368 BULLETIN OF THE BUREAU OF FISHERIES 



local stock is fully reestablished by the first weeks of spring when (judging from 

 the year 1920) Pleurobrachia may be fully as abundant locally as it is in summer. 

 Its appearances and disappearances are so sporadic, not only in the Gulf of Maine 

 but also in European waters, where it is an equally familiar member of the plankton 

 (Kramp, 1913), that a long-continued series of records of its occurrence will be re- 

 quired before its seasonal fluctuations can be outlined more definitely. 



We have no satisfactory data on the absolute numbers in which Pleurobrachia 

 occurs in the Gulf of Maine, but obviously with so large an animal it requires only 

 a fraction as many individuals for the tow-net catches to be measurable by quarts 

 as for creatures as small as copepods to yield very moderate catches. Furthermore, 

 quantitative, hauls often fail to afford a true estimate of the local abundance of these 

 ctenophores even when they are plentiful, for they are usually so streaky in their 

 occurrence that the vertical net may catch only a few (or even miss them altogether) 

 at a locality where the horizontal net with its longer journey through the water 

 takes them in multitudes. For example, the quantitative haul yielded Pleurobrachia 

 at the rate of only 220 per square meter of sea surface (less than 10 individuals per 

 cubic meter) off Yarmouth on April 13, 1920 (station 20102), although half an hour's 

 haul of the meter net at 25 meters brought back upwards of 4 liters of them, and a 

 net of 20 centimeters diameter captured 1 liter at the surface. Again, in Massa- 

 chusetts Bay on March 4, 1920 (station 20058), the vertical net did not yield a 

 single Pleurobrachia (though its catch otherwise showed it to be working properly) , 

 whereas many hundreds were taken in the horizontal haul from 30 meters. 



Economic importance. — Pleurobrachia is an important factor in the economy of 

 waters where it abounds, chiefly as a destroyer of smaller planktonic animals but also 

 in some small degree as food for certain fishes. Wherever these ctenophores swarm 

 they sweep the water so clean and they are so voracious that hardly any smaller 

 creatures can coexist with them. Copepods in particular are locally exterminated 

 in the centers of abundance for Pleurobrachia, though in their own turn they may 

 swarm nearby; and it is common to find these ctenophores packed with copepods or 

 with euphausiid shrimps and larval fishes ingested and partially digested. 



There is reason to believe, too, that Pleurobrachia is a serious enemy to the suc- 

 cessful reproduction of sundry fishes (e. g., cod and haddock) by feeding on their 

 buoyant eggs (p. Ill), few of which can escape destruction in localities where cteno- 

 phores are numerous. Indeed, it is doubtful if more than a trifling proportion of the 

 fish eggs of any sort that are spawned on German Bank can survive there, with 

 Pleurobrachia so plentiful in that neighborhood the year round. In short, the local 

 abundance of the latter may well determine the productivity or otherwise of any 

 particular area in the Gulf as a nursery for gadoids or flatfish. Hence, it is fortunate 

 for the inhabitants of New England that the spawning ground for haddock on the 

 eastern part of Georges Bank seems practically free from Pleurobrachia. Neither 

 did we find it in any number on the haddock-spawning grounds off Massachusetts 

 Bay in May, 1920, notwithstanding its local abundance in the southern part of the 

 bay a few weeks earlier (p. 367), nor on the Isles of Shoals-Boon Island grounds in 

 April and May, 1913. 



