REPRODUCTION AND RECRUITMENT IN FRESHWATER FISH 293 



In the second and third periods a drought and some untoward deaths 

 during sampling precluded exact continuation of the experiment with the 

 same populations offish but growth, though rather slower, continued to be 

 negatively correlated with population density. The mortality was much less 

 than in the first period, but such as did occur tended to be heavier in the 

 denser populations. 



A combination of the data on mortality and growth for each period 

 allows the calculation of the production and its variation with the original 

 stocking density. The data for the first period, and calculated data for four 

 stocking densities for the whole period up to the beginning of September, 

 are shown in Table III and in Fig. 4. The calculated data for four stocking 

 densities are computed from graphs of mortality rates and growth rates 

 plotted against stocking densities for each of the three periods of the experi- 

 ment, using as a stocking density for each period the surviving density at the 

 end of the previous period. 



These data show considerable similarity to those obtained by Ricker & 

 Foerster for Cultus Lake salmon in the relationships between initial seeding 

 or stocking density and resultant production and numbers of survivors. 

 Although the two species are both salmonid fish, the sockeye fry feed on 

 plankton and are pelagic in a lake, but the trout fry feed on bottom and drift 

 fauna in a small stream, so their ecological niches are quite different. It does 

 seem then as if the observed effects of varying population density on the 

 production and yield might be typical of a wide range of kinds of young 

 fish populations. 



Inversely density dependent growth-rates are widely known for fish 

 populations but it is perhaps significant that they should be found to operate 

 so early in life. It has been postulated that natural population regulation in 

 fish occurs mainly in the adult phase through the effect of high population 

 density on adult growth rate and thus fecundity, but it appears that there 

 can also be intense intraspecific competition in the early stages with resultant 

 population regulation through density dependent mortahty as well as 

 inversely density dependent growth. 



DISCUSSION AND CONCLUSIONS 

 The examples of production estimates quoted above show that the whole 

 reproductive and recruit rearing phase of the life history can involve a large 

 part of the production offish. If the total production of gonad products and 

 the subsequent body growth up to maturity or recruitment is combined it 

 can then be compared both to the grand total production by a year-class 

 through the whole of its life, and to the biomass of such a year-class on 



