Prager and MacCall. Contaminant and climate effects on spawning of three pelagic fishes 



323 



\og(R/P) = a + ylnP + 6P+Ie,.v, + 8. 



(4) 



This differs from our original model, Eq. 3, only in including 

 the term containing the estimated parameter y. When y = 0, 

 this term drops out and the model is equivalent to Eq. 3. 

 When y < 0, the model is more strongly compensatory than 

 Eq. 3; when y > 0, the model is depensatory. In results of 

 fitting this model, the depensatory effect was estimated as 

 significant only in the absence of climate or combined vari- 

 ables <y= 1.57, P < 0.025 for a one-sided t-test). With the 

 introduction of climate variables, the depensatory effect 

 became smaller and not statistically significant (y=1.02, P < 

 0.19); with the introduction of the combined variables, the 

 depensatory effect disappeared (y= -0.53, the wrong sign for 

 depensation). In the model with a depensatory parameter, 

 correlations of the explanatory effect (summation term in 

 Eq. 4) with contaminants were within 2% of those obtained 

 from the model without the depensatory parameter. Of 

 course, the correct specification of spawning-success rela- 

 tionships may be more complex than even Eq. 4. None- 

 theless, these results indicate that a simple model including 

 depensation cannot account for the observed decline in 

 spawning success residual to stock-size effects. When this 

 decline, as modeled by our combined variables (Fig. 7b), is 

 combined with Ricker stock-size effects, the resulting model 

 fits well the observed data on Pacific sardine (Fig. 7c). 



Are there plausible mechanisms by which contaminants 

 may have contributed to the decline of the sardine popula- 

 tion? The high fat content of sardines, anchovy, and mack- 

 erel makes them likely to accumulate fat-soluble compounds 

 such as the organochlorines listed in Table 2. It is known 

 that DDT has caused widespread reproductive failures in 

 birds, including those in this geographical area (Anderson 

 et al. 1975). Also, fish have been observed to depurate PCBs 

 by transferring them into eggs (Binder et al. 1984). Since 

 the early stages of many fish species have exhibited reduced 

 survival rates after exposure to toxic compounds (Weis & 

 Weis 1989), plausible mechanisms do exist. The Pacific sar- 

 dine stock was under severe overfishing pressure at the 

 time of its collapse, and such pressure would increase its 

 susceptibility to environmental stress. The possibility that 

 contaminants may be implicated in the collapse of a major 

 marine fishery is a novel and startling one. 



Acknowledgments 



This work, including the reconstructions by Summers et al. 

 (1988), was funded by the NOAA National Ocean Service, 

 Ocean Assessments Division, Seattle, Washington. We grate- 

 fully acknowledge the support of Alan Mearns and Thomas 

 O'Connor of the Ocean Assessments Division. We thank 

 Arnold Mantyla of the Scripps Institution of Oceanography, 

 who corrected some of the climate data and provided addi- 



1960 



Figure 7 



Components I dashed lines) of a model of spawning 

 success (recruits/spawner) of Pacific sardine Sardinops 

 sagax stock off southern California. Each panel also 

 shows the observed spawning-success data (solid line) 

 after logarithmic transform, (a) Model component as- 

 sociated with estimated stock-size (depensatory) pa- 

 rameter; (b) model component associated with 

 estimated parameters corresponding to climate vari- 

 ability and contaminant loadings; (c) predicted values 

 from full model (intercept plus stock-size and environ- 

 mental effects). The environmental component is more 

 influential in the mid-1950s and later, when contami- 

 nant loadings generally increased. 



tional data to fill several gaps. J. Browder, V. 

 Restrepo, and W Richards reviewed the manu- 

 script in draft; the work was improved by their 

 suggestions. Two anonymous reviewers also 



