120 Reproduction, Competition, and Predation 



Experiments designed to expose the causes o£ reduced growth and 

 reproduction in crowded fishes have furnished evidence that where ade- 

 quate food was available, inhibited growth and reproduction was due to 

 ammonia and other material excreted into the water by the fishes them- 

 selves."^^' ^'^' ^^' ^- In most of the experiments in which conclusive results 

 were obtained, the fish were tropical aquarium fish, goldfish, European 

 carp, or one of the species of buffalo— fish that have a reputation for 

 intermittent production of year classes. 



Goldfish stocked in small ponds at the beginning of the growing season 

 at the rate of 200 4-ounce fish per acre produced large numbers of young, 

 while those stocked at the rate of 2000 or more of 4 ounces or larger 

 produced few or no young. "^^ Originally, it was thought that the goldfish 

 ate their own eggs and young in the ponds stocked with the larger num- 

 bers of adults; however, examination of adult females showed that eggs 

 were well formed but never laid. Later, it was discovered that each time 

 during the summer that adult goldfish were moved from their regular 

 ponds into new ponds freshly filled with water, that the fish spawned 

 within 48 hours. 



On the basis of these and other experiments. Swingle '^^ postulated the 

 presence of a repressive factor composed of a secretion or excretion pro- 

 duced by the goldfish themselves that inhibited final development and 

 deposition of eggs, although these eggs were already practically mature. 



Additional experiments showed that the inhibitory material was ex- 

 creted into the water by the goldfish and that when this water was 

 moved into new ponds it retained its ability to inhibit reproduction, even 

 when it was diluted 2:1 with fresh water. Similar tests using carp and 

 bigmouth buffalo gave results comparable to those from goldfish experi- 

 ments. 



Swingle believed that largemouth bass were affected by an inhibitory 

 factor. Certainly there is evidence that the production of young in this 

 species is never directly related to number of spawners; usually there 

 appears to be an inverse relationship between bass fry and number of 

 adults available for spawning.^^ 



It was also assumed that overcrowded bluegills depressed the produc- 

 tion of largemouth bass through the secretion of a repressive factor. If 

 this were so, why were bluegills able to build up overcrowded populations 

 and depress bass reproduction without curtailing their own reproduction? 

 More experimental work must be done before exact evidence is available 

 to prove or disprove this hypothesis, particularly when crowded bluegills 

 have been observed repeatedly to feed upon bass eggs and fry. 



Yashouv ^^ placed two small carp in each of a number of aquaria con- 

 taining 26-27 liters of water. These fish were fed 10 per cent of their 



