III. ECOSYSTEM MODELS AS A FISHERY MANAGEMENT TOOL 



There have been few attempts so far to apply full-scale ecosystem models/ 

 to fishery management problems. Full-scale means a quantitative understanding 

 of the dynamic processes controlling production rates of all the major biological 

 components including the linkages to the physical driving forces. Perhaps the 

 Andersen-Ursin model for the North Sea comes closest in this regard. However, 

 this model falls short of real predictive or even diagnostic power because we 

 do not have a quantitative understanding of the mechanisms controlling the 

 recruitment process in fish. We cannot link survival of young fish quantitatively 

 and mechanistically to either primary or secondary (zooplankton) production nor 

 to the circulation dynamics which drive the whole process of organic production 

 in the sea. In fact, we are just beginning to measure predation mortality on 

 pre-recruit fish by recruited sizes. Determining the relative importance of 

 this predation, which is applied largely to post-larval stages, requires that 

 we also measure larval mortality rates, a prodigious logistic problem. There 

 was a clear consensus in Panel A that more fundamental understanding of this 

 recruitment process is required before we can expect ecosystem models to achieve 

 predictive capability really useful for short-term fishery management needs. 

 The possibilities are perhaps somewhat better for helping evaluate long-term 

 strategies with ecosystem models, but even here the major priority is for better 

 quantitative understanding of ecological processes rather than development of 

 more and better ecosystem models. There are already plenty of models available. 

 Definitive input data is the critical need. 



In spite of these problems the Panel felt that there should be a continuing 

 effort toward the application of ecosystem models as a mechanistic framework 

 within which to help clarify the nature of processes controlling fish production. 

 For example, given qualitative but size-specific data on predator-prey interactions 

 in the natural environment and experimental estimates of energy requirements 

 for normal growth, it may be feasible to explore the robustness or reasonableness 

 of alternative hypotheses on pre-recruit mortality processes taking into account 

 time dependent aspects and physical dynamics affecting larval fish food pro- 

 duction. Furthermore, multispecies models, involving predator-prey interactions 

 properly scaled by at least an empirically based energy budget, may provide 

 some insight into the potential for affecting long-term average biomass and 

 species composition of finfish populations through alternative harvest strategies. 

 Ecosystem models may also be helpful in resolving resource allocation problems 

 (e.g., recreational vs. commercial fishery disputes over the relative impacts 

 of the respective fisheries on resources). 



Since an appropriate model depends critically on the available data as 

 well as the specific questions posed, the development of generic ecosystem 

 models would appear to be of limited value (except in very broad terms). On 

 the other hand, comparisons between different models using the same data base 

 might be helpful in some cases for clarifying properties of the models. It 

 would be perhaps more useful to apply a given model to two generally comparable 

 ecosystems which occur in different locations and which have independent measures 

 of the critical quantities (biomass, production, consumption) and mechanisms 

 (growth, mortality). 



On the critical problem of the recruitment process, empirical models have 

 clearly been useful for developing hypotheses, but understanding the mortality 

 mechanisms and evaluating them with dynamic models will be necessary for testing 



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