178 Discussion 



our impression is that looking over the whole range of growth 

 data for fish it is possible to see fairly characteristic sorts of growth. 

 However much food you give a small fish it never grows very much 

 bigger than its characteristic size in nature. This is a pretty general 

 statement, but despite the extent to which growth can be varied 

 experimentally, there does seem to be an overall pattern in nature 

 which is fairly consistent. 



Holt: This sexual difference in susceptibilities is found rather 

 commonly in experiments on the effects of water pollution and in 

 studies to improve the transport of live fish in closed containers ; the 

 males are usually more sensitive. Unfortunately I have no good data 

 with which to compare respiration rates of the sexes. Spawning 

 plays a more important part as a factor associated with death in 

 fish, than, for example, food supply. One can vary the growth rate 

 tremendously by adjusting the food supply, without changing the 

 mortality rate at all. But where death is caused through spawning 

 or is associated with it and maturity, it seems that the males are 

 more drastically affected than the females. Thus in one salmon 

 species all the males and most of the females die after spawning, but 

 some of the females return to the sea. 



Nigrelli: In aquarium management we constantly find, when we 

 autopsy fish, that there are about three females to one male, which 

 means that there is some sort of selection in the collection. 



Danielli : If the fish are dying as a result of spawning perhaps one 

 can change the situation. Death following spawning in salmon is 

 said to be due to some syndrome involving the pituitary. This, 

 surely, could be modified by appropriate hormone treatment. 



Nigrelli: Are there any comparative figures on population or 

 growth studies on salmon or trout in hatcheries and those under 

 natural conditions ? 



Holt: I do not know. 



Nigrelli: I think that in large hatcheries there is a lower infant 

 mortality rate than is found under natural conditions. 



Comfort: In these natural populations, is there always a tendency 

 for the growth to be smooth in outline ? Or does the same effect 

 occur in wild fish as I have produced by keeping them small arti- 

 ficially and then increasing the food supply ? 



Beverton: Yes; perch is the most notable example. You get that in 

 the wild, particularly where there is a marked change in feeding 

 habits as the fish grow bigger. For example, perch up to about 15 cm. 

 feed primarily on animal plankton such as freshwater shrimps, etc. 

 Above that size they change to exclusively carnivorous habits. If 

 the pond or lake has no supply of small fishjthey^ just stop growing. 



