SILLIMAN; EXPERIMENTAL EXPLOITATION OF FISH POPULATIONS 



Because of the great variability in the recruit- 

 ment data, group means were used for both 

 species. The basis of the grouping was an interval 

 of 5 g (0.0-4.9, 5.0-9.9, etc.) in the adult tank stock 

 weight. Data used in calculating recruitment 

 cur\'es were pairs consisting of the mean adult 

 stock weight and mean recruit weight for each 

 group. 



The stock-recruitment data for the guppy 

 (Figure 6) could be fitted by a Ricker (1958) curve 

 of the type: 



where Rj^+i is recruitmen|: during brood interval 

 N + 1 and 5^ is mean adult stock during brood 

 interval N, both in grams. Fitting of the curves 

 shown (Figure 6) was by least squares to the rec- 

 tilinear logarithmic form of the relation: 



log,^jv+i/ S^ = \og,a - hS^. 



Values of the constants are given in Table 6. 



Data for the swordtail (Figure 7) did not con- 

 form well to the Ricker relation, as shown by the 

 parabolic nature of the points for the competing 

 stock, and were fitted better by a simple parabola: 



symbols as above. Curves shown (Figure 7) were 

 fitted directly to the grouped data by least squares. 

 Values of the constants are in Table 6. 



Comparison of results for the two species 

 (Figures 6, 7; Table 6) reveals both similarities and 

 differences. Despite the different types of 



Independent 



Table 6.-Stock-recruitment data comparing populations of the 

 two species_with data from the relations: guppy, /?^ ^ j = 



aSj^fCxp (-65^); swordtail, R^ 



= aSj^j - bS 



'In fitting curves, data were grouped by 5-g intervals of S,^. 

 2Guppy, 3 \Nk; swordtail, 4 wk. 



recruitment curves, there is the suggestion in both 

 species that maximum recruitment occurs at in- 

 termediate rather than very high or very low adult 

 stock levels, as has been observed for other species 

 (Ricker 1958; Silliman 1969b). Also, recruitment 

 for both species was depressed by competition. 

 However, the depression was greater for the 

 swordtail, as shown in the comparison of standard- 

 ized recruitment (last column. Table 6). This is in 

 keeping with the finding to be reported below that 

 the guppy is more productive than the swordtail. 

 This finding is also supported by the fact that 

 guppy recruitment was greater than swordtail 

 recruitment in both independent and competing 

 situations (last column. Table 6). 



Some additional information on recruitment 

 was obtained from a study of count discrepancies. 

 Because all additions to and removals from the 

 adult tanks were recorded, it was possible to cal- 

 culate an "expected" count (the previous count 



Swordtail A, a C 



Independent 



Figure 6. -Stock-recruitment relation for the guppy. Data 

 grouped by 5-g intervals of S^ . N is the number of the 3-wk 

 brood interval. 



Figure 7.-Stock-recruitment relation for the swordtail. Data 

 grouped by 5-g intervals of Sfj . N is the number of the 4-wk 

 brood interval. 



879 



