DYNAMICS OF CERTAIN NORTH SEA FISH POPULATIONS 255 



specific in their feeding habits, their preferred food being Oikopkura which 

 is not particularly abundant in the spawning area when the larvae are present 

 and is rather patchily distributed; Shelbourne (1957) has shown, in fact, that 

 at times when the abundance of Oikopkura is lowest the condition of the 

 larvae is poorest. 



Intraspecific competition among fish larvae for a limited food supply 

 could act as a compensating mechanism in several ways. Insufficient food 

 could be a direct cause of death, and it is a common experience when 

 attempting to rear marine fish such as plaice from eggs that adequate 

 feeding by the newly hatched larvae is critical for their survival. The effect 

 might also be indirect; a shortage of food that is not severe enough to cause 

 death by starvation but affects adversely the condition of the larvae would 

 be expected to make them more susceptible to other factors which might 

 not have otherwise been lethal. Thus Shelbourne (1957) has shown that 

 starving plaice larvae may die through a loss of their osmoregulatory powers, 

 so that physical factors such as temperature and perhaps salinity, that are not 

 directly density-dependent in their action, might become so in conjunction 

 with a lowering of the condition of the larvae caused by competition for a 

 limited food supply. This is an example of the point made by Chitty (i960) 

 in reconcihng the apparently conflicting ideas of Nicholson and of Andre- 

 wartha & Birch, that causes of death which in themselves are not directly 

 density-dependent may become so indirectly if the susceptibility of the 

 population to those causes is influenced by its density. Ivlev (1955) has made 

 a detailed experimental study of what he terms 'the ecology of starvation' 

 and has shown, for example, that fish whose condition has been lowered by 

 a shortage of food become more vulnerable not only to adverse physical 

 conditions of the environment but also to predators ; again, this means that 

 competition among the larvae for food may cause predation to have a 

 compensatory action without there necessarily being any change in the 

 number of predators. Even if shortage of food did no more than delay the 

 attainment of metamorphosis and so prolong the duration of the pelagic 

 phase, it would allow those causes of death which operate during the larval 

 phase to take a correspondingly greater toll and so to have, indirectly, a 

 compensating effect. 



CONCLUDING REMARKS 



Although it has been possible to eliminate some of the mechanisms that, in 

 theory, might be responsible for or contribute to the marked degree of 

 compensation that seems to characterize the long-term dynamics of the 

 North Sea plaice population, it is clear that no firm positive conclusions can 



