NATURE ADRIFT 



much dependent on the number of eggs laid as on what happens to them 

 afterwards. It is most unhkely that egg production could ever be too severely 

 curtailed by overfishing: if fish were as scarce as that it would not be econo- 

 mical to catch them. No marine fish are therefore likely to become extinct 

 through exploitation as actually occurred with the dodo and nearly with 

 the bison. 



How is the survival rate of the young fish affected by the plankton con- 

 ditions? There arc three main ways. First: the young fish after hatching feed 

 in the plankton and there must be enough food of the right type available 

 for them. Second: the eggs and the young fish are ideal food for a host of 

 carnivores in the plankton, including other fish. Third: being part of the 

 plankton they are drifted by the currents, perhaps to unsuitable areas for 

 their later development. The chances of surviving all these hazards is indeed 

 small. 



Taking each of these in turn we can look at them more closely. 



When the larvae hatch and first start to feed they are very small, only a 

 few millimetres long and they cannot be expected to hunt far and wide for 

 their food. Food must be available in plenty in the plankton and it must be of 

 the right type. Small fish select their food but only within certain limits, 

 some being less particular than others, and they feed largely on what is 

 available. However, some organisms are too spiny or awkwardly shaped, 

 some too big or too active to be caught, or of inadequate food value if 

 not actually unpalatable, so that mere availability is not everything. Whilst 

 some plant material may be taken in the very earliest stages, animal 

 material is also necessary. The most favoured food is undoubtedly the early 

 nauplius stages of copepods, which are feeding on the spring outburst of the 

 plant growth and are most abundant then. It is therefore not surprising that 

 cod roe is at its best in the fishmongers in late winter, for this is just before 

 the eggs are spawned to develop and hatch when the wealth of spring food is 

 available. 



The larvae are planktonic for three months or so and during all this time 

 they need planktonic food. Is it any wonder that all the millions of larvae 

 cannot find food all the time and so there are large losses? In an ordinary 

 year these losses amount to about lo per cent per day, but they can be less or 

 a lot greater. Conditions that are not conducive to the production of good 

 food supplies are therefore partly responsible for poor brood survivals. 

 These conditions have already been referred to in the preceding chapters but 

 can be repeated very briefly. Insufficient mixing of the water-masses is one 

 of the most important causes of failure as this means inadequate nutrients for 

 the plant growth and so paucity of the right kind of animal plankton as food 

 for the young fish. 



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