UNSTRUCTURED MARINE FOOD WEBS AND 

 "POLLUTANT ANALOGUES" 



John D. Isaacs' 



ABSTRACT 



The several species of fish living in the Gulf of California have been shown to possess 

 quite different concentrations of cesium (and cesium in respect to potassium) than the 

 same species of fish living in the Salton Sea. The Salton Sea fish display simple trophic 

 steps of concentration, whereas those in the Gulf all show about the same levels. These 

 differences are reasonably well explained by simplified trophic models of the two envi- 

 ronments. The concentration factor found in the known and describable food chain of 

 the Salton Sea, applied to a model of an assumed unstructured food web in the Gulf, leads 

 to reasonable results. This suggests that study of the concentrations in marine organisms 

 of such natural trace substances as cesium may lead to an understanding of the trophic 

 position of the organisms, and hence constitute "pollutant analogues" that may yield a 

 better understanding of the existing or potential distribution of pollutants in marine 

 organisms. 



Young (1970) found that the cesium content of 

 the muscle of the several species of fish in the 

 Salton Sea (California) increased by about a 

 factor of 2 (average 2.15) in each of the suc- 

 cessive steps in the simple linear food chain ex- 

 isting in that isolated marine environment and 

 that the cesium/potassium (Cs/K) ratio simi- 

 larly increased by about a factor of 3 (average 

 3.1). Results both for normal and radioactive 

 cesium were in agreement. Young further dem- 

 onstrated that the entry of the cesium into the 

 food chain was via the phytoplankton, only. 

 However, Young found that the same several 

 species of fish in the Gulf of California displayed 

 no such successive steps of increase, but rather, 

 that the muscle tissue of all contained about the 

 same concentration of cesium (with the suppos- 

 edly more primary feeding mullets surprisingly 

 having a somewhat greater than average con- 

 centration of cesium) . Young's results are sum- 

 marized in Table 1, where it can be seen that 

 the Cs/K ratio is elevated by about a factor of 



^ University of California, San Diego, Scripps Institu- 

 tion of Oceanography, La JoUa, CA 92037. 



16 between the primary food and the fish in the 

 Gulf of California. 



Young suggested that these differences might 

 be the result of a complex food web in the Gulf 

 of California, and reported some evidence for 

 this in the stomach contents of these Gulf fish. 



DEVELOPMENT OF AN 



UNSTRUCTURED FOOD WEB 



MATRIX AND EQUATIONS 



The following is an attempt to develop a highly 

 simplified model of a generalized food web, to 

 discuss some of its implications, and to test its 

 validity by using real data. 



Let us first assume that in a food web each 

 transfer of organic material (or energy), or of 

 individual elements such as cesium, from one 

 component of the web to another can be char- 

 acterized by the following quantities: 



Ki = a. coefficient of conversion of matter (or 

 energy) in food into living tissue 



7^2 = a coefficient of conversion of matter (or 

 energy) in food into irretrievable 

 form (e.g., by respiratory combus- 

 tion or mineralization) 



Manuscript accepted May 1972. 



FISHERY BULLETIN: VOL. 70, NO. 3, 1972. 



1053 



