168 A CENSUS OF THE SEA [PART II 



by the cod, plaice, haddock, whiting, and so on; the average 

 duration of the incubation periods of the eggs belonging to these 

 species; the spawning periods; the characters of the eggs and 

 larvae ; the ratio of the sexes in the adult phases ; the rates of 

 growth ; the distribution, and so on. If it is possible to estimate 

 the actual numbers of the eggs belonging to edible species of fishes 

 inhabiting a sea of kno'svn area, which are produced during the 

 entire spawning season of the year, then it is also possible to esti- 

 mate the actual numbers of adult fishes of the same species in- 

 habiting the same area — ^not by attempting to calculate (as Haeckel^ 

 supposed that Hensen did) what numbers of adults would result 

 from the growth of these eggs, but by calculating w^hat numbers of 

 mature females had produced the eggs. This is what Hensen and 

 Apstein attempted in the "Nordsee Expedition'" of 18951 



Such an investigation presents fcAver difficulties than are en- 

 countered in other quantitative plankton researches. Fish eggs 

 are comparatively large organisms, and therefore nets of fairly large 

 mesh (No. 3 Miillergaze) can be employed so that the filtration 

 capacity of the apparatus can be calculated more accurately than 

 in the case of the finer nets employed to catch diatoms and pro- 

 tozoa ; the eggs studied are specifically lighter than water and 

 float comparatively near to the surface, so that it is easier to work 

 the net and obtain fair samples : and they are quite im motile and 

 are not heliotropic, and so are more equally distributed than are 

 copepods, diatoms or larvae. The spawning period of most sea 

 fishes lasts about two months, but there is always a maximum 

 during which more eggs are produced than at other times. The 

 incubation period of most fish eggs varies from a week to a fort- 

 night, according to the species and temperature of the sea. Sup- 

 pose that a plaice egg takes exactly fourteen days to develop : at 

 the end of this time the larva hatches out and the egg disappears. 

 The average number of plaice eggs found per square metre of sea 

 must then be doubled to give us the total average number pro- 

 duced during February. By making very frequent observations ; 

 by considering the incubation periods of each species of egg cap- 

 tured : and by taking into account the influence of temperature on 



^ In the Plankton-Studien. 



•- Wiss. Meeresunt. Kiel Komm. Bd. ii. Heft 2, 1897. 



