264 DISCUSSION 



working with trout find that even when food is abundant a proportion of 

 the slower growing animals stop feeding, languish and die. 



W. E. Frost: I'm afraid this certainly occurs in some hatcheries! Could 

 the cause of mortality among some young fish be however taken one stage 

 further — and looked at in terms of behaviour ? Are plaice territorial on the 

 sea bottom ? If they are always chivvying one another for niches on the sea 

 bed, the weaker might be displaced to poor feeding stations. Trout are 

 certainly territorial in this way — are plaice ? 



R. J. H. Beverton : We know very little about the first few months in 

 the life of plaice after they have settled. It would be surprising if they were 

 territorial at that age. They are very widespread all along the coastal belts, 

 and while they might be territorial on a micro-scale, they are all very much 

 at the mercy of the currents. 



J. G. Skellam : It has been indicated that there are four levels of abstrac- 

 tion in the series: model — laboratory experiment — field experiment — 

 observation of nature. The laboratory experiment appeals to the theorist 

 because the correspondence between mathematical model and controlled 

 experiment can often be very close indeed. Though it may be reasonably 

 safe to extrapolate the general pattern of results, caution is required in using 

 the parameter values. Experience gained in the industrial field shows that 

 scale factors do affect the parameter values, for it is known that the optimum 

 conditions for a small-scale laboratory experiment are sometimes markedly 

 different from those which apply to a full-scale industrial plant. My view is 

 that we need to have work done at all four levels. 



L. B. Slobodkin: Laboratory experiments certainly have many of the 

 properties of mathematical models, but the animal is left with freedom to 

 think of things that would never occur to mathematicians ! 



REFERENCE 



Sette, O. E. (1943). Biology of the Atlantic mackerel {Scomber scomhrus) of North America. 

 Part I. Early life history, including the growth, drift and mortality of the egg and larval 

 populations. Fish. Bull, U.S., 50, No. 38, 149-237. 



