use of shrimp trawls with a reduced catch efficiency for fish, despite a 

 reduction in the rate that dead fish were returned to the system to close the 

 nutrient cycle (table 4). Annual gross primary productivity was slightly 

 decreased when half the bycatch was utilized, decreasing the quantity of dead 

 fish returned to the model system. However, annual gross primary productivity 

 did not increase when trawls that reduced fish catchability relative to that of 

 shrimp were used to decrease discards, despite the fact that nutrient regeneration 

 rates were highest under this condition (table 5). Perhaps this was because 

 saturation concentrations for nitrogen with respect to phytoplankton photosynthesis 

 were more frequently exceeded under the condition of half the shrimp trawl 

 efficiency for catching fish than under the present strategy for handling discards. 



The direct effect of discard rate on the standing stock of high-nitrogen 

 organic material was small, and the rate of zooplankton fecal pellet deposition 

 was so great by comparison that it overshadowed discarding as a source of high- 

 nitrogen organic material (table 6). Although decreasing shrimp trawl efficiency 

 for catching fish resulted in a decrease in the rate of deposition of dead 

 fish, this was greatly outweighed by an increase in zooplankton fecal pellet 

 production that was an indirect result of decreasing shrimp trawl efficiency 

 for catching fish. Both the rate of inflows and rate of outflows to the zooplankton 

 compartment were influenced by the discard rate and the bottomfish fishing 

 mortality rate (table 7). Predation of pelagic forage fish on zooplankton was 

 decreased when the standing stock of pelagic forage fish was depressed by 

 predation of marine mammals , which appears to have been due to the greater 

 abundance of bottomfish, which are also their prey (table 9). The rate of 

 fecal pellet production increased when zooplankton standing stock increased. 



Although predation by bottomfish did exert some detrimental influence on 

 shrimp stocks and shrimp harvests, this influence was minor when compared to 

 the influence of competition for common food between shrimp and bottomfish and 

 between shrimp and benthos (tables 10 and 11). An increased availability of 

 food for shrimp outweighed the increased competition for that food in the second 

 year (table 12). How the shrimp stock in the model system reacted to a change 

 in bottomfish standing stock was determined by the balance between production of 

 fecal pellets, which provide food for shrimp, benthos, and bottomfish, and the 

 pressure of competition for food with both benthos and bottomfish. 



Initially, when the fishing mortality of bottomfish was reduced with shrimp 

 trawls with half the efficiency for catching fish, shrimp standing stock and 

 the shrimp harvest reacted negatively. However, shrimp standing stock and the 

 shrimp harvest recovered when the supply of fecal pellets increased to the point 

 where an increased food supply overcame increased competition for food from the 

 larger standing stock of bottomfish. 



Under any management strategy, dead fish formed a small component of the 

 high-nitrogen organic compartment, relative to zooplankton fecal pellets, 

 which represented 82.5 percent of compartment standing stock at steady-state. 

 Furthermore, the nitrogen released from this compartment in decomposition was 

 infinitesimal compared to that released in the decomposition of low-nitrogen 

 organic material, and it was even small compared to that released in animal 

 excrement. Nevertheless, utilizing the bycatch, which decreased the rate of 

 discarding without increasing the standing stock of living bottomfish, did 

 affect shrimp stocks and shrimp harvests in the model system. Conclusions from 

 the simulations differ in this detail from conclusions of Browder (1981) and 



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