Many of the changes that are taking place today in the resource populations 

 are not always predictable, but at least many can be rationalized on the basis 

 of the model. It has become very clear, for example, that predation on the 

 smaller size of fishes has a great deal to do with the ultimate size of adult 

 populations. It is also becoming obvious that disease plays a far more 

 significant role than previously acknowledged. 



In any event, in the Northeast Fisheries Center we have one overall guiding 

 model but many variants, subsets and aspects of this model. There are three 

 general needs which may be categorized as follows : 



1. A critically important need is that for a relatively transparent 

 version in popular science terms. This version is one basis for communicating 

 to our constituents. These are usually static versions of the model and contain 

 information about the processes and rates in terms that every man can understand. 

 An example (fig. 1) of one such version — it speaks in terms of dollars rather 

 than grams of carbon per meter squared per year — was developed to illustrate 

 energy flow for an audience of non-biologists. 



2. The second need is to clarify the dynamics of the ecosystem and 

 ultimately to predict resources events. This version of the model is 

 characteristically scientific, opaque, and cumbersome to deal with unless you 

 are an expert. The dynamic model required in this instance has a minimum of 50 

 or more state variables. This number of variables is sufficient to overload 

 most computer main frames and accordingly it is less dynamic than we would 

 like, but it exists and it is used. 



3. From my point of view the most significant version of the model is 

 that one we use for communicating with our bosses. It is transparent, perhaps 

 deceptively so, but it does lead to the development of other kinds of information 

 that are necessary. 



Our simple form of the model can be stated as follows (fig. 2): 

 Change = Fishing (F) + Predation (P) + Disease (D) + Environment (E). 

 Let's have some definitions: 



Fishing (F). This component refers to all the activities associated with 

 population assessment. It includes survey cruises, age and growth studies, 

 analysis of landings data, and a great deal of analysis. 



Predation (P). The feeding of one species on another. We now can 

 demonstrate the likelihood that predations can and do structure the fish segment 

 of the ecosystem. The function is felt by man 2 or more years after the fact 

 since much of this predation takes place on the very young fish. 



Disease (D). Disease is an obvious "player" in the ecosystem. It's full 

 importance has only recently begun to be recognized. Fortunately, disease 

 phenomena are also relatively easy to deal with in the predictive mode once the 

 principles of their involvement are understood. Disease generally has the same 

 "future" significance that predation has. It manifests itself early and usually 

 on younger fish. Its effects peak several years after the initial "insult." 

 Disease is often directly linked to physical environmental change, both natural 

 and man-caused. 



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