Reproduction 99 



largemouth begin spawning in late May or June in the north, but some- 

 what earher in the south. The water temperature seems to have an im- 

 portant eflFect in initiating spawning activity of all the centrarchids. Also, 

 sudden drops in water temperature associated with periods of cold spring 

 weather may stop all spawning activity and sometimes kill the embryos 

 alreadv in the nests. 



Bullheads and catfish also sweep out nests and ofiFer protection to 

 embryos and young fish. Channel catfish spawn during the period from 

 late May to mid-July in Missouri,^^ with a peak occurring in early June 

 and one in late June. Bullheads usually spawn during late May and early 

 June as far north as central Iowa and Illinois. 



Hybridization 



Hybridization within certain families of warm-water fishes is not un- 

 common. Hybrids have been observed in many groups of fishes including 

 the Cyprinidae (minnows) and Esocidae (pikes), and they are relatively 

 common in the Centrarchidae (sunfishes). The artificial production of 

 hybrids in the laboratory is not difiicult if one is able to obtain ripe or 

 nearly-ripe individuals of species capable of hybridization. In nature, hy- 

 brids may be produced accidentally, and they are much more common 

 among fresh-water fishes than among salt-water species. ^^ However, as 

 the stimulus of a ripe male appears to be necessary in some species to 

 induce the females to release eggs, the possibility seems remote that 

 sperms from an outside source might fertilize the eggs thus released. 

 There are recorded instances, however, of female carp releasing ripe eggs 

 without the stimulus of a male fish. No similar behavior has been reported 

 for members of the sunfish family, although it is difficult to see how ripe 

 females could retain all of their eggs, due to the pressure within the 

 fishes' abdomen. Probably most of the unspawned eggs are resorbed. 



Among closely related genera "mating behavior patterns" and "con- 

 sciousness of species" may fail to prevent a certain amount of promiscuity, 

 particularly in crowded populations where there is competition for 

 spawning ground space. Under these conditions, hybrids may be as 

 common as one in 10 of the young produced in a spawning season, al- 

 though the production of hybrids rarely exceeds 1 or 2 per cent. 



Some use has been made of hybridization in fish management, e.g., 

 hybridization of the northern pike, with the muskellunge, to produce the 

 "silver musky" ( which is highly regarded by some fishermen ) , or the use 

 of hvbrid sunfish to produce larger individuals with a low reproductive 

 potential. 



Several studies of natural and artificial hybrids ( Figure 5.3 ) of various 

 species of sunfishes have been made.^^^ 37, 48, 54, cs, is Sunfish hybrids have 

 been reported between species in the following hst: green sunfish, blue- 



