370 



Part III. — Fifteenth Annual Report 



offshore for spawning appears to be determined chiefly by the physical 

 conditions in relation to the safety of the floating eggs and their transport 

 to the places most suitable for the welfare of the young fishes derived 

 from them. If, for example, plaice were to spawn close to the shore on 

 the east coast, a very large proportion of the eggs would inevitably 

 suffer destruction by being stranded on the beaches before the embryonic 

 fishes inside them were far developed, or by being carried into water of 

 insufficient density, as at the mouths of estuaries, when they would sink 

 to the bottom ; and yet the habitat of the young of no fish is more 

 characteristically restricted to the shallow inshore waters than in the 

 case of the plaice. The plaice spawns early in the year when the water 

 is cold, and it possesses a relatively large egg, both conditions being 

 associated with a prolonged period of development and therefore a pro- 

 longed flotation. On the other hand, the dab or the sprat, which spawn 

 later, when the water is warmer, and which possess a relatively small 

 egg, shed their eggs to a considerable extent within the territorial 

 waters, the period of development being comparatively brief. Since the 

 two factors mentioned are so important in connection with the carriage 

 of floating eggs by currents, namely, the size of the egg and the tem- 

 perature of the water during the period of flotation, it is necessary to 

 consider them in some detail. 



The spawning period or season for all the more important species of 

 the food-fishes is now well determined. In the following table I give 

 the limits of the spawning season, the diameter of the egg, the mean 

 number of eggs produced by one female, and the duration of the period 

 of development within the egg of a number of species, as stated by 

 different observers : — 

















Mean 

 Dia- 

 meter 

 of Egg 

 in Milli- 

 metres. 



Mean 









Number 



Duration of Period 



Species. 



Spawning Season. 



produced 

 by one 

 Female. 



of Development within 

 the Egg. 



Plaice 



Middle of Jan. -middle of May. . 





300,000 



20 days at 41° F. 



Common Dab 



End of Feb. -middle of July . 



0-85 



106,000 



3 days at 45°-60° F. 



Flounder . 



Beginning of Feb. -end of June . 



0-92 



1,411,000 



3§ days at 54° C. 



Long Rough Dab 



End of Jan. -middle of May . 



11 



44,000 



14 days in March. 



Lemon Dab 



End of April-end of Aug. 



is 



411,000 



6 days at 53° F. 



Turbot 



Beginning of April-end of July . 



102 



8,500,000 



6-7 days in June. 



Brill . 



Middle of March-end of June 



1-3 



825,000 



8-9 days in May 



Cod . 



End of Jan. -beginning of June . 



1-38 



4,530,000 



15 days at 43° F. 



Haddock . 



Middle of Jan. -end of May . 



145 



450,000 



15 days at 43° F. 



Whiting . 



Beginning of March-end of Aug. 



12 



250,000 



10 days at 46° F. 



Saithe 



January- end of April . 



116 



6,500,000 



12 days in Feb. 



Ling . 



End of Mar.-end of Aug. 



1-08 



18,500,000 



9 days 



Gurnard 



Middle of April-end of Aug. 



145 



256,000 



6-7 days 



Sprat . 



Middle of Mar.-middle of Aug. . 



10 







The observations made by several investigators, and specially by Mr 

 Dannevig at Dunbar, have shown that, at the same temperature, pelagic 

 eggs with small yolks hatch more rapidly than those whose yolks are 

 large, and that the development or hatching of the eggs of a given 

 species is accelerated by an increase of temperature and retarded by a 

 decrease of temperature. It is obvious from the variations in the period 

 of development of the eggs of different species, as shown in the above 

 table, that they will be in very different degrees subject to the trans- 

 porting power of sea currents ; and that, other things being equal, 

 they will be carried very different distances before hatching occurs. 

 Exact observations are wanting as to the duration of the period of 

 development of the eggs of many species under different temperatures, 

 but sufficient is known to enable some conclusions of importance to be 



