NELSON ET AL.: LARVAL TRANSPORTOFfifl£VOOft77A TYRANNUS 



Comparison with Pacific Sardine 



Computed survival indices allow comparisons 

 between the Pacific sardine and Atlantic 

 menhaden, in addition to those detailed by 

 McHugh (1969). Radovich (1962) presented data 

 for Pacific sardine showing the effect of good, 

 average, and poor environmental conditions on 

 the spawner-recruit relationship. He used 

 maximum and minimum parabolas based on 

 highest and lowest recruitment years and iden- 

 tified the area between the curves as indicative of 

 the effects of the environment as well as spawning 

 stock size on recruitment. A similar approach, 

 modified by using the right-hand skewed Ricker 

 curve yields similar results (Figure 7). Year clas- 

 ses used in the computation of the maximum and 

 minimum recruitment curves for Atlantic 

 menhaden were not selected for high and low 

 recruitment as was done by Radovich, but were 

 selected because they represented extremes in the 

 variation of transport factors. The maximum 

 recruitment curve was developed from year-class 

 size during the 3 yr of highest (3^700 metric tons/ 

 skm) southern onshore transport (1955, 1958, 

 1969). Similarly, the minimum recruitment curve 

 was computed from year-class size during the 3 yr 

 of lowest (<100 metric tons/s-km) onshore 

 transport (1959, 1963, 1967). The two curves 

 represent a wide range of environmentally in- 

 duced fluctuation around the stock and re- 

 cruitment curve calculated from the 1955-70 data 

 base. No statistical significance can be attached to 

 the upper and lower curves because each is based 

 on three data points. However, the figure indicates 

 the range of variance that masks the density- 

 dependent function if pertinent environmental 

 factors are not identified and weighted for effect at 

 various stock sizes. The greater slope of the 

 maximum curve is of particular interest, indicat- 

 ing a significant loss of potential recruits in good 

 environmental years if adequate stock size is not 

 maintained. 



Additional parallels can be drawn between 

 Pacific sardine and Atlantic menhaden spawner- 

 recruit relationships during periods of overfishing 

 and low survival. A comparison of spawning stock 

 size and year-class size for the two species linked 

 in chronological order shows striking similarities 

 (Figure 8). In each case, there was a period of 

 several years at high stock size in which the size 

 appeared to be near or past the maximum needed 

 to produce large numbers of recruits. A series of 



|333-S]/55 



SIZE OF SPAWNING STOCK (NO OF EGGS x 10") 



FIGURE 7. — Ricker spawner-recruit relationships calculated for 

 years of good and poor environmental conditions. The upper 

 curve is calculated from observed recruitment during the three 

 greater years of Ekman transport, the middle curve is calculated 

 from the 16-yr data set, and the lower curve is calculated from 

 observed recruitment during the three lesser years of Ekman 

 transport. 



good year classes ( 1937-39 for sardine; 1955, 1956, 

 and 1958 for menhaden) was followed by a series of 

 poor survival years (1940-45 for sardine, 1959-64 

 for menhaden). These reductions in recruitment, 

 combined with excessive fishing pressure, reduced 

 spawning stock size drastically, leading to a re- 

 stabilization of stock and recruitment around 

 small stock levels. In the case of menhaden, the 

 5-yr period of decline reduced the spawning stock 

 size by an order of magnitude. By 1966, spawning 

 potential had dropped to a low of 5 x 10 12 eggs 

 from the 1961 high of 165 x 10 12 . The parallel 

 between the two sets of data is a cause for concern, 

 because the decline and apparent restabilization 

 of Pacific sardine stocks was followed by a com- 

 plete collapse of the fishery. Henry (1971:23) in his 

 analysis of the decline of the Atlantic menhaden 

 fishery stated, "I am concerned that the stocks of 

 Atlantic menhaden may have been reduced to a 

 level that is having an adverse effect on recruit- 

 ment." Clark (1974:14), in a study of the effects of 

 schooling on population dynamics on small school- 

 ing species (as in the case with Atlantic menha- 

 den), concluded that, "A commercial fishery based 

 on such a species might be expected to experience a 

 rather spectacular population collapse, which 

 could be brought on either as a direct result of an 

 increased fishing effort which suddenly trans- 

 forms the system into an unstable mode, or as an 

 indirect result of fishing which reduces resiliency 

 and renders the population vulnerable to the ef- 

 fects of random environmental fluctuations." The 

 possibility of a complete collapse in the Atlantic 



37 



