NATURE ADRIFT 



twenty-four days at 5°C, twenty days at 6°C, twelve days at io°C or ten 

 days at I2°C. The smaller the eggs the quicker they develop and hatch. 



The eggs hatch before the young fish have developed sufficiently to have 

 a inoutli so that for the first few days they continue to feed on the remnants 

 of the yolk carried under the body in the 'yolk sac'. When the mouth has 

 opened the young fish starts to feed, first on the phytoplankton and small 

 zooplankton animals like the earliest nauplii of the smaller copepods (Fig. 21). 

 As it gradually grows, larger and larger planktonic animals are taken. The 

 food taken is partly dependent on what is available and partly on selection by 

 the little fish themselves, which are sometimes quite choosy. Young plaice and 

 lemon sole are particularly partial to Oikopleiira (Plate XIX) which docs not 

 seem so attractive to young cod or haddock, although adult haddock have 

 been known to eat them in large numbers on occasions. The young Crustacea, 

 copepods especially, and many of the larval invertebrates discussed in the 

 preceding chapter, are widely acceptable. 



For several weeks, and sometimes months, these little fish will be part of 

 the plankton, feeding on it, being drifted by the water movements and being 

 eaten by other predatory species in the plankton. The economic significance 

 of this will be considered in Chapter 10, but it is obvious that there must be 

 very great losses. To make up for this the numbers of eggs laid by each 

 female fish at one spawning can be very large indeed, and vary with the amount 

 of care taken and with the size of egg in relation to the size of the fish. Shore 

 fishes with nests in which the eggs are guarded have comparatively few eggs, 

 a herring which lays its eggs in a carpet on the sea floor unattended lays about 

 50,000. Fish with planktonic eggs lay greater numbers; plaice which have 

 large eggs (i-8 millimetre in diameter) produce about j million, haddock lay 

 about ^ million eggs about 1-4 millimetre in diameter, cod which are larger 

 fish lay well over a million eggs about the same size as haddock eggs, and ling 

 with small eggs of only i millimetre ciiameter produce over 2 million ; even 

 greater figures have been quoted. The eggs of the same species of fish 

 tend to be smaller when the fish first spawns and increase in size as the fish 

 grows older and bigger. Egg size will affect survival as bigger eggs have more 

 yolk and give the young fish a better start in life. 



As there are 'lots of good fish in the sea' the total egg production is 

 enormous. Even in a relatively small area like the shelf water round the Faroe 

 Islands the annual production of haddock eggs alone is about 6,000,000,000,000 

 The plaice egg production in the southern North Sea is about the same figure, 

 more than half of which is in the Southern Bight. Pilchard eggs in the 

 English Channel in 1950 were estimated at 400,000,000,000,000. The usual 

 method of obtaining such estimates is by sampling the area with a series of 

 vertical hauls with a Hcnscn net (Chapter 2: Plate IV) from the bottom to the 



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