110 



TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



the Isle of Man was clearly not due to any scarcity of zoo- 

 plankton organisms. On July 16th and 17th, when the 

 phyto-plankton invasion was at its maximum and the hauls 

 with the fine nets contained from 11 millions to 15 millions of 

 Rhizosolenia stolterfolhii alone, besides large numbers of other 

 species, copepoda and Oikopleura were more abundant than 

 on August 6th. The mean number of copepoda per haul on 

 July 16th was 67,000 along with 5,000 Oikopleura and on the 

 following day 108,000 copepoda and again 5,000 Oikopleura. 

 On July 31st, when the invasion of phyto-plankton was 

 obviously dying out, the mean number of copepoda per haul 

 was 55,000 along with 1,000 Oikopleura. When the phyto- 

 plankton was at its maximum it completely obscured the 

 zoo-plankton organisms and a naked eye examination only of 

 the collections gave the impression that copepoda, etc., were 

 very scarce. It was only during the microscopic investigation 

 of the hauls that the true proportions of phyto-plankton and 

 zoo-plankton became known. 



The following table gives the mean numbers per haul of 

 zoo-plankton organisms present on the three dates in July and 

 on August 6, which were represented in the mackerel stomachs 

 of the latter date. The last column is the mean per fish for 

 comparison with the mean per haul. 





July 16 



July 17 



July 31 



Aug. 6 



Aug. 6 





1 



15 



13 







902 



13,000 



9,800 



21 



32 



2,250 



40,825 



5,375 



2 



2 



3 



5 



581 



10,275 



5,150 



61 







2,862 



89,340 



5,125 



0-5 



1 



0-5 



75 



785 



2,515 



2,470 



67 







715 



48,387 



1,185 



3 



33 



5 



96 



753 



5,468 



3,012 



101 



4 



3,186 



33,930 



1,020 



1 



Zoea 



10 



" Mysis " 



18 





1 



Calanus 



Pseudocalanus 



Temora 



Centropages 



5 

 33 



217 



2 



1-5 





52 





20 





717 







