Reproduction 97 



External Sex Characteristics 



In many kinds of fresh-water fish, secondary sex characteristics develop 

 as the adult fishes approach the spawning season. These characteristics 

 make it possible to separate the sexes, either through differences in 

 coloration (Figure 5.2), or through structural differences (such as tuber- 

 cules on the heads of some male cyprinids or a complete change in body 

 appearance in some male channel catfish and salmonids). In many 

 species, male secondary sex characteristics are so definite as to make 



Figure 5.2. Sexual dimorphism in hybrid crappies. 

 During the spawning season the male crappie (lower) 

 becomes very dark, particularly on the head, ventral 

 side, and fins. 



accurate sexing of males easy; in others only long experience can develop 

 proficiency in sex determination. An ability to separate the sexes may be 

 useful and important when stocking new or renovated waters because it 

 allows one to set up balanced or unbalanced sex ratios of selected kinds of 

 fish; also, one may be relatively certain of the sex of individuals released 

 in ponds for the production of hybrids between two closely related species. 



Spawning 



Nearly all of the warm-water fish begin spawning during the first 6 

 months of the year; some have short spawning periods lasting only a few 

 days; others may spawn for 1 or 2 weeks, while still others may spawn 

 intermittently over a period as long as 3 or 4 months. 



