EXPLOITATION EFFECTS UPON INTERSPECIFIC RELATIONSHIPS 



IN MARINE ECOSYSTEMS' 



Saul B. Saila and James D. Parrish" 



ABSTRACT 



Due to man's continuing efforts to extract greater harvests of marine organisms from 

 the world ocean, it is becoming increasingly important to be able to predict the conse- 

 quences of exploitation on complex assemblages of organisms. These assemblages, or 

 ecosystems, consist of predator and prey organisms in various interacting combinations. 

 Preliminary evidence available from studies of marine invertebrate communities in coastal 

 areas has indicated that removal of grazing herbivores or predators at various levels 

 results in lower species diversity and greater instability of the ecosystem. In order to 

 permit a quantitative evaluation of the effects of various rates and types of exploitation 

 on interspecific relationships, model ecosystems were constructed utilizing a subset of 

 graph theory as applied to network analysis. A basic ecological trophic unit was for- 

 mulated, and these units were combined to form more complex model ecosystems. In par- 

 ticular, a hypothetical four species system of interacting predator and prey organisms 

 was analyzed to demonstrate the consequences of varying certain model coefficients, espe- 

 cially rates of exploitation. It was shown that nonselective exploitation tended to main- 

 tain stability of the system better than highly selective exploitation. A hypothetical 

 example of an empirical approach for examining changes in community structure was 

 also demonstrated. 



Much of the present theory of fisheries science 

 as well as many practical fisheries management 

 techniques are based on the concept of a single 

 species or unit stock (Beverton and Holt, 1957; 

 Ricker, 1958). This approach continues to be 

 useful in describing and predicting the behavior 

 of fisheries consisting primarily of a single spe- 

 cies. Recently, Walters (1969) developed a de- 

 terministic computer simulation model for deter- 

 mining optimum harvest strategies based on a 

 unit stock. However, modern fishing seems to 

 be progressing toward exploitation on many spe- 

 cies of the larger animals in aquatic ecosystems. 

 To some extent this is due to the tendency to- 

 ward reduction to meal of many species of fishes. 

 Some obvious areas for the future development 



' Part of this work is a result of research sponsored 

 by U.S. Department of Commerce, National Oceanic and 

 Atmospheric Administration, Office of Sea Grant, under 

 Grant #2-35190 and a National Institutes of Health Fel- 

 lowship, 5 FOl GM48175-02, General Medical Sciences 

 to JDP. 



^ Graduate School of Oceanography, University of 

 Rhode Island, Kingston, RI 02881. 



Manuscript accepted January 1972. 



FISHERY BULLETIN: VOL. 70, NO. 2, 1972 



of marine fisheries will be in tropical or sub- 

 tropical waters, such as parts of the Indian 

 Ocean as well as the southeast and southwest 

 Atlantic. These areas are characterized by a 

 relatively greater diversity of species than the 

 traditional fishing grounds of north-temperate 

 regions. It can reasonably be expected that 

 some species will be exploited intensively from 

 them. It therefore seems desirable to attempt 

 to better understand trophic relations of ecosys- 

 tems consisting of several interacting species in 

 order to develop management techniques for 

 these systems. This increased understanding 

 includes the eff"ects of various kinds of exploi- 

 tation strategies as well as environmental per- 

 turbations on these ecosystems. It is suggested 

 that stresses applied to ecosystems may produce 

 somewhat similar eff'ects, regardless of their 

 origin. 



Some of the large volume of literature con- 

 cerning diversity and stability in terrestrial as 

 well as aquatic ecosystems has been reviewed at 

 a recent symposium (Woodwell and Smith, 



383 



