92 Reproduction, Competition, and Predation 

 REPRODUCTION 



Most of the vertebrates that prey upon fishes are more recent (geo- 

 logically speaking) than their victims. Thus, it is conceivable that as pred- 

 ators of fishes e^'olved and accumulated, the reproductive potential of 

 fishes expanded to compensate for greater and greater losses through 

 predation, until the reproductive potential became very high. Less than 

 100 years ago in the United States man began to dominate other verte- 

 brates, and he purposely or inadvertently upset the normal relationships 

 between fish predators and their prey. The high reproductive potential 

 of a fish without an accompanying high predation rate became a detri- 

 ment to the well-being of a fish species in that too many individuals 

 survived for the available food supply. Tliis high reproductive potential 

 of fishes has remained unchanged, and its significance must be appreciated 

 in any plan for the management of a species. 



Reproductive Potential of Fishes 



Common warm-water fishes produce numbers of eggs in an inverse rela- 

 tionship to the amount of protection that they give the sex products after 

 they are released. In species such as the European carp which offer no 

 protection to its sex products, a single female may produce several 

 hundred thousand eggs. In contrast, the stickleback, which builds a com- 

 plicated nest of plant material and then actively guards it, may lay a few 

 hundred eggs. Between these extremes are the sunfishes depositing their 

 eggs in depressions which they have made in the river or lake bottom and 

 then attempting to guard against the predatory activities of their own 

 kind, and other kinds of aquatic predators. Here the number of eggs is 

 intermediate, ranging from 5000 to 10,000 in largemouth and smallmouth 

 basses to 20,000 to 50,000 in the crappies and larger sunfishes. Jenkins "^^ 

 cited an example in which 50 adult crappies with a reproduction potential 

 of 590,000 produced a population of one-year-old fish of 200,500. 



Actual counts of eggs produced by various kinds of warm-water fishes 

 have demonstrated that the reproductive potential of every species is 

 more than adequate to replace the losses of the adults that produced them. 

 For example, one pair of bluegills may easily produce 50,000 to 75,000 

 fertile eggs in a life time. The survival of only two of these embryos, 

 through development, hatching, and growth to sexual maturity, is neces- 

 sary to replace the loss of the parents. 



In order to investigate the success of natural reproduction of some 

 common nest-building fishes. Carbine ^^ siphoned off the fry from the 

 nests of largemouth bass, rock bass, common sunfish, and bluegills in 

 Deep Lake, Oakland County, Michigan, and made counts of the number 



