158 SIR W, A. HERDMAN : RESULTS OF CONTINUOUS 
form a very important part of the: food of migratory fishes such as the 
herring and mackerel, and of the younger stages of many if not most of 
the other edible fish. 
Out of the six most abundant species of Copepoda dealt with in the former 
paper on plankton published by the Linnean Society (* Spolia Runiana,” III. 
1918), I shall now select two—the small but very abundant Oithona helgo- 
landica and the much less numerous but far larger Calanus finmarchicus— 
for special examination over the series of years. There is no doubt that both 
are important food-matters in the sea. The average number, per haul, of 
Oithona is over 8000 and of Calanus about 260. 
Oithona is the most generally abundant Copepod throughout the year in 
the Irish Sea, but the months when it is taken in greatest numbers are June 
to November (incl.), with the maximum generally in July. There may 
also be a second maximum in October or November. 
Oithona had a run of four “strong” years, 1911 to 714 incl.; and of 
these *11 and '14 were the strongest, the largest single hauls being over 
225,000 on July 18th, 1911, and just under 200,000 on November 9th, 1914. 
There was another record year in 1919 when the maximum was unusually 
early (June-July) and the largest of any year in the series, averaging about 
100,000 per haul for two months—100,100 on June 2nd and 115,280 on 
July 31st. The poorest years have been the first three (1907-’9) and, more 
recently, 1916 and 717, 
Calanus finmarchicus is a northern oceanic form, and its centre of distribu- 
tion seems to be the North Atlantic to the south of Iceland. Although a few 
specimens are to be found in the plankton hauls all the year round in the 
Irish Sea, whenever large numbers appear suddenly, as they commonly do in 
summer or autumn, that may be taken as an indication of an invasion of 
oceanic water with some of its contained plankton. 
Apart from these periodic invasions, which are generally in July or early 
August, the highest numbers between May and October are generally between 
1000 and 4000. When the numbers suddenly run up to 20,000 or 50,000 
the cause is an invading swarm which has appeared, and which generally 
disappears again in a few days. In 1909 we have evidence of an unusually 
large swarm that entered Port Erin bay on July 17th and 19th, when hauls 
estimated at 20,000 each were obtained, while the official gatherings taken 
on July 15th and 21st gave no evidence of unusual numbers. Then, again, 
on July 11th, 1916, a swarm of Calanus appeared in the bay, when over 
12,000 specimens were taken in one net, while a few days before a similar 
haul gave only 10 specimens and another a few days after gave only 200— 
the swarm was rapidly disappearing. An unusually high and unusually 
early record was 50,720 on May 17th, 1912. There is evidence of other 
less-marked swarms on July 4th and 18th, 1911. 
