ABSTRACT 



Possible interactions between shrimp and bottomfish were incorporated into 

 a theoretical model of the near-shore ecosystem of the north-central Gulf of 

 Mexico. The model was used to simulate the changes in the standing stock and 

 harvests of shrimp that might be side effects of reducing the unwanted fish 

 caught in shrimp trawls by either of two methods: (1) decreasing the proportion 

 of the bycatch that is discarded (but keeping the total quantity caught the 

 same) and (2) reducing the total quantity caught (but discarding the same 

 proportion of the bycatch). Possible direct and indirect influences of each 

 change in harvesting procedure on food availability to shrimp, predation on 

 shrimp, and competition with shrimp were explored. Although discards are a 

 minor portion of the dead organic material used by shrimp for food, decreasing 

 the discard rate by utilizing the bycatch decreased the availability of food to 

 shrimp and its competitors in the model system. Using shrimp trawls with 

 reduced catch efficiency for fish relative to that for shrimp resulted in 

 increased total availability of food for shrimp but increased competition from 

 bottomfish for that food. The increased supply of the common food ultimately 

 outweighed increased competition from bottomfish, and the shrimp standing stock 

 and shrimp yield recovered in spite of a higher standing stock of bottomfish. 

 Predation on shrimp by bottomfish was a relatively minor influence compared to 

 the other two effects. Changes in standing stocks not directly connected to 

 shrimp were as important in determining the response of shrimp stocks to 

 different strategies for reducing discards as changes in standing stocks that 

 interacted directly with shrimp stocks as prey, predators, or competitors. 



INTRODUCTION 



A simulation model of the near-shore marine ecosystem of the north-central 

 Gulf of Mexico is under development at the Southeast Fisheries Center of the 

 National Marine Fisheries Service, Miami, Fla. The modeling effort is being 

 used to investigate the dynamics of interactions among economically important 

 species such as shrimp ( Penaeus spp.) and menhaden ( Brevoortia patronus ) that 

 are harvested from this highly productive area, which has been called "the 

 fertile crescent" (Gunter 1963). An immediate need of fishery managers is to 

 know the possible effects on shrimp of reducing the discarded bycatch of the 

 shrimp fishery. Fishing operations that use relatively nonselective gear such 

 as bottom trawls usually catch species other than those at which they direct 

 their effort. Incidentally caught, or "bycatch," species are sometimes sold, 

 but they are often dumped overboard when the amount is extremely large or when 

 their value is low relative to that of the target species; such is the case in 

 the Gulf of Mexico shrimp fishery. The fish discarded by the shrimp fishery 

 are almost always killed by the trawl or by handling on deck and are dead when 

 returned to the sea. 



The total ex-vessel value of shrimp is greater than that of any other U.S. 

 fishery species. More than half of the U.S. shrimp harvest comes from the 

 northern Gulf of Mexico. The weight of "discards" averages approximately 14 

 times the weight of the shrimp landed and amounts to about 400,000 metric tons 

 in the offshore area from Perdido Bay, Fla., to Point Au Fer, La., which has 

 been delineated as the "primary area" of concentration of these species for 

 purposes of making resource surveys. Atlantic croaker ( Micropogonias undulatus ) , 

 a sciaenid fish, makes up over 50 percent of the bycatch by weight. Five other 



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