Phytoplankton standing stock, (estimated from chlorophyll data) and net 

 primary productivity values were from offshore Mississippi Delta studies. The 

 Michaelis-Menten half-saturation constant relating phytoplankton production to 

 nitrogen concentration in the water came from a laboratory study with a coastal 

 diatom, using nitrate as the substrate. Phytoplankton respiration was estimated 

 from generalized information and is not specific to the area or the species 

 found there . 



Some assimilation coefficients were available from the literature, but 

 most were approximated, based on qualitative information. Selectivity 

 weighting factors of alternative food sources for benthic organisms, shrimp, 

 bottomfish, and large scavengers were weighted differently to reflect qualitative 

 information from the literature regarding feeding selectivity. High-nitrogen 

 organic material was weighted higher than low-nitrogen organic material for 

 both benthos and shrimp, assuming a preference by these groups for the high- 

 nitrogen material. The weighting factor for feeding by benthos on low- 

 nitrogen organic material was adjusted downward slightly to set the respiration 

 rate-coefficient for high-nitrogen organic material (calculated in the flow 

 balancing procedure) close to the value of the decomposition rate-coefficient for 

 fecal pellets that was calculated from Johannes and Satomi (1966); the model 

 value was 0.2071, as compared to the literature value of 0.1834. Benthos was 

 weighted equal to high-nitrogen organic for feeding by shrimp. Shrimp received 

 a low weighting relative to alternative prey of bottomfish to reflect the 

 infrequent occurrence of commercial penaeid shrimp in the stomachs of bottomfish 

 that has been noted by Sheridan et al. (1981). This would tend to minimize the 

 effect that predation by bottomfish could have on shrimp standing stock in the 

 model. Discards were weighted slightly more heavily than live fish in the food 

 flow to large scavengers, reflecting the fact that sharks and others in this 

 group have a scavenging tendency, although they also feed on live animals. 

 Marine mammals were weighted lower than live fish in the feeding flow to 

 scavengers because the size of adult marine mammals equals or exceeds that of 

 large scavengers, and adults are capable of protecting both themselves and 

 juveniles. Weighting factors were estimated on a log^g scale, as a first 

 approximation. Simulation tests indicated that weighting factors of this 

 magnitude were necessary for differences between simulations caused by differences 

 in weighting factors to be detected on the scale of the simulation graphs. 



SIMULATION TESTS 



Using the model, tests were made of the effect of alternative strategies 

 of handling bycatch on fishery yields and standing stocks. These strategies 

 were as follows: 



1. Present trawling and discarding practices. 



2. Half of bycatch not discarded (utilized). 



3. Shrimp trawl efficiency for catching fish cut in half. 



4. Shrimp trawl efficiency for catching fish halved and 

 directed fishing effort for bottomfish doubled. 



199 



