of the Fishery Board for Scotland. 



43 



during the greater part of larval life, the advantage of which is enormous. 

 The fecundity is indeed, in a certain sense, a measure of the natural 

 destruction that takes place in the individuals of a species. Since, on an 

 average, two parents require to produce eventually only two reproductive 

 adults in order to keep up the normal number of the species, it follows 

 that the product of all the other eggs, except those which give rise to the 

 two individuals in question, are destroyed at some period or other of their 

 lives ; from two or three million cod eggs only two reproductive indi- 

 viduals survive, on an average, to carry on the species and maintain its 

 normal numbers. 



It is well known that it is in the early stages that the greatest destruc- 

 tion occurs in nature, that is to say, in the embryonic or egg stage and in 

 the larval stage. There is not sufficient information to enable one to 

 apportion with exactness the relative destruction that takes place in the 

 egg stage and in the larval stage, or in the later stages, but there are 

 reasons which show that the natural loss of pelagic eggs must be very 

 large, as in the case of the plaice, the cod, and most of the food fishes. 

 The fecundity of fishes which have demersal eggs, lying on or attached to 

 the bottom, is very much less than the fecundity of fishes which produce 

 pelagic or floating eggs. Thus, the mean number of eggs among twelve 

 species investigated in which the eggs are demersal was 24,700, whereas 

 the mean number among twenty-three species with pelagic eggs was 

 2,388,000, or nearly 100 times greater. This enormous difference in the 

 fecundity cannot, however, be taken as an exact measure of the greater 

 natural destruction of pelagic eggs as compared with demersal eggs, for 

 some other influences are in operation ; but there is no doubt that a great 

 part of the difference is to be explained in this way, and that large 

 numbers of the pelagic eggs, at the mercy of the waves and currents, are 

 stranded (as Sars noted with those of the cod at the Lofoten Islands) or 

 otherwise destroyed. 



It would be of value in this connection if a large body of accurate 

 information existed to show the relative proportions of the eggs, larvae 

 and post-larvaa of the food-fishes existing under natural conditions in the 

 sea. It does not appear that such complete information has been pub- 

 lished, but there are many observations which clearly indicate that the 

 number of larvae within an area is much less than the number of eggs. 

 In looking over the lists of the tow-net collections of pelagic eggs and 

 larval and post-larval fishes made by the " Garland," and published in 

 various reports of the Fishery Board, the great preponderance of the eggs 

 is noteworthy ; but the relationship cannot be expressed in figures, because 

 exact numbers are not always given. In an extensive series of tow-net 

 collections made by Mr. Knut Dahl* in certain Norwegian fjords in 1904 

 an accurate record was kept of the number of pelagic eggs and of the 

 pelagic young fishes obtained, summarised as follows : — 



* " Undersogelser over Nytten af Torskeudklsekning i Ostlandske Fjorde." 

 Aarsberetning vedkommende Norges FisJcerier for 1906, pp. 93-97. 



[Table. 



