FISHERY BULLETIN: VOL. 84, NO. 2 



capable of taking advantage of those years when a 

 large harvestable stock is available (1971, 1978, and 

 1979). Total mortality rates (averaging 83% for age-1 

 fish and 95% for ages 2-4 fish) are extremely high. 

 Major expansions of the fleet and processing facil- 

 ities necessary to substantially increase the fishery's 

 share of population biomass would require enormous 

 capital investment. Based on results of the simula- 

 tion model, large increases in fishing effort would 

 also result in an overall average decline in landings 

 that would likely be followed by an economically 

 forced reduction in effort. Under present circum- 

 stances, we do not envision the sustained intensifica- 

 tion of effort necessary to drive the gulf menhaden 

 stock to biological extinction. 



The simulation model estimates that the effort cur- 

 rently applied in the fishery is probably very close 

 to that which is necessary to produce MSY (Fig. 9), 

 while it exceeds the necessary level in the catch- 

 effort production function (Fig. 8). Assuming that 

 the simulation model reasonably approximates 

 average conditions, some increase in overall yield 

 could be obtained through a modest increase in 

 effort, which has in fact occurred in more recent 

 years. 



Based on recruitment levels for 1964-77, it is evi- 

 dent that considerable variation will occur around 

 a long-term sustainable yield level, regardless of the 

 level of fishing mortality. We varied recruitment level 

 in the population simulation model through periods 

 of high (25 billion) and low (10 billion) levels of 

 recruitment to provide estimates of the yield from 



the fishery under good and poor recruitment 

 regimes, and to observe the rate of response to 

 recruitment changes. The results range from an ap- 

 proximate high of 757,000 t to a low of 303,000 t at 

 the high and low recruitment levels (Fig. 10). Since 

 only age-1 and age-2 fish predominate in the fishery, 

 only 2 years were required for the full impact of a 

 change in recruitment to be shown, with a majority 

 of the impact occurring in the first year. We then 

 allowed the average spawner-recruit relationship to 

 operate, stabilizing yield at 565,580 t. Actual low 

 yield predictions are probably underestimated, in 

 that fishing mortality increases in years of low stock 

 size, and the fishery would produce higher yield than 

 through the fishing mortality imposed under average 

 conditions. Nevertheless, these extremes are near 

 the actual ranges in yield observed in the fishery dur- 

 ing the study period (316,100-820,000 t) and should 

 provide reasonable estimates of mean yield and 

 range expected in future years. 



Since considerable variation does exist around the 

 spawner-recruit curve and simulations were all con- 

 ducted in deterministic fashion, the model was run 

 with recruitment varying randomly between the 

 recruitment extremes calculated from our data set 

 (7.5 billion-25.0 billion). The results of that simula- 

 tion (Fig. 10) provide a long-term (50 yr) average of 

 467,459 t, but it varies from 718,000 to 263,000 t. 

 We anticipate that the fishery will continue to 

 operate somewhat in this fashion, unless there is a 

 cyclic environmental or biological influence on 

 recruitment. 



\/ b. ' ' 



V/fl. -0- O -6- -O- O -D- O^-O^-oVo 



10 



15 



20 



25 



30 



35 



40 



45 



50 



YEARS 



Figure 10.— Annual yield of the gulf menhaden fishery projected by the population simulation 

 model when upper and lower values of recruitment from the 1964-77 year classes are inserted 

 (dashed line) and when recruitment varies randomly within limits of observed recruitment for 

 the same data set (solid line). 



324 



