DYNAMICS OF CERTAIN NORTH SEA FISH POPULATIONS 257 



phenomenon comes from certain experimental populations maintained under 

 conditions which cause certain compensatory mechanisms to become par- 

 ticularly pronounced, such as cannibalism in the case of the guppy populations 

 cited above and fouling of the medium in the case of certain insect popula- 

 tions kept under crowded conditions (Ricker, i954)- 



The indication that the compensatory processes in the plaice population 

 are of a kind which tend to result in a roughly constant number of individuals 

 surviving at the end of the phase of compensation, irrespective of the number 

 entering it, has a bearing on the kind of mechanism responsible. It seems that 

 to produce an end-result of this kind, the system must contain elements of 

 heterogeneity. For example, if all the larvae from a year's spawning were 

 competing on equal terms for a limited food supply, it would be expected 

 that above a fairly sharply defined limit of larval abundance there would be 

 mass starvation and something approaching a catastrophic mortality. The 

 fact that this does not appear to happen may be because the larvae are 

 hatching over a period of time, so that the larval population at any one place 

 and time consists of individuals at all stages of development from newly- 

 hatched larvae to those approaching metamorphosis. Thus it may be that 

 the larvae hatching during the first part of the season fmd sufficient food 

 awaiting them and survive well, whereas the later arrivals fmd a depleted 

 food supply and suffer accordingly; this could lead to a roughly constant 

 total number of survivors, depending on the variability of the food supply. 

 Again, if competition between larvae for food is indeed the basic mechanism 

 responsible for compensation, whether the effects are direct or indirect, the 

 fact that the competing larvae are in various stages of development may 

 result in the older and more active larvae surviving at the expense of the 

 younger and weaker ones. This might also be expected to reduce the likeli- 

 hood of a catastrophic mortaUty at high larval densities, especially when 

 taken in conjunction with the fact that the larger larvae are able to eat the 

 larger Oikopleura which the smaller larvae cannot. 



Whether the kind of compensatory mechanisms suggested here for the 

 pelagic larval stage of North Sea plaice are Hkely to be of general occurrence 

 in other marine fish populations cannot be estabhshed on present evidence. 

 A high larval mortality rate is probably common to all species which have a 

 high fecundity and whose eggs or larval stages are pelagic; but this is no proof 

 of compensation, as exemplified by haddock and sole where the larval 

 mortahty rate is probably of a similar order of magnitude to that of plaice. 

 What does seem to be emerging as something of a generahzation among 

 marine fish is that if compensation does occur it is not to be found to any 

 extent within the adult phase of the population as such. Neither does it seem 

 that fecundity alone is of general significance in this respect, despite the fact 



