228 Discussion 



Nigrelli: No critical work has ever been done to determine the 

 exact meaning of natural mortality. Fishery biologists lump many 

 things under this item. For example, epidemics of myxosporidiosis 

 are quite common but not recognized, and the disease affects young 

 populations much more than the old. Such an infection may be 

 diffuse. All it takes to kill off a population after an epidemic is a 

 slight change in environment, as the fish are already weakened by 

 the disease. These diseases are often self-limiting, and if the fish can 

 survive to, say, a year old, then they apparently can withstand 

 changes in temperature, or other factors including pollution. If the 

 fish die and are not examined for disease the death is then attributed 

 to natural mortality. 



Holt: There is a current work which is relevant to this question but 

 it is being done for another reason. Quite intensive population stud- 

 ies of parasitization are being carried out on cod and salmon in an 

 attempt to identify races or sub-populations. The results may well 

 help us to interpret the mortality rates of, say, different sub- 

 populations of the same species. 



Bourliere: Are there any indications about the rate of infestation 

 by the various parasites in wild fish? Is it as great as in captive ones? 



Gerking: There have been some studies made in Canada of Tri- 

 aenophorus (a tapeworm) infestation in whitefish, because it reduces 

 their market value. There have been attempts made to reduce the 

 incidence of this parasite by eliminating, as much as possible, the 

 intermediate hosts, and there has been partial success. 



Rockstein: What is the rate of growth of the common carp, and 

 its longevity? It is a fairly large fish, and it must therefore grow 

 rather rapidly. 



Nigrelli: I cannot tell you about that. 



Gerking: One of the most recent developments in the study of large 

 fish with rapid growth is some work on the sailfish in Florida waters 

 (de Sylva, D. (1957). Bull Mar. Sci. Gulf and Carib., 7, 1). It is a 

 well-known sport fish because it is large and can be caught on hook 

 and line. It was amazing to learn that these fish grow very rapidly, 

 living only four or five years, yet reaching 100 lb. 



Holt: Carp grow rapidly, but they have a low K; that is, their 

 increments in length tend to be constant, but they have a particul- 

 arly plastic growth. I think it is one of the few larger species in. 

 which there is some evidence of a post-reproductivx phase. 



Gerking: Is that well documented? 



Holt: No — except that I believe in pond culture it is the practice 

 not to keep large carp because their reproductive capacities are 

 beginning to fall off. 



