270 BULLETIN OF THE BUREAU OF FISHERIES 



Relationship to temperature and salinity. — The geographic distribution of P. parvus 

 in the ocean in general points to moderately high temperatures as most favorable 

 for it, justifying Farran's (1910) characterization of it as a tropical and temperate 

 species. The many records of it in the Red Sea, around Ceylon, and in the Malay 

 Archipelago, often from hauls no deeper than the intake pipe of a steamer's pump 

 (A.Scott, 1902), make it probable that no temperature ever prevailing in the open 

 sea is fatally high or even unfavorably so for it. Toward the other extreme, the 

 presence of P. parvus at so many localities in the Gulf of Maine in February and 

 March (table, p. 299) proves it able to survive cooling down to 3 to 5°. In fact, the 

 actual localities and depths of capture locate it in water fractionally cooler than 2° 

 at three different stations; 45 but most of these February-March records are from 

 localities where the temperature was above 3° at some level between the surface and 

 the bottom (stations 20044, 20045, 20046, 20048, 20054, and 20081). Specimens 

 drifting into colder regions or levels of the gulf in early spring may perish, as any 

 animal finding its optimum environment in high temperature probably would. 



Thus, the zone close to the coast may well be a death trap for this copepod during 

 the coldest season, but the stock living in the basin can avoid winter chilling by 

 sinking to the deeper levels, where it would not experience a temperature lower than 

 4 to 5° in most years. Therefore, it would not be surprising if more extensive study 

 proves its zone of maximum abundance in the gulf to he at a greater depth during 

 the coldest season than during summer and autumn. Tending to corroborate this 

 prediction is the fact that the richest catches for March and April (stations 20054 

 and 20115) were in vertical hauls from 250 and 295 meters, respectively, where the 

 temperature below 150 meters was 5° or higher; and that the vertical nets fished 

 through zones of water warmer than 10° (below 100 meters) at the localities of the 

 "rich" catches off the southwest slope of Georges Bank for February (stations 20044 

 and 20045). 



Previous records locate P. parvus in salinities higher than 40 per mille in the 

 Arabian Gulf and as low as 19.33 per mille in the Kattegat. In addition it appears 

 indifferently oceanic or neritic, occurring from the open sea, on the one hand, to 

 tide pools, on the other (fide Dr. C. B. Wilson). Therefore, it is not likely that the 

 variations in salinity which obtain in the Guff of Maine are an important factor in 

 influencing its distribution there. Perhaps no member of the crustacean plankton 

 of the open sea can accommodate itself to greater fluctuations in the salinity of the 

 water than this little copepod. 



Endemicity and immigration. — The spawning of P. parvus has not actually been 

 recorded in the Gulf of Maine, but the fact that the species occurs there throughout 

 the year and is about equally widespread from month to month, though with a 

 definite periodic cycle in its abundance and in the regularity of its distribution, is 

 strong evidence that P. parvus does reproduce successfully in the gulf, and that 

 enough of the stock survives the winter to multiply to the frequencies recorded for 

 summer and autumn. The monthly averages for the percentages of stations at 

 which the species has been taken and for the numbers of specimens per square 

 meter both point to May as the commencement of the breeding season in the gulf; 



« Station 20056, whole column cooler than 1.19°; station 20058, whole column 1.39 to 1.43°; station 20081, surface 1.95°. 



