process is a necessary abstraction of ecological modeling, representing an 

 attempt to balance among criteria of realism, precision and generality (Levins 

 1966). These fish groups are what Boling et al., (1975) referred to as "para- 

 species", defined consistent with modeling objectives. It is fortunate that 

 the fish assemblages in Chesapeake Bay's brackish SAV ecosystems are of relative- 

 ly low species diversity. In fact, 80-95 percent of the fish biomass in each 

 of the 3 categories (defined above) is comprised by 1-3 principal species 

 (Lubbers et al., 1981) with similar functional characteristics. The major 

 "Resident Fish" are Fundulus spp., Lucania parva and Apeltes quadracus ; 

 the most important "Schooling Fish" are Anchoa mitchilli and Menidia spp.; 

 "Predatory Fish" are dominated by Pomatomis saltatrix and Mo rone american a. 



The elements of nekton life cycles are included in the model by utilizing 

 special subroutines for spawning, recruitment and migration. Recruitment from 

 juvenile to adult age (size) classes is also represented in the model structure 

 for Schooling and Predatory Fish in terms of juvenile and adult numerical 

 abundance. Thus, issues of stock-recruitment and density-dependence can be 

 treated in the model, albeit at a coarse-grained level. The use of numbers and 

 biomass as distinct, but coupled, state variables allows considerable flexibility 

 and structural condensation while maintaining realistic model behavior. This 

 approach, which was used by Steele (1974) for zooplankton in his model of the 

 North Sea pelagic ecosystem, provides a means for tracking both energy flow 

 (as biomass) and population information (as numbers). Predator-prey relations 

 are often best described in terms of numerical abundance, while metabolic pro- 

 cesses are more a function of biomass. Traditional population models consider 

 numbers only (in separate age groups), while most ecosystem models utilize 

 biomass only. This model attempts to combine the strengths of both. 



The mathematical form of equations used in the model can be illustrated in 

 terms of Schooling Fish biomass and adult numbers. The temporal rat e-of -change 

 for biomass (Q35) is 



Q35 = assimilation - predation mortality - fishing mortality 



- spawning effort - respiration + immigration 

 -emigration, (6) 



while for adult numbers (Q36) the rat e-of -change is 



Q35 = recruitment from juveniles - predation mortality 



- fishing mortality + immigration - emigration. (7) 



Overall, the terms in the biomass equation utilize an interplay between variables 

 of biomass and number, where, for example, assimilation (a fixed fraction of 

 consumption) and mortality involve biomass and numbers for both prey and 

 predator, while the respiration term Involves only biomass (Q35). The terms in 

 Eq. 7 are (with the exception of recruitment) derived from those in Eq. 6, with 

 the reciprocal of average size used to convert from biomass units to numbers. 



The formulation for predation utilizes concepts of threshold densities 

 (e.g., Wiegert 1975), size-selective feeding (e.g., Brooks and Dodson 1965), other 

 criteria of selectivity (e.g., Ivlev 1961) and refuge provided by SAV structure 

 (Heck and Orth 1980). Predation is taken as the product of the predator activ- 

 ity (PRED) times prey availability (PREY). In the case of adult Schooling Fish, 



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