Discussion 229 



Comfort: Goldfish breeders, who are not scientists, say that very 

 often when a goldfish gets to about 16 years of age and is post- 

 reproductive, it improves in condition and size for show purposes, 

 and that a goldfish between the ages of 16 and 20 will often be a very 

 large and particularly glossy specimen for show purposes. 



Gerking: What was said about the carp is true of trout also. 

 Breeders do not keep old trout around because they are not efficient 

 reproductively. This is not due to a sterility factor, however, but it 

 reflects the growth of the gonad in relation to the weight of the trout. 

 This is a special case that I did not have time to mention. The 

 growth of the gonad is not proportional to the growth of the fish. 

 Therefore, older trout do not produce as many eggs as a younger one 

 does in proportion to the weight of the fish. It is more efficient for 

 the hatchery to keep younger fish for breeding than it is to keep the 

 same weight of older fish. 



Nigrelli: There are many parasites that affect the gonads. If 

 there is an epidemic of gonadal strigeidiasis in any population, it will 

 affect the reproductivity for that particular time. 



Holt: Would you suggest, then, that the older fish has had a longer 

 time to be exposed to infestation, and therefore might be more 

 likely to have a low rate of reproduction? 



Nigrelli: No, it would have to be an acute infection of some kind, 

 which could spread around by contact, particularly in close schools 

 of fish. Tuberculosis of the ovaries and testes will be found in 

 relatively older fish. 



Beverton: You gave us examples of fish living quite happily under 

 extreme environmental conditions. The opposite occurs as well. 

 We have done some work on the Arctic cod over the last few years, 

 and that is an interesting example of a fish which is living on the 

 borderline of its environmental tolerance. When it goes across that 

 border, in effect into the cold water from the Polar basin, the cold 

 water seems to cause quite a substantial mortality. The immediate 

 cause of death seems to be a breakdown in the osmoregulation, and 

 the blood saline of these fish that are caught in cold water is up to 

 four times the normal level. It is difficult to measure mortality in 

 that case, but the inference is that the cold had quite a substantial 

 effect. 



Danielli: If fish of the same age are put in different-sized con- 

 tainers, they grow to different sizes. What happens if they are put 

 into a larger container which is separated into two compartments, 

 one large and one small, with a perforated screen which allows the 

 passage of water but not of fish? 



Comfort: I have tried this with guppies. If you confine a fish in a 



