In making these tests, I assumed that the basic structure of the system 

 (i.e., the number of compartments and the links between compartments) did not 

 change as an adjustment to new conditions. In each test, no rate-coefficient 

 or other constant in the model program was changed other than the one (or in 

 test #4, two) changed to simulate the test condition. 



A sensitivity test was run to determine how changes in the selectivity 

 weighting factor of bottomfish for shrimp affected the behavior of the model 

 under alternative strategies of handling bycatch and how this might affect the 

 conclusions drawn from simulation results. A test was also made of the influence 

 on model results of the relative standing stocks of bottomfish and the other 

 groups. In a further test, marine mammals were made to feed on bottomfish 

 discards as well as living bottomfish, a feeding flow that was not included in 

 the original model. 



In the initialization routine, model coefficients were set at steady-state 

 to represent the present practice of handling discards. Under present practices, 

 all standing stocks were constant throughout the 5-year period of the simulation 

 (fig. 2). 



Figures 3 through 5 are simulation graphs of the management tests. These 

 graphs track the standing stock of each compartment for the 5 years following 

 imposition of new conditions. Under test conditions, standing stocks were 

 initially the same as under steady-state conditions, but changed in response to 

 the particular conditions imposed, moving toward a new steady-state (or, in 

 some cases, toward a stable oscillation). Shrimp stocks declined and leveled 

 at a new steady-state approximately 28 percent lower than the former one when 

 half of the bycatch was utilized rather than being returned to the system (fig. 

 3). When discards were reduced through the use of shrimp trawls with half the 

 efficiency for catching fish, shrimp stocks declined initially but rebounded to 

 present (steady-state) levels (fig. 4). When the combination of shrimp trawls 

 half as efficient in catching fish and a doubling of directed effort at bottomfish 

 was tested, shrimp standing stock declined by 10 percent (fig. 5). when shrimp 

 standing stocks declined, shrimp harvests declined, as indicated in table 2. 



Other standing stocks also responded to the tested changes in harvesting 

 practices. Marine mammals, zooplankton, and high-nitrogen organic material 

 increased when shrimp trawls that reduced fish catchability relative to shrimp 

 catchability were used, but decreased when the bycatch was utilized rather than 

 discarded. Contrarily, stocks of pelagic fish and migratory pelagic fish 

 decreased when shrimp trawls that reduced fish catchability relative to that 

 of shrimp were used but increased when the bycatch was utilized. Standing 

 stock of bottomfish and phytoplankton and the quantity of nitrogen in the water 

 also changed under different harvesting conditions, but these changes were so 

 small relative to the total quantity that they could not be seen on the graphs. 

 An idea of the effect of test conditions on these larger, more stable compartments 

 can be obtained by comparing total annual respiration of these compartments 

 during the different test simulations (table 3). Respiration is directly 

 proportional to standing stock and thus total respiration is an index of the 

 level of standing stock over a period of time. 



In the first sensitivity test, weighting factors were changed to indicate 

 less selectivity of bottomfish against shrimp relative to alternative prey. 

 The change in weighting factors increased the predation rate of bottomfish on 



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