ABSTRACT 



Our purpose in contructing this model is to summarize fishery and ecological 

 data; and by analyzing known and assumed trophic interactions, to determine the 

 role of yield species in the flow of organic matter through the system. This 

 effort aids in identifying fundamental ecological processes controlling system 

 production, and thus helps plan research to: (1) relate habitat type to fishery 

 production and (2) compare actual to potential fishery production. The ultimate 

 goal of this approach is to simulate how the system responds to changes in 

 trophic spectrum of the harvest. 



In developing the model we use three kinds of information: (1) commercial 

 and recreational fishery yield, (2) primary production, and (3) trophic 

 relationships (diet and conversion efficiency). The first two are available on 

 a regional basis and help determine the geographic boundaries of the system. 

 We consider only those species which are harvested close to shore and are 

 supported by food produced there. The boundaries encompass an ecologically 

 homogeneous area from Long Island to Georgia and out to 8 km. 



Our approach to structuring the system is to work down the food chain from 

 fishery yields to primary production, rather than the usual approach of trying 

 to balance inputs and outputs at each step up the chain. 



In this way we derived an estimate of material needed to drive the yield. 

 Trophic level of yield species is expressed as the Inverse of their production 

 cost, where cost is amount of primary production needed to produce one unit of 

 organism. Estimates of natural mortality were used to check the validity of 

 assumed trophic relationships. 



Figure 1 is a simplified picture of the system. It shows inputs and 

 outputs averaged over a 20-yr period. All units are kilograms of dry organic 

 matter. Much detail has been omitted in order to emphasize several major 

 characteristics of the system. 



Vascular plants (primarily marsh grasses) account for nearly as much 

 primary production as algae. Much of the primary production enters the food 

 chain through the bacterial-detrital complex. The fishery yield has a low 

 trophic structure; that is, most of the yield is near the base of this (tilted) 

 pyramid of increasing ecological cost. The width of the bars is proportional 

 to the yields, and the placement of the bars indicates the relative trophic 

 level. The yield of piscivorous fish is a relatively low proportion of the 

 total (7%) and could be supported by natural mortality of menhaden. 



Some of the preliminary conclusions from model analysis are: 



1) The total yield (143,000 MT dry weight) is significant. It equals 20 

 percent of the North Sea yield. 



2) The yield per square meter is high. It equals about 3 gm dry wt/m^ 

 which is about twice the North Sea. 



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