolf by the sailing fleet. The greater number of the steam drifters 

 at once left the Bristol Channel and commenced fishing in a semicircle 

 to the south and south-west of Scilly. The result was that moderate 

 but regular catches of ten thousand down were landed from this area, 

 the fishing although light being general. The stomach contents of 

 samples of these fish did not differ very materially from those of 

 Bristol Channel fish (see Samples Nos. 66, 67, Table III). 



A line of plankton samples was taken at this time from Plymouth 

 to the fishing area south-west of the Wolf, the chief aim being the 

 determination of the relative quantities of zooplankton and phyto- 

 plankton* present on the fishing area and adjacent waters. The 

 analyses of the tow-nettings taken on 23rd and 24th April during this 

 crviise (see Samples Nos. 11-20, Table I) are interesting when viewed 

 in relation to the distribution of shoaling mackerel at the time. 

 Briefly summarized, it appeared that from Plymouth to the Lizard 

 phytoplankton in every example was in excess of zooplankton 

 (Samples Nos. 11, 12, 13, and 20, Table I). In the single position in 

 Mount's Bay where tow-nettings were taken this was also the case 

 (Sample No. 19, Table I), whereas in samples taken, on the approach 

 to the fishing area and actually on the ground (Samples Nos. 14-17), 

 there was a rapid decrease in phytoplankton, leading to a reverse of 

 the former condition, i.e. to an excess of zooplankton over phyto- 

 plankton. It may be mentioned that the excess of phytoplankton 

 over zooplankton was largely, though not entirely, due to the 

 presence in the samples of large quantities of Phaeocystis glohosa, 

 Scherffel. 



* Throughout this paper the word "phytoplankton" must be understood as referring 

 to the larger organisms, .such as are recorded in the plankton tables of the International 

 Bulletin. No investigations have been made on the minute plankton organisms which 

 Lohmann has included under the term " microplankton." 



