FISHERY BULLETIN: VOL. 85, NO. 3 



Table 10. — Estimates of yield per recruit (grams) and mean yield 

 (thousands of metric tons) of Atlantic menhaden for three, 3-yr 

 intervals, maximum possible yield per recruit (Y/R) and yield with 

 the existing fishing pattern. 



classes), rate of fishing, geographical pattern of 

 fishing, age of recruitment, and numbers of re- 

 cruits. 



The general conclusion reached with these 

 analyses is that the stock suffers from growth 

 overfishing. To determine if a different initial 

 choice of a constant rate of M would alter this 

 conclusion, the relative biomass of a hypothetical 

 year class was estimated at specific ages with M 

 equal to 0.35, 0.45, and 0.55 and F equal to zero. 

 The growth equation for the 1970 year class (from 

 Table 3) and the annual weight-length expression 

 for 1972 (from Table 4) were used in these compu- 

 tations. The age of maximum biomass decreases 

 with increasing rates of M, as expected, but even 

 at M = 0.55 the age of maximum biomass exceeds 

 2.5 years (Fig. 20). This decrease (from about age 

 2.8 for M = 0.45) is insufficient to affect the con- 



clusion of growth overfishing. However, if the ini- 

 tial choice of M is too high, the analyses are un- 

 derestimating potential gains in total yield that 

 could be realized by decreasing fishing pressure 

 on younger ages. Similarly, if the choice of M is 

 too low, the analyses are overestimating potential 

 gains, but a net gain would still be realized within 

 the range of available estimates of M. 



Actual Yield by Year Class 



Using the estimates of numbers caught by age 

 and annual weighted mean weights at age, an- 

 nual landings were apportioned into biomass at 

 age landed and then summed by year class 

 through age 5. The calculations provide estimates 

 of yield by each year class. A plot of yield against 

 year-class size reveals lower than expected yields 

 from the 1975 and 1976 year classes, given their 

 magnitude (Fig. 21). This trend appears to have 

 started about 1973. 



Comparisons of growth and mortality patterns 

 were made of similar-sized year classes, 1955 and 

 1976, and 1956 and 1975, in search of causes of 

 the observed decrease in yield. Differences in fish- 

 ing mortality rates at age do not explicitly ac- 

 count for the dramatic differences in yield be- 

 tween the two pairs of similar-sized year classes 

 (Table 11). The 1955 year class was harvested at 

 a greater (less desirable) rate during the critical 



160 



M=0.35 



AGE IN YEARS 



Figure 20. — Age-specific relative biomass estimates of a hypothetical year 

 class of Atlantic menhaden in the absence of fishing, exposed to three rates of 

 natural mortality. If the year class was harvested instantaneously at any given 

 age, the corresponding ordinate value would represent yield per recruit in 

 grams. 



592 



