INTRODUCTION xiii 



but the true mathematical model may produce, by mathematical argument, 

 deductions and implications unlikely to have been discovered by other 

 means. Such deductions can then be tested against reahty, or used to make 

 practical recommendations and forecasts. The latter has been accomplished 

 successfully for fish popidations, and some of the main approaches to the 

 design and use of such models are discussed by Gulland, and in a more 

 general way by Watt. 



Another approach to understanding through simplification is by the use 

 of laboratory populations. Here environmental complexity can be reduced 

 to a minimum, and the combination of model and experiment used to test 

 theory as, for example, Slobodkin uses experiments with Daphnia and 

 Hydra to investigate the efficiency of exploitation. 



The ultimate aim for rational human use of a population must be to 

 obtain an efficient maximum (and economic) long-term yield. The simpler 

 fishery models are concerned only with the efficient utilization of the fished 

 stock, but more sophisticated and comprehensive considerations must include 

 the whole population in all life stages and eventually the whole biota. Long- 

 term trends and the factors controlling the numbers of young fish are 

 described by Beverton, while Le Cren discusses the efficiency of the repro- 

 ductive and recruit-rearing processes. 



In most population investigations there comes a time when consideration 

 has to be extended beyond intra-specific factors to inter-specific relationships 

 and to the environment. Little is known about the balance of closely related 

 species and how it may be affected by the exploitation of one or more of 

 them, but Lindstrom and Nilsson contribute a paper on the probable 

 competitive interactions of whitefish species in the same lake. In some 

 situations man fmds himself competing with predators for the same prey 

 species: such a situation would arise if African plains game were cropped as 

 a major protein source in the manner foreshadowed in Pearsall's paper. Such 

 a conflict can produce practical complications when the predators are 

 themselves of aesthetic or commercial value to man. The grey seal is con- 

 sidered by some naturalists to be worthy of protection, but Rae regards it 

 as a serious predator on valuable fish. Lockie attempts an assessment of the 

 relative importance of man and the grey seal as predators of salmon, and 

 Shearer illustrates one way out of such conflicts by describing some practical 

 measures to protect salmon nets from seals. The discussion following these 

 papers seems to indicate that more real information on the population 

 parameters of both seals and fish is needed before judgement can be given. 



One might think that nature conservation would invariably involve 

 protection against exploitation for the species or community that is being 

 preserved, but in practice some degree of cropping is often essential, 



