ponds. This incompatibility may be of a type that is 
expressed in some environments but not in others. 
The RB Fy hybrids were all males; so it was im- 
possible for these hybrids to produce an F> generation. 
The GR Fy] hybrids, with an approximate 50:50 sex 
ratio, produced a very small Fz generation in a gravel 
pit pond, fig. 4. Field observations revealed that their 
reproductive behavior was slightly abnormal. Males 
constructed nests and were extremely aggressive in 
guarding their territories. On two separate occasions 
a female was observed to approach a nest guarded by 
NBS PRL 
afi ah 
Ay A : 
Fig. 4. - 
8 
a male. The male faced the female and challenged her 
by flaring his opercular flaps. The female, which was 
greatly enlarged with eggs, swam slowly toward the 
male. The male left the nest and appeared to lead the 
female into it. The two fish started slowly swimming 
in small circles over the nest, with the female closer 
to the center. The male maintained a position slightly 
in advance of the female and, by constantly turning 
toward her, appeared to force her to swim within the 
limits of the nest. Before the spawning pair had time 
to make more than a few complete circles, nearby males 
\ 0.l-acre gravel pit pond that contained GR F] hybrids. 
