254 I^- J- H. BEVERTON 



between the causes of mortality is important in the search for possible 

 compensating mechanisms. Adverse water temperature, for example, acting 

 either directly or indirectly (e.g. through a disturbance of osmotic balance) 

 may well be responsible for a substantial part of the mortality of plaice larvae; 

 but unless the mortality rate it causes increases with larval density, and vice 

 versa, it cannot act as a compensatory mechanism. Predation is probably 

 another major cause of larval mortality; but again, for predation alone to be 

 an efficient compensating mechanism, the density of predators must be 

 closely correlated with that of the larvae. An obvious way in which this 

 could happen is through cannibalism of the larvae by their parents (Ricker, 

 1954), which was probably the main cause of compensation in the experi- 

 mental guppy populations studied by Silliman & Outsell (1958) and discussed 

 by GuUand (this Symposium). Cannibahsm of juveniles is known to occur 

 in a number of the more predatory marine species such as cod and hake, but 

 it is by no means clear whether cannibalism is of any general significance as 

 a compensating factor in natural fish populations. Cannibalism has not been 

 recorded in plaice where, in any event, the adults are demersal but the larvae 

 are pelagic. The possible compensatory nature of interspecific predation of 

 plaice larvae, however, certainly cannot be disregarded. Small planktonic 

 predators such as Sagitta may be able to reproduce in situ fast enough to have 

 some compensatory action, either through the direct effect on their reproduc- 

 tive rate of the contemporary supply of plaice larvae as food, or else indirectly 

 as the result of variations in the supply of other food organisms that also 

 influence the survival of the plaice larvae. Certain pelagic fish are present in 

 the plaice spawning area when the larvae are present, notably herring 

 returning from their spawning grounds at the entrance to the English 

 Channel. Plaice eggs have been recorded in herring stomachs (Hardy, 1924) ; 

 plaice larvae have not, but this may only be because most of the observations 

 on the food of herring refer to times and localities at which plaice larvae 

 would not be present, as there are many records of herring eating pelagic 

 larvae or post-larvae of other fish species (Savage, 1937). Whether the 

 abundance of plaice larvae is sufficient to cause herring or other fish to 

 aggregate on them is not known, but it may not be without significance that 

 in the years towards the end of the war and immediately after, when the 

 density of plaice larvae must have been at its highest and the degree of 

 compensation evidently most marked, herring were also several times more 

 abundant than usual in the Southern North Sea. 



Another important cause of death among fish larvae in many cases is 

 undoubtedly a shortage of food, and a quantitative relation between larval 

 mortality and abundance of food has been demonstrated for several species 

 (see Beverton & Holt (1957) for a summary of literature). Plaice are highly 



