NELSON ETAL.: LARVAL TRANSPORT OF BREVOORTIA TYRANNUS 



produce exceptional year classes and a propor- 

 tionally greater harvestable surplus at stock sizes 

 near the spawning optimum; and a series of poor 

 environmental years (1959-64), coupled with 

 excessive fishing pressure, will reduce stock size to 

 a level which produces little harvestable surplus. 



During the 16 yr covered by this study ex- 

 tremely large year classes were produced in 3 yr 

 (1955, 1956, and 1958). Favorable conditions in 

 1969 resulted in a high survival rate, but only 

 produced 2.7 billion recruits because of small 

 spawning stock size. In one other year (1966) 

 survival occurred that was greater than expected, 

 but at extremely low stock size. In the other 11 yr 

 recruitment was either near, or well below the 

 expected level, compounding the stock depletion 

 caused by excessive fishing pressure. The drastic 

 reduction in stock size resulted in a restabilization 

 of the stock-recruitment relationship around a low 

 stock level. This is evidenced by the steady decline 

 in catches from 1956 to a low of 162,000 metric 

 tons in 1969, followed by slightly higher catches in 

 succeeding years (Table 7). Extremely large 

 catches in the late 1950's are the result of the 

 unusual coincidence of 3 high survival years 

 within a 4-yr span. Average survival over the 

 16-yr period was much lower, and average year- 

 class size would be considerably smaller, even at 

 optimum spawning stock size. 



Schaaf and Huntsman (1972) gave MSY es- 

 timates for Atlantic menhaden of 600,000 metric 

 tons based on an equilibrium catch-effort curve 

 from historic data and 380,000 metric tons from a 

 population-prediction model. The population- 

 prediction model dampens the effects of large year 



classes and probably comes closer to representing 

 long-term MSY than the higher estimates. 



The maintenance of optimum spawning stock 

 size and several year classes in the spawning stock 

 is vital to insure adequate response to favorable 

 environmental conditions. Based on the estimated 

 survival rates over the 16-yr period, and the 

 optimum spawning stock size from the Ricker 

 function, surplus yield was calculated under 

 conditions which would maintain four spawning 

 groups (ages 3-6) in the populations. The calcu- 

 lation of surplus yield is based on an instantane- 

 ous natural mortality of 0.42 and fishing mortality 

 of 0.36 spread over 6 yr within a year class (ages 

 1-6) and assuming that one-half of the age-1 re- 

 cruits are vulnerable to the fishery. A full 

 complement of years 1-6, from year-class data 

 available after 1954, was not obtainable until 

 1961, when 6-yr-old fish were harvested from the 

 1955 year class. Under the conditions imposed on 

 the harvest, the allowable catch, computed for 

 1961-71, averaged 419,000 metric tons/yr (Table 

 7). Extremes in the allowable catch would have 

 ranged fron 227,000 to 633,000 metric tons, 

 depending on the size of year classes which con- 

 stituted stock size in a particular year. This catch 

 is similar to the MSY estimates of Schaaf and 

 Huntsman (1972), and was computed for a period 

 in which most of the year classes had less-than- 

 expected survival. The survival index was well 

 below 1.0 from 1959 to 1964, a period of six con- 

 tinuous years, and is reflected by the decline in 

 surplus stock during that period. Actual catches 

 made by the fishery from 1955 to 1971 (Table 7) 

 averaged approximately the same as MSY, but 



TABLE 7. — Catch of Atlantic menhaden at MSY for actual survival rates, 1955-70 year classes, fishery 

 landings, 1955-71, and predicted surplus from recruit-environment model. 



39 



