FISHERY BULLETIN: VOL. 84, NO. 2 



hypothesized that surface schooling species are more 

 susceptible to fishing effort than nonschooling 

 species, and indicators of abundance such as catch 

 and catch per unit effort (CPUE) are not reliable 

 when the "intrinsic schooling rate is greater than 

 the intrinsic (population) growth rate". Thus, severe 

 stock depletion could occur in the gulf menhaden 

 fishery before indications of such a situation were 

 evident from catch and CPUE data. The dynamic 

 aggregation process may be further aggravated 

 when the vessels are assisted by spotter aircraft 

 which greatly reduce search time. 



We have attempted to estimate characteristics of 

 the gulf menhaden stock, such as population size, 

 biomass, growth rate, spawner-recruit relationship, 

 and to determine characteristics of the fishery, such 

 as fishing mortality, catchability coefficient, yield- 

 per-recruit, equilibrium yield levels, and MSY. These 

 characteristics were determined through application 

 of cohort analysis, yield-per-recruit and spawner- 

 recruit models, and a deterministic simulation model 

 of the Gulf of Mexico population and fishery. Our 

 overall objectives are to evaluate the status of the 

 gulf menhaden stock, determine the impact of the 

 fishery, and provide an outlook for the stock and 

 fishery for resource managers and the purse seine 

 fishing industry in the Gulf of Mexico. 



GULF MENHADEN DATA BASE 



The National Marine Fisheries Service (formerly 

 Bureau of Commercial Fisheries) has maintained a 

 sampling program for gulf menhaden since 1964. 

 Details of the sampling methodology are given by 

 Nicholson (1978) and Huntsman and Chapoton 



(1973), and a description of the aging technique is 

 provided by Nicholson and Schaaf (1978). Vessel 

 landings by trip have been recorded, along with per- 

 tinent data on vessel size and characteristics. Overall 

 summaries of landings by year and nominal effort 

 (measured in vessel-ton-weeks) are available back to 

 1945, but the basis for the bulk of this analysis is 

 the catch and effort data (1964-79) and estimated 

 number of fish landed at age for these years (Table 1). 



WEIGHT-LENGTH RELATIONSHIP 

 AND GROWTH 



Estimates of growth rate are needed for yield 

 analyses and estimates of size at age are needed to 

 determine the spawner-recruit relationship. Al- 

 though some calculations use length and others 

 weight, all growth estimates were calculated for 

 length, and when required, weight was estimated 

 from the weight-length relationship. 



For each age group, there was no major systematic 

 variation in the mean length over the 15 yr period 

 (Fig. 1). In addition, no density-dependent correla- 

 tions were detectable for mean length at age on 

 stock size or on year-class size, estimated from the 

 subsequent cohort analysis. Hence there appeared 

 to be little, if any, potential gain in estimate accuracy 

 by computing and using year-class specific growth 

 rates when reconstructing the historical population 

 biomass and average size at age for the subsequent 

 spawner-recruit analysis, or to incorporate a density- 

 dependent growth function in the subsequent popula- 

 tion simulations for total yield. 



Estimates of overall mean length at age for each 

 quarter for the year classes that had passed com- 



Table 1. — Catch, effort, and estimated number of gulf menhaden landed at age for the 1964-79 fishing 



seasons (1964-78 for number at age). 



'Includes only vessels that fished 9 or more weeks. 



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