species, three of which are sciaenids , make up about another 30 percent (GMFMC 

 1980). A directed fishery harvests about 50,000 metric tons of these species 

 from the north-central Gulf of Mexico. They are used in the manufacture of pet 

 food and oriental fish paste (surimi), as well as being marketed fresh. Catch 

 rates and total landings of the bottomfish fleet have declined in recent years, 

 and some state fishery biologists and participants in the groundfish industry 

 feel that the decline is due to competition from an increasing number of shrimp 

 trawlers. 



Fishery managers charged with "optimizing yield" from our marine resources 

 need to know whether the discards are a necessary consequence of harvesting 

 shrimp, an economically valuable fishery product, or represent a waste that 

 should be eliminated, preferably without economic hardship to the shrimping 

 industry. A key question is whether killing fish in the shrimping operation 

 and throwing them back to sea may actually enhance the yield of the target 

 species by (1) reducing predation or competition, (2) providing a supplemental 

 food source, or (3) increasing nutrient regeneration and thereby stimulating 

 primary productivity, leading to a greater availability of shrimp food. If the 

 present way of handling discards enhances shrimp production, then reducing 

 discards could affect shrimp harvests detrimentally. 



There are two possible ways to reduce discards without reducing the level 

 of shrimping effort. One way is to catch fewer fish by using shrimp trawls 

 with a reduced efficiency for catching fish relative to that for catching 

 shrimp. This might increase the standing stock of living fish in the system 

 and decrease the standing stock of dead fish returned to the system. Another 

 way to reduce discards is to land a larger proportion of the bycatch. This would 

 reduce the quantity of dead fish returned to the system without increasing the 

 standing stock of living fish. These alternative approaches to reducing discards 

 might affect shrimp harvests differently because one approach increases the 

 standing stock of living fish and the other does not. The effects depend on 

 interspecies dynamics and nutrient cycling in the system. 



The ecosystem model integrates qualitative and quantitative information 

 about the system into a mathematical characterization that simulates interspecies 

 dynamics and nutrient cycling. Some simulations, using preliminary data, were 

 performed to determine the possible effect on shrimp of alternative strategies 

 to reduce discards. Modeling results indicate that the shrimp harvest could be 

 affected by either method of reducing discards. If shrimp trawls to reduce 

 fish catchability relative to that of shrimp were used, however, then an initial 

 decline in the shrimp harvest might be followed by a recovery to present levels 

 or slightly higher. In the simulation, the extent to which bottomfish selected 

 against shrimp in favor of alternative prey affected the recovery of shrimp 

 stocks. Indirect interactions among species were as important as direct 

 interactions in determining initial and long-term responses of shrimp stocks 

 to different strategies. 



Model results may be dependent upon certain parameters in the model that 

 determine the rate of nitrogen remineralization by living bottomfish. They may 

 also depend on the Michaelis-Menten half-saturation constant, which determines 

 the rate of phytoplankton production as influenced by the concentration of 

 nitrogen in the water. 



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