CHAPTER 7 



TJic yoiiii(^ fsli 



Herring congregate in shoals for spawning and lay their eggs in a thick 

 carpet, maybe a dozen eggs deep, glued to a stony area of the sea floor. Here 

 incidentally they arc a rich and easy source of food for other creatures, 

 especially the haddock. Skates and some dogfish lay their eggs in 'mermaids 

 purses', some shore fishes make 'nests' or lay their eggs in gelatinous masses, 

 but the eggs of by far the majority of marine fishes are freely floating in the 

 plankton. Mating takes place usually near the bottom and with a greater 

 degree of intimacy than has usually been supposed, the male shedding the 

 milt into the water in close proximity to the eggs as they are extruded. 

 Fertilization occurs freely in the sea water and usually well over 90 per cent 

 of the eggs are fertilized. They then float upwards as part of the plankton of 

 the upper layers and are drifted about by the currents. The eggs (Fig. 27: 

 Plates XXIII and XXXII) are usually spherical, but some — such as the anchovy 

 — are ovaL Some are characteristically sculptured, for example the lantern 

 fish, Myctoplniiu, and the dragonet, but most arejust plain transparent spheres 

 while alive and opaque white after death or preservation. (There is nothing 

 peculiar about this change which is essentially the same as the white of a hen's 

 egg changing from transparent to opaque white on cooking.) The egg is 

 filled with a clear yolk which is the food supply of the developing young 

 fish until after it hatches and can feed for itself This yolk may look homo- 

 geneous or be broken up into globules — a 'segmented' yolk. There may or may 

 not be an oil globule in the egg, but some, such as the eggs of the sole, have 

 numerous tiny globules. Each species offish lays its own type of egg within 

 a fairly close size range, although the eggs get smaller as the spawning season 

 progresses. Apart from the few sculptured or oval eggs, their identihcation 

 depends on size, on whether the yolk is segmented or not, if there is an oil 

 globule and, if so, its size in proportion to the total diameter of the egg. 

 The sizes overlap considerably so it is not always possible to identify newly 

 spawned fish eggs with certainty. As the embryo develops inside the egg its 

 own characteristic pigment patterns gradually appear making identihcation 

 much easier and more reliable. This takes about ten days in the cold waters 

 of the North Sea in March. The rate of development is very largely dependent 

 on temperature so that a fairly reliable indication of the age of an egg is 

 obtained from the state of development if the temperature is known. The 

 hatching time of plaice, for example, is roughly t20 'degree-days'; i.e. 



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