INTRODUCTION 



Fisheries ecosystems have been modelled since at least the turn of the century. 

 Nowadays the management of the world's fisheries is based, in part, on several 

 well established models of varying complexity; for example, Schaeffer's surplus 

 production model, Beverton and Holt's, or Ricker's, analytical models, and virtual 

 population analysis. These models are based on a species by species approach to 

 fisheries ecosystems, a fact which has led to criticism of their continued use, since 

 the realization of the importance of species interactions. 



To answer this criticism, models have been introduced which provide for inter- 

 action between the species in the management unit or ecosystem. This interaction 

 has most commonly taken the form of interspecific predation and these models 

 have undergone considerable development since their introduction to fisheries in 

 the late 60's and early 70's. Ursin (1982) summarizes the models which have been 

 applied to the management of marine fisheries. 



A consequence of the increased complexity of these models over the single 

 species approaches has been the requirement for more and more data and, when these 

 data are not available, their estimation or substitution with unproven formulae. 

 This has led to a reluctance to incorporate multispecies models into fishery 

 management decisions and occasionally their derision in the scientific press 

 (Gulland 1982), A diversity of available models has resulted, from the concise 

 multispecies virtual population analysis (e.g. Pope 1979) with biologically 

 identifiable data requirements, to the detailed analytical model of Anderson and 

 Ursin (1977) with requirements for parameters of ambiguous biological meaning. 

 Both approaches have their merits and it is only through the continued development 

 of models and their comparison that a holistic view of fisheries ecosystems will 

 be obta i ned . 



