FISHERY BULLETIN: VOL. 78. NO. 1 



Figure 3. — Maximum equilibrium 

 yields (x 10* g) from Schaefer surplus 

 production curves as a function of 

 fishing mortality for three hypothetical 

 fish species demonstrating the range of 

 r and K selection. 



40 .00 



36.10 



32 .20 . 



2G .30 



U3 



X; 24.40 



X 



o 



■^ 20.50 



16 .60 



> 



12 .70 



B.BO 



4 .90 



1 .00 



° SPECIES « 

 ' SPECIES B 

 ° SPECIES C 



0.00 .10 .20 .30 .40 .50 .60 .70 .BO .90 1.00 



FISHING MDRTPLITY (F) 



DISCUSSION 



Life history parameters vary in consistent pat- 

 terns. These patterns are explainable and predict- 

 able by the theoretical constructs of r and K selec- 

 tion. This is not a particularly new or unique idea 

 in fisheries biology. Beverton and Holt (1959) in- 

 vestigated a positive relationship between body 

 size and life span and between mortality and 

 growth rates. Gushing (1971) suggested that there 

 is a negative relationship between degree of den- 

 sity dependent regulation and fecundity. Alverson 

 and Carney ( 1975) have suggested a positive rela- 

 tionship between body size and the time when a 

 cohort maximizes its biomass. In population ecol- 

 ogy, similar relationships have been investigated 

 for zooplankton (Allan 1976), plants (Gadgil and 

 Solbrig 1972; MacNaughton 1975), and animals 

 (Smith 1954; Bonner 1965). All these empirical 

 observed trends in life history parameters, along 

 with the trends described here, are consistent with 

 r and K selection. 



It is important to reemphasize here the com- 

 parative nature of r andi^ selection. The r and K 

 continuum is a model and as such occurs only in an 

 idealized sense. The idealized r selected species 

 occurs in an ecological vacuum with no density 



effects and no competition. The idealized K 

 selected species occurs in a completely saturated 

 ecosystem where densities are high compared 

 with carrying capacities and competition for re- 

 sources is intense. The problem of applying this 

 model to any real situation is not a trivial one. 

 Species are not simply subjected to a single selec- 

 tive pressure, or even to a single set of selective 

 pressures. Because of this, r and /C concepts should 

 only be applied in a comparative sense between 

 groups of species that have some degree of func- 

 tional similarity. No species is r selected or K 

 selected in an absolute sense; it is only relatively 

 more r selected or K selected than some other 

 reference species. This theory will only have value 

 in a situation where the population dynamics of 

 one member of a species group are fairly well un- 

 derstood. 



The results of the model analysis give several 

 indications about the reaction to harvesting pres- 

 sure of species which are more or less r or K 

 selected. Fisheries based on more r selected 

 species will be more productive. They can be fished 

 at younger ages and at higher levels of fishing 

 mortality. Given a minimum population size, 

 these fisheries should also have a quicker recovery 

 from overfishing. Species which are more r 



