SEA- FISHERIES LABORATORY. Ill 



It is apparent from the above tables that the obvious 

 abundance of the zoo-plankton organisms noted by Professor 

 Herdman on August 6th was evidently not the entire cause 

 of the sudden, although late, invasion of mackerel in 1919. 

 The tables show quite clearly that the same zoo-plankton was 

 far more abundant three weeks earlier when no fish were being 

 caught. The presence of a very abundant phy to -plankton 

 along with the zoo-plankton may have had some relation 

 to the absence of the fish. There was a general scarcity of 

 mackerel in the northern parts of the Irish Sea in 1919. The 

 fishery off Walney was a complete failure, and phyto-plankton 

 was abundant in Barrow Channel up to the first week of August. 

 The fact that there was a sudden appearance of the fish just 

 as the invasion of phyto-plankton died out, indicates that 

 either the phyto-plankton itself or the causes which produced 

 and prolonged the huge invasion of Rhizosolenia, etc., had a 

 delaying influence on the approach of such pelagic fish as the 

 mackerel to our inshore areas. The prolific mackerel fishery 

 off Walney in 1913, lasted from the middle of June till the 

 beginning of September. It occurred when there was a pure 

 zoo-plankton which for a time consisted of ctenophora only. 



