Discussion 179 



But the data also show that this stunted growth does not seem to be 

 Associated with any marked increase in mortality rate. ~ '■ 



■^ Comfort: I think that is what I am going to find. 



Another point is that H. J. Van Cleave (1934; 1935. Ecology, 15, 

 17; 16, 101) suggested that the apparent senescence of some mol- 

 luscan species which are more or less indeterminate in growth is 

 actually a size effect, because there is no accumulation of animals in 

 the older age groups. In one of the freshwater limpets the question 

 arose whether, when they got beyond a certain size, their holding-on 

 mechanisms became inefficient or whether they were taken selectively 

 by predators (Hunter, W. R. (1953). Proc. zool. Soc. Lond., 123, 623). 

 Have you any instance of an adverse size effect in fish ? 



Beverton: I should have thought it usually worked the other way 

 in fish. For instance, they escape predation as they get bigger, 

 rather than the reverse, I would say. 



Comfort: What about catching them ? 



Beverton: That depends on the gear. Usually fishermen are after 

 the bigger fish and take steps to catch them, but not always. 



Holt: It is usually in the smaller fish, having rather high K values, 

 that the effect of reproduction on lifespan seems to be greatest, as 

 they die off quickly after reproduction. In fish like sturgeons which 

 grow to large sizes rather slowly there is no noticeable effect of re- 

 production on their mortality, even though they mature at a re- 

 latively small size. In middle-sized fish, such as the plaice, there is a 

 certain effect but not such a drastic one. Thus in large spawning 

 plaice there is a relatively great seasonal variation in the relation of 

 length to weight (what we call the "condition factor"), suggesting 

 that attainment of spawning condition becomes an increasing strain 

 on the fish as they grow older. 



Rockstein: Do land-locked salmon reproduce year after year ? 



Holt: Some land-locked populations do. 



Beverton: There are the "residual" and the "Ko Kanee" salmon 

 {Oncorhynchus nerka). Both are non-migratory, but whereas the 

 "residuals" are the progeny of anadromous parents, the "Ko 

 Kanee" is a self-maintaining stock which has no connexion with 

 either of the other two. 



Gerking: There are some land-locked Atlantic salmon that repro- 

 duce year after year. 



Rockstein: In a large reservoir in New York State we have brown 

 trout, also called salmon trout, and these can be caught in all sizes 

 depending on how successful the first year stock is in eluding the 

 angler. It appears from their annual movements into the lake in 

 spring and out again in the fall that they are spawning each year. 



