EXPERIMENTAL EXPLOITATION OF COMPETING FISH POPULATIONS 



Ralph P. Silliman' 



ABSTRACT 



Populations of the guppy, Poecilia reticulata, and the swordtail, Xiphophorus maculatus x X. helleri, 

 were grown both independently and in competition under controlled conditions. Independent popula- 

 tions were permitted to grow for about a year and then successively exploited at two different rates for 

 each species. In the control (unfished) pair of competing populations, both species grew for about 30 

 weeks, followed by decline and extinction of the swordtail and fluctuations in the guppy. Similar initial 

 growth in the test pair was followed by exploitation of both species at various combinations of rates. 



Measures of recruitment were available as weights of juveniles returned to adult tanks from separate 

 nursery tanks. Data from fitted curves showed that guppy recruitment exceeded that of the swordtail 

 under both independent and competing conditions. Depression of recruitment by competition was 

 greater in the swordtail than in the guppy. 



A mathematical model for competing populations consisted of a pair of differential equations 

 including elements of the Volterra competition formulae and the Fox exponential surplus-yield model. 

 By using the exploitation rates applied in the experiments, and constants from the independent 

 populations, the model was applied to biomass data from the control pair of competing populations. 

 Successive trials resulted in a reasonably good fit, and competition coefl^cients from this were used to fit 

 data from the exploited test pair. Yield isopleths calculated from the fitted model showed that maximal 

 yields were obtained when exploitation for the swordtail was lower than for the guppy, suggesting 

 lower productivity in the swordtail. The maximum sustainable yield represented about 20% of food 

 placed in tanks, and indicated at least as great efficiency from competing populations as from 

 independent ones. 



Results from the experiments clearly suggest that exploiting both members of a competing pair is 

 preferable to exploiting either alone, provided fishing rates are adjusted in relation to the productivity 

 of each species. 



Classical studies of fishery dynamics, such as those 

 discussed in the works of Beverton and Holt (1957) 

 and Ricker (1958), deal mostly with single popula- 

 tions treated as if they existed independently. 

 Fishery biologists have come to recognize, 

 however, that in many situations the fish stock 

 cannot be so treated (Larkin 1963; Murphy 1973). 

 The exploited population of interest is inter- 

 dependent with others (which may be either 

 exploited or unexploited) through competitive or 

 predator-prey relations. Any effect of exploitation 

 on one stock may produce a reaction in another, 

 resulting in readjustments in both populations, 

 and invalidating the expected response to exploi- 

 tation based on single-species dynamics. 



A familiar example of an apparent competitive 

 situation is contained in the population histories of 

 the Pacific sardine, Sardinops sagax, and the 

 northern anchovy, Engraulis mordax, off the 

 coast of California. The sardine suffered a cata- 



'Northwest Fisheries Center, National Marine Fisheries Ser- 

 vice, NOAA, Seattle, Wash.; present address: 4135 Baker NW, 

 SeatUe, WA 98107. 



strophic decline in the mid-1940's, followed by an 

 increase in the anchovy. An analog computer 

 model of Silliman (1969a, b) demonstrated that at 

 least part of the change in the anchovy population 

 size could be simulated with data on the sardine 

 population size and the differential equations of 

 Volterra (1928). Murphy (1973) provided recent 

 verification of the sardine-anchovy relation and 

 suggested that similar relations may prevail in the 

 Japanese and South African sardines. 



Laboratory experiments on the exploitation of 

 self-sustaining fish populations have been report- 

 ed fairly extensively (Silliman and Outsell 1958; 

 Silliman 1968; Nagoshi et al. 1972). Experiments 

 with competing populations have included such 

 diverse organisms as yeast cells (Cause 1932); 

 Protozoa (Cause 1934); Daphnia (Frank 1957); 

 beetles (Park 1962); and warblers (MacArthur 

 1958). To the best of my knowledge, however, 

 exploitation of competing laboratory fish popula- 

 tions has not previously been reported. 



The purpose of the experiments reported below 

 was to ascertain experimentally the reaction of 

 two competing fish populations to exploitation. 



Manuscript accepted January 1975. 

 FISHERY BULLETIN: VOL. 73, NO. 4, 1975. 



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