havior was ever observed in the mixed populations in the 
indoor pools. A high degree of compatibility among the 
outdoor populations was indicated by the fact that (1) shin- 
er production was greater in mixed populations than when 
shiners were alone and (2) the presence of tilapia and/or 
shiners did not limit catfish production. 
Direct observations of the species when mixed in the out- 
door pools were limited by the tendency of the catfish to 
hide when the water was clear enough for good visibility, 
and by phytoplankton blooms. All species normally fed at 
the surface, but catfish maintained alone were sometimes 
slow to rise to the food. When mixed with tilapia, however, 
they commonly rose immediately and competed vigorously 
with the tilapia for the food. Coexisting shiners stimulated 
the same response but to a lesser degree. All three species 
were stimulated to a faster and more vigorous feeding re- 
sponse when associated than when alone and able to feed at 
their leisure, and such stimulation led to a more efficient 
use of food. 
EXPERIMENTS IN 1969* 
First Series 
Methods.—Our first series in 1969 involved 21 pools 
having three replications of each of seven types of popula- 
tions. The catfish were stocked on May 29 and the shiners 
on June 4, 1969. We used two levels of densities which 
* All 1969 experiments were conducted in outdoor pools. 
doubled those used in 1968. The maximum populations of 
32 catfish and 180 shiners were equivalent to 17,760 catfish 
and 99,900 shiners per acre. The two species were again 
separated in some pools and mixed in others, as illustrated 
in Table 4. 
Mean weights of fish in the original stocks were about 
2.73 grams (173 per pound) for shiners and about 13.0 
grams (35 per pound) for catfish. Final censuses were made 
on September 10, 1969, providing growth periods of 104 
days for catfish and 98 days for the shiners. Tilapia were 
not used in 1969 because they were not essential to the stu- 
dy. Again, the catfish received pellets and the shiners re- 
ceived meal. All fish were fed twice a day for either 5 or ¢ 
days each week at a total daily ration approximating ° 
percent of their weight. The populations were sampled by 
seine, and fish weights were measured every 14 days. Al 
pools received nearly continuous aeration. 
Results.—Table 4 presents the average final standin; 
crops in replicated pools for the two species, both individ 
ually and combined, along with seasonal ranges and mean 
of various environmental parameters. Data for some popu 
lations were unusable because of excessive mortalities, du 
principally to failures in aeration, but usable data wer 
obtained from at least two replications of each of the seve 
experimental populations. We had believed that in dov 
bling the densities we would assure that carrying capacitic 
would be attained prior to our termination of the exper 
ments in September, but an inspection of the growth curv 
in Fig. 3 shows that rates of growth had not declined t 
Taste 4.—Means of final standing crops, in pounds per acre, for various mixed and unmixed populations of channel catfish (CC) an 
golden shiners (GS) when maintained outdoors for about 100 days in 10-foot diameter plastic pools, together with ranges and means } 
pH and parts per million of turbidity, ammonia, and free carbon dioxi 
de based on determinations made twice weekly over the same per 
od. 
Range (in parens) and Mean 
Type of Usable i : Turbid- 
Popula- Replica- Final Weight (Ibs/A) ity NH CO: 
tion 4 tions > CC GS Both pH (ppm) (ppm) (ppm) 
c 
CF 2 1,629 1,344 2,973 (7.20-9.00) (48-340) (0-1.90) (0-17.62) 
7.89 151 0.728 3.44 
d 
2 1,918 1,136 3,054 (7.49-9.01) (18-410) (0.17-1.31) (0-12.86) 
Wee, TA 0.660 2.67 
16 | 16 
4 | + 2 2,154 1,022 3,176 (7.31-8.80) (34-600) (0.30-1.97) (0-12.38) 
20 | 90 TATE 207 0.671 3.61 
16 
+ 3 1,049 709 1,758 (7.42-8.30) (2-57) (0.05-.85) (0-13.33 
20 7.78 29 0.413 er i 
2 1,238 687 1,925 (7(37-8.5) (17-150) (0.10-1.09) (0-9.52) 
7.84 89 0.523 PL iui 
(2) 2 2141 (7.40-8.84) (8-211) (0.13-1.26) (0-9.52) 
7.81 75 0.588 2.63 
3 = 1,260 (7.35-8.88) (14-260) (0.04-1.47) (0-11.43 
7.85 105 0.649 1.78 
@ Populations contain shiners in lots of 90 or 180 and catfish in lots of 16 or 32. 
bIndicates whether usable data were available from two or three replicates. 
© Designates pool containing 32 catfish and 180 shiners intermixed. 
d Designates pool containing 32 catfish separated from 180 shiners by a net. 
8 
