shrimp by a factor of 100. Simulation results indicated that shrimp standing 

 stock and the shrimp harvest were more sensitive to the reduction in bottomfish 

 fishing mortality rate caused by trawls with a reduced efficiency for catching 

 fish when predator selectivity against shrimp was eliminated. When there was 

 no selectivity against shrimp by bottomfish (weighting factors set at 1 and 

 predation rate increased by a factor of 100), the shrimp stock did not fully 

 recover in the 5-year period of the simulation (fig. 6). 



In a second sensitivity test, the initial standing stock of bottomfish was 

 reduced by one-half, which changed the relationship of standing stock of this 

 compartment to that of the others. This change made little difference to the 

 response of the model system to management tests, except that, under conditions 

 that induced expansion of bottomfish standing stock, the expansion occurred 

 more rapidly. For the third sensitivity test, a feeding link was established 

 between discards and marine mammals. Discards were weighted equally with other 

 mammal food sources. This change made almost no appreciable change in simulation 

 results, except that the oscillation in marine mammals that typified the 

 management-test simulations was eliminated from the simulation of the effect of 

 utilizing half the bycatch. 



The simulation plots revealed oscillations in many of the standing stocks 

 under non-steady-state conditions. These oscillations were intrinsic to the 

 model system and not due to oscillating system inputs, because all inputs to 

 the model system were constants. The frequency of oscillation of the marine 

 mammal standing stock (fig. 3) is obviously an artifact of the model and cannot 

 reflect the real world, because marine mammals have slow maturation rates and 

 long gestation periods and the standing stock of marine mammals obviously would 

 not fluctuate several times a year. Fluctuations such as those seen in other 

 standing stocks of the model system are plausible, although, in the real world, 

 seasonal variations in solar radiation and river discharge would superimpose 

 seasonal cycles on any intrinsic cycles of the system. 



DISCUSSION 



To help understand the basis for the changes observed in the plots, 

 calculations were made of total annual inflows and outflows to several of the 

 compartments under the test conditions. These annual totals are shown in tables 

 4 through 12. 



Model simulation results indicate that reducing discards by either proposed 

 strategy could have some detrimental effect on shrimp harvests initially, but 

 readjustments in the system will allow the shrimp stock and shrimp harvest to 

 recover within 2 years under the option of using trawls that reduced fish 

 catchability relative to that of shrimp, if predation pressure of bottomfish 

 on shrimp is low (selectivity weighting factor against shrimp of 0.01 or less). 

 The simulations of the model suggest that bottomfish harvesting strategies 

 could influence shrimp by more mechanisms than those considered in the question 

 originally posed in the introduction. Some of the influences observed in the 

 simulations were counter to what was expected. 



Although nitrogen remineralization was decreased when the discard rate was 

 reduced through bycatch utilization, the rate of nitrogen remineralization was 

 greatest when bottomfish fishing mortality was reduced by one-half through the 



207 



