FISHERY BULLETIN: VOL. 78, NO. 1 



the habitat the species occupies (Southwood et al. 

 1974; Southwood and Comins 1976). If mortality 

 factors in an environment are variable and/or un- 

 predictable, then their effects are likely to be less 

 selective in terms of population size or of the 

 phenotype involved. Under these circumstances, 

 individual competitive fitness is of relatively less 

 importance. The best strategy would be to place 

 maximal resources into reproduction and produce 

 as many offspring as possible (r selection). 



The contrasting situation is an environment in 

 which mortality factors are stable and/or predict- 

 able. Mortality under these circumstances will re- 

 sult in strong selection for individual fitness and 

 there will be pronounced differences between their 

 effects on different phenotypes. In these stable 

 environments, the optimal strategy would be to 

 produce offspring with substantial competitive 

 ability {K selection). Due to the previously as- 

 sumed relationship between fitness per offspring 

 and the number of offspring produced, this also 

 means the production of fewer offspring. 



The two situations described above are end 

 points of a spectrum. Species will always have a 

 number of different selective pressures operating 

 on them, both spatially and temporally. This is 

 particularly evident in aquatic organisms which 

 characteristically go through several life history 

 stages. This again emphasizes that the concept of 

 r and K selection should be applied only in a 

 comparative sense. Finally, comparisons must be 

 made between species of a similar ecological na- 

 ture. Comparisons between species of different 

 ecological types is meaningless since fundamen- 

 tally different types of selective factors will be 

 operating in those cases. 



r AND K SELECTION IN 

 MARINE FISHES 



Natural selection will favor nonreproductive ac- 

 tivities at the expense of reproductive activities 

 only when they enhance reproduction at later 

 stages in the life history and thereby maximize 

 overall survival (Crow and Kimura 1970). 

 Changes in allocation of a species' resources from 

 reproductive to competitive activities will only 

 occur in habitats where competitive activities en- 

 hance the survival of future offspring. The result 

 of this is that organisms under different selection 

 pressures will have characteristic life history pat- 

 terns. An r selected species will have life history 

 strategies which tend toward productivity. Thei^ 



selected species will have life strategies which 

 tend toward efficient exploitation of a specific 

 limiting resource (Pianka 1974). Therefore, 

 specific combinations of population parameters 

 can be identified as being characteristic of an r 

 strategist, while the opposing combination would 

 be characteristic of aK strategist. 



A species which is exposed to a large component 

 of nonselective or catastrophic mortality (i.e., an r 

 strategist) would be selected for characteristics 

 that would increase productivity. Increasing pro- 

 ductivity through reproductive activity generally 

 implies: 1) early maturity, 2) rapid growth rates, 

 3) production of larger numbers of offspring at a 

 given parental size, and 4) maximum production 

 of offspring at early age (Gadgil and Bossert 1970). 

 Other characteristics which are results of the allo- 

 cation of large portions of resources to reproduc- 

 tive activity are: 1) small body size, 2) high rates of 

 mortality, and 3) shorter life span (Pianka 1974; 

 Gadgil and Solbrig 1972). In terms of commonly 

 measured population parameters in fishery biol- 

 ogy, an r selected species would have: 1) a low age 

 at first maturity, 2) a high value ofk from the von 

 Bertalanffy growth equation, 3) a small Lx from 

 the von Bertalanffy growth equation, 4) high rates 

 of instantaneous natural mortality (M), and 5) low 

 maximum age. 



Even in environments with predictable mortal- 

 ity sources, increased allocation of resources to 

 competitive activities will only occur when two 

 prerequisites are met (Schaffer and Gadgil 1975). 

 The first is that reproductive potential increases 

 with some function of age. The second is that there 

 is some additional mortality risk associated with 

 reproduction. Under these assumptions, the attri- 

 butes associated with aK strategist would be: 1) 

 delayed maturity, 2) reduced growth rates, 3) low 

 mortality rates, 4) large body size, and 5) longer 

 life span. Again in terms measured in fishery biol- 

 ogy, aK selected species would have: 1) a high age 

 at first maturity, 2) a low k from the von Ber- 

 talanffy growth equation, 3) a large L-x from the 

 von Bertalanffy growth equation, 4) lowM, and 5) 

 a high maximum age. 



Using these life history correlates of r and K 

 selection (summarized in Table 1), it is possible to 

 predict the signs of a correlation matrix between 

 life history parameters (Table 2). The predicted 

 matrix can be compared with actual matrices cal- 

 culated using Spearman's rank correlation 

 coefficient. This coefficient only assumes that the 

 observed data are mutually independent and come 



