MULLIN and CONVERSE: EUPHAUSIID AND ZOOPLANKTON BIOMASSES 



more abundant following the initiation of the 

 fishery — could have been overshadowed by, 

 e.g., 1) replacement of whiting by the increase of 

 some other species of predator on euphausiids, 

 even a species which is also prey for large whit- 

 ing (Livingston 1983); 2) differential removal of 

 large whiting by the fishery, leaving smaller 

 whiting whose preference for euphausiids as 

 food exceeded that of the larger fish, so that 

 predation pressure did not decrease dramatically 

 because of altered age structure of the whiting 

 population; or 3) replacement of predator-limita- 

 tion by food-limitation of euphausiid biomass. 



We tried to minimize the effect of response 1) 

 by restricting the post-1965 analysis to the years 

 immediately following the initiation of the fish- 

 ery, on the theory that this might have repre- 

 sented a period of abundant euphausiids before 

 the ecosystem returned to a new equiUbrium 

 through the increase of a new, major zooplank- 

 tivore. Unfortunately CalCOFI coverage of the 

 northern California Current in the summers of 

 1967 and 1968 was very small. 



Response 3) is possible (indeed, euphausiid 

 biomass may never have been limited by Pacific 

 whiting's predation), but the increase in biomass 

 of small zooplankton in the northern California 

 area in 1966-69 (Fig. 3) suggests a food supply 

 which could have supported an increased bio- 

 mass of euphausiids — an increase which was not 

 reahzed. 



In considering the possible responses of the 

 zooplanktonic community to the Pacific whiting 

 fishery, it is worth remembering that there have 

 been natural fluctuations in the whiting popula- 

 tion as great as those caused by fishing. Ocean- 

 ographic variation in the whiting's spawning 

 area is important, higher temperatures being 

 associated with greater, and more variable, re- 

 cruitment (Swartzman et al. 1983; Bailey and 

 Francis 1985). Judging from scales collected in 

 an anoxic basin, whiting was much more abun- 

 dant off Southern California in the 30 years 

 around 1900 than in recent years (Soutar and 

 Isaacs 1974). Such fluctuations in the stock of 

 whiting are therefore only a manifestation or 

 symptom of more general environmental varia- 

 tion in the California Current. 



Overall, our results indicate that a major en- 

 vironmental perturbation, such as El Niiio, acts 

 on the California Current's ecosystem as a whole 

 (though the mechanism of action may differ 

 geographically; Roesler and Chelton 1987) and 

 modifies the components we studied in similar 

 ways. The system seems to adjust to more local, 



specific modifications, such as anthropogenic 

 changes in biomass and age structure of one 

 predator, so that widespread effects on plank- 

 tonic prey populations are difficult to detect. 



ACKNOWLEDGMENTS 



We thank Elisabeth McEvoy and Michael 

 Lindsey for the tedious analyses of many zoo- 

 plankton samples, W. G. Pearcy for generously 

 supplying data, Kevin Bailey and Paul Smith for 

 advice, and two reviewers for comments. This 

 work is a result of research sponsored in part by 

 U.C.S.D. Academic Senate grants, and by 

 NOAA, National Sea Grant College Program, 

 Department of Commerce, under grant number 

 NA85AA-D-SG140, project number R/F-111, 

 through the California Sea Grant College Pro- 

 gram, and in part by the California State Re- 

 sources Agency. The U.S. Government is 

 authorized to reproduce and distribute for gov- 

 ernmental purposes. 



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